Originally Posted by
Salusa
As a side note VHA blades seem to be very "rigid".
They don't have the same flex in them as Bell OEM blades.
Strange for composite blades which I would naturally assume would have more "flex" than OEM metal blades?
Our pilots have been experimenting with the "vertical hop" when low on fuel and no pax during mainly mountain flying...
Yes, my gut feeling was also that composite blades would be more flexible. But then I did a quick check of material comparative properties. Flexibility dictated by a parameter called Young's Modulus, or Modulus of Elasticity, "E". For Aluminium alloy, being the skin of a regular blade that is around 70 GPa. For unidirectional Carbon Fibre Epoxy Resin I see figures in the range of 135 to 183 GPa. So if the 'skin' of the composite blades has same thickness as alloy blades, it would have around double or more the stiffness. But I imagine the composite skin is much thicker than the alloy skin so stiffness of blades will be even more times that of the OEM ones.
The discussion of "vertical hop" gives me the feeling the composite blades might be experiencing some sort of natural frequency excitation as they do their 360 degrees through a massively changing inflow from advancing to retreating. That may just not have been the case with the less stiff OEM blades. Worth watching the slow-mo GoPro footage from Chuck Aaron of a flexing blade (along with cyclic pitch changes) on the Red Bull Bo105 (with rigid rotor head) at:
(you can see when blade is in aft position when the tail boom / tail rotor sweep into view). Perhaps it would be worth VHA doing the same visualisation with some of their blades under the conditions described and then repeat visualisation with regular OEM blades as a comparison.