In article ,
(Gregory W Shaw) writes:
Peter Stickney wrote:

Great work Greg, and mighty close. (You forgot to factor in the
increased temperature at the lower altitude, which will reduce power
somewhat. It's one of those things where the 90/90 rule comes in -
teh first 90% of the accuracy in the analysis takes up teh first 90%
of the effort, and the last 10% takes up the other 90%!


Thanks Peter,

I did take temp into account, that dropped power from 2070 to 2030 hp @
500 ft. Although I did fubar it a little, I used 5800 ft for the base
temp rather than 5750 ft, that would change power to 2033 hp instead of
2032 hp.

(sqrt (276.86 / 287.36)) * 2071 = 2032 hp @ 500 ft.

The change from 500 ft to SL drops power down to about 2026 hp. It looks
like I'm about 1% over published figures. Given the amount of slop
involved all around I'll take that. Particularly for something I can do
with a standard atmosphere chart and a $2.00 calculator in about 1
minute.

It's certainly within the difference that you're going to find
between individual engines. ANd therefore, more than accurate enough.

The temperature factor that I was considering, though, was within the
supercharger, and, to split it a bit more, the temperature addition
contributed by the individual stages, with teh intercooling between
the Aux and Mainstage factored in. (Then there's the difference in
impeller efficiency that occurs as the conditions change - If you're
not careful, it can drive you sane! It's that old 90-90 rule again. )
It wasn't the HP value that I was getting different, but the altitude.
Even that was well within tolerance, so I'd say our models agree.

Not Criticism at all, but Congratulation.

I have seen two different methods of calculating temp affects. I am
using (sqrt (old abs temp/ new abs temp)) * hp

Which is the closest one, although there are aberrations. The
published data for teh V1650-7 (The engine used on later P-51Bs and
the P-51D, don't match up. Even the Specific Engine Characteristics
table in the Pilot's Operating Handbook doesn't seem quite right.

I have also seen simpler version of old abs temp / new abs temp * hp

Using that method I come up with 1996 hp @ 500 ft and 1989 hp @ SL. It
could be that simple, a difference in calculation methods.

The Standard Atmosphere of that time was a bit different, as well,
which could also account for it. THe thing with trying to nail down
these numbers is that they aren't that exact in reality. Every
engine's different, every engine wears differently, and every day is
different. They're never that close.

My spreadsheet is a bit more complicated, it takes blower power into
account as well. And being able to see hp/MAP at multiple altitudes
simultaneously allows me to do some curve fitting that makes for a bit
better accuracy.

Good show. I've some similar tools, myself. (Of course). It's turned
out to be a necessity in sorting out the Variable Speed blowers that
the Germans, and later, Pratt & Whitney used. The normal way to
presenting the performance numbers for tham is just too abstract, and
so it requires a lot of backfitting to sort them out.

I have used it for a number of engines successfully. Given two data
points, generally military power and WEP, I can typically get it to
match within .5 in Hg and 1-2 hp at all altitudes I have published data
for. Given the accuracy of the starting data and all the other slop that
is probably about as accurate as possible.

That's excellent. We'll have to compare notes sometime.

Thanks for the additional Merlin & Griffon data, I'll add it to my
stash.

Plenty more if you need it, Greg, just send me a list, and I'll se
what I can do.

--
Pete Stickney
A strong conviction that something must be done is the parent of many
bad measures. -- Daniel Webster