On Thu, 2009-12-03 at 17:18 +0000, Stuart Buchanan wrote: > 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
Hmm, climb too high and cruise too low... Someone installed a climb propeller on our aircraft? :D For example: http://forums.cessnaowner.org/read/1/7599 "I have a 172H and this summer I had the pitch changed from cruise to climb. I lost approx 2 knots but saw a noticeable increase in takeoff and climb performance. " We're 10% off max cruise, and 25% up on climb performance.. Our propeller model really starts to fall off around an advance ratio of 0.60. Advance ratio is Speed / ((propeller revolutions/time)*(propeller diameter)) Some advance ratio calculations for our c172: 107 knot/ ((2500/min)*75 in) ~= .69 120 knot/ ((2500/min)*75 in) ~= .78 120 knot/ ((2700/min)*75 in) ~= .72 60 knot/ ((1000/min)*75 in) ~= .97 ------------------------------------------------------------------------------ Join us December 9, 2009 for the Red Hat Virtual Experience, a free event focused on virtualization and cloud computing. Attend in-depth sessions from your desk. Your couch. Anywhere. http://p.sf.net/sfu/redhat-sfdev2dev _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
