On 12/17/2008 12:18 AM, Ron Jensen wrote:
> I've made the constant "C" a configurable item, but I don't have a good
> name for it. The patch calls it <gagg-c> but there must be a better
> name... I just can't think of one.
Physically, this C represents internal friction. So, maybe
"gagg_friction" or some such???
> The Gagg-Farrar equation uses a constant to express the power drop off
> of a piston engine as air density decreases.
>
> sigma= air_density / standard_density
>
> phi = ( sigma - C ) / ( 1 - C )
>
> power *= phi
>
> In early testing it seems to work well. I used rho_air_manifold for
> air_density and replaced suction_loss, which was based in part on
> throttle position, with phi. This may resolve one of Torsten's issues
> with the FGPiston model.
There seems to be a consensus that this engine model would
benefit from some TLC.
The Gagg-Farrar equation is delightfully simple and reasonable.
However, AFAICT it assumes the throttle is wide open. That
makes sense for the original applications intended by Gagg
and Farrar ... but I don't think it does what FGPiston needs
to get done.
In particular, I call attention to the lines in front of the
patch Ron contributed:
suction_loss = RPM > 0.0 ? ThrottlePos * MaxRPM / RPM : 1.0;
if (suction_loss > 1.0) suction_loss = 1.0;
MAP = (something) * suction_loss;
First of all, regarding this quantity "suction_loss", it's a
misnomer to call it a "loss" ... because as this quantity
becomes greater the MAP becomes greater.
Terminology aside, I don't think we should be multiplying by
throttle/RPM ... instead we should be dividing by (1+RPM/throttle)
roughly speaking. This is much better behaved, especially
at low RPM.
Furthermore, these lines seem to be missing a rather important
dependence on ambient pressure. This is related to the fact
that while dynamic viscosity is insensitive to pressure and
density, the kinematic viscosity is directly sensitive.
I wrote up a first draft of the algebra involved in making a
quasi-plausible model of this stuff. It can be found at
http://www.av8n.com/fly/throttle.htm
This includes a graph of the non-decreasing power versus
revs curve that we discussed yesterday.
The analysis is not 100% complete; in particular it does
not include the frictional terms that Gagg-Farrar features.
I reckon if we add that in, we might get something pretty
nice. In particular, the Gagg-Farrar result should emerge
as a corollary of the more general analysis, applicable to
the special case where the throttle is wide open.
------------------------------------------------------------------------------
SF.Net email is Sponsored by MIX09, March 18-20, 2009 in Las Vegas, Nevada.
The future of the web can't happen without you. Join us at MIX09 to help
pave the way to the Next Web now. Learn more and register at
http://ad.doubleclick.net/clk;208669438;13503038;i?http://2009.visitmix.com/
_______________________________________________
Flightgear-devel mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/flightgear-devel
