D3, we missed each other there. For reference i hope you don't mind if i put highlights of your PM on this post. I think this will increase interest in your model:
Originally Posted by Delta3
I think my model is best classified as : Blade element augmented with impuls theory. That is iterations are performed to balance impuls(=induction) and local aerodynamics on each full rev, according to Glauert.
Near ground I put an extra rule on (OGE) impuls theory consistent with Leishmans formula's (p187), which is backed up by experimental data:
Tige/Toge = 1 / ( 1 - (R/4z)²)
no CFD calculation of the wake is done.
To improve precision, I can add aeroelasticity and tips losses (this can already be activated), but compairing during check mode of the program those reveils differences that are of second/third order and as such of less "full heli dynamic" interest.
Interesting that the altimeter goes down 20'. Don't forget that the altimeter will only read correct when the flow past the static port is the same velocity as the free field. Although i would have thought the downwash velocity would increase the altimeter reading, not decrease it. On the other hand if you had sideslip, from say cross wind, the static port would pick up some dynamic pressure (no instrument is perfect).
I think we have to consider other effects here too. There will be a lag which causes the altimeter to low pass the pressure signal. This is caused not least by the capacity in the VSI.
The other assumption in the aerodynamic model is that the freefield flow is static. It is quite possible for an acoustic wave to build up under the rotor, since the rotor would literally act as a very large diameter loudspeaker as you changed collective. In fact it would not supprise me if a microphone nearby would pick up a subaudio pressure wave, every time collective is changed. This is obviously nibling away at the incompressibility assumption.
I think the best test would be for a 2nd pilot to determine how an altimeter set for Qfe would vary in a low hover into wind, but then if the downwash is not over the static port, does the test count? The more i think about it the more i realise that Mathew's comment about instrumentation accuracy is the limitation. There may just be no practical way to determine if the air just under the rotor compresses slightly in a stable hover...
