Andy Ross wrote

> 
> Vivian Meazza wrote:
> > However (and there's always a however), I can't land the
> > thing. Closing the throttle and pulling back the propeller pitch
> > control doesn't reduce the power enough. I reasoned that there was too
> > much boost with the throttle closed, (currently set at 10%,
> > AFAICS). 10% of the supercharger output at sea level before the
> > wastegate is a big number
> 
> Yeah, that's a good point.  The idle tuning of the engine was done
> before turbocharging was added, and doesn't really map well to the new
> regime.  As a near-term workaround, you can always pull the mixture
> way back to reduce power to near-zero.

Unfortunately, the Hurricane had/has automatic mixture control!

> I'm wary of adding a magic number to the configuration files for this,
> though.  I wonder if there's a saner way of calculating an appropriate
> idle power dynamically from the input values...

That would be good ... if we knew or could deduce the HP. Meanwhile, it's
quite easy to tune the minimum output using the so called 'magic number'

> A related issue that stands to be fixed is that the current code
> doesn't really model a gear-driven supercharger properly.  With a
> centrifugal compressor like this, the output pressure gain is a direct
> funcition of RPM.  The existing code tries to model an exhaust-driven
> turbo charger by simply multiplying the input MP by the turbo-mul
> factor, which isn't the same thing.  The behavior will match at
> maximum power output, but be off in the middle and at the low end.

Working on it right now. BTW the Merlin had a Roots type displacement
compressor. I'm looking at some representative pressure ratio curves wrt
rpm. I'm testing out a supercharger which varies output with engine rpm,
modified by throttle.

> 
> > One other 'however': the property mp-inhg seems to be bound to the
> > supercharger output before the wastegate is applied.
> 
> Really?  PistonEngine.cpp:112 looks like it is setting the current MP
> based on the turbo output.  Maybe there's a different bug somewhere
> (units conversion, maybe?)
>

The property is bound to the variable _mp, while the wastegate is applied to
the variable mp. If the property is bound to mp, it gives the right output.
The problem is entirely in the binding - the turbo code works correctly. 

Regards,

Vivian



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