. Profile Power

Profile power in forward flight has the same source as it did in hover: the air friction drag of the individual blade elements. In the derivation of the blade element equations in the following section, it will be shown that the profile torque/solidity coefficient has the form:

c8/a» = _^(1 + M2)

The H-force coefficient due to the drag of the blade elements will be shown to be:

where cd is the average value, which may be assumed to be the same as that found in hover for this analysis. Figure 3.8 shows this component of power for the example helicopter as a function of forward speed.

Characteristics of the Total Main Rotor Power Required Curve

The total power required for the main rotor out of ground effect is:

The total main rotor power required for the example helicopter is plotted in Figure 3.8. At hover it has zero slope since the rotor cannot distinguish between forward flight and rearward flight. The power required at moderate speeds is less than at hover because of the rapid decrease in induced power. This leads to the observation that it is possible to fly a helicopter in forward flight that does not have enough power to hover provided that some means is available to achieve the takeoff. These means might include a strong wind or the use of ground effect. At some forward speed, the power required rises rapidly as a result of the cubic relationship of parasite power with speed. The location of the minimum power "bucket” depends on the disc loading and the cleanliness of the design. It varies from about 40 knots for "dirty,” low-disc-loading helicopters to 100 knots for clean, high-disc-loading aircraft.

The energy method that produced the power required curve of Figure 3.8 is only a first approximation. It tends to underestimate power required at high speeds. For the results of the more sophisticated blade element method, see Figure 4.38 of Chapter 4.