David Luff: I've put Phil's temperature correction back in since I'm not

David Luff: I've put Phil's temperature correction back in since I'm not
implicitly correcting for that yet.  The plane now makes it to 11000ft on
autopilot *slowly*.  I think I'll have to look at the prop parameters
next - altering the diameter and blade angle just slightly can have
quite an effect.  If you want to play then try changing the values
prop_diameter and blade_angle in FGNewEngine::init.  Keep blade
angle between 20 and 25 (since those are the only values for which
I've entered data and I interpolate between them) and ignore
FGProp1_Blade_Angle since that's an old variable that isn't used.  I
really ought to read all the engine and prop parameters from file to
avoid recompilation when tweaking !