[Flightgear-devel] JSBSim Functions for Propwash and Downwash [was: Clarification on YASim input]

[Ron J. <wi...@je...>]

On Saturday 18 June 2011 21:00:41 Jon S. Berndt wrote:
> Here's an example from Hal's P-51D Mustang. This is from an old version, so
> it may have changed by now, but it illustrates the approach. In the
> <aerodynamics> section of the config file - but outside of any <axis>
> element - is this definition of qbar due to propwash:
>
> [Note: <v> is shorthand for "value", and <p> is shorthand for "property".]
>
> <function name="aero/thrust-qbar_psf">
>   <product>
>     <v> 0.5 </v>
>     <p> atmosphere/rho-slugs_ft3 </p>
>     <pow>
>       <sum>
>         <p> velocities/u-aero-fps </p>
>         <product>
>           <p> propulsion/engine/prop-induced-velocity_fps </p>
>           <v> 2.0 </v>
>         </product>
>       </sum>
>       <v> 2.0 </v>
>     </pow>
>   </product>
> </function>

I recently added this function to the A6M2 Zero. I placed it on CYdr (rudder 
yaw) and dCLdf (flap lift delta) as well as CMde. It makes the plane fly 
much, much better during the initial takeoff roll. It probably should be used 
with CMdf (pitch due to flaps) as well.

But why are you multiplying propulsion/engine/prop-induced-velocity_fps by 2?

> [Note that while this function definition is named "aero/coefficient/Cmde"
> it is not really a coefficient, but an actual moment in units of foot
> pounds. The "coefficient" part of the name is a holdout from years past.]

I prefer to read this as "The reconstituted coefficient" or "The formula 
containing the coefficient" :)
 
Seriously, I was never confused by the function naming. It helped me along 
when I was first learning JSBSim, because those are the terms all the 
textbooks use. Look at FDMs like the c182rg that have had everything renamed 
to the marginally pendantically correct "FLDe" instead of "CLDe" and "MmDe" 
instead of "CmDe". I find it ugly and potentially confusing to new users.

Thanks,
Ron