To: Dave Jackson. .. .Let’s see how much flack this attracts:. .. .I was told by (name on request) that on blades that are symmetrical (NASA 0012 used on early helicopters) the aerodynamic center is located mainly on the blade chord line which makes them stable in flight. However, on cambered blades (like an airplane wing), the aerodynamic center is located outside of the blade, which makes them somewhat unstable. With increased pitch they tend to climb and with decreased pitch they tend to dive and not necessarily where and when the pilot commanded them when he introduced cyclic pitch.. .. .On a conventional control system the blade is firmly anchored and any tendency to climb or dive is reacted by the blade attachment and the pitch horn / pitch link which results in the twisting of the blade. (This is a supposition on my part).. .. .The Apache suffers from this problem because of the unstable attachment of the blade to the head. Using the analogy of a three-legged milking stool it is stable with three legs but if you remove one leg then it becomes unstable. On the Apache the blade is restrained from flying outward by series of straps that are anchored at the rotorhead and attached to the blade at the other end. The only other point of attachment is the pitch horn / pitch link which anchors the system to the swashplate. In the case of the Apache when pitch is added or subtracted the blade will feather by flexing or twisting the attachment straps. The front strap will work against the rotorhead while the rear strap, which is not anchored down, will flex upward. The continued instability of the blade will cause the rear strap to continually strike the rotorhead causing the straps to break. This is a major maintenance driver on the Apache.