CLIMB ALONG INCLINED TRAJECTORY

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§ 50. General Characteristics of the Climb Regime /10^

Along an Inclined Trajectory

Climb along an inclined trajectory is rectilinear flight of the helicopter with constant velocity and constant angle relative to the horizontal plane.

In this flight regime the helicopter is subject to the weight force, main rotor and tail rotor thrust forces, and the parasite drag force (Figure 67). To determine the flight conditions the helicopter weight force /10- can be broken down into components: G^ perpendicular to the flight path, and

G^ parallel to the flight path and directed opposite the motion.

The main rotor thrust force can also be resolved into the components Y (lift force), P (propulsive force), Sg (side force). The conditions for steady climb are expressed by the equalities

— (rectilinear flight and constant climb angle);

— (constant velocity);

— (absence of lateral displacment of the helicopter);

— (absence of helicopter rotation about its center of

gravity).

Y = G

G„ + X 2 par

T„ = S t. r s

EM = 0 eg

CLIMB ALONG INCLINED TRAJECTORY

Figure 67. Forces acting on helicopter in climb.

§ 51. Thrust and Power Required for Climb

The thrust required for climb can be found by using the diagram of the forces acting on the helicopter (Figure 67),

(31)

where

Gi = G cos 0.; Gz — Gsin9.

Comparing (31) with (26) , we can say that the helicopter parasite drag in climb is practically equal to the parasite drag in horizontal flight at the

same speed. For example, for the Mi-1 in climb (V ^ = 85 km/hr; 0^9°) the

slight increase is explained by the fact that during climb the air flow approaches the fuselage at a large negative angle, which leads to increase of the parasite drag force coefficient from 0.009 to 0.0097. The lift force is less during climb than in horizontal flight. The propulsive force during climb will be greater than the propulsive force in horizontal flight by the

magnitude G sin 0. Consequently, in (31) the first term of the radicand is smaller than in (26), while the second term is larger than the second term of (26). Therefore, the thrust required for climb along an inclined trajectory is practically the same as the thrust required for horizontal flight at the same speed.

If a helicopter can hover, then it can climb along an inclined trajectory. This conclusion is confirmed by the fact that excess thrust appears in the forward flight regime (Figure 68). Consequently, even with some thrust deficiency for hovering (dashed curve in Figure 68), climb along an inclined trajectory at a speed greater than the minimal horizontal flight speed is possible. This circumstance is utilized in the running helicopter takeoff.