VERTICAL CLIMB

To climb vertically the total lift force required, once the ascent is established at a steady rate, is slightly greater than for hovering, the difference being only the difference in drag of

t

Fig. 15.2 In hovering, rotor lift must equal weight plus drag.

the fuselage in the rotor slipstream, which is faster than when hovering (Figure 15.4). However, the engine power required is considerably more. To initiate the climb, either the rotor speed must be increased, to produce more lift, or the angle of attack of all the blades must be increased. Many successful model helicopters have flown with the rotor speed as the only control for climbing or descending. The disadvantage is that the engine and rotor cannot respond instantly to commands so there is a delay with each change as the rotor accelerates or decelerates. More commonly now, and universally with full-sized helicopters, the lift is governed by collective pitch control (Figure 15.5, p.221).

The collective pitch control rotates all the blades simultaneously-to a greater pitch so that they present a higher angle of attack to the air. Since they work at an increased lift coefficient, the strength of the vortices at the tip of the blades increases at once. The profile drag may also increase, depending on the aerofoils section and its drag bucket (see Chapter 9). In any case, there is a marked increase of drag and torque. To keep the rotor turning sufficiently fast to produce the required additional lift to start the ascent, more engine power must be used. In full-sized helicopters it is usual to couple the collective pitch control to the engine throttle to ensure that this extra power is provided automatically when required. Without this, it is easy for the pilot, by coarse use of collective pitch, to slow the rotor down and this can result in less, rather than more, lift force and a descent instead of a climb.

Once the vertical climb has been established, the motion causes further changes of the relationship of pitch angle to airflow, just as a propeller in forward flight has the relative angle of its blades changed by the forward flight velocity.