Re: [Flightgear-devel] ground effect +- h_b-mac-ft +- documentation

[Stuart B. <stu...@ya...>]

John Denker wrote:

> On 12/01/2009 12:47 PM, Stuart Buchanan asked:
> 
> > You suggest two possible explanations. Not having access to a C-172,
> > nor enough flight time to be able to guess which is correct, I'm not
> > sure there's much I can sensibly do without making it worse. If you
> > can suggest which is wrong, I'll see what I can do. My guess would be
> > idle RPM, given your previous comments that the aircraft if
> > over-powered.
> 
> Those are good questions.  I don't yet know for sure
> whether there is too much power and/or too little drag.

I took the opportunity to check the PoH against the simulator experience. While I didn't go as far as getting the OAT exactly right, the errors I came across were fairly signficant (using a HUD to get accurate altitude/TAS etc.)

As I think you've noted before, the climb rate is too high - I consistently see 1000ft/min up to 8000ft ASL, instead of approx 800ft/min at sea level, and 400ft/m at 8000ft ASL.

In contrast, the cruise speed is a bit too low - I don't recall what I saw at sea-level, but at 8000ft ASL, I saw 107 KTAS rather than 120 KTAS (though as that was very close to the IAS, it may be that the environment was not quite right).

I'm not sure what to make of this. Perhaps the drag and power should be reduced, or possibly the alpha drag needs to increase?

I suspect I need to use JSBSim directly to tune these parameters better.

-Stuart