Change of Forward Speed

One of the most important of the rotor’s flapping characteristics is that caused by a change in speed. To illustrate, let us examine a lifting rotor in a wind tunnel as the tunnel is started. The tunnel speed adds to the velocity on the advancing blade, thus increasing its lift, but subtracts from the velocity on the retreating blade thus decreasing its lift. The advancing blade accelerates up about its flapping hinge, but at the same time the blade is being rotated toward the nose. As a result, the advancing blade reaches its maximum upward flapping angle over the nose and the retreating biade reaches its maximum downward flapping angle over the tail. The rotor trims itself to an equilibrium position when the flapping velocities on the advancing and retreating blades are just enough to change the angles of attack to compensate for the change in dynamic pressure. Thus the flapping will increase as the tunnel speed increases. The magnitude of the flapping is a function of the lift of the rotor. If the blades initially had no lift, the unbalanced dynamic pressure would cause no flapping. The rearward tilt of the rotor thrust vector produces a nose-up pitching moment with respect to the helicopter’s center of gravity, as shown in Figure 7.4. The resultant change in pitching moment as a function of forward speed is known as speed stability> and is one of the most important differences between a helicopter and an airplane, which has no corresponding change in pitching moment with respect to speed.

In free flight, the change in longitudinal flapping with increasing speed is stabilizing since it produces a nose-up moment that causes the helicopter to pitch up and to slow down to its original speed. In some cases the effect of a horizontal stabilizer carrying positive lift, or the interference effects of the front rotor on the rear rotor of a tandem rotor helicopter, can overpower the natural speed stability

of the rotors and produce negative speed stability. In this case, an increase in speed produces a nose-down pitching moment. This causes the helicopter to go into a dive in which the speed and the nose-down pitching moment increase as a pure divergence. This characteristic is, of course, undesirable from a flying-qualities standpoint; but pilots an learn to fly even such unstable aircraft, or the characteristic can be changed by methods which will be discussed. It will later be shown that in all flight conditions too much positive speed stability is as bad as too much negative speed stability and that in hover the optimum condition is one of neutral speed stability. A manifestation of positive speed stability is the requirement for the pilot to move the cyclic stick forward as he increases speed to keep the helicopter trimmed. If the speed stability is negative, the pilot will push the stick forward to initiate an increase in speed; but when he finally trims at the new speed, the stick will be further aft than when he started.