While testing our version #1 rotor blade in the spring & summer of 1995, a blade weave problem was evident. See Slide Show section of web site. The problem was well pronounced. Even with the collective and cyclic locked out to provide a very stiff control linkage, we could not spin the rotor over 500 RPM without a blade weave developing.

A blade weave is a relative low frequency oscillation of the blade pitch caused when the blade aerodynamic center (AC) is ahead of the blade dynamic center of gravity (DCG). The blade AC is located approximately ¼ of the blade width from the leading edge. The AC spanwise position depends on the blade taper and has to be calculated. Blade DCG is influenced by the spinning of the blade and has to be calculated. DCG is not at the same position as the blade static center of gravity (SCG, point at which the blade could be statically balanced).

The blade weave frequency is a function of blade mass and the stiffness of the rotor linkage (both collective and cyclic). The frequency for our version #1 rotor blade occurred at about 0.4 cycles per revolution of the rotor or 1 cycle per 2.5 rotations.

Should the blade AC be ahead of the blade DCG, then when the blade sees a disturbance it will try to diverge and pitch up or down. It is kept from doing so because of the pitch linkage. However before this diverging pitch change is stopped, the pitch linkage is loaded up and stretched. Now once the blade pitching stops, the stretched linkage pulls the blade back and it pitches in the opposite direction until the pitch linkage is again loaded up. If the blade oscillation amplitude does not dampen out, then a blade weave will develop which shows up as a sever stick shake. The stiffer the pitch linkage the higher the blade weave frequency and the higher the blade RPM can turn before a blade weave will develop.

All moveable flight control surfaces such as a rudder, horizontal stabilizer or stabilator, ailerons, and rotor blades need to have their Center Gravity (CG) ahead of their aerodynamic center to be stable. Some flight surfaces such as wings or props can operate with their CG behind their aerodynamic center if they have high torsional stiffness (wing) and a stiff control linkage (prop) as well as a dynamic pressure that is not too high; dynamic pressure being a function of velocity squared.

Another possible candidate for blade oscillations is blade flutter. Blade flutter is an oscillation condition of the rotor blades that generally occurs at a higher frequency than the rotor's RPM. Blade flutter frequency is a function of blade mass and the torsionall stiffness of the blade. Our rotor blades are torsionally very stiff (4 layers of +/- 45 Carbon cloth are used in the rotor blade's skin) and thus we have never measured any blade flutter tendency.

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