To: Dave Jackson
The first operational helicopters had symmetrical airfoils in that the camber above and below the chord line was the same. When the designers departed from that philosophy and made helicopter blades like airplane wings they started to have problems. What I was taught in tech school and on the job at various helicopter firms the cambered blade as opposed to the symmetrical blade will try to climb when cyclic pitch is increased and tend to dive when cyclic pitch is decreased. This would lead me to believe that this condition precipitated the inclusion of dynamic vibration suppression systems on some helicopters. One helicopter that suffers from this phenomenon is the Apache. On the Apache the blades are attached to the head by a strap pack in the form of the letter “A”. The blade is attached to the apex of the A and the two legs of the A are attached to the rotorhead. This in itself is a very flimsy attachment so the only rigid attach point is the pitch horn / pitch link attachment to the swash plate. As mentioned previously the blade will try to climb and dive with the respective inputs of the pitch horn. Because the pitch link and the forward attach point of the A to the rotorhead are forward of the pitch axis, the two points tend to stabilize the blade but the rear attach point does not provide any stability other than the centripetal / centrifugal forces acting on the attach point. When the blade tries to dive the rear portion of the strap pack will lift up and then will be forced down when the blade climbs. This banging against the lower plate of the rotorhead causes the outside elements of the strap pack to fatigue and eventually fail resulting in a very high maintenance item.