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XyZspineZyX
10-14-2003, 08:18 AM
Hi everyone,

I cam across an interesting issue while trying to analyze P-51 performance. And that being the 2 stage supercharger, and its quantitative effect on available power at altitude.

for a piston engine not using any kind of supercharger the available power can be given roughly by:

P = n*throttle(%)*Ps*(density of air at the altitude/density of air at sea level)

where Ps is max power available at sea level, and n is the propeller efficiency factor. Consequently your available power would decrease with height. But the supercharger rams more air into the engine, increasing power. How much more air is getting to the engine? I think that's where manifold pressure comes in. In a "training" film about P-51B, they say that at 20,000 feet supercharger switches to 2nd stage high blower - and the manifold pressure goes up to 60(mm Hg?). Now my question is how to translate the indicated manifold pressure to the actual pressure of the air coming to the engine(and hence density, assuming the air temperature is not increased as it passes through supercharger). I know some of you may be actual pilots or otherwise know about this. So any input would be appreciated!

Message Edited on 10/14/0302:47AM by KrasniyYastreb

XyZspineZyX
10-14-2003, 08:18 AM
Hi everyone,

I cam across an interesting issue while trying to analyze P-51 performance. And that being the 2 stage supercharger, and its quantitative effect on available power at altitude.

for a piston engine not using any kind of supercharger the available power can be given roughly by:

P = n*throttle(%)*Ps*(density of air at the altitude/density of air at sea level)

where Ps is max power available at sea level, and n is the propeller efficiency factor. Consequently your available power would decrease with height. But the supercharger rams more air into the engine, increasing power. How much more air is getting to the engine? I think that's where manifold pressure comes in. In a "training" film about P-51B, they say that at 20,000 feet supercharger switches to 2nd stage high blower - and the manifold pressure goes up to 60(mm Hg?). Now my question is how to translate the indicated manifold pressure to the actual pressure of the air coming to the engine(and hence density, assuming the air temperature is not increased as it passes through supercharger). I know some of you may be actual pilots or otherwise know about this. So any input would be appreciated!

Message Edited on 10/14/0302:47AM by KrasniyYastreb

XyZspineZyX
10-14-2003, 08:45 AM
Good question, but you have to say why or what you need such information for, otherwise I don't know how to answer.

But I pretty sure the indicated manifold pressure is very close to the actual, even up there.

The turbo is compressing the thinner surrounding air into the engine to create an artificial higher density inside the engine, similar to the engine operating at lower altitudes, which makes the indicated manifold pressure close to actual...

I hope that was the answer you were looking for??

rgds

XyZspineZyX
10-14-2003, 08:51 AM
"assuming the air temperature is not increased as it passes through supercharger"

in fact the temperature rises dramatically. This is why intercoolers and water injection help so much.

XyZspineZyX
10-14-2003, 09:03 AM
Why I am asking is because I'm trying to analytically find the minimum and maximum speeds(at least the theoretical ones) for the P-51 at different altitudes by plotting the Power required(vs velocity) curve against the power available at that altitude.

I guess what I'm looking for is what the indicated manifold pressure actually measures(it can't measure the real pressure since at sea level it is 760mm HG!), and how i can use that to calculate the density of the air coming into the engine, and hence the available power.
Of course, if someone happens to have some Power vs Altitude graphs for the supercharged V-1650-7 Merlin, that would solve the entire problem!

Message Edited on 10/14/0303:08AM by KrasniyYastreb

XyZspineZyX
10-14-2003, 09:06 AM
Manifold pressure is the pressure as measured in the intake manifold.

Remember to deduct the loss of power from driving the charger too. It is significant.

I'd guess that the MAP is measured in inHg rather than mm, BTW. /i/smilies/16x16_smiley-wink.gif 60 mm Hg would be 2.36 inHg, or 80 hPa. Roughly equivalent to 17700 meters above sea level, real altitude... most recips would perform rather poorly, I think. /i/smilies/16x16_smiley-very-happy.gif

Edit: The brits, and possibly others as well, seemed to favour measuring not the absolute manifold pressure but the manifold pressure compared to SLS. One additional caveat.

The air temperature will increase through the supercharger, but you should be able to safely assume isentropic compression, i e no heat goes in or out of the air. That way, everything gets a lot easier to calculate.

Cheers,
Fred

"If we are an arrogant nation, they will resent us. If we are a humble nation, but strong, they will welcome us."
- George W. Bush, during his campaign. No comment.

(Quote brought back by popular demand - RBJ missed it so much he mailed me about it)

Message Edited on 10/14/0308:09AM by effte

XyZspineZyX
10-14-2003, 10:14 AM
Well, 760 mmHg is equivalent to 29.92 inches of Hg. . . Manifold pressure is given in inches instead of millimeters.

Basically manifold pressure is simply the indication of engine power in aircraft equipped with adjustable pitch props. The super charger is a device which compresses the air entering the intake and makes the engine "think" it is operating at a lower altitude. Typically a normally aspirated, non-turbo/super charged engine begins to lose its maximal power output above 6,500 to 8,000 feet (1,981 to 2,439 meters), so a super charger will permit a given power setting at higher altitudes. (there is also an increas in performance when below this critical altitude, but this is not the reason for super or turbo charging as the available power belwo these altitudes is usually quite high without the help)

I do have the P-51 Pilot operations books, however I don't have the one with the performance charts here so for now try to use these (hard to read, but if you know what your looking for they get the job done).

http://www.zenoswarbirdvideos.com/More_P-51_Stuff.html

The information that I have commited to memory is this:
Max take-off MP: 61 inches MP
Normal operating range: 20-36 inches MP
Max indicated airspeed 505 MPH/440 Knots/808 Kmh (true, or speed over the ground will be higher when at altitude)
Max Takeoff RPM 3,000
Normal operating range: 1,600-2,400 RPM

To better understand what manifold pressure is a measure of, and how it relates to aircraft performance take a look here.
http://www.avweb.com/news/columns/182081-1.html

S!
TX-EcoDragon
Black 1

Reserve Pilot Aircraft #2 of Gruppo 313
Pattuglia Acrobatica Virtuale
http://www.pav-amvi.it

http://www.calaggieflyers.com/

Message Edited on 10/14/0301:30AM by TX-EcoDragon

XyZspineZyX
10-14-2003, 10:18 AM
effte wrote:
- Remember to deduct the loss of power from driving
- the charger too. It is significant.

I recall reading 6% as a good estimate for this recently.

Kernow
249 IAP

XyZspineZyX
10-14-2003, 04:40 PM
- Of course, if someone happens to have some Power vs
- Altitude graphs for the supercharged V-1650-7
- Merlin, that would solve the entire problem!

There are various charts out there, but some don't make it clear wether they are taking into account ram air pressure. There is a Russian chart that Isegrim might have, which iirc does not take ram into account.

Another source is the Spit IX test reports at

http://www.fourthfightergroup.com/eagles/spittest.html

in particular, look at the Spit LF IX with Merlin 66, as that's closest to the 1650-7.

http://www.fourthfightergroup.com/eagles/bs543.html

It doesn't give power outputs, but the climb/speed performance does indicate the power available.

I think the Mustang had a bit better ram recovery, but that's variable between individual examples of the same plane.

A British chart gives the following figures:

1590hp 0 ft
1660hp at 9,800ft
1480hp at 13,000ft
1530hp at 20,000ft
1400hp at 22,500ft

It's a straight line chart, and those figures give the peaks and troughs, so plot those figures and you should have the rest. The figures are approx, so don't extrapolate much above 22,500ft.

All figures are with 400mph ram, and at 18lbs boost (67" in MAP)

- Why I am asking is because I'm trying to
- analytically find the minimum and maximum speeds(at
- least the theoretical ones) for the P-51 at
- different altitudes by plotting the Power
- required(vs velocity) curve against the power
- available at that altitude.

The problem you'll have with that is power varies per aircraft, due to the ram recovery. That shouldn't amtter too much, but the more you extrapolate, the greater the error that can creep in.

Engine charts can be misleading, because they may not show ram, or show a different level of ram recovery to what is available in the particular plane.

- I guess what I'm looking for is what the indicated
- manifold pressure actually measures(it can't measure
- the real pressure since at sea level it is 760mm
- HG!), and how i can use that to calculate the
- density of the air coming into the engine, and hence
- the available power.

The indicated manifold pressure does measure the pressure of air coming in to the engine.

The Merlin 66/V-1650-7 ran at 18lbs boost pressure in British terms, which means 18lbs per square inch above normal sea level pressure. In other words, absolute pressure was 14.9 (sea level air pressure) + 18lbs added by the supercharger, which means a pressure of 33 lbs/sq in (roughly). That pressure could be maintained up to 10,000ft or so, at which point manifold pressure begins decreasing.

At 16,000ft the higher speed supercharger gear cuts in, and pressure jumps back up to 18lbs, which is maintained to 22,000ft or so. Above that, the supercharger can no longer maintain the rated pressure, and it drops again.

However, if you look at the Spitfire charts, you can see that at 34,000ft, the supercharger can still maintain 5.7 lbs boost, which means 14.9lbs (sea level pressure) + 5.7lbs. It's not until close to 40,000ft that intake pressure drops below normal sea level pressure.

XyZspineZyX
10-14-2003, 10:49 PM
Thanks for your help everyone. Now I have calculated/estimated the boost provided by the supercharger based on the Spit data. The problem now is that while pressure in the intake manifold does not decrease significantly with altitude, ambient temperature does, which leads to a value for the density of air coming into the engine to be greater than that at sea level! This of course gives an available power at 25,000ft that is greater than that at sea level.

Using the top speed of the Mustang 437 mph at 25,000ft, the engine power to achieve that speed was calculated to be 1052Hp - which I took to be the max available power at that height. This leads to the conclusion that the temperature of the air coming into the engine must increase to 74 degrees Celsius as it passes throught the supercharger, to maintain the corresponding air density. However if I do the same calculation for sea level, I get a temperature increase of only 3-4 degrees. So I still don't actually know how much power is developed at a given altitude.

XyZspineZyX
10-14-2003, 11:14 PM
Yes, the air pressure inside the merlin, and most of the high power engines of WWII is considerably greater than that at sea level. The concept is simply, more air in the chamber, more material to combust.

For example, the R-2800 requires 51 inches of pressure to operate at peak horsepower, reguardless of the altitude. Racing Merlins have been run at over 120 inches of boost. granted, if you have even a minor hicup in the antidetonant system, your engine is likely ot be all over the place, at such high power ratings, but it can be done.

For some good columns on manifold pressure, fuel mixture, and turbo chargers see here:

Engine operation (manifold pressure, props, mixture, and all three in conjunction):
http://www.avweb.com/news/columns/182081-1.html
http://www.avweb.com/news/columns/182082-1.html
http://www.avweb.com/news/columns/182084-1.html
http://www.avweb.com/news/columns/182085-1.html

Turbo compressors:
http://www.avweb.com/news/columns/182102-1.html
http://www.avweb.com/news/columns/182103-1.html
http://www.avweb.com/news/columns/182104-1.html
http://www.avweb.com/news/columns/182105-1.html
http://www.avweb.com/news/columns/182106-1.html
http://www.avweb.com/news/columns/182107-1.html

Leaded fuels, and antidetonents, and detonation:
http://www.avweb.com/news/columns/182132-1.html
http://www.avweb.com/news/columns/182149-1.html

Enjoy.

Harry Voyager

http://groups.msn.com/_Secure/0YQDLAswcqmIpvWP9dLzZVayPXOmo6IJ16aURujNfs4dDETH84 Q6eIkCbWQemjqF6O8ZfvzlsvUUauJyy9GYnKM6!o3fu!kBnWVh BgMt3q2T3BUQ8yjBBqECLxFaqXVV5U2kWiSIlq1s6VoaVvRqBy Q/Avatar%202%20500x500%20[final).jpg?dc=4675409848259594077

XyZspineZyX
10-17-2003, 03:09 PM
I have almost solved the puzzle. I need one more piece. Either the intercooler/aftercooler efficiency, or the induction air temperature at full power at some altitude. Even one data point would suffice. Anybody have a P-51 manual or some dusty book lying around?

Message Edited on 10/17/0310:35AM by KrasniyYastreb