Our heavyweight helicopter equal in the world does not have
In Rostov started production of the most load-lifting rotary-wing car The Russian holding «Helicopt[...]
HELICOPTER TAKEOFF AND LANDING. Takeoff
Helicopter takeoff is an unsteady accelerated flight mode. During takeoff the velocity varies from V = 0 to the velocity at which steady-state climb is established. This climbing speed is usually equal to the economical horizontal flight speed. Depending on takeoff weight, airfield altitude above sea level, and presence of obstacles, the takeoff may be performed helicopter-style, airplane-style, and helicopter-style with or without utilization of the "air cushion."
Sometimes the helicopter travels over the ground prior to or during takeoff, i. e., taxiing is performed. Helicopter taxiing differs significantly from airplane taxiing.
Helicopter taxiing characteristics. Taxiing is accomplished by means of the propulsive force P, which balances the wheel friction force F (Figure 87a). The reactive moment of the main rotor is balanced by the thrust moment of the tail rotor. The basic differences in helicopter taxiing are:
(1) Presence of a large lift force, which is a component of the main rotor thrust and reduces the wheel pressure force on the ground, i. e., reduces the support reaction. As a result, wheel friction on the ground is reduced, and the possibility of helicopter overturning is increased;
(2) Presence of side forces: tail rotor thrust and the side component
of the main rotor thrust (Figure 87b). These forces develop large overturning moments about the wheel support points, which balance one another. But if there is a change of one of the side forces the overturning moment is unbalanced and can cause the helicopter to overturn (Figure 87c);
(3)
 |
 |
 |
A large nose-down moment develops as a result of the propulsive force P, which creates high loads on the landing gear wheels (wheel).
t. r
‘ t. rt? ,
/ї •?
Therefore, helicopter taxiing must be performed more carefully than airplane taxiing. The taxiing speed must not exceed 10 – 15 km/hr. The surface of the area over which taxiing is performed must be smooth. Taxiing in a strong crosswind is not permitted, since this can lead to overturning of the helicopter.
Helicopter-style takeoff is the primary takeoff mode (Figure 88). In this takeoff a vertical liftoff is made and check hovering is performed at a height of 1.5 -2m (operation of the main rotor, engine, and equipment is checked). Then the helicopter is transitioned into climb along an inclined trajectory with simultaneous increase of the speed. In this process "sinking"
 |
|
|
|
|
|
 |
|
Figure 88. Helicopter-type takeoff.
of the helicopter is possible, i. e., a reduction of the altitude, and sometimes the wheels may even come in contact with the ground. This phenomenon is caused by tilting the main rotor coning axis forward to develop the propulsive force P, the result being a decrease of the vertical component of the main rotor thrust. Therefore, along with tilting of the main rotor coning axis forward, there must be an increase of the thrust force by increasing the rotor pitch.
The takeoff is considered terminated when the helicopter reaches a height of 20 – 25 meters or is above the surrounding obstacles. At this time the acceleration, i. e., the increase of the velocity along the trajectory to the optimal climbing speed, which corresponds to the minimum level flight power, is also terminated. But this type of takeoff cannot he performed if:
the helicopter is overloaded (insufficient engine for hovering outside the "air cushion" influence zone);
the air temperature is high (reduced engine power);
the takeoff is made from a high-altitude airfield (low air density at the given altitude so that insufficient engine power is available). Under
these conditions an airplane-type takeoff is made.
Airplane-type takeoff. During the airplane-type takeoff the helicopter accelerates on the ground, then lifts off and transitions into a climb along an inclined trajectory (Figure 89). In this takeoff use is made of the primary advantage of main rotor operation in the forward flight: increase of
the thrust developed by the rotor with increase of the velocity of the air stream approaching the main rotor (see Figure 68).
|
Figure 89. Airplane-type takeoff. |
As a result of the thrust increase there is an increase of the lift force. /143 When it becomes somewhat greater than the weight force, the helicopter lifts from the ground and transitions into a climb along an inclined trajectory with further increase of the flight speed. We see from the power required and available curves for horizontal flight (see Figure 63a) that the power required for horizontal flight decreases markedly for even a small speed increase. If takeoff is impossible at V = 0 because of insufficient power, at a speed of 40 – 50 km/hr considerable excess power is developed, which then makes it possible for the helicopter to transition to the climb regime with simultaneous increase of the flight speed.
An airfield or at least a small smooth area is required for the airplane – type takeoff. The ground run during takeoff with flight weight exceeding by
10 – 15% the normal takeoff weight for helicopter-type takeoff is 50 – 100 meters. In this case the liftoff speed is 50 – 70 km/hr (with acceleration during the ground run 2.2 m/sec ) and the ground run time is 7 – 10 seconds.
The ground run is performed on all wheels of the landing gear. Some helicopters (the Mi-6, for example) perform the last part of the ground run on the nosewheel. When using this ground run technique the acceleration is increased as a result of the inclination of the fuselage longitudinal axis and the resulting increase of the propulsive force P. The helicopter takeoff is considered complete when a safe height (25 m) and a velocity along the trajectory close to the economical speed for horizontal flight have been reached.
Helicopter-type takeoff utilizing the air cushion. Vibrations may arise during airplane-type takeoff ground run on an uneven surface. Then the takeoff is made using the air cushion (Figure 90). In this takeoff the helicopter lifts off vertically, utilizing the increased main rotor thrust in the air cushion influence zone (the distance from the main rotor plane of revolution to the ground does not exceed R).
After liftoff and hovering in the air cushion zone, the helicopter is transitioned into forward flight i. e., flight at low height with increase of the speed. During the transition maneuver the influence of the air cushion diminishes with increase of the speed, but the forward flight effectiveness increases; therefore, the main rotor thrust force increases, which makes it possible to transition the helicopter into a climb along an inclined trajectory. In order to perform such a takeoff it is necessary to have a sufficiently smooth area, i. e., there must not be any large ditches or dropoffs, where the influence of the air cushion disappears.
In certain cases none of the techniques examined above are applicable because of obstacles surrounding the area. Then takeoff is made without
utilization of the air cushion, i. e., liftoff and check hovering are performed and then a vertical climb is initiated. At a height of 5 – 10 meters above the surrounding obstacles the helicopter is transitioned into climb along an inclined trajectory with simultaneous acceleration to the economical velocity. Vertical takeoff is rarely used, since it requires high power and is performed in the danger zone. If sufficient power is not available, yet takeoff must be made, the helicopter weight should be reduced.
