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YankeeDoodle

Is the P-40 too slow?

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Yet when the British carried out a series of flight tests in late 42-early 43 specifically to find out what was the maximum boost that could be used on the F.3.R model, i.e. the same engine used in the 1942 P40E, they stated the maximum boost was 56":

 

 

 

 

 

...The level speed measurements with the F.21.R engine installed were first made using a boost limitation of 51" Hg. Measurements were made subsequently using full throttle down to ground level to determine the maximum boost and airspeeds obtainable. With the supercharger gear changed to that of the F.3.R, level speeds were measured using various boost pressures up to a maximum of 56" Hg.

 

 

http://www.wwiiaircraftperformance.org/mustang/ap222.html

 

 

How about re-read the sentence. Test to determine maximum boost obtainable was done on the F.21.R engine. F3R was tested up to maximum, not using full throttle at ground level, but adjusted to a maximum of 56" which coincidently corresponds to the maximum in the pilots notes

FTH of 56" for the F3R on the Mustang is 8,000ft as shown by the graph

ap222speed.gif

 

If you extrapolate the boost line so you get FTH boost at sea level like you can see on the F21R you would get in the high 60s to low 70s inches of MP

Edited by RoflSeal

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Revised 1944 engine limits on all V-1710-73 - K series P-40  (over 60 in MP) and V-1710-81/99 M/N series P-40 (over 57 in MP) series engines clearly state-

 

exceeding these limits will result in cylinder head or other engine part failure

 

AN-VV-F-781 amendment 5 1944 (and other docs)

 

The -39 earlier engines that we have were considerably less developed than the later models with differences to block crankshaft and many other improvements,

 

to say nothing about some of the isolated 'high boost' escapades being on high octane fuel not available in Russia

 

Cheers, Dakpilot

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Done a few checks and I think this is what is exactly happening with P40: 100% throttle is 70" MP, not 56" as I thought. After 10" on MP gauge (@ 30% throttle), to gain extra 5" you have to open throttle by 5%, too. At 50" MP throttle is around 80% - extrapolating to 100% MP should rise to... 70" MP. Max emergency power as stated in specification (at 56" MP) is approx 85% throttle then.

 

Looks like things are alright after all. We just have an option to over-boost the over-boost, blowing up the engine shortly, as it probably should.

 

I think your extrapolation, if linear, is suspect.

I don't think mass flow is linear with throttle butterfly angle, either with or without forced induction.

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But if you have a few data points and have worked a least-squares fit then I guess that would answer it - for the game implementation at least. It would be hard, experimentally to account for ram air effect at various airspeeds without quite a lot of messing about. Might be an interesting exercise.

Edited by Dave

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That's the report I was talking about earlier, at 2960 feet it's 344 mph (553.6 km/h) at 57", the 323 mph value is with a lower manifold pressure which isn't specified, note it says 1125 HP in comparison to the 1415 HP of the 57" setting at the same altitude

 

Ah yes, didn't notice that. 344 mph (553 km/h) at 1415 hp at 2960 ft would equate to about 329 mph (530 km/h) at SL @ 57" MP.   Quite abit faster that I'd expect and an odd outlier, but then again they weren't TAS readings and it was during testing of an experimental radiator and at a weight of just 7400 lbs.

 

Meanwhile a later test of production aircraft showed a top SL speed of 314 mph and 352 mph at 9,200 ft before the carburettor malfunctioned and boost pressure fell. The only other difference was 500 lbs in weight.

 

Thus performance at 57" MP for a P-40E is probably most likely ~315 mph at SL at normal weight, which jives well with the listed performance at 44.5" MP of 287 mph at SL for a full weight fighter.

Edited by Panthera

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Looking att the attached chart it looks like RoflSeal and Ehret have a good point in that the pedal to the metal with no boost control at 3000 rpm points to 70” or at least seems to converge to around 1750 hp at full throttle SL. As I understand it this is also what Ehret’s measurements in-game and extrapolation results in and in that case I guess one should not be too surprised if you in-game blow your engine pretty fast if you apply full throttle at SL. ;)

 

 

post-23617-0-77052700-1516112870_thumb.jpg

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How about re-read the sentence. Test to determine maximum boost obtainable was done on the F.21.R engine. F3R was tested up to maximum, not using full throttle at ground level, but adjusted to a maximum of 56" which coincidently corresponds to the maximum in the pilots notes

FTH of 56" for the F3R on the Mustang is 8,000ft as shown by the graph

ap222speed.gif

 

If you extrapolate the boost line so you get FTH boost at sea level like you can see on the F21R you would get in the high 60s to low 70s inches of MP

 

 

two points:

 

1. if it was that well known that the Allison could be run successfully at 70" boost, why are they limiting the boost to 56"? The whole point of the test was to find out what was the maximum boost and speed available. 

 

2. according to the Hazen letter, it was physically impossible to run a 66" or 70" boost with a 8.8 gear ratio (i.e. the P40E engine) unless you overrev the engine past 3000 rpm, so no you cannot extrapolate, there is a physical limit.

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66" manifold pressure was certainly possible in the P40E at high speeds, low altitude, and especially with over-rev of the engine.

 

probably if you push the boost to maximum and push the RPM past 3,000 rpm, but how long would the engine live and how is that usable in combat?

 

again, according to the Hazen letter, at 60" boost with a 8.8 blower gear, you are at the "structural limit" of the engine. So pushing the boost past 60" and also overreving the engine past 3,000 RPM, you run a very serious risk of seriously damaging the engine.

 

 

 

 

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2. If you have ram you don't need to over-rev the engine. The chart you quoted for instance shows 41" boost@16k and 56" boost@8k, with a more than linear increment as altitude increases. Conservatively this aircraft with this engine would obtain 71" of boost at sea level and full throttle.

 

1. 70" is above engine design limit (as has been pointed out by Venturi). It can go well under certain circumstances, it work OK under even more circumstances, but under the wrong ones it will blow up your engine in less than a minute. And even if these circumstances only covered 1% of the operational conditions, it would in real life be unacceptably high. You'd never clear the engine for such a setting.

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probably if you push the boost to maximum and push the RPM past 3,000 rpm, but how long would the engine live and how is that usable in combat?

 

In BoX..? It's a very useful trick (up to 30s for 1.7K HP) even if difficult to pull off! P40 driver with stopwatch can deny a 109 rope'n'dope or catch/runaway from a 190 in dives...  :lol:

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2. If you have ram you don't need to over-rev the engine. The chart you quoted for instance shows 41" boost@16k and 56" boost@8k, with a more than linear increment as altitude increases. Conservatively this aircraft with this engine would obtain 71" of boost at sea level and full throttle.

 

1. 70" is above engine design limit (as has been pointed out by Venturi). It can go well under certain circumstances, it work OK under even more circumstances, but under the wrong ones it will blow up your engine in less than a minute. And even if these circumstances only covered 1% of the operational conditions, it would in real life be unacceptably high. You'd never clear the engine for such a setting.

 

Yet we have 109's not being able to run at 1.42ata for more than that before the engine blows :-P Kind of ridiculous :D

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I think we at least can conclude that the maximum boost is not the "maximum permitted" of 56 inHg, but rather at the very least 60 inHg. Would be nice to get a developer statement though, since it'd be possible that they aren't aware of the engine speed limitations having an effect on the maximum attainable boost.

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Guest deleted@117422

I think we at least can conclude that the maximum boost is not the "maximum permitted" of 56 inHg, but rather at the very least 60 inHg. Would be nice to get a developer statement though, since it'd be possible that they aren't aware of the engine speed limitations having an effect on the maximum attainable boost.

+1 

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