FORWARD FLIGHT PERFORMANCE

Power Required

Charts representing the nondimensional forward flight performance of isolated rotors and the iterative procedure for determining the trim conditions for the entire helicopter were developed in Chapter 3. These charts and procedures have been used for the example helicopter with appropriate accessory and transmission losses to calculate the engine power required in level forward flight for several gross weights. The results are presented in Figure 4.38. The basic curves show the power required assuming an advancing tip Mach number of 0.7 and the supplementary curves give the additional compressibility corrections as a function of temperature and gross weight based on actual advancing tip Mach number and the method of Figure 3.43, including the secondary effect of increased tail rotor power due to increased main rotor torque.

Also included in the figure are lines representing "upper and lower stall limits” where the increments of the profile torque coefficient, Cg/o0, at the most

critical azimuth position are 0.004 and 0.008, respectively. These limits are from the isolated rotor charts of Chapter 3, corrected for the twist effect as outlined in the discussion of those charts. The presence of these lines is not truly a limit, but simply a warning that many rotors suffer from high blade loads, high control loads, high vibration, and/or erratic flapping due to retreating blade stall in this regime. On the other hand, many rotors have proved relatively stall-tolerant to much higher levels. The aerodynamicist should assume that the rotor designers and dynamicists will provide such a rotor and that he can ignore the stall limits for purposes of estimating performance. An optimistic discussion of the aerodynamic capability of rotors will be found in reference 4.25.

The power required curves of Figure 4.38 as shown apply directly to sea level, standard, conditions but can also be used at other conditions, as follows:

Notes: • Example Helicopter

• Chart Method of Chapter 3

• Out of Ground Effect

• Basic Curves Based on M^qq = 0.7

True Speed, knots

FIGURE 4.38 Normalized Engine Power Required in Forward Flight

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