Hi all,

I have a question related with the subroutine “attach” of the dynamic stall analysis included in Aerodyn (BEDDOES). I’ve been going throught the code, to actually understand what it does, and I’ve tried to relate it with the original model propossed by Beddoes and Leishman, see: (" A Semi-Empirical Model for Dynamic Stall", J.G. Leishman and T.S. Beddoes, Journal of the American Helicopter Society, Vol.34-3). The fact is that, of course, the subroutine is completelly consistent with the model, as expected. The only difference with the orginal model, is a function defined as SAT, which apparently reduces the pitch rate of the profile(alfa dot) by around 0.1 on every time step, and has a very strong impact on the final results. There is no comment about it in pierce’s thesis.

My question is: does anybody know exactly why is that function included on the code? and what is its purposse?

Thanks a lot.

Hi Aleixo,

The SAT function is included in the routine only to limit the extremum “alpha dot” that is allowed in the calculations. SAT is a two slope saturation function. At low values the input value is unchanged, at higher absolute values the input is reduced depending on the “slope” value of the function. Alpha dot can become unreasonably high under conditions such as a parked rotor with an unusual yaw error. The SAT function was designed to allow alpha dot values within reason to pass through unchanged, while limiting the unreasonably high values and preventing excessive aerodynamic coefficient values.