Re: Rules of thumb...

On Monday 20 June 2005 15:51, 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.

Incredibly high pitch.  If you look at a good fast shutter speed photo 
of a Bear (Tu-95) in flight from the side, you see nearly the entire 
propeller blade.  But it's also got a wide pitch range.  Low Pitch is 
down around 25 or 30 degrees, IIRC. That should make the low speed 
efficiency fairly good. 

> 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.

Interesting. It would be interesting to see what happens at negative 
AoAs, as well. (For example, when a TF-104, with the big canopy, 
exceeds Mach 1.8,/36,000', the wing's at a Negative AoA.  The 
airplane's actually getting pretty much all of its lift from the 
expansion over the canopy.  

-- 
Pete Stickney

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