On Fri, 2009-12-11 at 10:44 -0700, John Denker wrote: > On 12/03/2009 10:18 AM, Stuart Buchanan wrote: > > >> 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.) > > The current c172p is better but still not great. > > One thing I noticed: With the throttle all the way back: > -- In flight at 60 kias : tachometer = 1000 rpm > -- Parked, 0 kias : 900 rpm
J=V/nD so, In flight J = 60 knot/ ((1000/min)*75 in) ~=0.97 Parked J = 0.0 Cp @ 0 = 0.058, Cp @ 1 = 0.020 E.g., it takes about 3 times more power to turn the prop parked at 900 rpm than in-flight at 60 kcas and 1000 rpm. > This tells me the effect of the prop loading the engine > is not being modeled correctly. So, given a Cp and RPM we can estimate the required horsepower... P = Cp*rho*n^3*d^5 ((0.00238 slug/ft3) * ( 900/min)^3 * (74 in)^5) * 0.058 ~= 7.6 hp ((0.00238 slug/ft3) * ( 700/min)^3 * (74 in)^5) * 0.058 ~= 3.6 hp ((0.00238 slug/ft3) * (2700/min)^3 * (74 in)^5) * 0.058 ~= 204 hp Since our engine only reaches 160 hp@ 2700 rpm we can't spin 2700 static RPM, we can only reach 2300-2400 rpm. This is in-line with my understanding of the performance of this aircraft. > The parked tach reading should be quite a bit lower. Please define "quite a bit lower." > ======================== > > Also, from a standing start, with the throttle all the > way back, the model will accelerate to 30 knots in a > few thousand feet. This is wildly unrealistic. I did not observe this in JSBSim stand-alone. I let the sim run for 30 seconds at 900 rpm and only reached 9 knots, after 5 minutes the speed was only 26 knots. Perhaps you had a tail wind or were headed down hill? That said, reducing idle engine power to 4.4 hp yielded a propeller RPM of 730 and a 300 second speed of 0.3 knots. > Changing the throttle detent so that the throttle can > be more fully closed would help with this, and maybe > also with the static tach reading (above). Since thrust is directly related to RPM, yes, reducing RPM will reduce thrust here, as noted above. > But there may also be issues with the prop efficiency at ultra-low > airspeed (high blade angle of attack). Low blade angle of attack? Increasing airspeed increases blade AoA assuming RPM is held constant. > In RL the prop > efficiency is markedly reduced under such conditions. Depends on the propeller, actually. Ron ------------------------------------------------------------------------------ Return on Information: Google Enterprise Search pays you back Get the facts. http://p.sf.net/sfu/google-dev2dev _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
