Re: Rules of thumb...

On Monday 20 Jun 2005 20:51, Andy Ross wrote:
> Lee Elliott wrote:
> > I found that by lowering it from the typical cruise speed of
> > 431kt @ 36000ft to 300kt @ 1000ft has improved the take-offs
> > no end
>
> Yeah, 431 knots with a prop this size is a *really* high pitch
> (most aircraft would call this "feathered").  Placing the peak
> efficiency there means that lower pitches (i.e. takeoffs) are
> going to land on a much less favorable part of the efficiency
> curve.
>
> With a constant speed prop, you have a lot of latitude here. 
> The governor will find the best pitch for you in most cases. 
> The code is written from an energy balance principle anyway,
> so you can't get non-physical behavior.  The worst* failure
> mode of a bad propeller definition is one that is slightly
> more efficient (or much *less* efficient) than it should be.
>
> * Well, no.  If you lie about the power numbers, you can end
> up bolting a Cessna prop to a giant turbine (or vice versa);
> the engine will spin to ridiculous RPMs and the solver will
> freak out.  But the power value is a really easy number to get
> right.
>
> > and it's also allowed me to reduce the camber on the wing
> > which was causing sudden pitch-downs at low altitude/low
> > weights/high-speeds.
>
> This has been a long-standing bug.  I *think* it's in
> Surface.cpp, and happens due to a discontinuity in the lift
> curve at zero (?) AoA. That theory matches all the reports,
> anyway.  But I've stared at the code repeatedly and can't find
> it.  If anyone gets a simple configuration that shows this
> effect really well, let me know.
>
> > However, I've not been able to get a config working if I
> > specify a condition lever anywhere and I imagine that this
> > points to a serious problem in the config.
>
> This feature isn't mine. :) But reading it, it looks pretty
> simple; it just modulates the idle speed of the engine.  It
> shouldn't make any difference to the output at full throttle.
>
> I'm equally unsure of exactly what the condition lever is
> supposed to do, and I suspect it's very engine-dependent; this
> implementation looks safe, though.
>
> Andy

Thanks for the insights.

Does the code include the prop-pitch governor?  I was sort under 
the impression that it did but that didn't seem to gibe with 
controlling the prop rpm using the pitch control.  Am I in fact 
doing what the governor does?

LeeE

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