> From: "Vivian Meazza"
> Jim wrote
> > Hi Andy,
> > 
> > On the p51d fdm configuration, it looks like the substantial change was
> > actually increasing the turbo multiplier from 2.0 to 5.5,  and not
> > reducing the cruise speed as stated in the CVS log of March 23.  The
> > cruise speed change does have an effect, but it is fairly small.
> > 
> > The problem with putting the turbo multiplier up in that range is the
> > manifold pressure output is directly multiplied by that number.  So full
> > throttle produces an output of 164 inHG manifold pressure.  We should be
> > seeing about 61 inHG at sea level for this engine.
> > 
> > Setting this multiplier lower to get the correct manifold pressure with
> > turbo at sea level should reduce the maximum flight level for the aircraft
> > since the second stage turbo cannot currently be modeled.   On the other
> > hand, using this lower value should NOT produce incorrect lower altitude
> > performance since all the data I'm using is for below the 20,000 ft
> > altitude where the second stage kicks in.  The drag numbers calculated by
> > YASim should be more or less correct up to at least up to 20,000 ft where
> > the second stage would be kicking in.
> > 
> > If there is a problem that setting the multiplier to 5.5 fixes,  I suspect
> > it is in the FDM design and not the P51D configuration.  Any ideas how we
> > can fix or work around this?
> > 
> Where do you get your numbers from for the boost? There are some
> contemporary figures around for the Rolls Royce built versions of the
> Mustang engine which indicate that the turbo multiplier should indeed be
> around 5:
> http://www.spitfireperformance.com/jf934climb.jpg
> Otherwise, you won't get the correct high-altitude performance. In the
> figure above extrapolate the boost curves back to sea-level, ignoring the
> effect of the Boost Controller (aka wastegate), and you will see that 5 is
> about right - even a bit more for the later Merlins. Don't forget that us
> Brits work in psi gauge, while the ex-colonies work in psi absolute: same
> thing +- 1 atmosphere. 
> Although there is a bug in the current code which gives an incorrect readout
> of boost pressure (I forwarded a correction to Andy some weeks ago), the
> existing code gives pretty good results if you plug in the numbers right out
> of the book. I have just done it for the Merlin XX and was very impressed by
> the accuracy.
> Of course, we still have to model the gear-driven supercharger, but again I
> have forwarded some code to Andy which does this. We are still waiting to
> finalise some curves to match supercharger output. I've got some good-enough
> results here.

Hi Vivian,

This sounds very interesting.  I think I'll wait and see what Andy does with 
your patches before "fixing" the P51D again.



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