Thanks again Nick. Certainly overturns another misunderstanding i had about blade 5P modes (or 5/4 wavelength eigenmodes) being significant. Then again the 15% hinge offset will push blade 5/4WL above rotor 5P, by between 1.1 to 2.7 times (say 1.7 or 8.6P). A combination of this and good internal damping lets 5 bladed system work.
Albeit not aero (
) i can definately concur the importance of seat dynamics in perceptions of overall vehicle tranfer function. As part of a small team i am the dynamics and durability engineer (as well as designer, draughtsman, general dogsbody and teaboy
). I like to amaze folks with how well a BIG truck actually can ride&handle (and oversteer if nobody is watching
).
I would imagine that composites make life a little easier, because of the construction techniques allowing good joins between beam members, and the material having inherent damping properties. A larger machine with more panel area, and tough weight requirements, introduces all sorts of new eigenmodes which in the auto industry can be dealt with by swaged stiffeners. Composites allow thicker/stiffer sandwich construction, with some of the panels on EH101 proto being amazing (Heli museum UK).
Edit (24/01/07):
Using flexural waves (frequency dispersive bending wave) for blades in combination with rotating frequency gives:
Rotating_Blade_Freq = SQRT( (N^2 x Static_Nat_Freq )^2 + ( N x Rotor_Freq)^2 ) ; where N = blade N/4 wavelength mode.
For Comanche example this would gives "actual" flexural modes within ranges:
1/4 mode: 1.12P - this mode causes the swashplate lead angle
3/4 mode: 3.36P to 5.44P (1.12 to 1.81 x 3P) - say 4.28P
5/4 mode: 5.60P to 13.56P (1.12 to 2.71 x 5P) - say 8.71P
7/4 mode: 7.84P to 25.69P (1.12 to 3.67 x 7P) - say 14.19P
The range is because the calc does not consider construction, so that lower range is for stressed skin only and upper range is for solid section. This is not confirmed in FE, but would explain why high offset results in such a smooth helicopter. Nick, i imagine Bill Welsh is pretty knowledgeable...
Mart