The invention relates to transport engineering and concerns an air propulsion device made in the form of a propeller with restrictions on its diameter, and a method for increasing the thrust force and efficiency of the propeller. The method consists in choosing by calculation the optimal number of monoplane basic propeller blades that provide the maximum coefficient of performance (COP) and the tractive force corresponding to this efficiency. Determine the difference between the required and calculated traction forces. The resulting difference is compensated by thin-walled polyplanar working surfaces attached to the base blades mainly on the side facing in the flight (rise) direction, provided that the peripheral speeds of the ends of the base propeller blades and polyplanar working surfaces do not exceed: D n max ≤6000, where D is limited (given) diameter of the swept area, m; and n max - maximum propeller speed, rpm. The propeller for implementing the method contains wide base blades and symmetrical transverse profiles of the blades. Polyplane working surfaces are made, for example, in the form of a grid with a set of flat mutually perpendicular plates and mounted on the base blades, starting from the end of the blade. EFFECT: increase in thrust force and increase in propeller efficiency. 2 n. and 10 z.p. f-ly, 1 tab., 6 ill.
The invention relates to transport engineering and relates to an air propeller made in the form of a propeller.
The state of the art is defined by virtually monoplane propellers with a limited number of large diameter blades. The way to increase the propeller thrust force is associated with an increase in the number of propeller blades, and an increase in efficiency is achieved by reducing the resistance of additional aerodynamic bearing (working) surfaces in the air in the subsonic speed range of the end elements of the propeller blades.
An analogue of the implementation of this method are the so-called tandem screws and lattice wings. As you know, the main disadvantage of these wings is a lower lift-to-drag ratio than monoplane ones at subsonic speeds. The bearing, stabilizing and steering lattice surfaces are of the greatest practical importance.
Description of the prototype and its shortcomings
The closest analogues include multi-blade propellers of a significant diameter of aircraft and rotorcraft bearing and lifting devices of helicopters, which are distinguished by exceptionally large diameters and low transverse rigidity. With design restrictions on the diameter of the propeller and the associated decrease in the swept area, the propeller traction force (lift force) is reduced, which is the main disadvantage of small-diameter monoplane propellers. The disadvantage of multi-bladed propellers is the decrease in propeller efficiency associated with an increase in the aerodynamic drag of such propellers in the axial direction. A decrease in propeller efficiency leads to an increase in fuel consumption, a decrease in flight range, an aircraft carrying capacity, etc. The disadvantage of rotary-wing structures is heterogeneous longitudinal and transverse vibrations, which reduce the safety of helicopter operation, especially in rough terrain and in mountainous conditions (the phenomenon of "ground resonance", etc.).
The technical problem to be solved by this method is to increase the thrust force of the propeller while reducing the length of its base blades and increase its efficiency with the corresponding fuel economy. An increase in thrust and an increase in efficiency is achieved due to additional polyplanar bearing surfaces attached to the bearing surfaces of the base blades and ensuring the achievement of the required and, above all, economical modes of movement of the aircraft, for example, the cruising speed of an aircraft or the rate of climb of a helicopter. This takes into account that the power absorbed by the screw is proportional to the third power of the number of revolutions of the screw and the fifth power of the screw diameter. At constant values of the number of revolutions, the diameter of the propeller and the angle of attack of the blades, the absorbed power increases approximately in proportion to the ratio of the developed surface of the base propeller blades to the size of the area swept by it. As a result, the power absorbed by the propeller is approximately proportional to the number of base propeller blades, unless additional polyplane devices increase the developed surface of the base propeller blades. At the same time, the problem of limiting the propeller diameter arises on seaplanes and some machines belonging to non-traditional aircraft designs, for example, to air vehicles, i.e. to vehicles converted into an aircraft according to RF patent No. 2169085.
The essence of the method for increasing the thrust force and efficiency of a multi-blade propeller with a limited (given) diameter of the swept area of the propeller is that the calculation method selects the optimal number of monoplane basic propeller blades that provide maximum efficiency and the traction force corresponding to this efficiency at a given subsonic, for example, cruising speed flight of an aircraft, determine the discrepancy between the required and calculated thrust forces, compensate for the resulting discrepancy with thin-walled polyplanar bearing (working) surfaces attached to the base blades mainly on the side facing in the direction of flight (ascent) provided that the peripheral speeds of sound do not exceed the base speeds of the base propeller blades and polyplanar load-bearing surfaces:
D·n max ≤6000, where D is the limited (given) diameter of the swept area in m, and n max is the maximum rotation speed of the propeller, rpm. These essential features of the method are complemented by the organization of the interaction of the propeller with the power plant (engine).
In order to minimize the fuel consumption of the power plant, the propeller rotation speed, which ensures the cruising mode of the flight of the aircraft, is selected corresponding to the number of revolutions of the power plant with a minimum specific fuel consumption, with a tolerance of minus 5-10%.
In order to expand the speed modes of the aircraft, the propeller rotation speed corresponding to the cruising mode of the flight of the aircraft is selected less than the number of revolutions of the power plant at the minimum specific fuel consumption, provided that the corresponding specific fuel consumption of the power plant will not exceed the minimum specific fuel consumption more than, than 5-10%.
In order to expand the speed modes of the aircraft, the propeller rotation speed corresponding to the maximum speed mode is selected to be greater than the number of revolutions of the power plant at the minimum specific fuel consumption, provided that the corresponding specific fuel consumption of the power plant will not exceed the minimum specific fuel consumption by more than by 5-10%.
At the same time, in the cruising mode of flight and at higher speeds limited by the maximum number of propeller revolutions according to the above hyperbolic dependence on the propeller diameter D, there are close to optimal ratios of the geometric parameters of polyplanar bearing surfaces:
b pp ≤b lv, where b pp and b lv are the chords of the plans and the base propeller blade, respectively; Where 
c - plan thickness; 
Where 
t - step plans; λ pp =l/b pp, where l is the length of the plans, l=(0.2-0.3) D; ν=6°-10°, where ν is the sharpening of the edges of the plans; m≤8, where m is the number of plans in one cage at m≤4, for m=5-8 the polyplane device can be placed on both sides of the base blade; H=(m+1)·t, where H is the height of the polyplane device.
The aerodynamic loads on the plans, the increase in thrust and efficiency depend on these parameters.
The given parameters are shown in Fig.1, 2, 3, 5, 6. Therefore, the total number of plans for this method 
where k is the number of basic propeller blades.
At the same time, the essence of this method, expressed by the relationship of the kinematic characteristics of the propeller and the power absorbed by it with the energy characteristics of the power unit (engine), is represented, under certain assumptions, by the following power balance:
N t \u003d N e -N in, where N t is the power of the propeller thrust at a given flight speed, Ne is the power of the power unit in the corresponding flight mode and Nv is the power absorbed by the propeller and expended on aerodynamic resistance to rotation of the propeller. In this case, the fuel consumption of the power unit is given through the specific fuel consumption g e (g/l.c. hour): G t =ge ·N e ·10 -3 , where G t is the hourly fuel consumption (kg/h). These relationships displays Fig.4.
Therefore, the ratio of N t to the sum of N t +N in makes it possible in principle to determine the efficiency of the method and device for its implementation, both by calculation and by experiment. An increase in the thrust force by several times based on the polyplanar design of the propeller is accompanied by an increase in its aerodynamic drag. However, the increase in the corresponding absorbed power lags behind the increase in the thrust force and N t, so the efficiency tends to increase at the indicated values of t, i.e. number of planes in a polyplane propeller device.
Thus, this method of increasing the thrust force and the efficiency of the propeller determines a higher level of technology in the area under consideration, the possibility of its industrial production and use in different operating conditions. This gives grounds for protecting the method as industrial property.
Connection of the essential features of the method with the solution of the technical problem
The reasons for limiting the diameter of the propeller may be the heterogeneous requirements of the production and operational use of the aircraft. In any case, taking into account the tactical and technical requirements and the associated propeller diameter lead, in the case of monoplane propeller blades, to a decrease in traction or pushing force. This makes it necessary to increase the number of propellers and, accordingly, the power plants on the aircraft to ensure the required flight speed and rate of climb. However, unforeseen technical inconsistencies may arise. For example, on seaplanes, the placement of propeller-driven units on the wings causes an increase in the diving moment from the thrust forces, which requires appropriate compensation for the oppositely acting moments of the stabilizer. Consequently, the presence of the mentioned contradictions entails additional problems of a technical and economic nature. In connection with the above, the use of polyplanar propeller blades can make it possible to most simply and rationally solve the main technical problem, namely, to eliminate the occurrence of technical contradictions. Among them, we note the following: along with the thrust force, the problem of efficiency usually arises, which in turn is associated with the consumption and supply of fuel on board the aircraft, limiting the carrying capacity or flight range, etc. Based on a systematic approach to designing and creating a competitive object of aircraft technology apparatus, the priority is the perfection of the necessary energy conversion processes with maximum efficiency, which for propellers depends on the number of its blades (k). With an increase in the number of blades from the minimum 2-3, the propeller efficiency increases and reaches a maximum at the number k=5-7, and with a further increase in k, the efficiency begins to decrease.
There is reason, first of all, to find the number k corresponding to the maximum efficiency, and then increase the thrust force of the propeller by increasing the number of thin-walled bearing (working) plates of polyplane surfaces on the base blades, without increasing the projection of the propeller on the swept area and without significantly increasing the frontal the resistance of the propeller and the aircraft, as well as without increasing the air resistance to the rotation of the propeller.
Thus, the first essential features of the subject matter of the invention are causally related to the solution of the technical problem of the method.
The most important essential features are the features of thin-walled polyplanar bearing surfaces, which ensure the minimization of their resistance and the energy absorbed by the propeller, i.e. reducing the resistance of the propeller and increasing its efficiency. The latter causal relationship is especially pronounced when comparing this method with the method of using tandem screws. The same purpose is the limitation (prevention) of supersonic peripheral speeds at the ends of the propeller blades, expressed by the above numerical condition, which limits the maximum number of propeller revolutions in hyperbolic dependence on the propeller diameter.
To clarify and confirm the above provisions, let us consider the results of design calculations of a propeller for a heavy air vehicle (mass G = 3 tons). Engine power according to the above-mentioned RF patent No. 2169085 N max ≤600 hp The propeller diameter (D) limit on this aircraft is due to the fact that it must not exceed the overall width of the air vehicle, otherwise the air vehicle will not be certified for road operating conditions by road safety requirements. For this reason, the diameter of the propeller must be limited (D≤2m).
Let's evaluate the calculated thrust force of the propeller (T p) and its efficiency (η) for the specified cruising speed v=200 km/h and propeller rotation speed n=2000 rpm for a number of values of the number of basic propeller blades k=4-8 .
| N (hp) | k | Tr (kg) | η (%) |
| 400 | 4 | 399,0 | 0,74 |
| 6 | 407,9 | 0,755 | |
| 8 | 405,9 | 0,752 | |
| 500 | 4 | 491,0 | 0,727 |
| 6 | 497,5 | 0,737 | |
| 8 | 489,9 | 0,726 | |
| 600 | 4 | 565,4 | 0,698 |
| 6 | 581,5 | 0,718 | |
| 8 | 579,0 | 0,715 |
The calculations were made by A.N. Kishalov.
The obtained data give grounds to choose the calculated number k=6 blades and thrust equal to T p =581.5 kg for a monoplane propeller. However, in order to maintain the cruising flight mode at the specified n, a thrust force equal to T t >T p is set according to the technical requirements. Therefore, the discrepancy ΔT=T t -T r ≈T r ≈0.5 T t.
To obtain a given T t, an additional area of polyplanar working surfaces is required, approximately equal to the area of the base blades. By distributing this area over the partial length of the monoplane blades, we obtain recommendations for further design calculations and for the manufacture of prototypes of a polyplane propeller device. At the same time, we note that there is a reserve for increasing the thrust force with an increase in the number of revolutions of the propeller up to n max, providing the corresponding maximum speed. Comparing the condition that the circumferential speeds at the ends of the propeller blades do not exceed the speed of sound, we can make sure that it is feasible: n max =6000·D -1 =3000 rpm.
Consequently, the designed aircraft has a margin for increasing the flight speed by increasing the number of revolutions of the propeller in excess of that which provides the cruising flight speed. From the data presented, it can also be seen that, if necessary, it is permissible to try to obtain a higher traction force at lower propeller speeds by increasing the number of polyplanar bearing surfaces.
Therefore, the use of this method also presents a potential opportunity to reduce the operating costs of fuel, when the specified number of revolutions of the screw, corresponding to the cruising speed n k, does not coincide within certain limits with the number of revolutions of the power plant, corresponding to the minimum specific fuel consumption.
Thus, this method expands the limits of the integrated increase in the efficiency of the "power plant - propeller", or in short "engine - propulsion", increases the tractive force T and efficiency, is of industrial importance and can be submitted for patent protection of industrial property.
We specify the cause-and-effect relationships of the features and technical results of the method.
The possibility of increasing the efficiency is due to the fact that the increase in thrust is achieved by using thin-walled polyplanar bearing surfaces with significantly lower aerodynamic resistance than the base propeller blades. Therefore, the increase in traction force is proportional to the increase in the number of polyplanar working surfaces, while the air resistance of the propeller increases in a much smaller proportion. This changes the balance of power and propeller efficiency in the desired direction according to the following expression:
η=1/,
N t is the power of the thrust force of a monoplane propeller;
N in - power absorbed by a monoplane propeller;
ΔT is the coefficient of increase in the thrust force of the propeller with a polyplanar device;
ΔC - coefficient of increase in the resistance of the propeller with a polyplanar device;
At the same time, the assumption is made that the known components of the generalized efficiency of a monoplane propeller retain their values when the propeller is equipped with a polyplane device.
Of course, it must be taken into account that the power absorbed by the screw is proportional to the third power of the number of revolutions of the screw, as noted above. For this physical reason, variations in the number of revolutions of the propeller have a significant effect on variations in the power absorbed by the propeller. So, within the limits of a change in the number of revolutions by 5-10%, the absorbed power changes by 16-33%. The use of this phenomenon makes it possible to obtain additional opportunities for increasing the efficiency and reducing the specific fuel consumption of the power plant, mainly for a propeller operating at a constant speed, corresponding, for example, to the minimum specific fuel consumption.
Thus, the presented method of increasing the thrust force and efficiency of the propeller, taking into account the characteristics of the speed characteristics of the power plant and the characteristics of fuel consumption, opens up additional opportunities for improving the efficiency of the engine-propulsion system.
To implement the presented method of increasing the thrust force and efficiency, a propeller device is required, mainly with wide base blades and a symmetrical transverse profile of the blades. In order to reduce the absorbed power and increase the efficiency of polyplanar bearing (working) surfaces, they are made, for example, in the form of a grid with a set of flat mutually perpendicular plates and are installed on a limited length of the base blades, starting from the end of the blade. Base blades are made with a variable thickness not exceeding from end to end (i.e. from butt to end) 10% of the blade chord length, the total thickness of additional plans does not exceed 1/3 of the base blade thickness, and the thickness of plan plates and the distance between them must refer to the chord b pp, as indicated above, i.e. 
a 
moreover, the front and rear edges of the additional plans are pointed.
At the same time, the level of technology increases due to the consideration of the circular motion of the polyplane device. The plates connecting the polyplanar bearing (working) surfaces are made cylindrical with the radii of the cylindrical surfaces equal to the distances of these plates from the center of rotation of the propeller, and the generatrices of the cylindrical surfaces are inclined from the normal to the plane of symmetry of the base blade by the average angle of attack of the base blades, which is set in the cruising flight mode , and the inclination of the generatrices of the cylindrical surfaces is performed in the direction of rotation of the screw (towards the rotation of the blade), as a result, these generators are parallel to the axis of rotation of the screw.
In order to ensure short runs during takeoff and landing of the aircraft, the plans of the polyplane device are set with their own angles of attack at the neutral position of the base blade.
In order to increase the efficiency of the polyplane device at all stages of flight from takeoff to landing, the plans are made profiled along the base blade at a distance of 30-50% of the chord from the leading edge of the plan with the bend of the plan plate at an angle γ, and with the possibility of increasing the bend angle in the direction towards the axis of rotation screw in accordance with the dependence of the angle on the length of the plan l p and the distance from the end of the base blade l i by the expression:
tgγ i =(l+l i /l n)tgγ o , where γ o is the initial bending angle of the plan formed with the plane of the asymmetric cross section of the base blade and determined at the end of the base blade (see Fig.5 and 6).
In order to increase the rigidity of the plans in the longitudinal and transverse directions and reduce the noise level, mainly for the asymmetric profile of the base blade, the plans are made curvilinearly corrugated, and the radii of curvature change along the base blade and are equal to the distances of the top of each individual corrugation to the center of rotation of the screw.
In order to ensure rigidity and strength, reduce vibrations and noise levels, the non-separable connection of the plans between themselves and the base blade is carried out mainly by high-temperature soldering, and with amorphization of the solder layers, for example, sharp deep cooling.
At the same time, in order to ensure maintainability and improve adaptability to various operating conditions, the propeller is equipped with interchangeable (modular) polyplane devices removable from the base blades, which differ in the degree of increase in thrust, for example, for flat and high-mountain conditions. This circumstance is important both for the method developed for patenting and for the device for its implementation.
Given the possibility of using this method and device in various climatic and weather conditions, in order to combat icing, the propeller is equipped with an anti-icing device.
Description of the device in statics.
Figures 1, 2, 5 and 6 show the structure of the base blade of a propeller with a polyplanar device, which serves to increase thrust and efficiency. Figure 1 shows the following structural elements of the device: 1 - base blade with an axis of rotation 2, 3 - plate plan, 4 - cover clip plans, 5 - connecting plates plans.
The base blade on these figures of the propeller is presented in a neutral position in the absence of an angle of attack (α=0). The polystan device installed on the base blade contains plan plates with additional working surfaces, which increase the total aerodynamic bearing surface with the base blade. The placement of the polyplane device, starting from the end of the base blade, increases its efficiency both in statics and in dynamics. In statics, a polyplane device reduces the mass of this device in comparison with a monoplane device of a larger diameter propeller. This advantage is especially evident on rotary-wing machines and, which is very important, due to the shortening of the blades, the safety of helicopters increases in operating conditions in rough terrain when flying at low altitude, maneuvering, etc.
At the same time, the polyplane device increases the longitudinal and transverse rigidity of the base blade, which makes it possible to reduce its thickness, and in dynamics it increases the traction force while minimizing the total mass of the polyplane device and the base blade. In fact, the polyplanar device, together with the base blade, forms a truss-type building structure, which accordingly sharply increases the natural oscillation frequencies of the blade, reduces the oscillation amplitudes and internal stresses in the construction materials. This increases the fatigue strength of the propeller.
The increase in the effective area of the polyplane device can be carried out both on one side (figure 1 and 5), and on both sides (figure 6) of the base blade of the propeller, starting from the end and ending at a length of 0.5-0.7 of the length of the base blades to reduce the stress concentration from the action of two-sided unidirectional loads.
A symmetrical profile with respect to the chord of the blades (FIGS. 5a and 6a) is preferable to an asymmetric one (FIGS. 5b and 6b).
According to the data, the manufacturing technology of lattice wings represents non-separable designs of polyplane devices, given their one-time use in most cases (for example, in rocket science). On the contrary, a reusable aircraft is characterized by long-term reusable use, which causes interest in demountable structures, for example, not only maintainability should be taken into account, but also the possibility of using polyplane devices of different power, taking into account various operating conditions and requirements for polyplane propellers. So, for example, for flat and high mountain conditions, polyplane devices can differ in the number and size of plans in order to ensure the greatest efficiency of the aircraft, fuel economy requirements, etc.
Therefore, on a single base propeller, different polyplane modules can be used, which are removable and replaceable.
Thus, in development and R&D, different designs of polyplane propellers should be presented. This circumstance should be reflected in the patent protection of this method and device for its implementation.
The use of lattice wings for the circular movement of the propeller blades complicates the task of providing a minimum air resistance of a polyplane structure. The fact is that with a rectilinear movement of the plates of plans can be flat and mutually perpendicular. For a propeller, connecting plates installed perpendicular to the propeller blade cannot be flat, because. in this case, they will increase the air resistance to the rotation of the propeller (i.e., they will “rake” the air, tell it to move along the blade, increasing turbulence in the zone of polyplanar bearing surfaces).
To eliminate this unfavorable phenomenon, the plates connecting the polyplanar surfaces must be profiled, namely
cylindrical, with radii of curvature R 1 , R 2 ,...R n equal to the distances of these plates from the center of rotation of the screw (figure 2). With this design of the plates connecting the plans, they will provide minimal resistance to the rotation of the propeller.
In addition, the same plates must take into account the rotation of the base propeller blades at the angle of attack to create a thrust (lift) force. The solution to this problem should be sought for propellers with variable angles of attack of the blades, taking into account the duration of the operation of the propeller with one or another angle of attack. In most cases, the most correct will be the orientation to the angle of attack corresponding to the cruising mode of the flight of the aircraft. Accounting for the angle of attack of the base blade in the formation of the connecting plates must ensure that the generatrix of the cylindrical surface is parallel to the axis of rotation of the propeller. Then the minimization of the air resistance of these plates during circular motion (within the circle swept away by the propeller) will be ensured.
Thus, in the end, the connecting plates of a polyplanar device will be an oblique cut of a cylindrical surface, i.e. actually a segment of an elliptical surface. This circumstance must be taken into account, starting with the manufacture of the very first prototypes of a propeller according to this method. In fact, these clarifications refer to the "know-how" of the method for increasing the thrust and efficiency of the propeller, as well as the device for its implementation.
Along with the specifics of the formation of connecting plates, it is necessary to take into account the conditions for increasing the efficiency of the bearing surfaces of the plans in interaction with the base blades.
These conditions are associated with three restrictions: first, the distance from the base blade to the next bearing surface of the plan depends mainly on the thickness of the plan plates, their ratio should be about 1:10 (optimum); the second - the angle of attack of the plan β i must be greater than the angle of attack of the base blade α by several degrees (1°-5°), Fig.5 and 6; third - the same can apply to the angles of attack of the following plans, i.e. β i -β i-1 =1°-5°.
The latter limitation may also be related to the difference in circumferential speeds at the end of the propeller blade and in the middle part of the blade, in the zone of which it is recommended to complete the polyplanar device, where the mentioned ratio of 1:10 is reduced to 1:8.
The theory and practice of using polyplane devices of lattice wings shows that the thinner the plans, the less distance between them is required to obtain the necessary lift (or control). The limitation is the strength and rigidity of the plans, as well as the technological problems of the reliability of joints made by soldering. At the same time, a negative side effect is the expansion of the spectrum of sound phenomena with a decrease in the rigidity of thin planes.
The use of corrugated plates with depressions and protrusions, commensurate with the thickness of the plans, instead of flat plates, is advisable, because. this dramatically increases the rigidity of the plans. This can be estimated by methods of the theory of elasticity, by determining the so-called cylindrical stiffness of the corrugated plates.
From the use of corrugations, one can also expect a positive aerodynamic effect: laminarization of the adjacent layers of the air flow along the corrugation, delaying the separation of the flow from the surface of the plan when the flow is unsteady and its speed increases.
The noted signs and features of the corrugated plates make it possible to slightly increase the angles of attack of the plans.
Therefore, taking into account the heterogeneous phenomena associated with the operation of a polyplane device makes it possible to improve and increase the efficiency of the propeller device, i.e. the propulsion of the aircraft.
Thus, this method of increasing the thrust force and efficiency is feasible with the presented propeller device for various operating conditions of aircraft, helicopters, air vehicles and other aircraft equipment.
The operation of the polyplane propeller device is considered with automatic speed control, which ensures the most economical use of all engine power in various flight modes. At the same time, the operating conditions of a polyplane propeller are considered from the standpoint of meeting the requirements of short takeoff and landing, minimizing the moments of inertia and gyroscopic moments when controlling the propeller thrust vector .
It is known that self-adjusting propellers optimize takeoff and climb conditions and increase the limit of achievable speed. This is explained in the "engine-propulsion" system in Fig.4. In addition, it should be taken into account that self-adjusting propellers improve the controllability of the aircraft-convertible vehicle in on- and off-road driving on steep grades, especially when using thrust vector control.
However, given the priority requirements of high fuel efficiency and increase efficiency, basically the operation of the polyplane propeller device is considered at constant speed in area A (figure 4), corresponding to the highest efficiency and fuel economy in cruising flight mode. This provision remains valid when setting and solving the problem of short takeoff and landing of an aircraft. At the same time, when solving the problem of short takeoff, the operation of a propeller with a polyplanar device can be considered in the zone of higher speeds, limited by the external characteristic of the engine and zone B (Fig. 4) of the absorbed power of the propeller, i.e. at (Ne-Nv)≥Nt.
When setting the task of ensuring a short takeoff and landing (in interaction with the wheel brakes of the landing gear), the run to the stop of the aircraft can be reduced by almost half. In this case, the braking force developed by the propeller may exceed the thrust force. Accordingly, the power absorbed by the screw also increases. Comprehensive improvement of the thrust and thrust characteristics of a propeller with a polyplane device is more easily achieved with a two-sided placement of a polyplane device, i.e. on both sides of the base blade (figure 6). In cases where it is necessary to compensate for the gyroscopic moments of the propeller when controlling, for example, the thrust vector, it is advisable to use two oppositely rotating propellers, which limits the conditions for placing polyplanar devices. To overcome this technical contradiction, profiled plan plates can be used, which are schematically reflected in Figs. 5b and 6b.
Thus, a polyplane device, along with an increase in the traction characteristics of a propeller of a limited diameter, expands the possibilities for improving other important functional and operational properties of an aircraft. In this case, the operation of the polyplanar device is associated with less resistance to the rotation of the propeller, i.e. causes less absorbed power of the screw, which increases its efficiency.
Let us consider in more detail the operation of the base blade with a two-sided and symmetrical arrangement of the polyplanar device (upper and lower, Fig.6a). In this case, when α=0 and the rotation of the propeller at the angles of attack of the upper β i and lower β j plans, there are practically balanced multidirectional forces T i and T j . With an increase in the fuel supply to the engine and an increase in the number of revolutions, automatic control ensures the rotation of the base blade by an angle α and, at the same time, brings the entire polyplane device to positive angles of attack and, accordingly, to unidirectional forces T. The total thrust force of the base blade and plans increases rapidly, as a result, the takeoff is accelerated and shortened. When landing, everything happens in reverse order. After passing the neutral position (α=0) of the base blade and plans of the polyplanar device, negative angles of attack and negatively directed forces T arise, i.e. braking forces. Thus, the braking process is intensified, and the stopping distance is shortened by two to three times compared to braking with only wheel brakes.
The complexity of the air flow in a polyplanar device has at least three effects: 1 - the possibility of increasing the thrust force of the propeller and its efficiency; 2 - the possibility of creating a braking force by the propeller when changing the direction of its rotation with a constant angle of attack of the base blade, or with a constant direction of rotation, but with a change in the angle of attack of the base blade from positive to negative, and 3 - the possibility of icing of the propeller and polyplane device due to the effect Joule Thompson.
If the first two effects should be attributed to the positive results of this method and device, then the third phenomenon requires measures to combat icing, mainly at high humidity and relatively low temperatures, for example, in mountainous and high-altitude operating conditions of the aircraft. The latter, however, requires experimental verification, since the inevitable vibrations of the structural elements of a polyplane device can have a destructive effect on the ice cover (crust) on thin-sheet plans. The increase in traction force and efficiency, as well as the possibility of shortening several times the run during takeoff and landing of an aircraft based on this method and device should be attributed to a higher level of aviation technology and the need to protect industrial property.
LITERATURE
1. Belotserkovsky S.M., Frolov V.P. and other Lattice wings. - M.: Mashinostroenie, 1985. 320 p.
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3. Goshek I. Aerodynamics of high speeds. - M.: Izd-vo inostr. lit., 1954. 547 p.
4. Petrakov V.M., Frolov V.P., Tsipenko V.G. Polybody aircraft. Auth. certificate No. 2111896 RF, MKI V64S 35/00.
5. Lukanin V.N., Derbaremdiker A.D. RF patent No. 2169085, IPC B60F 5/02, 1999, B.I. No. 17, 2001.
6. Belotserkovsky S.M., Kamnev P.I. Lattice wings in rocket science, cosmonautics, aviation. / Ed. Belotserkovsky S.M., Frolov V.P., Podobedova V.A., Plaunova V.P. - M.: Novy Tsentr, 2007. 407 p.
7. Kurochkin F.P. Design and construction of aircraft with vertical takeoff and landing. - M.: Mashinostroenie, 1977. 223 p.
List of drawings
Fig.1. Structural diagram of a propeller blade with a polyplanar device at the end of the blade: 1 - basic propeller blade; 2 - the axis of rotation of the propeller and the direction of the thrust force T; 3 - plan plate; 4 - cover clip plans; 5 - connecting plates of plans; D - propeller diameter; 1 and H are the length and height of the polyplane device, respectively.
Fig.2. View of the base blade with a polyplane device along the axis of rotation of the screw: 6 - corrugated surface of the cover of the holder of plans and radii of curvature R 1 and R 5 of the end walls of the holder of plans and connecting racks of plans R 2 , R 3 , R 4 ; b - blade width (chord). Designations 1-5 are shown in Fig.1.
Fig.3. Types of polyplane devices: a - frame; b - cellular; c - combined.
Fig.4. An example of an external speed characteristic of the power of a piston internal combustion engine of an aircraft with a characteristic of specific fuel consumption and a scheme for coordinating the optimal operating modes of the engine-propulsion system and minimizing fuel consumption with increased efficiency of the propeller: Ne - engine power depending on the speed of rotation of the crankshaft (n) ; g e - characteristic of specific fuel consumption;
A - the zone of economical operation of the propeller in the cruising flight mode of the aircraft; B - the area of characteristics of the power absorbed by the propeller, depending on the speed of rotation of the engine crankshaft.
Fig.5. Structural schemes of a one-sided installation of a polyplanar device on the blades: a - blades with a symmetrical cross section and b - with an asymmetric cross section: 1 - base blade; 7 - the center of the axis of rotation of the base blade at the angle of attack of the blade: +α during takeoff and flight and -α during landing and braking after touching the reference surface with a constant direction of rotation; 3 - plans polyplane device facing the flight (rise) and installed at their own angles of attack β i and β m to the horizontal; 8 - direction of rotation of the screw (see figure 1); T l - thrust force of a separate propeller blade. Designations 1-3 are shown in Fig.1.
Fig.6. Structural schemes of two-sided installation of a polyplane device on the base propeller blades: a and b - respectively with a symmetrical and asymmetric cross-section of the blades; 1-7 the same as in Fig.1 and 5; 9 - plans of a polyplane device installed on the opposite side of the blade at angles and
1. A method for increasing the thrust force and efficiency of a multi-blade propeller with a limited given diameter of the swept area of the propeller, which consists in choosing the optimal number of monoplane basic propeller blades by the calculation method, providing maximum efficiency and the traction force corresponding to this efficiency at a given subsonic, for example, cruising speed flight of an aircraft, determine the difference between the required and calculated thrust forces, compensate for the resulting difference with thin-walled polyplanar load-bearing (working) surfaces attached to the base blades mainly on the side facing in the direction of flight (ascent), provided that the peripheral speeds of the ends of the base propeller blades do not exceed the speed of sound and polyplane load-bearing surfaces: D n max ≤6000;
where D is the limited (given) diameter of the swept area, m,
and n max - maximum propeller speed, rpm.
2. The method according to claim 1, characterized in that the speed of rotation of the propeller, which ensures the cruising mode of the aircraft, is selected corresponding to the number of revolutions of the power plant with a minimum specific fuel consumption with a tolerance of minus 5-10%.
3. The method according to claim 1, characterized in that the propeller rotation speed corresponding to the cruising mode of the aircraft is selected less than the number of revolutions of the power plant at a minimum specific fuel consumption with the condition that the corresponding specific fuel consumption of the power plant will not exceed the minimum specific fuel consumption fuel more than 5-10%.
4. The method according to claim 1, characterized in that the propeller rotation speed corresponding to the maximum speed mode is selected to be greater than the number of revolutions of the power plant at the minimum specific fuel consumption, with the condition that the corresponding specific fuel consumption of the power plant will not exceed the minimum specific fuel consumption fuel more than 5-10%.
5. A propeller for implementing the method according to any one of claims 1 to 4, mainly with wide base blades and a symmetrical transverse profile of the blades, characterized in that polyplanar bearing (working) surfaces are made, for example, in the form of a lattice with a set of flat mutually perpendicular plates and are installed on a limited length of the base blades, starting from the end of the blade, while the base blades are made with a variable thickness not exceeding 10% of the blade chord length from end to butt, the total thickness of additional plans does not exceed 1/3 of the base blade thickness, and the thickness of the plates plans and the distance between them should be related as: 1: (90 ± 10), and the front and rear edges of additional plans are pointed.
6. The propeller according to claim 5, characterized in that the plates connecting the polyplanar bearing (working) surfaces are made with radii of cylindrical surfaces equal to the distances of these plates from the center of rotation of the screw, and the generatrices of the cylindrical surfaces are inclined from the normal to the plane of symmetry of the base blade to the average angle of attack of the base blades, which is set in the cruising flight mode, and the inclination of the generatrices of the cylindrical surfaces is performed in the direction of rotation of the propeller (towards the rotation of the blade), as a result, these generatrixes are parallel to the axis of rotation of the propeller.
7. The propeller according to claim 5, characterized in that the plans of the polyplane device are installed with their own angles of attack with the base blade in the neutral position.
8. The propeller according to claim 7, characterized in that the plans are profiled along the base blade at a distance of 30-50% of the chord from the leading edge of the plan with the plan plate bending at an angle γ, and with the possibility of increasing the bending angle in the direction towards the axis of rotation of the screw in accordance with the dependence of the angle on the length l n of the plan and the distance l i from the end of the base blade according to the expression: tgγ i =(l+l i /l n) tgγ o , where γ o is the initial bending angle of the plan formed with the plane of the asymmetric cross section of the base blade and defined at the end of the base blade.
9. The propeller according to claim 8, characterized in that the plans are curvilinearly corrugated, and the radii of curvature vary along the base blade and are equal to the distances of the top of each individual corrugation to the center of rotation of the screw.
Purpose and types of aircraft power plants.The power plant is designed to create the thrust force necessary to overcome drag and ensure the forward movement of the aircraft.
The traction force is generated by an installation consisting of an engine, a propeller (propeller) and systems that ensure the operation of the propulsion system (fuel system, lubrication system, cooling system, etc.).
At present, turbojet and turboprop engines are widely used in transport and military aviation. In sports, agricultural and various purposes of auxiliary aviation, power plants with piston internal combustion aircraft engines are still used, which convert the thermal energy of the burning fuel into the rotational energy of the propeller.
On Yak-18T, Yak-52 and Yak-55 aircraft, the power plant consists of an M-14P piston engine and a V530TA-D35 variable-pitch propeller.
Many sports aircraft use Rotax engines:
PROPELLER CLASSIFICATION
Screws are classified:according to the number of blades - two-, three-, four- and multi-bladed;
according to the material of manufacture - wooden, metal, mixed;
in the direction of rotation (view from the cockpit in the direction of flight) - left and right rotation;
by location relative to the engine - pulling, pushing;
according to the shape of the blades - ordinary, saber-shaped, spade-shaped;
by types - fixed, unchangeable and variable step.
The propeller consists of a hub, blades and is mounted on the engine shaft with a special bushing.
Fixed pitch screw has blades that cannot rotate around their axes. The blades with the hub are made as a single unit.
fixed pitch screw has blades that are installed on the ground before flight at any angle to the plane of rotation and are fixed. In flight, the installation angle does not change.
variable pitch screw It has blades that, during operation, can, by means of hydraulic or electric control or automatically, rotate around their axes and be set at the desired angle to the plane of rotation.
Rice. 1 Air fixed-pitch two-blade propeller
Rice. 2 Propeller V530TA D35
According to the range of blade angles, propellers are divided into:
on conventional ones, in which the installation angle varies from 13 to 50 °, they are installed on light aircraft;
on feathered - installation angle varies from 0 to 90°;
on brake or reverse propellers, have a variable installation angle from -15 to +90 °, with such a propeller they create negative thrust and reduce the length of the aircraft run.
The propellers are subject to the following requirements:
the screw must be strong and weigh little;
must have weight, geometric and aerodynamic symmetry;
must develop the necessary thrust during various evolutions in flight;
should work with the highest efficiency.
On the Yak-18T, Yak-52 and Yak-55 aircraft, a conventional paddle-shaped wooden two-bladed tractor propeller of left rotation, variable pitch with hydraulic control V530TA-D35 is installed (Fig. 2).
GEOMETRIC CHARACTERISTICS OF THE SCREW
The blades during rotation create the same aerodynamic forces as the wing. The geometric characteristics of the propeller affect its aerodynamics.Consider the geometric characteristics of the screw.
Blade shape in plan- the most common symmetrical and saber.
Rice. 3. Shapes of the propeller: a - blade profile, b - blade shapes in plan
Rice. 4 Diameter, radius, geometric pitch of the propeller
Rice. 5 Helix development
Sections of the working part of the blade have wing profiles. The blade profile is characterized by chord, relative thickness and relative curvature.
For greater strength, blades with variable thickness are used - a gradual thickening towards the root. The chords of the sections do not lie in the same plane, since the blade is made twisted. The edge of the blade that cuts through the air is called the leading edge, and the trailing edge is called the trailing edge. The plane perpendicular to the axis of rotation of the screw is called the plane of rotation of the screw (Fig. 3).
screw diameter called the diameter of the circle described by the ends of the blades when the propeller rotates. The diameter of modern propellers ranges from 2 to 5 m. The diameter of the V530TA-D35 propeller is 2.4 m.
Geometric screw pitch - this is the distance that a translational propeller must travel in one complete revolution if it were moving in air as in a solid medium (Fig. 4).
Propeller blade angle - this is the angle of inclination of the blade section to the plane of rotation of the propeller (Fig. 5).
To determine what the pitch of the propeller is, imagine that the propeller moves in a cylinder whose radius r is equal to the distance from the center of rotation of the propeller to point B on the propeller blade. Then the section of the screw at this point will describe a helix on the surface of the cylinder. Let's expand the segment of the cylinder, equal to the pitch of the screw H along the BV line. You will get a rectangle in which the helix has turned into a diagonal of this rectangle of the Central Bank. This diagonal is inclined to the plane of rotation of the BC screw at an angle . From the right-angled triangle TsVB we find what the screw pitch is equal to:
(3.1)
The pitch of the screw will be the greater, the greater the angle of installation of the blade . Propellers are subdivided into propellers with a constant pitch along the blade (all sections have the same pitch), variable pitch (sections have a different pitch).
The V530TA-D35 propeller has a variable pitch along the blade, as it is beneficial from an aerodynamic point of view. All sections of the propeller blade run into the air flow at the same angle of attack.
If all sections of the propeller blade have a different pitch, then the pitch of the section located at a distance from the center of rotation equal to 0.75R, where R is the radius of the propeller, is considered to be the common pitch of the propeller. This step is called nominal, and the installation angle of this section- nominal installation angle .
The geometric pitch of the propeller differs from the pitch of the propeller by the amount of slip of the propeller in the air (see Fig. 4).
Propeller pitch - this is the actual distance that a progressively moving propeller moves in the air with the aircraft in one complete revolution. If the speed of the aircraft is expressed in km/h and the number of propeller revolutions per second, then the pitch of the propeller is H P can be found using the formula
(3.2)
The pitch of the screw is slightly less than the geometric pitch of the screw. This is explained by the fact that the screw, as it were, slips in the air during rotation due to its low density relative to a solid medium.
The difference between the value of the geometric pitch and the pitch of the propeller is called screw slip and is determined by the formula
S= H- H n . (3.3)
SPEED OF MOVEMENT AND ANGLE OF ATTACK OF PROPELLER BLADE ELEMENT
The aerodynamic characteristics of propellers include the angle of attack and propeller thrust.The angle of attack of the elements of the propeller blade called the angle between the chord of the element and the direction of its true resulting movement W(Fig. 6).
Rice. 6 Installation angle and angle of attack of the blades: a - angle of attack of the blade element, b - speed of the blade element
Each element of the blade performs a complex movement, consisting of rotational and translational. The rotational speed is
Where n With- engine speed.
forward speed is the speed of the aircraft V . The farther the blade element is from the center of rotation of the propeller, the greater the rotational speed U .
When the propeller rotates, each element of the blade will create aerodynamic forces, the magnitude and direction of which depend on the speed of the aircraft (the speed of the oncoming flow) and the angle of attack.
Considering Fig. 6a, it is easy to see that:
When the propeller is rotating and the forward speed is zero (V=0), then each element of the propeller blade has an angle of attack equal to the installation angle of the blade element ;
With the translational movement of the propeller, the angle of attack of the propeller blade element differs from the angle of inclination of the propeller blade element (becomes smaller than it);
The angle of attack will be the greater, the greater the installation angle of the propeller blade element;
The resulting speed of rotation of the propeller blade element W is equal to the geometric sum of the translational and rotational velocities and is found according to the right triangle rule
(3.5)
The greater the rotational speed, the greater the angle of attack of the propeller blade element. Conversely, the greater the forward speed of the propeller, the smaller the angle of attack of the propeller blade element.
In reality, the picture is more complicated. Since the screw sucks in and rotates the air, throws it back, giving it additional speed v, which is called suction speed. As a result, the true speed W" will differ in magnitude and direction from the suction speed, if they are added geometrically. Therefore, the true angle of attack " will be different from the angle (Fig. 6, b).
Analyzing the above, we can conclude:
at forward speed V=0 the angle of attack is maximum and is equal to the angle of installation of the propeller blade;
with an increase in translational speed, the angle of attack decreases and becomes less than the installation angle;
at high flight speed, the angle of attack of the blades can become negative;
the greater the speed of rotation of the propeller, the greater the angle of attack of its blade;
if the flight speed is constant and the engine speed decreases, then the angle of attack decreases and may become negative.
The conclusions drawn explain how the thrust force of the fixed-pitch propeller changes with a change in flight speed and number of revolutions.
propeller thrust occurs as a result of the action of aerodynamic force R on the element of the propeller blade during its rotation (Fig. 1).
Expanding this force into two components, parallel to the axis of rotation and parallel to the plane of rotation, we obtain the LR force and the force of resistance to rotation X propeller blade element.
Summing up the thrust force of the individual elements of the propeller blade and applying it to the axis of rotation, we obtain the thrust force of the propeller R .
The propeller thrust depends on the propeller diameter D, revolutions per second n, air density and calculated according to the formula (in kgf or N)
Where - propeller thrust coefficient, taking into account the shape of the blade in plan, the shape of the profile and the angle of attack, is determined experimentally. The propeller thrust ratio of the Yak-18T, Yak-52 and Yak-55 - V530TA-D35 aircraft is 1.3.
Thus, the thrust force of a propeller is directly proportional to its coefficient, the air density, the square of the propeller revolutions per second, and the propeller diameter to the fourth power.
Since the propeller blades are geometrically symmetrical, the magnitude of the resistance forces and their removal from the axis of rotation will be the same.
The force of resistance to rotation is determined by the formula
(3.7)
Where Cx l - drag coefficient of the blade, taking into account its plan shape, profile shape, angle of attack and surface finish ;
W - resulting speed, m/s;
S l - blade area;
TO - the number of blades.
Fig.1 Aerodynamic forces of the propeller.
Rice. 2. Operating modes of the propeller
The force of resistance to the rotation of the screw relative to its rotation creates a moment of resistance to the rotation of the screw, which is balanced by the torque of the engine:
M tr =X V r V (3.8)
The torque generated by the engine is determined (in kgf-m) by the formula
(3.9)
Where N e- effective engine power.
The considered mode is called the propeller positive thrust mode, since this thrust pulls the aircraft forward (Fig. , a). When the angle of attack of the blades decreases, the forces decrease. R and X(reduced propeller thrust and braking torque). It is possible to achieve a situation where P=0 andX= R. This is the zero thrust mode (Fig. , b).
With a further decrease in the angle of attack, a mode is reached when the propeller begins to rotate not from the engine, but from the action of the forces of the air flow. This mode is called self-rotating propeller or autorotation (Fig. , c).
With a further decrease in the angle of attack of the elements of the propeller blade, we obtain a mode in which the resistance force of the propeller blade X will be directed in the direction of rotation of the screw, and at the same time the screw will have a negative thrust. In this mode, the screw rotates from the oncoming air flow and rotates the engine. The engine is spinning up, this mode is called windmill mode (Fig., d).
Self-rotation and windmill modes are possible in level flight and in a dive.
On the Yak-52 and Yak-55 aircraft, these modes manifest themselves when performing vertical figures down at a small pitch of the propeller blade. Therefore, when performing vertical figures downwards (when accelerating over 250 km / h), it is recommended to tighten the propeller by 1/3 of the stroke of the lever by controlling the pitch of the propeller.
PROPELLER THRUST DEPENDENCE ON FLIGHT SPEED.
With an increase in flight speed, the angles of attack of the propeller blade, fixed pitch and fixed, rapidly decrease, propeller thrust drops. The greatest angle of attack of the propeller blade will be at zero airspeed at full engine speed.Accordingly, the propeller thrust decreases to zero and then becomes negative. The motor shaft spins. To prevent spinning of the screw, reduce the engine speed. If the engine is not throttled, it may be destroyed.
The dependence of the V530TA-D35 propeller thrust on the flight speed is shown in the graph in Fig. 7. To build it, the thrust of the propeller is measured at different speeds. The resulting graph is called the thrust characteristic of the power plant.
Rice. 7 Characteristics of the M-14P power plant in terms of thrust (for H = 500 m) of the Yak-18T, Yak-52 and Yak-55 aircraft with the V530TA-D35 propeller
INFLUENCE OF FLIGHT HEIGHT ON PROPELLER THRESHOLD.
Finding out the dependence of thrust on flight speed, the operation of the propeller at a constant height with a constant air density was considered. But when flying at different altitudes, air density affects the thrust of the propeller. With an increase in flight altitude, the air density decreases, respectively, the propeller thrust will also decrease proportionally (at constant engine speed). This can be seen from the analysis of formula (3.6).PROPELLER BRAKING TORQUE AND ENGINE TORQUE.
As previously discussed, the drag torque of the propeller counteracts the torque of the motor.In order for the screw to rotate at constant speed, it is necessary that the braking torque M t, equal to the product 
, was equal to the engine torque M cr, equal to the product of F d ,.
those. M t \u003d M cr or \u003d F d (Fig. 8).
Rice. 8 Propeller braking torque and engine torque
If this equality is violated, then the engine will reduce speed or increase.
An increase in engine speed leads to an increase in M cr and vice versa. The new equilibrium is established at the new engine speed.
POWER REQUIRED TO ROTATE THE PROPELLER
This power is expended to overcome the forces of resistance to the rotation of the propeller.The formula for determining the power of the propeller (in hp) is:
(3.10)
Where - power factor depending on the shape of the propeller, the number of blades, the angle of installation, the shape of the blade in plan, on the operating conditions of the propeller ( relative step)
From formula (3.10) it can be seen that the required power for the rotation of the propeller depends on the power factor, on the speed and flight altitude, revolutions and diameter of the propeller.
With an increase in flight speed, the angle of attack of the propeller blade element, the amount of air thrown back and its speed decrease, therefore, the power required to rotate the propeller also decreases. As the flight altitude increases, the air density decreases and the power required to rotate the propeller also decreases.
With an increase in engine speed, the resistance to rotation of the propeller increases and the power required to rotate the propeller increases.
The propeller, rotated by the engine, develops thrust and overcomes the drag of the aircraft, the aircraft moves.
The work done by the thrust force of the propeller in 1 second. when the aircraft is moving, is called thrust or the net power of the propeller.
The thrust power of the propeller is determined by the formula
(3.11)
Where P in is the thrust developed by the propeller; V is the speed of the aircraft.
With an increase in altitude and flight speed, the thrust power of the propeller decreases. When the propeller is in operation, when the aircraft is not moving, maximum thrust is developed, but the thrust power is zero, since the speed of movement is zero.
PROPELLER EFFICIENCY.
DEPENDENCE OF EFFICIENCY ON ALTITUDE AND FLIGHT SPEED
Part of the engine rotational energy is spent on rotating the propeller and is aimed at overcoming air resistance, swirling the ejected jet, etc. Therefore, the useful second work, or the useful traction power of the propeller, n b, there will be less engine power N e spent on the rotation of the propeller.The ratio of useful propulsive power to the power consumed by the propeller (effective engine power) is called the coefficient of performance (efficiency) of the propeller and is denoted . It is determined by the formula
(3.12)
Rice. 9 Power characteristics of the M-14P engine of the Yak-52 and Yak-55 aircraft
Rice. 10 Approximate view of the curve of change in available power depending on airspeed
Rice. 11 Altitude characteristic of the M-14P engine in modes 1 - takeoff, 2 - nominal 1, 3 - nominal 2, 4 - cruising 1; 5 - cruising 2
The value of the efficiency of the propeller depends on the same factors as the propulsive power of the propeller.
The efficiency is always less than unity and reaches 0.8 ... 0.9 for the best propellers.
Np- required power.
To reduce the speed of rotation of the propeller in the engine, a gearbox is used.
The degree of reduction is selected in such a way that in the nominal mode the ends of the blades are flowed around by a subsonic air flow.
Rice. 12 Power characteristics of the M-14P engine of the Yak-52 and Yak-55 aircraft
Rice. 13 Approximate view of the curve of change in available power depending on airspeed
Rice. 14 Altitude characteristic of the M-14P engine in modes 1 - takeoff, 2 - nominal 1, 3 - nominal 2, 4 - cruising 1; 5 - cruising 2
The plot of available effective power versus flight speed for Yak-52 and Yak-55 aircraft is shown in Fig. 9.
Graph Fig. 10 is called the characteristic of the power plant in terms of power.
At V=0, Np=0; at flight speed V=300 km/h, Np==275 hp (for the Yak-52 aircraft) and V=320 km/h, Np=275 l. With. (for the Yak-55 aircraft), where Np- required power.
With increasing altitude, the effective power decreases due to a decrease in air density. The characteristic of its change for the Yak-52 and Yak-55 aircraft from the flight altitude H is shown in Fig. eleven.
Rice. 15 Altitude characteristic of the M-14P engine in modes 1 - takeoff, 2 - nominal 1, 3 - nominal 2, 4 - cruising 1; 5 - cruising 2
With increasing altitude, the effective power decreases due to a decrease in air density. The characteristic of its change for the Yak-52 and Yak-55 aircraft from the flight altitude H is shown in Fig. eleven.
VARIABLE PITCH SCREWS
To eliminate the shortcomings of fixed-pitch and fixed-pitch propellers, a variable-pitch propeller (VSP) is used. Vetchinkin is the founder of the VIS theory.REQUIREMENTS FOR VISH:
VISH should set the most favorable angles of attack of the blades in all flight modes;Remove the rated power from the engine over the entire operating range of speeds and altitudes;
Maintain the maximum value of the efficiency factor over the widest possible range of speeds.
The blades of the VISH are either controlled by a special mechanism, or are set to the desired position under the influence of forces acting on the propeller. In the first case, these are hydraulic and electric propellers, in the second - aerodynamic ones.
hydraulic screw - a propeller, in which the change in the angle of installation of the blades is carried out by the pressure of the oil supplied to the mechanism located in the propeller hub.
electric screw - a propeller, in which the change in the angle of installation of the blades is made by an electric motor connected to the blades by a mechanical transmission.
Aeromechanical propeller - a propeller, in which the change in the angle of installation of the blades is carried out automatically - by aerodynamic and centrifugal forces.
The most widely used hydraulic VISH. An automatic device in variable-pitch propellers is designed to maintain a constant set speed of the propeller (engine) by synchronously changing the angle of inclination of the blades when changing the flight mode (speed, altitude) and is called a speed constancy controller (RPO).
Rice. 16 Operation of V530TA-D35 variable pitch propeller at different flight speeds
RPO, together with the mechanism for turning the blades, changes the pitch of the propeller (the angle of inclination of the blades) in such a way that the revolutions set by the pilot using the VIS control lever remain unchanged (given) when the flight mode changes.
In this case, it should be remembered that the revolutions will be maintained as long as the effective power on the engine shaft N e is greater than the power required to rotate the propeller when the blades are set to the smallest angle of inclination (small pitch).
On Fig. 16 shows a diagram of the operation of the VIS.
When changing the flight speed from takeoff to maximum in level flight, the angle of installation of the blades increases from its minimum value min up to maximum Max (big step). Due to this, the angles of attack of the blade change little and remain close to the most advantageous.
The work of the VIS during takeoff is characterized by the fact that the entire engine power is used during takeoff - the greatest thrust is developed. This is possible provided that the engine develops maximum speed, and each part of the propeller blade develops the greatest thrust, having the least resistance to rotation.
To do this, it is necessary that each element of the propeller blade work at angles of attack close to critical, but without stalling the air flow. On Fig. 16, but it can be seen that the angle of attack of the blade before takeoff (V=0) due to the flow of air at a speed V slightly different from the angle of inclination of the blade by the value f min. The angle of attack of the blade corresponds to the magnitude of the maximum lifting force.
The resistance to rotation in this case reaches a value at which the power expended on the rotation of the screw and the effective power of the engine are compared and the revolutions will be unchanged. With an increase in speed, the angle of attack of the propeller blades decreases (Fig. 16, b). The resistance to rotation decreases and the propeller becomes lighter, as it were. The engine speed should increase, but the RPO keeps them constant by changing the angle of attack of the blades. As the flight speed increases, the blades turn to a greater angle. Wed .
When flying at maximum speed, the VIS must also provide the maximum thrust value. When flying at maximum speed, the angle of inclination of the blades has a limiting value pmax (Fig. 16, c). Therefore, with a change in flight speed, the angle of attack of the blade changes, with a decrease in flight speed, the angle of attack increases - the propeller becomes heavier, with an increase in flight speed, the angle of attack decreases - the propeller becomes lighter. RPO automatically translates the propeller blades to the appropriate angles.
As the flight altitude increases, engine power decreases and the RPO reduces the angle of inclination of the blades to facilitate engine operation, and vice versa. Consequently, the RPO keeps the engine speed constant with a change in flight altitude.
During landing approach, the propeller is set to a small pitch, which corresponds to the takeoff speed. This makes it possible for the pilot, when performing various maneuvers on the landing glide path, to obtain takeoff power of the engine with an increase in speed to maximum.
The bladed propeller of an aircraft, it is also a propeller or a bladed machine, which is driven by the engine. With the help of a screw, the torque from the engine is converted into thrust.
The propeller acts as a propeller in aircraft such as airplanes, cyclogyros, gyroplanes, snowmobiles, hovercraft, ekranoplans, as well as helicopters with turboprop and piston engines. For each of these machines, the screw can perform different functions. In airplanes, it is used as a main rotor, which creates thrust, and in helicopters, it provides lift and taxiing.
All aircraft propellers are divided into two main types: propellers with variable and fixed pitch. Depending on the design of the aircraft, propellers can provide either push or pull thrust.
When rotating, the propeller blades capture air and produce its rejection in the opposite direction of flight. A low pressure is created in front of the screw, and a high pressure zone behind. The thrown air acquires a radial and circumferential direction, due to this, part of the energy that is supplied to the propeller is lost. The very swirling of the air flow reduces the streamlining of the apparatus. Agricultural aircraft, when working on fields, have poor uniformity in the dispersion of chemicals due to the flow from the propeller. A similar problem is solved in devices that have a coaxial screw layout, in this case compensation occurs using the operation of the rear screw, which rotates in the opposite direction. Similar propellers are installed on aircraft such as the An-22, Tu-142 and Tu-95.
Technical parameters of propellers
The most significant characteristics of the propellers, on which the thrust force and the flight itself depend, of course, are the pitch of the propeller and its diameter. Pitch is the distance a propeller can travel by being screwed into the air in one complete revolution. Until the 30s of the last century, propellers with a constant rotation pitch were used. Only in the late 1930s, almost all aircraft were equipped with variable-pitch propellers.
Screw parameters:
The diameter of the propeller circle is the size that the tips of the blades describe when rotating.
The pitch of the screw is the actual distance traveled by the screw in one revolution. This characteristic depends on the speed of movement and revolutions.
The geometric pitch of the propeller is the distance that the propeller could travel in a solid medium in one revolution. It differs from the tread of the propeller in the air by the sliding of the blades in the air.
The angle of location and installation of the propeller blades is the inclination of the blade section to the real plane of rotation. Due to the presence of twist of the blades, the angle of rotation is measured along the section, in most cases it is 2/3 of the entire length of the blade.
The propeller blades have a front - cutting - and a trailing edge. The cross section of the blades has a wing-type profile. In the profile of the blades there is a chord, which has a relative curvature and thickness. To increase the strength of the propeller blades, a chord is used, which has a thickening towards the propeller root. The section chords are in different planes, since the blade is made twisted.
The propeller pitch is the main characteristic of the propeller, it primarily depends on the angle of the blades. Pitch is measured in units of distance traveled per revolution. The more pitch the propeller makes in one revolution, the more volume is discarded by the blade. In turn, an increase in pitch leads to additional loads on the power plant, respectively, the number of revolutions decreases. Modern aircraft have the ability to change the inclination of the blades without stopping the engine.
Advantages and disadvantages of propellers
The efficiency of propellers on modern aircraft reaches 86%, which makes them in demand by the aircraft industry. It should also be noted that turboprops are much more economical than jet aircraft. Nevertheless, the screws have some limitations both in operation and in the constructive plan.
One of these limitations is the "locking effect", which occurs when the diameter of the screw increases or when the number of revolutions is added, and the thrust, in turn, remains at the same level. This is due to the fact that sections with supersonic or transonic air flows appear on the propeller blades. It is this effect that does not allow aircraft with propellers to reach speeds higher than 700 km / h. At the moment, the fastest car with propellers is the domestic model of the Tu-95 long-range bomber, which can reach speeds of 920 km / h.
Another disadvantage of screws is the high noise level, which is regulated by ICAO world standards. The noise from the screws does not fit into the noise standards.
Modern developments and the future of aircraft propellers
Technology and experience allow designers to overcome some of the noise problems and increase traction beyond the limitations.
Thus, it was possible to bypass the locking effect due to the use of a powerful turboprop engine of the NK-12 type, which transmits power to two coaxial propellers. Their rotation in different directions made it possible to bypass locking and increase traction.
Thin saber-shaped blades are also used on propellers, which have the ability to delay the crisis. This allows you to achieve higher speeds. This type of propellers is installed on the An-70 aircraft.
At the moment, developments are underway to create supersonic propellers. Despite the fact that the design is being carried out for a very long time with considerable cash injections, it has not been possible to achieve a positive result. They have a very complex and precise shape, which greatly complicates the calculations of designers. Some off-the-shelf propellers of the supersonic type have been shown to be very noisy.
Enclosing the propeller in a ring - an impeller - is a promising direction of development, since it reduces the end flow around the blades and the noise level. It also improved security. There are some aircraft with fans that have the same design as the impeller, but are additionally equipped with an airflow direction apparatus. This greatly improves the efficiency of the propeller and the engine.
PROPELLER THEORY
Introduction
The propeller converts the rotational power of the engine into forward thrust. The propeller pushes the air mass back, creating a reactive force that pushes the aircraft forward. The thrust of the propeller is equal to the product of the mass of air and the acceleration given to it by the propeller.
Definitions
propeller blade It is a load-bearing surface similar to an airplane wing. Definitions such as chord, profile curvature, relative profile thickness, relative elongation are similar to the definitions for an aircraft wing.
The angle of installation of the propeller blades ( blade angle or pitch )
This is the angle between the blade chord and the plane of rotation. The installation angle decreases from the root of the blade to the tip, because the circumferential velocity of the blade section increases from the butt to the tip. The angle of installation of the blade is measured in a section located at 75% of its length, counting from the butt.
Screw pitch ( geometric pitch )
This is the distance that the propeller would travel in one complete revolution if it were moving through the air at the angle of the blades. (You can imagine the pitch of a screw as the movement of a bolt twisting along a thread, but we will not use this analogy further)
Blade geometric twist ( blade twist )
The sections of the blade, located closer to its tip, cover a greater distance in one revolution. In order for the pitch of the screw to be the same for all sections of the blade, the angle of installation of the sections gradually decreases from the butt to the tip.
The angle of installation of the blades on many propellers can vary. When the angle of the blades is small, they say that the propeller is in fine pitch mode, and when, on the contrary, it is in large pitch mode (coarse pitch).
tread screws (effective pitch or advance per revolution)
In flight, the propeller does not cover a distance equal to the pitch of the propeller in one revolution. The actual distance traveled by the propeller depends on the speed of the aircraft and is called the propeller pitch.
Screw slip ( slip )
The difference between pitch and lead of a screw is called the slip of the screw.
Helix angle ( helix angle )
This is the angle between the actual trajectory of the propeller section and the plane of rotation.
Angle of attack(α)
The trajectory of the movement of the section of the blade in the air determines the direction of the oncoming air flow. The angle between the chord of the blade section and the direction of the oncoming flow is the angle of attack of the blade section. The angle of attack is affected by the peripheral speed of the section (rotor speed) and the true speed of the aircraft.
Fixed pitch propeller ( fixed pitch propeller )
The figures show the operation of a fixed pitch propeller under changing flight conditions. An increase in the true speed of the aircraft at a constant propeller speed (peripheral sectional speed) reduces the angle of attack of the propeller. Increasing the propeller speed at a constant true airspeed increases the angle of attack of the propeller.
Aerodynamic forces arising on the propeller
A propeller blade is a load-bearing surface similar to an airplane wing. When it moves through the air at a certain angle of attack, then aerodynamic forces are created on it in the same way as on a wing. Between the surfaces of the blade there is a pressure difference. The surface of the blade where more pressure is created is called the working surface of the blade (pressure face or thrust face). When the propeller creates direct thrust, the back (flat) surface of the blade is the working one. The pressure difference creates a total aerodynamic force, which can be decomposed into two components, thrust and rotational resistance.
Propeller thrust
thrust is the component of the total aerodynamic force perpendicular to the plane of rotation. The thrust force is unevenly generated along the length of the blade. It is minimal at the tip of the blade, where the pressure drop between the surfaces disappears, and also decreases in the butt due to the low circumferential velocity. The thrust creates a bending moment on each blade, trying to bend them with their tips forward. (A force equal and opposite in direction to the propeller thrust pushes the air back.)
Torque of resistance to rotation
The force of resistance to rotation of the propeller on the shoulder from the axis of rotation to the point of application of the full aerodynamic force creates a moment of resistance to rotation. A moment equal in magnitude and opposite in direction acts on the aircraft, trying to rotate it about the longitudinal axis. Also, the moment of resistance to rotation creates bending moments on the propeller blades, trying to bend them against the direction of rotation.
The centrifugal twisting moment of the blade ( centrifugal twisting moment )
The lateral components of the centrifugal forces "A" and "B" create a moment relative to the axis of change in the angle of the blade, trying to reduce the pitch of the propeller.
Aerodynamic twisting moment of the blade ( aerodynamic twisting moment )
Since the center of pressure is located ahead of the axis of change in the angle of installation of the blade, the total aerodynamic force creates a moment tending to increase the pitch of the propeller.
The aerodynamic moment counteracts the centrifugal twisting moment, but is weaker than it.
propeller efficiency
The efficiency of the propeller is determined by the ratio of the traction power and the power supplied to the propeller from the engine. The thrust power of the propeller is determined by the product of the propeller thrust by the true speed of the aircraft, and the engine power is determined by the product of the engine torque by the angular velocity of the propeller.
propeller efficiency = propulsion power / engine power
Dependence of propeller efficiency on flight speed
It was shown above that as the flight speed increases, the angle of attack of the fixed-pitch propeller blades decreases. This leads to a decrease in propeller thrust. At some speed, this angle will decrease so much that the propeller thrust will decrease to zero. This means that the efficiency of the screw will also become zero.
For a fixed pitch propeller, there is only one speed at which the blades will flow around at the most favorable angle of attack and the efficiency of the propeller will be maximum. (at constant angular velocity of rotation)
With a further decrease in the speed of the aircraft, the angle of attack of the blades increases. The thrust of the propeller increases, but the product of thrust and speed (traction power) begins to fall. At zero speed, the thrust of the propeller is maximum, but the propeller does not produce useful work, so its efficiency is again equal to zero.
The efficiency of a fixed pitch propeller varies greatly with airspeed.
As can be seen from the figure, using a variable pitch propeller (blade angle), it is possible to achieve its efficient operation in a wide range of flight speeds.
Fixed-pitch propeller with the ability to change the angle of the blades in the hub when servicing on the ground.
A propeller with a choice of three fixed blade angles in flight. The small pitch propeller is set for takeoff, climb and landing. During cruising flight, the propeller is set to the high pitch position. In case of engine failure, the screw is set to the vane position.
Variable pitch propeller (constant speed propellers).
On modern aircraft, propellers are installed that automatically maintain a given speed by changing the angle of the blades. This allows you to maintain high efficiency over a wide range of speeds, improve takeoff and climb performance and ensure fuel economy in cruise flight.
variable pitch propeller
The figure shows a typical propeller and engine control panel on small piston aircraft. All levers are in the takeoff position (far forward).
The propeller speed control is set to maximum speed.
Moving the middle lever back will decrease propeller speed.
Note: An analogy can be drawn between a propeller speed control lever and a gear lever in a car.
The maximum propeller speed is first gear in the car.
The minimum propeller speed is fifth gear in the car.
The figure shows the operating conditions of the propeller at the beginning of the runway runway. The propeller revolutions are maximum, the translational speed is low. The angle of attack of the blades is optimal, the propeller works with maximum efficiency. As the speed increases, the angle of attack of the blades will decrease. This will lead to a decrease in thrust and resistance to rotation. At constant engine power, engine speed will increase. The propeller speed control will begin to increase the pitch of the propeller blades to prevent the propeller speed from increasing. Thus, the angle of attack of the blades will be kept at optimal values all the time.
The figure shows the operating conditions of the propeller when flying at high speed. As true airspeed increases, the propeller speed control constantly increases the pitch of the blades, maintaining a constant angle of attack.
The figure shows the operation of the propeller in cruise flight. Optimum power and propeller speeds are specified in the flight manual. It is generally recommended to first reduce the engine power and then reduce the propeller speed.
Throughout the flight, the constant speed controller controls the pitch of the propeller blades to maintain the desired speed. At least it tries to achieve it.
If the torque from the engine disappears (idle mode or failure), then the regulator, trying to maintain speed, reduces the angle of the blades to a minimum. The angle of attack of the blades becomes negative. Now the total aerodynamic force on the propeller is directed in the opposite direction. It can be decomposed into the negative thrust of the propeller and the force tending to spin the propeller. The propeller will now turn the engine.
On a twin-engine aircraft, if one engine fails, if the propeller of the failed engine autorotates, then the climb characteristics and flight range deteriorate very much and the control of the aircraft becomes difficult due to the additional turning moment. Also, the rotation of a failed engine can lead to its jamming or fire.
feathering
When the propeller blades turn to an angle of attack of zero lift, the force that rotates the propeller disappears and the propeller stops. The drag (negative thrust) of the propeller is reduced to a minimum. This greatly improves climb performance (in case of failure of one of the two engines), since the climb gradient depends on the difference between the thrust of the engines and drag.
Also, feathering the propeller blades reduces the turning moment from the failed engine. This improves the controllability of the aircraft and lowers the minimum evolutionary speed in the event of a V MC engine failure.
On single-engine aircraft, propeller feathering is not provided. However, in the event of an engine failure, it is possible to significantly reduce the negative thrust of the propeller. To do this, the screw speed controller is set to the minimum speed. In this case, the screw will be set to the maximum pitch position.
This allows you to increase the lift-to-drag ratio of the aircraft, which will reduce the altitude loss gradient in gliding with a failed engine. The engine speed will also decrease due to a decrease in the force tending to spin the screw.
If you turn the propeller speed control to increase the speed of rotation, then the effect will be the opposite.
Power take-off from engine to propeller
The propeller must be able to absorb the full power of the engine.
It must also operate at maximum efficiency over the entire operational range of the aircraft. The critical factor is the speed of flow around the tips of the blades. If it approaches the speed of sound, then the phenomena associated with the compressibility of air lead to a decrease in thrust and an increase in the moment of resistance to rotation. This significantly reduces the efficiency of the propeller and increases its noise.
Limiting the speed of flow around the tips of the blades imposes restrictions on the diameter and angular speed of rotation of the propeller, as well as on the true flight speed.
The propeller diameter is also limited by the requirements of a minimum clearance to the surface of the airfield and the fuselage of the aircraft, as well as the need to install the engine as close to the fuselage as possible in order to reduce the turning moment in case of failure. If the engine is located far from the longitudinal axis of the aircraft, then it is necessary to increase the vertical tail to ensure the balance of the aircraft in case of engine failure at low speed. All of the above shows that it is impractical to ensure that the propeller consumes all the available engine power by simply increasing its diameter. Often this is achieved by increasing the fill factor of the propeller.
Propeller fill factor ( solidity )
This is the ratio of the frontal area of all the blades to the area swept by the propeller.
Methods for increasing the fill factor of the propeller:
Increasing the chord of the blades. This leads to a decrease in the relative elongation of the blade, which leads to a decrease in efficiency.
Increasing the number of blades. The power take-off from the engine increases without increasing the speed of the flow around the tips and reducing the relative elongation of the blades. An increase in the number of blades over a certain amount (5 or 6) leads to a decrease in the efficiency of the propeller.
Prop thrust is created by throwing a mass of air back. If the fill factor of the propeller is excessively increased, then the mass of air that can be accelerated as it passes through the propeller will decrease. To effectively increase the number of blades, coaxial screws are used that rotate on the same axis in opposite directions.
Moments and forces generated by the propeller
The screw creates moments in all three axes of the aircraft. The reasons for these moments are different:
screw reaction heeling moment
gyroscopic moment
wake helical moment
moment due to asymmetrical flow around a propeller
Note: Most modern engines are equipped with clockwise rotating propellers (when viewed from the rear). On some twin-engine aircraft, a counter-clockwise rotating propeller is installed on the right engine to eliminate the disadvantages associated with the appearance of a critical engine (see chapter 12).
Heeling moment of propeller reaction
Since the propeller rotates clockwise, an equal and opposite torque acts on the aircraft.
When the aircraft is taking off, the left pneumatic will carry a greater load, which will create more rolling resistance. Therefore, the aircraft will tend to turn to the left. In flight, the aircraft will tend to roll to the left. This moment will be most noticeable at maximum propeller thrust and low flight speed (low efficiency of the rudders).
The heeling torque of the propeller reaction is practically absent for coaxial propellers rotating in opposite directions.
The original text says that twin-engine aircraft with co-rotating propellers have no heeling torque until one of the engines fails. This is not true. In theoretical mechanics it is said that the total moment acting on a rigid body is equal to the algebraic sum of the moments lying in the same plane. That is, the moment of reaction of the propellers will act on the aircraft, regardless of the number of operating engines, and if all the propellers rotate in the same direction, then the moments will add up.
Gyroscopic moment
A rotating propeller has the properties of a gyroscope - it seeks to maintain the position of the axis of rotation in space, and in the case of the application of an external force, a gyroscopic moment appears, tending to turn the axis of the gyroscope in a direction that differs by 90 ° from the direction of forced rotation.
It is convenient to determine the direction of action of the gyroscopic moment using the following mnemonic rule. Imagine yourself sitting in the cockpit of an airplane. The plane of rotation of the engine (propeller) will be depicted as a circle, and the direction of rotation - by arrows along the circle.
If one arrow is drawn from the center of the circle in the direction of movement of the aircraft nose, then the second arrow, directed tangentially to the circle in the direction of rotation of the engine (propeller), will show the direction of the additional (precessional) movement of the aircraft nose, caused by the action of the gyroscopic moment of the engine (propeller).
The gyroscopic moment appears only when the aircraft rotates in pitch and heading.
Coaxial propellers have no gyroscopic moment.
Wake helical moment
The propeller throws back a swirling jet of air, which, rotating around the fuselage, changes the flow around the keel. Since the screw rotates clockwise, the jet flows around the keel at an angle to the left, causing a lateral force on it to the right.
The helical moment from the propeller wake creates a yaw moment to the left. The amount of torque depends on the operating mode of the engine and the speed of the propeller.
You can reduce the helical moment with:
using coaxial screws
installation of a fixed compensator on the rudder
installing the engine with a small lapel of the propeller axis to the right
setting the keel at a slight angle to the left
Moment caused by asymmetrical flow around the propeller
In flight, the propeller axis is deflected from the direction of the oncoming flow by the angle of attack. This leads to the fact that the descending blade flows around at a greater angle of attack than the ascending one. The right side of the propeller will generate more thrust than the left side. Thus, a yaw moment to the left will be created.
This moment will have the greatest value at the maximum engine operation mode and the maximum angle of attack.
Influence of atmospheric conditions
Changes in atmospheric pressure and/or temperature result in a change in air density.
This affects:
engine power at constant throttle position
moment of resistance to rotation of the screw.
An increase in air density leads to an increase in both of these parameters, but the engine power increases to a greater extent.
Influence of air density on the operation of a fixed-pitch engine
An increase in density leads to an increase in propeller speed and vice versa.
Influence of air density on the moment of resistance to rotation (required motor torque) of a fixed pitch propeller
An increase in density leads to an increase in the moment of resistance to the rotation of the screw and vice versa.
This is a separate independent unit, or rather a whole bladed unit. It is the propeller for the apparatus on which it is installed, that is, it turns the engine power into traction and, ultimately, into movement.
The man has long been paying attention to the screw. The first theoretical evidence of this is still in the manuscripts and drawings of Leonardo da Vinci. And practically it was first used (for meteorological instruments) by M. V. Lomonosov. at first it was installed on airships, later and to this day on airplanes and when using engines. It is also used on ground vehicles. These are the so-called hovercraft, as well as snowmobiles and gliders. That is, its history (as well as the history of all aviation :-)) is long and fascinating, and it seems that it is far from over.
As for the theory and principle of action ... I wanted to start drawing vector diagrams, and then I changed my mind :-). Firstly, not that site, and, secondly, I have already described all this, and even :-). Let me just say that the propeller blades have an aerodynamic profile, and when it rotates in the air, the same picture arises as when the wing moves.
Aerodynamic force (picture from previous article :-))
All the same, the same bevel of the flow, only now the lifting force becomes the propeller thrust, forcing the aircraft to move forward.
There are, of course, and their own characteristics. After all (more precisely, its blades) in comparison with makes a more complex movement: rotational plus translational forward movement. And in fact, the theory of the propeller is quite complicated. However, for a fundamental understanding of the issue, all that has been said is quite enough. I will dwell only on some features. I note, by the way, that there are not only pulling propellers, but also pushing propellers (such, by the way, were on the Wright brothers' plane).
The propeller of the German airship SL1 (1911) with a diameter of 4.4 m.
Propeller for A400M transport aircraft.
Transport aircraft A400M.
When the propeller rotates and simultaneously moves forward, each of its points seems to move in a spiral, and the propeller itself seems to be “screwed into the air”, almost like a screw into a nut or a screw into a tree. The analogy is very significant :-). It looks like a thread of a bolt-nut pair. Each thread has a parameter such as a pitch. The larger the pitch, the more stretched the thread, and the bolt is screwed into the nut faster. The concept of pitch also exists for the propeller. In fact, this is such an imaginary distance that a propeller rotating in the air will move when it is turned one revolution. In order for it to “screw in” faster, it is necessary that the force pulling it (the thrust of the screw, the very analogue of the lifting force) be greater. Or all, respectively, vice versa. And this can be achieved by changing the value of the analogue of the angle of attack, which is called the angle of the propeller blade, or simply the pitch of the propeller. The concept of propeller pitch exists for all types of propellers, for airplanes and for helicopters, and the principle of their operation is, in general, the same.
Hercules C-4 transporter of the Krolev Air Force parked with propellers in vane mode.
The first propellers on airplanes had a fixed pitch. But the fact is that any screw has such a parameter as efficiency, which evaluates the efficiency of its work. And it can change depending on the change in flight speed, engine power, and the drag of the propeller affects this. Here, in order to maintain efficiency at a sufficient height, a pitch change system was invented (as early as the 30s of the 20th century) and propellers with variable pitch in flight (VISH) appeared. Now, depending on the flight mode set by the pilot, the propeller pitch can change. In addition, there are usually two more special modes. Reversible - to create when the aircraft is braking on the ground and vane, which is used when turning off (often emergency) the engine in flight. Then the blades are set "downstream" so as not to create unnecessary resistance to flight.
The propeller diameter and pitch are the main technical parameters of a propeller. There is also such a thing as twist. That is, each blade is slightly twisted along its entire length. This is done again so that at the same power the blade creates the most thrust.
American experimental aircraft Bell X-22 with impellers 1966
French experimental aircraft with NORD 500 CADET impellers. 1967
1932 Italy. Experimental aircraft with impeller "Flying Barrel"
Modern screws are generally quite diverse in their design. The number of blades can vary (on average from 2 to 8). can be both pulling and pushing. A screw is also called a propeller. This is an old name and comes from the Latin prōpellere, which means to drive, push forward. Now, however, another word has come into use. This is the word impeller. It means "impeller" and they called it a certain type of propeller enclosed in an annular shell. This allows you to increase the efficiency of its work, reduce losses and increase safety. However, such aircraft are only at the stage of experimental development.
The main speed range for the use of propellers is limited to speeds of 700-750 km / h. But even this is a fairly high speed, and various technical tricks are used to ensure stable and efficient operation throughout the entire range. In particular, multi-blade propellers with saber-shaped blades are being developed, work is underway on supersonic propellers, and the above-mentioned impellers are being used. In addition, the so-called coaxial screws have been used for a long time, when two propellers rotate in different directions on the same axis. An example of an aircraft with such propellers would be the fastest aircraft powered by turboprops, the Russian strategic bomber Tu-95. Its speed (max.) is 920 km/h.
Strategic bomber TU-95.
Unfortunately, , especially in combination with , still has a limited scope. Of course, where short-haul aircraft are so needed, the so-called he will still show himself. But nevertheless, he, together with his companion piston engine, has already lost the height-speed-range competition. But more on that in another post...
Photos are clickable.
