Category HELICOPTER AERODYNAMICS

Purpose of Main Rotor Hub Horizontal Hinges

The horizontal hinge (HH) has its axis in the plane of rotation of the hub, perpendicular to the longitudinal axis of the. blade (Figure 35a). The blade thrust develops a moment which rotates the blade about this hinge. The thrust moment M.^, = Та causes rotation of the blade relative to this hinge, /50

and this means that the moment is not transmitted to the hub (the thrust overturning moment is eliminated) (Figure 35b).

When the horizontal hinge is used, the thrust-force bending moment at the root of the blade becomes zero, thus unloading the root section; the blade bending is reduced and therefore blade fatigue stresses are reduced and blade service life is increased. The vibrations caused by azimuthal variation of the blade thrust-force moment are also reduced. Summarizing, we can say that the horizontal hinges are intended to:

eliminate the main rotor thrust overturning moment in the forward flight regime;

relieve the blade root section of the thrust bending moment;

reduce fatigue stresses in the blade and vibrations caused by azimuthal variation of the blade thrust moment.

In addition, the horizontal hinges simplify control of the main rotor and helicopter, improve helicopter static stability, and reduce the magnitude of the azimuthal blade thrust variation.

Factors limiting maximal helicopter horizontal flight speed and ways to increase this speed

Answer 1. The maximal horizontal flight speed of a helicopter is limited by the engine power available. Flight at speeds higher than the maximal is not possible, since more power is required for such flight than the engine develops. Powerful gas turbine engines are installed on the new helicopters, for example, the Mi-2, Mi-6, and Mi-8, to increase the maximal flight speed.

Answer 2. With increase of the horizontal flight speed, the forward tilt of the main rotor plane of rotation must be increased. This leads to reduction

of the main rotor angle of attack. As a result of the reduction of the angle

of attack, the rotor thrust force decreases. This is then the reason for the maximal speed limitation. To increase the maximal speed we must increase the main rotor rpm, which requires the more powerful gas turbine engines.

Answer 3. The rotor thrust force must be increased in order to increase the flight speed. If the thrust force is increased by increasing the rpm, local compression shocks develop on the blades. If we increase the thrust force by increasing the main rotor pitch, then the angles of attack of the blade elements at the 270° azimuth increase. The speed is limited by the onset

of blade stall at angles of attack above the critical value. To increase the

speed the main rotor must be unloaded by installing an additional wing or thrusting propellers, i. e., compound helicopters such as the Mi-6, Kamov rotor-wing, and so on must be constructed.

Purpose and Principle of the Main Rotor – Tilt Control System

The main rotor tilt control is designed to control the collective and

cyclic pitch. This system is used to control the main rotor thrust force in magnitude and direction. Therefore, the tilt control is the most important unit of the helicopter control system. There are three types of main rotor tilt controls: ring, "spider," and crank types. The latter type of tilt

control is used only on two-rotor helicopters with side-by-side positioning of the rotors.

The

ring type tilt control can be used on

all helicopters. It includes

(Figure

105):

(1)

movable ring (plate);

(2)

fixed ring;

(3)

slider;

(4)

universal hinge or cardan;

(5)

scissors or bellcrank;

(6)

vertical control rods.

The movable ring of the tilt control rotates relative to the fixed ring. It is driven from the main rotor hub by means of a scissors. On the

Подпись: /169movable ring there are levers which are connected with the blade pitch control horns by means of vertical links. The fixed ring is connected with the slider by means of a universal, which consists of a ring and two mutually perpendicu­lar shafts. The universal permits the tilt control rings to deflect in any direction. If the plane of the rings is perpendicular to the main rotor shaft axis, when the movable ring rotates the vertical links will not have any vertical displacement and the blade pitch will not change. Consequently, in this case the rotor will not have any cyclic pitch change.

If the plane of the rings is tilted forward, the vertical links will be at the lowest position at the 180° azimuth and the blades will have the mini­mal pitch at this azimuth. At the 360° azimuth the links occupy the highest

Purpose and Principle of the Main Rotor - Tilt Control System

Figure 105. Ring-type tilt control: 1 – slider; 2 – lateral

control lever; 3 – fixed ring; 4 – movable ring (plate);

5 – lever on movable ring; 6 – vertical link; 7 – blade pitch change horn; 8 – axial hinge; 9 – vertical hinge;

10 – horizontal hinge; 11 – scissors; 12 – universal ring;

13 – universal longitudinal axis; 14 – longitudinal control lever; 15 – cyclic pitch lever; 16 – collective-throttle lever.

position and the blade pitch will be maximal. From 0 to 180° azimuth the links move downward and the blade pitch decreases. From 180 to 360° azimuth the links move upward and the pitch increases. Cyclic pitch change is accom­plished in this way. We mentioned previously that as a result of cyclic pitch change the cone-of-revolution axis is tilted in the direction of minimal pitch. This means that, in order to tilt the cone axis in any direction, the plane of rotation of the movable ring of the tilt control must be tilted in

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this same direction. The tilting is accomplished with the aid of two levers on the fixed tilt control ring. The control system levers are connected with the cyclic pitch stick, located in the cockpit. When the stick is moved forward, the motion is transmitted to the longitudinal control lever on the fixed ring of the tilt control, and the fixed ring rotates around the trans­verse axis of the universal so that its leading edge descends.

This means that the pitch is minimal at the 180° azimuth, and the cone-of – revolution axis is tilted forward. When the cyclic pitch stick is moved aft, the leading edge of the tilt control ring rises and the pitch is minimal at the 0° azimuth and the cone-of-revolution axis tilts aft. The conclusion is that when the stick is deflected the cone axis tilts in the same direction. When the stick is moved to the right or left the motions are transmitted from the stick to the lateral control lever, and the tilt control ring is rotated about the longitudinal axis of the universal. In this case our previous conclusion still holds: when the stick is deflected the main rotor

cone-of-revolution axis deflects in the same direction. Control of the cyclic pitch and direction of the thrust force vector is accomplished from the cockpit with the aid of the cyclic pitch stick.

On the tilt control slider, there is a lever which is connected by the control system with the collective-throttle lever located in the cockpit.

Подпись:When the collective-throttle lever is moved up, the tilt control slider rises. All the vertical links move upward together and rotate all the blades to a higher incidence angle. In this way the collective pitch is increased. When the collective-throttle lever is moved down, the tilt control slider lowers and the main rotor collective pitch decreases. The tilt control slider is connected by the control linkage with the stabilizer rotation lever. There­fore, change of the collective pitch is associated with change of the stabil­izer incidence angle. On some helicopters the main rotor collective pitch control is coupled with the tail rotor pitch control.

When this coupling is used on helicopters with right-hand rotation of the main rotor, increases of the collective thrust leads to increase of the pitch and of the tail rotor thrust force, and control of the helicopter is made easier. With increase of the main rotor collective pitch, there is an increase of its reactive moment, which causes the helicopter to turn to the left. This rotation is compensated by increase of the tail rotor thrust.

We have examined a very simple tilt control scheme of the ring type.

The real tilt control has two essential characteristics which must be mentioned. The first characteristic is that the hinges of all the levers on the tilt control movable and fixed rings are located in the same plane, which passes through the point of intersection of the universal axes. This arrangement of the hinges makes possible independence of the action of the longitudinal and lateral control of the helicopter. The second characteristic amounts to the following.

In the functional schematic (Figure 106) the longitudinal and lateral control levers are located on the fixed ring opposite the universal axes.

In the real tilt control these levers are located at some angle relative to the universal axes, which is called the control lead angle (x) • In the absence of lead the cone axis will not deflect in the direction of cyclic control stick deflection, rather at some angle ahead in the direction of rotor rotation. This lag in the deflection of the cone-of—revolution axis is associated with the inertia of the blades.

At the 180° azimuth the pitch is minimal, but the blade flapping angle will not be minimal, since the blades will continue to flap down by inertia.

This means that the cone-of—revolution axis tilts in the direction of the 210° azimuth rather than in the direction of the 180° azimuth, i. e., the deflection of the cone-of-revolution axis will not coincide with the stick deflection, and this makes control of the helicopter more difficult. There­fore, the tilt control ring is deflected with a lead angle x = 25 – 30°, which then leads to coincidence of the deflection of the stick and the main rotor cone-of-revolution axis.

Figure 106. Main rotor control lead:

Purpose and Principle of the Main Rotor - Tilt Control System1 – main rotor shaft; 2 – slider;

3 – universal ring; 4 – universal lateral axis; 5 – tilt control fixed ring; 6 – bearing balls; 7 – tilt control movable ring; 8 – longitudinal control lever; 9 – universal longitudinal axis; 10 – lateral control lever;

X-l ~ longitudinal control lead angle;

^2 ~ lateral control lead angle.

Classification of Helicopters

The basic classification of helicopter types is that of the number of main rotors and their disposition. According to the number of main rotors, it is possible to classify helicopters as single rotor, dual rotor and multi­rotor types.

Single rotor helicopters appear in many varieties. Helicopters of the single rotor scheme have a main rotor, mounted on the main fuselage and a tail rotor mounted on the tail structure (see Figure 3). This arrangement, which

was developed Ъу B. N. Yur’yev in 1911, provides a name for one classification.

The basic merit of single rotor helicopters is the simplicity of con­struction and the control system. The class of single rotor helicopters includes the very light helicopters (flight weight about 500 kgf), and very heavy helicopters (flight weight greater than 40 tons). Some of the deficien­cies of the single rotor helicopter are:

Large fuselage length;

A significant loss of power due to the tail rotor drive train (7 – 10% of the full power of the engine);

A limited range of permissible centering;

A higher level of vibration (the long transmission shafts, extending into the tail structure, are additional sources of spring oscillations).

Dual rotor helicopters appear in several arrangements.

Rotors arranged in tandem; this is the most prevalent arrangement (Figure 5a)

Rotors in a transverse arrangement (Figure 5b);

A cross connected rotor scheme (Figure 5c);

A coaxial rotor arrangement (Figure 5d).

The basic merits of helicopters with a tandem rotor arrangement are:

Wider range of permissible centering;

Large fuselage volume; which allows it to contain large-sized loads;

Increased longitudinal stability;

Large weight coefficient.

Helicopters with a tandem arrangement of rotors can have one or two engines, which are located in the forward or aft parts of the fuselage. These helicopters have the following serious deficiencies:

Classification of Helicopters

Figure 5. Dual rotor helicopters.

A complicated system of transmission and control; /8

Adverse mutual interaction between the main rotors which causes, in addition, a loss of power;

Complicated landing techniques are required in the autorotation regime of main rotors.

The following advantages are attributed to helicopters with a transverse arrangement of rotors:

Convenient utilization of all parts of the fuselage for crew and passengers, since the engines are located outside the fuselage;

Absence of harmful interaction of one rotor with the other;

Higher lateral stability and controllability of the helicopter;

The presence of an auxiliary wing, where the engines and main rotors are located, allows the helicopter to develop a high speed.

Deficiencies of these helicopters are as follows:

A complicated system of control and transmission;

An increase in size and structure weight due to the presence of the auxiliary wing.

Dual rotor helicopters with cross connected rotors have a considerable advantage over helicopters with transverse rotors; they do not have an auxil­iary wing, which reduces the size and structure weight. But, at the same time, with these advantages there is a deficiency, — a complicated transmission /9

and control system.

These helicopters are not produced in the Soviet Union. They are en­countered, on occasion, abroad.

The basic advantage of dual rotor helicopters with coaxial rotors is their small size. Their disadvantages:

Complicated structure;

Deficient directional stability;

Danger of collision of the rotor blades;

Considerable vibration.

In the Soviet Union, there are only light helicopters with this rotor arrangement.

Multi-rotor helicopters are not widely used in view of their complex construction.

In all dual-rotor helicopters, the main rotors rotate in opposite direc­tions. In this way the mutual reactive moments are balanced, and the necessity of having a tail rotor is eliminated. Thus the power loss from the engine is reduced.

PRINCIPLES OF HELICOPTER FLIGHT

§ 1. Brief IWstory of Helicopter Development

і

The idea of creating a flying apparatus with an aerial screw, which ]_3

created a lifting force, was suggested for the first time in 1475 by Leonardo de Vinci. This idea was too premature owing to the impossibility of technical realization of the project and opposition by religious opinions. The idea was buried in the archives. A sketch and description of this flying apparatus was displayed in the Milan library and published at the end of the 19^ century.

In 1754, M. V. Lomonosov substantiated the possibility of creating a heavier than air flying apparatus and built a model of a dual rotor helicopter with the rotors arranged coaxially.

In the 19^ century many Russian scientists and engineers developed projects for flying machines with main rotors. In 1869, electrical engineer A. N. Lodygin proposed a projected helicopter powered by an electric motor.

In 1870 the well known scientist M. A. Rykachev was engaged in the develop­ment of propellers.

Metallurgist-scientist D. K. Chernov devised a helicopter scheme with longitudinal, transverse, and coaxially arranged rotors.

Numbers in the margin indicate pagination in the original foreign text.

At the end of the 19^ century, the development of flying machines engaged the attention of the distinguished Russian scientists D. I. Mendeleyev, К. E. Tsiolkovskiy, N. Ye. Zhukovskiy and S. A. Chaplygin. A period of indepth scientific substantiation of the idea of flight with heavier than air flying machines began.

A close associate of N. Ye. Zhukovskiy, B. N. Yur’yev, in 1911 proposed a well-developed single rotor helicopter project with a propeller for direc­tional control and also a fundamental arrangement for helicopter control, that of automatically warping the main rotor. After the Great October Socialist Revolution, when our country began to develop its own aviation industry, work on the creation of a helicopter was continued.

In 1925, in TSAGI, an experimental group for special constructions was organized under the leadership of B. N. Yur’yev This group was engaged in the development of a helicopter.

In 1930 the first Soviet helicopter was built, the TSAGI 1-EA (Figure 1). LA This helicopter was tested by the engineer responsible for its construction, Aleksey Mikhaylovich Cheremyukhin. Cheremyukhin set a world record altitude of 605 m in this helicopter.

PRINCIPLES OF HELICOPTER FLIGHT

PRINCIPLES OF HELICOPTER FLIGHT

Figure 1. TSAGI 1-EA Helicopter.

In 1948 the single rotor helicopters Mi-1 and Yak-100 were built. As a result of the State trials, the helicopter Mi-1 proved to have the most satis­factory characteristics and it was accepted for mass production.

In 1952 the helicopter Mi-4 was built, which, for that time, had a very large useful load. The same year saw the completion and first flight of the tandem arrangement dual rotor helicopter, the Yak-24, "Flying Wagon" designed by A. S. Yakovlev (Figure 2).

PRINCIPLES OF HELICOPTER FLIGHT

In 1958 the heavy helicopter Mi-6 was constructed which, up to the J_5_

present time, has no equal abroad.

In 1961 the helicopters Mi-2 and Mi-8 (Figure 3), which have gas turbine engines, were built. At the present time they are in mass production and they will gradually replace the Mi-1 and Mi-4 helicopters.

The ability of a helicopter to fly vertically, and the possibility of motion in every direction, makes the helicopter a very maneuverable flying machine, and since it can operate independent of airfields its boundaries of utilization are considerably widened.

PRINCIPLES OF HELICOPTER FLIGHT

Figure 3. Mi-8 single rotor helicopter.

At the present time helicopters are found in more and more wider applica­tion in the national economy. They appear as a basic means of conveyance in locations where it is impossible to utilize ground transport or fixed wing airplanes. Helicopters are utilized in civil construction work and to rescue people and property at times of various natural calamities. Lately helicopters are being widely used in the rural economy. From the examples given, it can be seen that the possibilities of utilizing helicopters as flying machines are far from exhausted.

The Helicopter and its Basic Components

Principles of Flight

A helicopter is a heavier than air flying machine that has a lifting force created by a main rotor according to aerodynamic principles.

The basic components of a helicopter are as follows:

Main rotor. Put in motion by the power plant (engine).

Fuselage. Intended for accomodation of crew, passengers, equipment and cargo.

Landing gear, that is, arrangement intended for movement over the ground J6_ or for parking.

Tail rotor. Provides directional equilibrium and directional control of the helicopter.

Propulsion system which sets in motion the lifting and tail rotors and auxiliary systems.

Transmission transfers the torque from the power plant to the main and tail rotors.

All components of the helicopter are attached to the fuselage or are set in it.

Flight is possible for a flying machine if there is a lifting force counterbalancing its weight. The lifting force of the helicopter originates at the main rotor. By the rotation of the main rotor in the air a thrust force is developed perpendicular to the plane of rotor rotation. If the main rotor rotates in the horizontal plane, then its thrust force T is directed vertically upwards (Figure 4a), that is, vertical flight is possible. The characteristics of the flight depend on the correlation between the thrust force of the main rotor and the weight of the helicopter. If the thrust force equals the weight of the helicopter, then it will remain motionless in the air. If, though, the thrust force is greater than the weight, then the helicopter will pass from being motionless into a vertical climb. If the thrust force is less than the weight, a vertical descent will result.

The plane of rotation of the main rotor with respect to the ground can be inclined in any direction (Figure 4b, c). In this case the rotor will fulfill a two-fold function; its vertical component Y will be the lift force and the horizontal component P — the propulsive force. Under the influence of

The Helicopter and its Basic Components

Figure 4. Principle of flight controls of a helicopter, a – vertical flight; b – horizontal flight forwards; c – horizontal flight backwards.

this force the helicopter moves forward in flight. JJ_

If the plane of the main rotor is inclined backwards, the helicopter will move backwards. (Figure 4c). The inclination of the plane of rotation to the right or to the left causes motion of the helicopter in the corresponding direction.