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.

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

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.

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.

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.


Definitely follow up with a visit to the Fourth Fighter Group Web
page. Mike Williams has done a fantastic job of collecting up data on
this subject and others, and in presenting it to us. Much of the data
is directly from Flight Test Reports of the A&AEE and Central Fighter
Establishment. You can't get any better than that.
It's well worth the time spent there.

I haven't visited there in about 6 months or so. I need to go back and
see what new stuff he has. Great resource.

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

Greg Shaw