6. Autorotation

Autorotation is an emergency mode. It can arise if the engine stops in flight (usually without the pilot's desire), when the rotor is not driven by the engine and begins to rotate by aerodynamic forces resulting from rate of oncoming airflow through the rotor [1, 3].

#### 6.1. Vertical autorotation

In the case of vertical autorotative descent (without forward speed) without wind, the forces that cause a rotation of the blades are similar for all blades, regardless of their azimuth position [2].

During vertical autorotation, the rotor disk is divided into three regions (as illustrated in Figure 16a): driven region, driving region, and stall region. Figure 17 shows the blade sections that illustrate force vectors. Force vectors are different in each region, as the relative air velocity is lower near the root of the blade and increases continually toward its tip. The combination of the inflow up through the rotor with the relative air velocity creates different aerodynamic forces in each section along the blade [2].

Figure 16. Autorotation regions in (a) vertical descend and (b) forward autorotation descend.

In the driven region, illustrated in Figure 17, the section aerodynamic force T acts behind the axis of rotation. This force has two projections: the drag force D and lift force L. In this region, the lift is offset by drag, and the result is a deceleration of the blade rotation. There are two sections of equilibrium on the blade—the first is between the driven area and the driving region, and the second is between the driving region and the stall region. At the equilibrium sections, the aerodynamic force T coincides with the axis of rotation. There are lift and drag forces, but neither acceleration nor deceleration is induced [2].

In the driving region, the blade produces the forces needed to rotate the blades during the autorotation. The aerodynamic force in the driving region is inclined slightly forward with respect to the axis of rotation. This inclination provides thrust that leads to an acceleration of the blade rotation. By controlling the length of the driving region, the pilot can adjust the autorotative rpm [2].

In the stall region, the rotor blade operates above its stall angle (maximum angle of attack), causing drag, which tends to slow rotation of the blade.

Figure 17. Force vectors in vertical autorotation.

Figure 15 shows the effects of this on the power required to hover. If the hover height in ground effect must be maintained, the aircraft can only be kept at this height by reducing the angle of attack (AoA) so that the total reaction produces a rotor lift exactly equal and opposite to weight. It shows that the angle of attack is slightly less, the amount of total rotor thrust is the same as the gross weight, the blade angle is smaller, the power required to overcome the reduced rotor drag (or torque) is less and the collective control lever is lower than when

These conclusions are also true to flight in ground effect other than the hover, but the effect is

Autorotation is an emergency mode. It can arise if the engine stops in flight (usually without the pilot's desire), when the rotor is not driven by the engine and begins to rotate by aerody-

In the case of vertical autorotative descent (without forward speed) without wind, the forces that cause a rotation of the blades are similar for all blades, regardless of their azimuth position [2]. During vertical autorotation, the rotor disk is divided into three regions (as illustrated in Figure 16a): driven region, driving region, and stall region. Figure 17 shows the blade sections that illustrate force vectors. Force vectors are different in each region, as the relative air velocity is lower near the root of the blade and increases continually toward its tip. The combination of the inflow up through the rotor with the relative air velocity creates different aerodynamic

namic forces resulting from rate of oncoming airflow through the rotor [1, 3].

Figure 16. Autorotation regions in (a) vertical descend and (b) forward autorotation descend.

hovering out of ground effect. Therefore, there is better lift/drag ratio.

smaller.

6. Autorotation

38 Flight Physics - Models, Techniques and Technologies

6.1. Vertical autorotation

forces in each section along the blade [2].

#### 6.2. Autorotation in forward descend

Autorotative force in forward flight is produced in exactly the same scheme as when the helicopter is descending vertically in still air. However, because of the forward flight velocity there is a loss of axial symmetry in the induced velocity and angles of attack over the rotor disk. This tends to move the distribution of parts of the rotor disk that consume power and absorb power, as shown in Figure 16b. A small section near the root experiences a reversed flow; therefore, the size of the driven region on the retreating side is reduced [1].

### 7. Helicopter stability and control

#### 7.1. Helicopter stability

Helicopter stability means its ability in the conditions of external disturbances to keep the specified flight regime without pilot management [3, 5].

Let us consider the longitudinal motion of a helicopter on the hovering regime (Figure 18). The weight of the helicopter W, attached to the helicopter's CG, is balanced by the thrust force of the main rotor T, applied at a point removed on the vertical axis by the distance z (see Figure 18a).

Recall that a helicopter, like any aircraft, is considered statically stable, if it after a deviation from the steady flight regime tends to return to its original position. Suppose, for example, that as a result of the action of a wind gust U the thrust T is deflected backward (see Figure 18b). The tilt of the thrust will result in the appearance of a horizontal component D acting backwards and a longitudinal pitching moment M = Dz. Under the action of the horizontal component, the helicopter will start to move back with a speed Vx, and under the action of the moment M it will start to rotate relative to the roll axis, increasing the pitch angle with the angular velocity q (see Figure 18c).

Both effects: both the translational velocity and the rotation of the fuselage, and hence the axis of the rotor, will cause the resultant forces T on the rotor to tilt to the same side, opposite to the original inclination. This will cause the appearance of a horizontal component and a longitudinal moment, already oppositely directed, due to which the helicopter will tend to return to the initial pitch angle and to zero forward speed. This means that the helicopter is statically stable in pitch angle and hover speed. Its static stability is due to the properties mentioned above: speed stability and damping.

Figure 18. Longitudinal motion of the helicopter in hover.

Consider, however, the further movement of the helicopter. The inclination of the resultant in the direction of parrying disturbance is too great because of the presence of velocity stability. It leads to the fact that the helicopter in its movement to the initial position skips the equilibrium position and deviates in the opposite direction, but already by a large magnitude. The motion of the helicopter takes the character of oscillation with increasing amplitude. The aircraft, which in the free disturbed motion ultimately leave the initial equilibrium state, is called dynamically unstable. Thus, a helicopter on a hovering regime is dynamically unstable.

6.2. Autorotation in forward descend

40 Flight Physics - Models, Techniques and Technologies

7. Helicopter stability and control

angular velocity q (see Figure 18c).

above: speed stability and damping.

Figure 18. Longitudinal motion of the helicopter in hover.

specified flight regime without pilot management [3, 5].

7.1. Helicopter stability

Figure 18a).

Autorotative force in forward flight is produced in exactly the same scheme as when the helicopter is descending vertically in still air. However, because of the forward flight velocity there is a loss of axial symmetry in the induced velocity and angles of attack over the rotor disk. This tends to move the distribution of parts of the rotor disk that consume power and absorb power, as shown in Figure 16b. A small section near the root experiences a reversed

Helicopter stability means its ability in the conditions of external disturbances to keep the

Let us consider the longitudinal motion of a helicopter on the hovering regime (Figure 18). The weight of the helicopter W, attached to the helicopter's CG, is balanced by the thrust force of the main rotor T, applied at a point removed on the vertical axis by the distance z (see

Recall that a helicopter, like any aircraft, is considered statically stable, if it after a deviation from the steady flight regime tends to return to its original position. Suppose, for example, that as a result of the action of a wind gust U the thrust T is deflected backward (see Figure 18b). The tilt of the thrust will result in the appearance of a horizontal component D acting backwards and a longitudinal pitching moment M = Dz. Under the action of the horizontal component, the helicopter will start to move back with a speed Vx, and under the action of the moment M it will start to rotate relative to the roll axis, increasing the pitch angle with the

Both effects: both the translational velocity and the rotation of the fuselage, and hence the axis of the rotor, will cause the resultant forces T on the rotor to tilt to the same side, opposite to the original inclination. This will cause the appearance of a horizontal component and a longitudinal moment, already oppositely directed, due to which the helicopter will tend to return to the initial pitch angle and to zero forward speed. This means that the helicopter is statically stable in pitch angle and hover speed. Its static stability is due to the properties mentioned

flow; therefore, the size of the driven region on the retreating side is reduced [1].

The given case relates to the helicopter's movement on the pitch angle on the hover. The roll motion on the hover has a similar character. The difference here is manifested only in the period and the degree of growth of oscillation, which depend on the moments of inertia of the helicopter, different in pitch and roll.

The helicopter is neutral in the yaw angle and the altitude on the hover. This means that the helicopter does not tend to keep a given course angle or a given flight altitude. At the corresponding disturbances these parameters will change. But their change will continue only as long as the perturbation is working. At the end of the disturbance, the course angle and altitude will not change.

It can be said that the helicopter is stable with respect to the yaw rate and the vertical speed. This stability is explained by the fact that the main rotor at an increase of the airspeed in a direction opposite to the thrust reduces its thrust, and conversely, when this speed decreases increases the thrust, thus creating a damping force in the direction of the axis of rotation. Therefore, the tail rotor creates a large damping yaw moment on the helicopter, and the main rotor—a damping force for vertical helicopter movements.

In forward flight, the efficiency of helicopter control and the derivatives of the damping moments and moments of stability with respect to the main rotor speed vary insignificantly. However, the moment derivative with respect to the angle of attack, which for the main rotor corresponds to the instability, begins to play an important role. This instability can be compensated if the fuselage of the helicopter has a stabilizer, which improves the desired degree of stability in the angle of attack. But it is difficult to provide satisfactory longitudinal stability even with well-designed stabilizer. That's why the modern helicopters are equipped with electronic stabilization.

In the forward flight, the roll movement is strongly connected with the yaw movement, just as it does on the airplane. These two movement types are therefore referred to as one, "lateral" movement of the helicopter. The own lateral motion of a single-rotor helicopter during a forward flight, as a rule, is periodically stable. In the low-speed modes, while the relationship between the roll and yaw movements is still small, and the roll motion, like the hovering, is unstable, the lateral motion of a single-rotor helicopter is unstable.

Static stability of helicopters with two main rotors differs slightly from the stability of the helicopter with one main rotor. The tandem main rotor helicopter has a significantly greater longitudinal static stability, and the coaxial main rotor helicopter has a greater lateral stability. This is explained by the change of main rotors thrust at a disruption of the equilibrium.

So, the helicopter, essentially, cannot maintain a steady flight regime. The pilot, piloting the helicopter, continuously has to act on the helicopter's controls and create control moments, under which the helicopter to maintain the specified flight regime.

#### 7.2. Helicopter control

Control characteristics refer to a helicopter's ability to respond to control inputs and so move from one flight condition to another [6]. There are four basic controls used during flight. They are the collective pitch control, the throttle, the cyclic pitch control, and the antitorque pedals (Figure 19).

#### 7.3. Collective pitch control

The collective pitch control changes the pitch angle of all main rotor blades. The collective is controlled by the left hand (Figure 19). As the pitch of the blades is increased, lift is created

Figure 19. Basic helicopter controls.

causing the helicopter to rise from the ground, hover or climb, as long as sufficient power is available.

The variation of the pitch angle of the blades changes the angle of attack on each blade. The change in the angle of attack causes a change in the drag, which reflects the speed or rpm of the main rotor. When the pitch angle increases, the angle of attack increases too, therefore the drag increases, and the rotor rpm decreases. When the pitch angle decreases, the angle of attack and the drag decrease too, but the rotor rpm increases. To maintain a constant rotor rpm, which is specific to helicopters, a proportional alteration in power is required to compensate for the drag change. This is achieved with a throttle control or a correlator and/or governor, so that the engine power can be regulated automatically [2].

#### 7.4. Throttle control

So, the helicopter, essentially, cannot maintain a steady flight regime. The pilot, piloting the helicopter, continuously has to act on the helicopter's controls and create control moments,

Control characteristics refer to a helicopter's ability to respond to control inputs and so move from one flight condition to another [6]. There are four basic controls used during flight. They are the collective pitch control, the throttle, the cyclic pitch control, and the antitorque pedals

The collective pitch control changes the pitch angle of all main rotor blades. The collective is controlled by the left hand (Figure 19). As the pitch of the blades is increased, lift is created

under which the helicopter to maintain the specified flight regime.

7.2. Helicopter control

42 Flight Physics - Models, Techniques and Technologies

7.3. Collective pitch control

Figure 19. Basic helicopter controls.

(Figure 19).

The purpose of the throttle is to regulate engine rpm if the system with a correlator or governor does not maintain the necessary rpm when the collective is raised or lowered, or if those devices are not installed, the throttle has to be moved manually with the twist grip to maintain desired rpm. Twisting the throttle outboard increases rpm; twisting it inboard decreases rpm [2].

The correlator is a device that connects the collective lever and the engine throttle. When the collective lever raises, the power automatically increases and when lowers, the power decreases. The correlator maintains rpm close to the desired value, but still requires an additional fine tuning of the throttle. The governor is a sensing device that recognizes the rotor and engine rpm and makes the necessary settings to keep rotor rpm constant. Under normal operation, once the rotor rpm is set, the governor keeps the rpm constant, and there is no need to make any throttle settings. The governor is typical device used in turbine helicopters and is also used in some helicopters with piston engines [2].

#### 7.5. Cyclic pitch control

The rotor control is performed by the cyclic pitch control, which tilts the main rotor disk by changing the pitch angle of the rotor blades. The tilting rotor disk produces a cyclic variation of the blade pitch angle. When the main rotor disk is tilted, the horizontal component of thrust moves the helicopter in the tilt direction.

Figure 20 shows the conventional main rotor collective and cyclic controls. The controls use a swash plate. The collective control applies the same pitch angle to all blades and is the main tool for direct lift or thrust rotor control. Cyclic is more complicated and can be fully appreciated only when the rotor is rotating. The cyclic operates through a swash plate (Figure 20), which has non-rotating and rotating plates, the latter attached to the blades with pitch link rods, and the former to the control actuators [7].

#### 7.6. Antitorque pedals

Two anti-torque pedals are provided to counteract the torque effect of the main rotor. This is done by increasing or decreasing the thrust of the tail rotor (Figures 19 and 21). The torque varies with changes in main rotor power; therefore, the tail rotor thrust is necessary to change

Figure 20. Rotor control through a swash plate.

Figure 21. Tail rotor pitch angle and thrust in relation to pedal positions during cruising flight.

too. The pedals are connected to the pitch change device on the tail rotor gearbox and enable the pitch angle of the tail rotor blades to increase or decrease [2].

## 8. Flight performance

The pilot's ability to determine, in advance, the helicopter's flying characteristics is of utmost importance. It is very important to determine what maximum weight the helicopter can carry before take-off, if the helicopter can safely hover at a given altitude and temperature, what distance is needed to climb above the obstacles, and what is the maximum climb rate [2].

#### 8.1. Factors affecting performance

There are many factors that influence a helicopter's performance in flight. The most important ones are: altitude, including pressure altitude and density altitude, helicopter gross weight, and the wind.

#### 8.2. Altitude

One of the most important factors in helicopter performance is the air density, which decreases with a gain in altitude. The effect of altitude is shown in Figure 22a. Increasing density altitude increases the power required in hover and lower airspeeds. At higher airspeeds, the results of lower air density result in a lower power requirements because of the reduction of parasitic drag. A higher density altitude also affects the engine power available. The power available at a higher density altitude is less than that at a lower one. As a result there is a decrease in the excess power at any airspeed [1].

#### 8.3. Weight

Increases in aircraft gross weight go hand in hand with requirements for higher angles of attack and more power. As shown in Figure 22b, by increasing the weight, the excess power becomes less, but it is particularly affected at lower airspeeds because of induced drag [1].

High gross weight also affects of the maximum height at which the helicopter can operate in ground effect for a given power available. Under these conditions, the heavier the helicopter is, the lower the maximum hover altitude is [3].

#### 8.4. Wind

too. The pedals are connected to the pitch change device on the tail rotor gearbox and enable

The pilot's ability to determine, in advance, the helicopter's flying characteristics is of utmost importance. It is very important to determine what maximum weight the helicopter can carry before take-off, if the helicopter can safely hover at a given altitude and temperature, what distance is needed to climb above the obstacles, and what is the maximum climb rate [2].

the pitch angle of the tail rotor blades to increase or decrease [2].

Figure 21. Tail rotor pitch angle and thrust in relation to pedal positions during cruising flight.

8. Flight performance

Figure 20. Rotor control through a swash plate.

44 Flight Physics - Models, Techniques and Technologies

Wind direction and velocity also affect hovering, takeoff, and climb performance. Translational lift occurs any time when there is relative airflow over the rotor disk. This explains whether the relative airflow is caused by helicopter movement or by the wind. With the increase in the wind speed, the translational lift increases, therefore less power is required in hovering [2].

Besides the magnitude of wind velocity, its direction is essential. Headwind is the most desirable because it gives the greatest increase in performance. Strong crosswind and tailwind require the

Figure 22. Power required and power available at (a) different altitudes, and (b) different weights.

more tail rotor thrust to maintain the directional control. The increased tail rotor thrust takes away a power from the engine, and therefore will have less power available to the main rotor, which produces the required lift. Some helicopters have a critical wind azimuth limits and the manufacturer presents maximum safe relative wind chart. If the helicopter operates above these limits, it can cause a loss of tail rotor control [2].

#### 8.5. Performance charts

When developing performance charts, aircraft manufacturers make some assumptions about the operating helicopter conditions and the pilot's ability. It is supposed that the helicopter is in good operating condition and the engine is able to develop its rated power. It is assumed that the pilot performs normal operating procedures and he has average flying abilities [2].

With these assumptions, the manufacturer develops performance data for the helicopter taking into account the flight tests. But the helicopter is not tested under all conditions shown on the performance chart. Instead, an evaluation of the specific data is performed and the remaining data are obtained in mathematical way [2].

Generally, the charts present graphics related to hover power: in ground effect (IGE) hover ceiling vs. gross weight, and out of ground effect (OGE) hover ceiling vs. gross weight. The exact names of these charts may vary by different helicopter manuals. These are not the only charts, but these charts are perhaps the most important charts in each manual—they help to understand the amount of power which the helicopter have to have under specific operating conditions (altitude, gross weight, and temperature).
