|
From: Marius N. <ma...@u-...> - 2002-09-18 19:26:39
|
Hello guys, I was looking at the piston engine model in FGPiston.cpp. I am not a = propulsion guy but there are some things that don't look right. For = example, in FGPiston::doEnginePower(void), the power at current altitude is computed as Power% =3D (6e-9 * (MAP * = RPM)^2 + (8e-4 * MAP*RPM) - 1 + (TSL - T) * 7/120. The first part of the = equation looks like some kind of interpolation function of manifold = pressure and engine speed which provides a %Power approximation for a = specific engine. However, if you grab the McCormick book (Aerodynamics, = Aeronautics and Flight Mechanics) and look at the engine curves for = O-360-A (page 288) you can see that BHP at sea-level is pretty much = linear with the MAP, so I think the second order term in MAP does not = make sense. The second part of the equation I assume is for correcting = power with altitude and it certainly looks bogus to me. Instead, the = engine output power should be proportional with the density ratio - that = is Power =3D PowerSL * (p/pSL) * (TSL/T)^0.5 I am assuming that this method of estimating the %Power was chosen to = make it easier for the users to set the engine parameters by editing a = single number - the max power, but at the expense of the model realism. = I would be interested in modifying the JSB piston engine model, such = that it uses 2-D look-up tables of Power and FuelFlow as functions of = MAP and RPM which are then specified in the engine xml file. We do have = the data for the O-360 from the McCormick book. Can we get the data for = other Lycoming engines as well ? I guess that remains to be seen. ------------------------------------------- Marius Niculescu Unmanned Dynamics LLC (541) 308-0894 http://www.u-dynamics.com |