HELICOPTER DESCENT ALONG INCLINED TRAJECTORY

§ 54. General Characteristics of the Descent Regime /10{

Rectilinear flight at constant velocity along an inclined trajectory is termed the helicopter descent regime with operating engine. A characteristic of this regime is the possibility of controlling the vertical rate of descent and the speed along the trajectory by varying the power supplied to the main rotor.

In this regime the following forces act on the helicopter: weight,

main rotor thrust, parasite drag, and tail rotor thrust (Figure 70).

The helicopter motion takes place along a trajectory which is inclined to the horizon at the angle 0, termed the descent angle.

We resolve the weight force G and the main rotor thrust force T into components perpendicular and parallel to the flight trajectory. We obtain the weight force components G^ = G cos 0 and G^ = G sin 0. The main rotor thrust components will be the lift force Y perpendicular to the flight trajectory, and the force P parallel to this trajectory. The force P may be directed

X X

either opposite the helicopter motion direction or in the direction of this motion.

The direction of the force P^ depends on the position of the cone axis and the main rotor plane of rotation. If the cone axis is perpendicular to

the trajectory, then P = 0. If the cone axis is inclined aft relative to the perpendicular, then P^ will be directed opposite the helicopter motion and will retard this motion. If the cone axis is tilted forward, the force P^ will be directed along the motion and together with the component G2 will be a propulsive force. The cone axis direction is connected with the position of the rotor plane of rotation and, consequently, with the main rotor angle of attack. Most frequently, the main rotor angle of attack is close to zero or has a small negative value. During flight with a large descent angle, the angle of attack is positive and the force Px is directed opposite the motion.

Steady state descending flight is possible under the following conditions

The first condition assures rectilinear flight and constant descent angle. Consequently, by varying the lift force Y we can alter the helicopter descent angle. When the lift force is increased, the descent angle decreases, and vice versa. The second condition assures constant helicopter speed. Let us compare these conditions with those for climb along an inclined trajectory.

The first condition is the same for descent and climb. The second condi­tions differ fundamentally from one another: in climb, the propulsive force

is the main rotor thrust component P, while in descent this force will be the weight force component G^. The thrust force component P^ may be either a part of the propulsive force or a part of the retarding force, depending on the position of the main rotor cone axis. The third and fourth descent conditions are analogous to the same conditions for the other flight regimes.