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Old 22nd Oct 2016, 11:17
  #153 (permalink)  
dClbydalpha
 
Join Date: Jan 2011
Location: on the cusp
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AnFI - I don't need to "measure how much". I've posted something that readers of this thread can go check for themselves, think about and come to their own conclusions if they want to.

Ascend Charlie - I agree it is evident to anyone with experience. However to the casual observer, or someone who is ab initio, the simplicity of AnFI's original assertion, accompanied by shiny maths and glittery technical words, may seem quite compelling. This is my concern.


The concept is, from my interpretation of the original posts, as follows


Lift is proportional to rho and v squared
Inertial forcings is proprtional to mass, radius and v squared

given a constant density, mass and radius, and substituting in rotor speed for v then through the mathematics of the equation for coning angle, it is possible to "approximately" cancel out rotor speed leaving a constant relationship between lift and coning angle.

The concept is explored within the context of limiting thrust from the disc, i.e. As rotor speed slows, it is necessary to apply more pitch to the blade to derive the same lift, so you hit CLmax "earlier", beyond that the blade stalls and can no longer sustain the lift required, but the inertial forces are lower due to the reduction in rotor speed. Both the aerodynamic and the inertial forces are proportional to rotor speed squared, and so, according to AnFI's proposition there is approximately an "ultimate coning angle" fixed irrespective of rotor speed.


This concept is used in the original original post to try and explain why by looking at the coning angle it can be determined when a pilot ran out of lift capability during a manouevre and it isn't necessary to concern yourself with rotor speed. But if we turn it on its head things are more worrying

If the CLmax is reached approximately at an "ultimate coning angle" irrespective of rotor speed then the disc is essentially self-limiting. The inertia will balance out the aerodynamics, any attempt to put more pitch on when at that "ultimate coning angle" will result in loss of lift as CLmax is exceeded. What is true for the disc must be true also for the blade, therefore an "ultimate coning angle" must also represent an ultimate self-limiting flap angle ... all independent of rotor speed.

The proposition ultimately leads to a concept that any rotor is self-limiting in flap allowing any amount of blade pitch to be applied at whatever Nr% and the physics just takes care of it as CLmax is always reached at approximately the same angle, close enough that it doesn't matter.

I'm not going to recommend any pilots give it a go on rotor run down that's for sure.
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