Coaxial helicopter scheme.

Today there are three main schemes, that are used in the global helicopter constructure: single-screw, coaxial and longitudinal schemes, and the majority of helicopter vehicles are built on the base of one-screw scheme. The pioneers of the helicopter constructure knew very well the advantages of the coaxial scheme of the flying vehicle. However, foreign engineers managed to execute wholesale manufacturing and vast exploitation only of single-screw helicopters with steering screw. This scheme is called the classic one. Single-screw helicopters are very popular in Russia too.

In 1947 Nikolay Kamov started working on designing helicopters of the coaxial scheme in practice. For the last 50 years the team of the company «Kamov» created and put into the mass production the following spectrum of coaxial helicopters: Ka-10, Ka-15, Ka-18, Ka-25, Ka-26, Ka-27, Ka-29 and world-known Ka-32 and Ka-50.

The domains of the usage of coaxial helicopters were determined by their pecularities,that is to say small size, high thrust-to-weight ratio and maneurability, aerodynamic symmetry. These features provided convenient deployement on the tiny-sized runway platforms of the various ships. In utopias of taking off and landing on the shaking deck as well as flying over the sea the unique qualities of coaxial helicopters were brightly shown. Ka-25 and Ka-27 were widely used on the navy ships. Ka-26 and Ka-32 were used in civil aviation. Thanks to the high efficiency of work these helicopters are appreciated abroad as well.

In the early 80s the company «Kamov» created a new modern helicopter of the coaxial scheme Ka-50, designed in the interest of the millitary aviation to perform millitary issues.The flying tests confirmed high flight, technical and maneuverable characteristics of the aircraft and showed its advantages in comparison with the helicopters made on the base of the single-screw scheme. The coaxial helicopters of the company «Kamov» started being talked about in the world, arising various disputes and discussions.

This is why it is especially important to carry out the objective comparative analysis of the features of the helicopters of the coaxial and single-screw schemes.

Compensation of the jet moments of the coaxial bearing screws.

The features of the coaxial helicopters are connected with the realization of the essentially new way of compensation of the jet moment of the bearing screws in comparison with the single-screw helicopters. The jet moments of the coaxial helicopter’s screws are mutually counterbalanced directly on the axis of their rotation. On the helicopter of the single-screw scheme itis required to create a side force of the steering screw applied onthe fuselage for the compensation of the jet moment of the bearing screw.

Designers of coaxial helicopters, in essence, created a new type of the bearing system without the jet moment. The jet moments on the screws are automatically compensated throughout the whole flight without any intervention of the pilot. Due to this, the change in power on the screws of the coaxial helicopter doesn’t lead to disbalance of the helicopter in the traveling relation. In the established flight the top and lower screws of the coaxial helicopter have the zero total jet moment. When moving pedals there arises a difference of the jet moments thanks to which the control the of helicopter on the track is exercised.

The way of performing the compensation of the jet moment, used by the single-screw helicopter demands constant attention of the pilot and regulation of draft of the steering screw for balancing of the helicopter during the flight.

Power opportunities

From the power point of view such decisions at which the power of the power plant goes mainly on useful needs are optimum for the aircraft. For the helicopter it is creation of necessary lifting and propulsive forces on the set mode of the flight.

In the single-screw helicopter the part of power is spent on the drive of the steering screw which creates draft force, used for compensation of a torque of the bearing screw. These expenses make up to 10-12% of the power coming to a shaft of the bearing screw and represent pure losses of the helicopter.

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fig. 1

Whereas on the coaxial helicopter all free power of the power plant is used for the drive of the bearing screws, that is to say for the formation of carrying power. Thus the jet moments are mutually counterbalanced. Therefore, there aren’t any direct power consumption on compensation of the jet moments. Besides, in the hanging mode coaxial screws have positive impact oneach other, that also leads to economy of power. This circumstance is illustrated in fig. 1 which contains the scheme of the air stream going from the top and lower screws of the helicopter which is on the hanging mode. As the stream from the top screw is narrowed in the plane of the lower screw for 15-20%, the lower screw has opportunity to carry out an additional suction of air. In general it increases the section of a stream and reduces power costs to create the carrying power. Besides, thanks to an opposite direction of rotation of the screws on the coaxial bearing system, the energy costs of twisting of a stream significantly decrease that also leads to the decrease in unproductive losses of power.

Results of flight tests and other experimental materials testify that the efficiency of the coaxial bearing screws on average by 1,06-1,1 times (for 6-10%) is higher, than single that is visible in fig. 1. Considering economy of the power going for compensation of the jet moment (10-11%), we receive that in general the efficiency of coaxial helicopters is 16-22% higher, than the single-screw ones. The listed power features provide for the coaxial scheme essential advantages in a ceiling of hanging and in a vertical speed of lifting.

At first sight it seems that due to existence of a twin-screw column coaxial helicopters have to have the bigger front resistance, than the single-screw aircrafts. However, on flight tests this advantage of the single-screw helicopters in used power wasn’t shown. It can be explained by the following factors:
— beneficial mutual effect of the coaxial bearing screws in progress (effect of «a biplane box», providing noticeable economy of part of the located power of the power plant which goes for creation of carrying power and equivalent used inductive power),
— additional costs of power of the drive of the steering screw by the single-screw helicopters;
— additional resistance of the steering screw of the single-screw helicopter, especially taking into account an interference of the steering screw and a tail beam of the helicopter;
— additional harmful resistance of a fuselage of the single-screw helicopter in flight with sliding as it is more preferable for the pilot to pilot the helicopter without list;
— significant reduce of the front resistance on the coaxial helicopter (for example, on Ka-50 — the chassis, recomposing during the flight).

Weight-dimension characteristics

The coaxial design allows to reduce sizes and the mass of the helicopter and it gives it a number of advantages. For comparison purposes weight-dimension characteristics of coaxial and single-screw helicopters with the steering screw it is neccessary to consider two cases: the first when coaxial and single-screw helicopters have the same flight weight and the identical located power of the power plant, and the second when coaxial and single-screw helicopters have identical diameters of screws.

In the first case the use of the coaxial bearing system allows to reduce overall dimensions of the helicopter by 35-40% in comparison to the single-screw one. In the second case the smaller aerodynamic quality and additional losses of power on the drive of the steering screw in the single-screw helicopter stanf for the smaller value of flight weight. Due to the steering screw the dimension sizes of the single-screw helicopter are 20% more that of the coaxial one.

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fig. 2

Thesmall size of a glider of the coaxial helicopter and concentration of heavy units near the center of masses lead to noticeable reduction of the moments of inertia in respect of vertical and cross axes (fig.2) that plays an important role in providing high controllability and maneuverability.

Stability and controllability

The most important feature of the coaxial helicopter significantly improving characteristics of stability and controllability is its aerodynamic symmetry. In development and formation of aircraft industry designers repeatedly addressed to aerodynamically symmetric schemes. Aerodynamic symmetry of the aircraft provides a number of important flight features, and the main thing, simplicity of management. In this plan the aircraft construction development example is very evident: planes were projected and under construction only the symmetric.

The helicopter of the single-screw scheme is aerodynamically asymmetrical aircraft with inherent only to it a number of characteristics. In helicopter engineering it was taken for granted as an inevitable payment for simplicity of the technical solution. However the history of development of helicopter engineering showed that this simplicity isn’t actual. Creation of the efficient steering screw and the transmission having the acceptable resources and characteristics — represent an extremely complex problem which still remains actual.

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fig. 3

Aerodynamic symmetry of the coaxial scheme of the helicopter is provided by the lack of the jet moment on its body, relative proximity of the top and lower screws and their beneficial effect on each other, which leads to a small difference of their drafts in the balanced situation. The side forces of screws directed to the opposite sides counterbalance each other, and the cross moment, arising because of a small rating of screws, is small. Thanks to the absence of the steering screw in the coaxial helicopter there is no constantly operating variable side force. The design of coaxial helicopters provides a harmonious combination of management efficiency and aerodynamic damping that causes good characteristics of controllability. Fig. 3 represents characteristics of controllability of a number of helicopters in the cross channel according to the standard of Aviation management of army of the USA A05-ZZS «Requirements to manual control for military helicopters» for the hanging and flight mode at a small speed. The schedule of dependence of a constant delay time on characteristic frequency displays various levels of controllability of helicopters. It is visible that the Ka-50 helicopter according to the characteristics corresponds to the level of controllability 1 (excellent characteristics of controllability) «Requirements» of AV5-ZZS. Thus it has essential advantages in comparison with other helicopters in size of delay and in frequency.

Thanks to aerodynamic symmetry on the coaxial helicopter there are practically no communications between longitudinal and lateral motion, moreover, independence of control paths and simplicity of piloting is provided. Control of such helicopter is available to pilots of average qualification.

Aerodynamic symmetry substantially changes helicopter’s «face». Absence of the variable (on the flight modes) moment of roving and side force, operating on a fuselage, improves characteristics of stability and controllability, increases safety of flight and facilitates the solution of fighting tasks in extreme conditions. On the coaxial helicopter there are no communications between change of power of engines (the general step of screws) and the traveling and aileron control. On the single-screw helicopter all maneuvers in the course of which the operating mode of engines changes (dispersals and braking, changes of height of flight, «hill» and a dive, fighting turns, etc.) are followed by traveling rebalancing and need of parrying of the arising side forces by the use of the list and sliding. Due to the lack of symmetry, continuous change of interrelation between the movement in the vertical and horizontal planes piloting of the single-screw helicopter becomes complicated and demands more care in training.

Coaxial helicopters on simplicity of management are comparable to planes for initial training. At the same time according to flight characteristics of stability, controllability and maneuverability they have superiority over the single-screw helicopters.

«The flight on the coaxial helicopter is possible and, in essence, is carried out by reflex, just as walking, releasing all human resources for the solution of a tactical task» — the Honored test pilot, the Hero of the Soviet Union N. P. Bezdetnov wrote. You couldn’t say better about controllability of the helicopter of the coaxial scheme.

Maneuverability

In the course of manoeuvering two problems are solved: an exit to the set direction and provision of the demanded difference of heights on the relations to the purpose. Of course, in practice these problems are solved together with other issues, but for the best understanding of the physical nature of the phenomena we will consider them separately.

Manoeuvres with change of the direction of the movement can be executed as under the influence of an overload (we will talk about it separately), and without it. Manoeuvres at which the longitudinal axis of the helicopter is guided in the set direction as a rule, by input of the helicopter in deep sliding, are called «flat». Specific features of the helicopter are ability to make «flat» turns, realized on hanging and when moving with small speeds of flight. The most known «flat» manoeuvre is the helicopter’s turn at the track in the hanging mode. It should be noted that intensity of modern fight and need of achieving tactical superiority for a fighting situation demand expansion of range of speeds, on which «flat» manoeuvre can be used.

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fig. 4

Thanks to the constructive features the coaxial helicopter has unlimited opportunities in the realization of a «flat» turn, surpassing possibilities of the single-screw helicopters. Features of a design provide concentration on the coaxial screw of all major functions: creation of carrying and propulsive (driving) power, longitudinal, aileron and traveling control and management of the general step. Traveling control of helicopter with the coaxial bearing screw is exercised by using a difference of the moments of rotation on the top and lower screws (fig. 4).

It leads to that the control system in general becomes almost independent of a sliding corner. This circumstance, and also lack of the steering screw allows the coaxial helicopter to carry out a «flat» turn with big corners of sliding. For the single-screw flat helicopter the turn is essentially impossible. On the single-screw helicopter admissible corners of sliding are significantly limited to existence of the steering screw. The matter is that change of a corner of sliding leads to change of an angle of attack of the steering screw, conditions of its work and the swing movement of its blades, especially at big speeds of flight. The increase in amplitude of the movement of blades of the steering screw over admissible is a direct threat to security of flight. It is caused by the fact that on steering screws there are no distortion machine guns, and prevention of excessive growth of amplitude of the swing movement is provided only with the regulator of a wave, the opportunities of which are limited. Therefore with a growth of amplitude of the swing movement over the admissible blows of blades of the steering screw about a beam are possible. Besides, with growth of amplitude of the swing movement the loadings in elements of a construction of the steering screw grow as well, which also imposes restrictions on sliding corners.

On coaxial helicopters considerable corners of sliding are admissible. As we already spoke, it is explained by lack of the steering screw and independence of system of traveling management of a sliding corner. The tail plumage of the coaxial helicopter doesn’t impose any restrictions on sliding corner size as it is calculated on change of a corner of sliding in range up to 180 °.

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fig. 5

The «flat» turn was fulfilled on the millitary Ka-50 helicopter. Thus in the range of speeds from 90 to 100 km/h this manoeuvre can be carried out within slidings (turns at the rate) up to 180 ° both to the right, and to the left, and at big speeds (up to 230 km/h) — in limits of 90, thus the list of the helicopter is close to zero. The «flat» turn on big corners (fig. 5) is especially fighting manoeuvre and provides the direction of the motionless weapon of the helicopter towards the purpose in the shortest possible time. Use of a «flat» turn gives to the millitary helicopter unlimited advantages in fight against any land or air situated opponent.

Due to the lack of the steering screw by coaxial helicopter there is an opportunity to carry out manoeuvres with bigger efficiency which is realized by a deviation of pedals to an emphasis with the greatest possible speed and creation of angular speeds without any restrictions.

On the single-screw helicopter efficiency of traveling management is superfluous.

It is connected with need of ensuring traveling balancing for all range of change of power of the power plant. However this efficiency can’t be completely realized in flight — Restriction of angular speeds of rotation by these helicopters is caused by need of prevention of hit of the steering screw in the mode of a vortex ring, and also conditions of durability of a tail beam, the steering screw and transmission.

On coaxial helicopters traveling management is harmonious and balanced, thus optimum degree of efficiency of traveling management is provided. The wheel of the direction increases efficiency of traveling management in proportion to the increasing aerodynamic moments of a glider at increase in speed of flight. Piloting the coaxial helicopter, pilots quickly get used to new conditions and are convinced that by this it is possible to carry out the manoeuvres inaccessible for the single-screw helicopter.

On the modes of landing close to the self-rotation mode, and with an invariable frequency of rotation of the bearing screws in coaxial helicopters, some decrease in efficiency of traveling management is noted. Necessary degree of efficiency of traveling management on these modes is provided also thanks to direction wheels. For increase in efficiency of traveling management on the coaxial helicopters also change-over of frequency of rotation of the bearing screws with its reduction by 3-4% is used.

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fig. 6

The absence of the steering screw in coaxial helicopter gives to the pilot the chance to exercise control at the rate by a deviation of pedals to an emphasis with the greatest possible speed that provides the shortest time of a turn for this corner. It is well visible in fig. 6 where movement parameters when coaxial and the single-screw helicopters perform the turns are reflected in the hanging mode. The coaxial helicopter has big advantage in contrast to the single-screw one in the rate of increase and in the maximum size of angular speed of a turn, and also a large supply of traveling management on hanging, including on a static ceiling, irrespective of barometric height. This advantage passes into significant tactical superiority and provides a prize in a competition.

On the single-screw helicopter with growth of the flight height or with growth of temperature of external air because of reduction of surplus of power of the power plant and increase in a step of the steering screw the located course of traveling management decreases, and consequently, the efficiency of the turn on a hanging ceiling where all located power is used decreases as well so the single-screw helicopter can’t make turns without loss of height.

Manoeuvres with the use of overloads

Manoeuvres with use of vertical and tangential overloads provide the change of a trajectory and speed of flight of the helicopter. In accordance with the accepted terminology, these manoeuvres are usually subdivided on vertical («hills», a dive, etc.), horizontal (bends, the forced bends, dispersals, braking, etc.), spatial (the ascending and descending spirals, fighting turns, turns on «hill», turns on a dive), etc.

Maneuverable opportunities of helicopters are characterized, on the one hand, by the admissible level of overloads, and with another — ability effectively to realize them. Parameters of the bearing system for coaxial and single-screw helicopters of one class define from the identical level of admissible overloads. Comparison of the located overloads demands special consideration.

Manoeuvres in the vertical plane

When performing vertical manoeuvres the flight speed that affects characteristics of maneuverability changes significantly. Getting the helicopter to the set vertical overload is provided, as a rule, with an increase in a corner of pitch and, respectively, an angle of attack of the bearing screw — thus growth rate of an overload is directly connected with growth rate of a corner of pitch, that is — with opportunities of system of longitudinal management, its efficiency and hipness. The efficiency of longitudinal management is higher, the corner of pitch and an overload change quicker. Thus during growth of an overload the speed of flight doesn’t manage to decrease significantly that increases efficiency of manoeuvre. In case of insufficient efficiency of helicopter control when performing manoeuvre the speed of flight decreases quicker, than the overload grows, thus there can be problems with achievement of the set overloads.

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fig. 7

Efficiency and power of longitudinal management of the coaxial helicopter is much higher than of the single-screw one. It is provided by the smaller moments of inertia (see fig. 2] and the big located moments of management, which can be explained by great value of shoulders of forces applied to plugs of the top and lower screws concerning the center of mass of the device. In fig. 7 statistical dependences of the maximum located longitudinal acceleration on the mass of coaxial and single-screw helicopters are shown — Thanks to the bigger power of management by coaxial helicopter the increase in a corner of pitch and an overload occurs much quicker therefore the exit to the maximum overload is carried out quicker and with insignificant reduction of speed of flight. In the single-screw helicopter due to the fact that the process of a conclusion to an overload will drag out, noticeable falling of speed takes place and, therefore, the smaller level of the maximum overload is reached. Thus, the coaxial helicopter, having bigger efficiency and a capacity of longitudinal management, has significantly the big located overloads. Input of the coaxial helicopter in a dive is carried out more effectively and more safely, than of the single-screw one. The matter is that at the moment of input in dive it is required to give the handle from itself, thus the vertical overload significantly decreases, there is the corresponding curvature of a trajectory and the angular speed of a fuselage on a dive grows, in the course of clearing of this angular speed for transition to the established dive the pilot assumes the handle.

Thus the swing movement of blades develops quicker, than the angular speed of a fuselage changes. If the change of angular speed of a fuselage is insufficient because of the small efficiency of longitudinal management (as, for example, at the single-screw helicopter), owing to the oncoming relative traffic of a tail beam and blades their dangerous approchement and even impact is possible. By helicopters of the coaxial scheme the similar thing is impossible. Thus, performance of manoeuvres with the decrease of a vertical overload by coaxial helicopter is more effective and it is safe.

Manoeuvres in the horizontal plane

Among horizontal manoeuvres it is expedient to distinguish manoeuvres rectilinear and curvilinear. Dispersals and braking in the horizontal plane are most often carried out, especially when performing a fighting task in flights by the earth. Possibility of fast movement by the earth from one point to another sharply increases survival of the helicopter when performing military operations.

As an example we will consider dispersals from the hanging mode (any direction- forward, back, to the right, to the left). These manoeuvres can be valued by the following parameters:
the maximum acceleration of dispersal when using surplus of power, the most admissible speeds of movement and time of an exit to these speeds.

Bigger surplus of power due to the lack of the steering screw and higher aerodynamic quality of the coaxial screws in comparison with the single-screw allow the coaxial aircraft to carry out dispersal from the hanging mode with the maximum acceleration and much quicker to disperse to the set speed. It raises both fighting opportunities of the rotary-wing aircraft, and its survival.

The most admissible speeds of flight sideways and back too characterize maneuverability as in the result they define speed of movement of the helicopter from one point to another what is especially important in the conditions of fight. Speed of movement of the coaxial helicopter in any direction from the mode of hanging is limited only to the maximum located courses in a control system of screws. In the single-screw helicopter existence of the steering screw imposes essential restriction on movement speed sideways from the hanging mode owing to possibility of hit of the steering screw in the mode of a vortex ring.

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fig. 8

It is necessary to stop especially on curvilinear manoeuvres in the horizontal plane. The possibility of performance essentially new maneuver — «funnels» (rice, 8) on the coaxial helicopters should be noted here. The tactical sense of application of «funnel» is that at its performance the helicopter can keep a long time in an aiming zone of the land purposes and make firing on them, despite of the negative corner of pitch. On the single-screw helicopter keeping a negative corner of pitch leads to dispersal, leaving from the purpose and to new calling on it that reduces probability of defeat of the purpose.

«Funnel» — is especially a fighting manoeuvre which is carried out at a speed of 100-180 km/h with a negative corner of pitch till 30-35 ° and, in essence, is a side bend at which angles of heel and pitch interchange the position. When performing manoeuvre the making draft of the screw, parallel to the horizontal plane, is directed to the funnel center. This force is counterbalanced by the inertial forces arising at the movement of the helicopter on a trajectory close to circular to a sliding corner 90e. Thus, performance of «funnel» by coaxial helicopter is based on its ability to make deep slidings and movements sideways with big speeds.

Fighting manoeuvre which is applied for the fast change of the direction of the movement, is the forced bend too. — It can be effective at attack of the land purposes and in air fight at attack on counter courses (see fig. 5). On coaxial helicopters big intensity of performance of the forced bends is provided significantly. It is explained by lack of restrictions on the angular speed of rotation and possibility of performance of the forced bend with deep (to 60 °) sliding. The coaxial helicopter possesses these opportunities thanks to lack of the steering screw.

Coaxial helicopters have advantages and when performing all spatial manoeuvres, especially when performing manoeuvres like turn on «hill» when it is necessary to gather big angular speeds and to use deep slidings.

Except above-mentioned manoeuvres, on coaxial helicopters such aerobatic manoeuvres as a slanting loop, somersaults, the ascending barrel, etc. are successfully carried out. At their performance on coaxial cars corners of pitch reach 90 °, lists — 130-140 °.

Autorotation

The analysis of the statistical materials received on the basis of flight tests shows that at identical load of square meter blown away by the bearing screw the coaxial helicopters have smaller minimum vertical speeds of decrease on the autorotation mode, than the single-screw helicopters. It is explained by existence of the two-plan effect, reducing inductive losses of power, in the coaxial bearing system . Besides, on the autorotation mode, despite small draft, the steering screw of the single-screw helicopter consumes a certain power that also leads to increase in vertical speed of decrease in the single-screw helicopters.

Comparison shows that the minimum vertical speed of the millitary coaxial helicopter having load on the blown away area of 57 kgfs/m’, V-10 ° ’ more, than the vertical speed of the single-screw helicopter with loading 43 kgfs/m’. However this difference doesn’t affect landing characteristics of helicopters for the following reasons:
— thanks to aerodynamic symmetry of the coaxial helicopter, absence in control paths of cross communications like «the general step — the pedal» doesn’t come essential disbalance of the car in space upon transition from motor flight to the autorotation mode;
— landing speeds of coaxial helicopters in the mode of autorotation are about 15 km/h less, than of the single-screw ones. It is explained by lower (on 20-30 m) vigorous alignment of cars with big (to 10 °) pitch corners that is provided by higher power of longitudinal management and smaller dimensions of a glider. Smaller landing speeds increase safety of landing, especially by a cross-country terrain.

Traveling controllability of coaxial helicopters in the mode of autorotation is provided at the expense of developed vertical tail group and a difference of torques on screws. «The guide to flight operation» contains recommendations about reduction of frequency of rotation of the bearing screw for 3-4% for the performance of planning on autorotation and for landings with small forward speeds of the movement. At preservation of speed of planning it brings to decrease of 2-3 m/s of vertical speed of decrease. The difference of the moments of resistance to rotation which is simultaneously arising on the bearing screws leads to increase in efficiency of traveling management and improvement of landing characteristics.

Flight on Flight in the mode of a «vortex ring».

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fig. 9

In thecompany «KAMOV», in LII, GNIKI VVS, TsAGI was carried out a large volume of model and flight researches of coaxial helicopters and screws on the mode of «a vortex ring». In the course of flight tests of coaxial aircraft this mode was for the first time studied by the test pilot of our experimental design bureau D-K, Efremov on the helicopter Ka-15. Further this mode was carried out at tests on the Ka-25 helicopters (in GNIKI) and Ka-26 (in LII).

In fig. 9 there are the results of test studies of the mode of «a vortex ring» in a wind tunnel on coaxial and single screws and in flight experiment on coaxial helicopters.

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fig. 10

From the obtained data itfollows:
— the upper bounds of zones of «a vortex ring» for coaxial and single screws practically coincide (see fig. 9). Thus the right and lower borders of a zone where signs of this mode are rather weak, for coaxial screws are slightly wider;

— the upper bound of a zone of «a vortex ring» significantly depends on load of square meter of the blown away area of the bearing screw (fig. 10). Thus with increase in loading admissible value of vertical speed of decrease in the rotary-wing aircraft in flight operation grows. On coaxial helicopters with loading more than 40-50 kgfs/m’ the admissible vertical speed of decrease at small speeds of planning is the same, as on the single-screw helicopters (not less than 5 m/s);

— hit of the coaxial helicopter in the mode of «a vortex ring» and a car conclusion from it are safe, as well as on the single-screw helicopter. For a conclusion of the rotary-wing aircraft of any scheme from the mode of «a vortex ring» it is necessary to have a certain stock of height.

Safety of flights

In questions of flight safety a human factor is especially important. Coaxial helicopters are safer than the single-screw ones as they are easier in management, have the best characteristics of controllability and maneuverability and high aerodynamic quality.

The coaxial helicopter with smaller dimensions in comparison with the single-screw helicopter of the same class, is safer when maneuvering near obstacles and in small heights. In view of the fact that dimensions of the coaxial car are defined by diameters of the bearing screws, in the course of flight near obstacles damage of tail group of the coaxial helicopter is almost impossible. However even damage or loss of plumage, for example, at rough landing, has no essential impact on safe completion of flight as traveling controllability is provided with coaxial screws. By the single-screw helicopter at damage and loss of the steering screw the situation close to the catastrophic is created.

When comparing safety of flight of coaxial and single-screw helicopters opponents often pay attention to danger of impact of blades by coaxial helicopters. It should be noted that the problem of rapprochement of blades with elements of a design is equally actual both for coaxial, and for the single-screw helicopters and is solved by known methods. It should be noted that on the single-screw aircraft cases of impact of blades of the bearing screw with a tail beam, a crew cabin, and also the steering screw with a trailer beam are also recorded.

The bearing screws of coaxial helicopters are designed taking into account the provision of the demanded safety of flight. Besides, in the course of design of the helicopter constructive stocks between blades of the lower screw and elements of a design of the helicopter are provided. The distance between blades of the top and lower screws and between blades of the lower screw and elements of a design of the helicopter is measured with high precision in the course of flight tests in all range of the operational modes of flight, including when performing all manoeuvres. These measurements are taken by means of the special equipment. On the base of measurements and generalization of results of tests of coaxial helicopters on all operational modes including when performing aerobatic manoeuvres, constructive measures for prevention of dangerous rapprochement of blades of the top and lower screws, and also blades of the lower screw with glider design elements are developed.

Eduard PETROSYAN, deputy chief designer

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