Angle of Attack Stability

The stability with respect to angle of attack is evaluated in flight by trimming for level flight and then entering a steady, descending turn at the same speed without changing collective pitch. The extra rotor thrust required to support the weight and centrifugal force will come from the increased angle of attack produced by the rate of descent. If during the turn, the longitudinal control is aft of the level flight position, the helicopter is said to have positive maneuver stability, which is another way of saying that it has positive angle of attack stability. The procedure for calculating trim conditions can be used to investigate the sign and magnitude of this stability. The first step is to calculate the trim conditions in a dive at constant collective pitch (using the method outlined in Chapter 3) of a helicopter whose effective gross weight is equal to its actual gross weight multiplied by the load factor, corresponding to the turn. The second step is to correct the calculated longitudinal cyclic pitch to account for the effect of pitch rate (using the procedure in Chapter 7). This process has been done for the example helicopter in a 1.3-g descending turn at 115 knots with collective held at the level flight value. The results are presented in Table 8.7.

The more forward position of the stick in the turn than in level flight is an indication that this particular helicopter does not have a large enough horizontal stabilizer to give it positive angle-of-attack stability. This might be considered to be a design deficiency that would be corrected in an actual program by increasing the stabilizer area or by installing a "black box” stability augmentation system that could use a signal from an angle of attack sensor to move the swashplate nose down or to change the stabilizer incidence nose up so that the pilot would have to use aft stick to compensate. For a helicopter with either inherent or artifically achieved

positive angle of attack stability, an inadvertant pitching motion which produced a nose-up angle of attack would result in a stabilizing nose-down control response without pilot action. Design decisions in this matter involve tradeoffs between the weight of an adequately sized stabilizer and the weight of an auxiliary stabilizing system. Also entering into the study are considerations of safety involving possible shutdowns or hardovers of the black box system.