[Jsbsim-devel] Piston engine model

[Marius N. <ma...@u-...>]

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