In article ,
(Gregory W Shaw) writes:
Others have already hit on what effect higher octane ratings had. Peter
Stickney will probably have one of his great replies coming along soon
too. But here is a quick rundown on what 104/150 octane should provide
for a Merlin 266.

The US had an empirical formula for calculating MAP limits at different
PN. It is a little conservative, but gives a good ballpark figure.

((old MAP -7) * new PN/old PN) +7 = new MAP

So, 66.6 in Hg on 100/130 octane would give:

((66.6 - 7) * 150/130) +7
59.6 * 1.154 + 7 = 75.76 in Hg

The RAF actually used +25 psi, about 80.9 inches.

We know the Merlin 266 was rated at 66.6 in Hg, 1705 hp @ 5750 ft in low
blower. That is enough information to approximate how much power the
engine provides at any altitude.

We also know static pressure at 5750 ft is approx 24.20 in Hg. So,
dividing 66.6 by 24.20 gives us approx 2.75 for the pressure ratio that
the Merlin 266 provides in low blower.

Multiplying static pressure by the pressure ratio gives the manifold
pressure available at any altitude. 80.9 in Hg would be attainable up
to about 500 ft unrammed, and approx 82.4 in Hg at SL.

Since we know it produces 1705 hp @ 66.6 in Hg we can figure how much
it makes at 80.9 in Hg. 1705 * 80/66.6 gives about 2071 hp. Then you
have to take the difference in temp into account. Sqrt of absolute temp
at 5750 ft / absolute temp at 500 ft times 2071 hp.

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

(I'm using the 1976 standard atmosphere for all calculations, older
atmosphere models might provide slightly different figures)

This should be accurate +- about 1%. You can do the same thing for just
about any engine, provided you have an accurate base altitude, power and
MAP rating to start with. I cheated and created an Excel spreadsheet
that does all the work for me.

You need to make sure and use static ratings, a lot of RAF ratings are
with 350 or 400 mph RAM which will screw things up. RAM will cause a
higher rated altitude from the ram pressure, but lower power due to
compression heating.

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%!

I've been able to dig up the manufacturer's numbers, as reported in
_Aircraft_Engines_of_the_World_, 1946.

For the Merlin 66, Standard Day, No Ram.
The Combat Ratings in Low Blower we
1705 HP @ 5,750', 3000 RPM/+18 Boost
2000 HP @ SL, 3000 RPM/+25 Boost

These are very close to your numbers, and the effect to the ambient
temperature on the charge air temerature probably make up the
differnce.

Just for the record, here are the numbers in High Blower:
1580 HP @ 16,000', 3000 RPM/+18
1860 HP @ 10,500', 3000 RPM/+25

Other Merlins were also rated for 3000R/+25 on teh 150 PN fuel.
The Merlin 24, with s single-stage, 2-speed blower produced a Combat
Power of:
Low Blower: 1640 HP @ 2,000'; 3000 RPM/+18
1730 HP @ 0'; 3000 RPM/+20.5 (The supercharger
couldn't produce +25# of Boost at Sea Level)
High Blower: 1500 HP @ 9,500'; 3000 RPM/+18
1780 HP @ 4,000'; 3000 RPM/+25

The Merlin 113/114 Series was also re-rated with 150 PN
I'm not sure what their boost limit was on 100/130 fuel, so I'll leave
it out, for now.

Merlin 130 Series engines were also able to use 150 Octane fuel:
Combat Power for a Merlin 130 was:
Low Blower: 1830 HP @ 5,500'; 3000 RPM/+20
2020 HP @ 1,500'; 3000 RPM/+25
High Blower: 1690 HP @ 18,000'; 3000 RPM/+20
1845 HP @ 14,250'; 3000 RPM/+25


Two-Stage supercharged Griffon engines (60 series) were also rated
with 150 PN.
Combat Power for a Griffon 69 was:
Low Blower: 2000 HP @ 6,750'; 2750 RPM/+21.0
2300 HP @ 500'; 2750 RPM/+25
High Blower: 1810 HP @ 21,000'; 2750 RPM/+21.0
2060 HP @ 15,750'; 2750 RPM/+25

TO show you what the effects are of some other approaches, here are
the numbers for an ADI equipped Packard Merlin, the V1650-9 used on
the P-51H:
War Emergency Power:
Low Blower: 1600 HP @ 11,800'; 3000 RPM/67"
1930 HP @ 10,100'; 3000 RPM/80"
High Blower: 1330 HP @ 23,000'; 3000 RPM/67"
1639 HP # 23,500'; 3000 RPM/80" (That's what the
sources say - quite frankly, the altitude number has
to be bogus. It should be around 18,800')

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.

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