Airframe Derivatives

Fortunately, the airframe derivatives are somewhat more straightforward to evaluate than the rotor derivatives. Sometime in the latter stages of the design process, wind tunnel results should be available for determining static derivatives directly. Up to that time, the equations of Chapter 8 can be used with the following methods.

Horizontal Stabilizer

Equations for the steady X and Z forces of the horizontal stabilizer were presented in Chapter 8. They can be used almost as is for obtaining derivatives (see tables

9.10 and 9.11). The exception is the addition of a term to the equation for stabilizer angle of attack to account for the time required for the main rotor downwash to reach the stabilizer during nonsteady conditions. The equation now becomes:

TABLE 9.10

Nondimensional Horizontal Stabilizer Derivatives

Value for

Example Helicopter

-.00515 -.00076 .00077 .00108 .00076 .00331 -.00014

and the time to double amplitude is:

_ .693