Could you have designed a better P-39?

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Thank you. The article I read said the decreased engine life was due to overboosting and/or overrevving (I can't specifically) but, it was an interview with a pilot and that was probably just his opinion/assumption. Sounds like the actual culprit was poor oil quality.

"At 66", the overboosting produced 1745 hp at SL and 1770 hp at 2000 ft. The engines were also subject to over-revving.

At 70", the overboosting at 3200 rpm produced 1780 hp at 3000 ft.

Allison was worried that the overboosting with the 8.8 blower ratio would produce a 'bad precedent' once engines with a 9.6:1 supercharger ratio entered service. However, the engines with the low alt ratio were "apparently" standing up well to the overboosting. Allison recommended an emergency rating of 60", producing 1570 hp for the engine, for the 8.8:1 engines and 57" for the 9.6:1 engines.


EDIT: Did some more checking and it looks like just a week after the Allison/GM memo, the -39 was re-rate to a maximum of 56" (1470 hp at SL) and the -73 was re-rated to a maximum of 60" (1550 hp at SL). Looks like the engines with the 9.6:1 supercharger ration were limited to a maximum of 57" (1360 hp at SL, 1480 hp at 10,000 ft) at least until April 1944."

If the Allisons were standing up to that level of power, why didn't Allison crank them up like that from the factory?
 
pinsog said:
If the Allisons were standing up to that level of power, why didn't Allison crank them up like that from the factory?
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Sounds like they did if they had to establish a WEP setting as well as take off power.

Recognizing their conservative ratings, it may have been done so production could be kept up with demand, especially at a time when
the war was not decided.
We look at it in hind-site, but its probably a fair enough reason not knowing what future engine demands would be and for which aircraft. This was pre-packard merlin.
 
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pinsog said:
Does anyone have horsepower figures of P39 vs Spitfire vs Me109 at various altitudes?
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Disclaimer: I did not rectified discrepancy PS vs. HP here. The lines for V-1710 are not set in stone: for any pixel of those lines that might be wrong, I am to blame.
 

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No doubt it would be tight. I think the superchargers could fit relatively easy and cooling air supply could be provided by enlarging the air intake. Fitting the intercoolers would be a challenge. But the P-39 needed this to be a real player in Europe and the Pacific.
 
davparlr said:
But the P-39 needed this to be a real player in Europe and the Pacific.
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It depends on your expectations.
I was hoping to come up with something universally accepted to be superior to the P-40.
Altitude issues aside, I'm having a hard time understanding why the P-39 "concept" should not be able to achieve that.
I understand that the execution left a lot to be desired.
But why can't the general concept be made to work?
Does the driveshaft add so much weight to make the concept ineffective?
 
Meaning to show that besides engine development, the Germans were unique in their ability to think 'outside' the box when it comes to power development. Would be interesting to show the comparitive results to the normal DB 601 DB 605 engines ran with B4 / C3 fuels. And maybe apply that to a theoretical P-39 build.
 
gjs238 said:
But why can't the general concept be made to work?
Does the driveshaft add so much weight to make the concept ineffective?
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the drive shaft added about 100lbs. 50lbs for the drive-shaft itself and about 50lbs for the more rigid fuselage needed to keep the propeller in alignment with the engine. The initial "concept" did not include either selfsealing tanks/armor or the wing guns. Bell was also off by a good 10% on weight even not including guns. the plane was "sized" for a much smaller weight than it turned out. The prototype did have tankage for 200US gallons but increasing weight and the change to self sealing tanks cut the fuel capacity to 120 gallons. Long thin tanks suffer more from thick walls than fat tanks. Bell didn't like or want the wing guns which were not in the initial proposals. throw in 245-265lbs of armor and BP-Glass and the planes weight was really spiraling.
Part of the "concept" is that the engine goes on or close to the center of gravity which is supposed to help maneuverability, maybe it does but it also means that there is no room left (or darn little) to put consumable items like fuel and ammo on the center of gravity to avoid trim changes as the load is used up.
Building the smallest airframe they thought they could get away with also cut down on the volume to add things later.

P-63 shows what was needed to fit in what was wanted and even it is tight. Trying to get P-63 performance (load carrying ability and flight characteristics) from the smaller airframe doesn't seem likely or Bell probably would have done it.
 
Hi, Ratsel,
That was the only chart providing graphs for 601/605 that I've managed to find, but if you have another one or more, than please, post them And some data about usage of GM-1, other than occasional instances?
As one can see, I didn't bothered with turboed Allison either - fellow member asked about real P-39s,109s Spits.
 

Germans didn't think 'outside' the box when it came to power development. they chose a different approach to begin with and not all that different from their early V-12 engine ( the big BMW) or the Hispano V-12. Large displacement, low rpm, low boost, low weight for displacement engines. Once on that path a lot of the development becomes rather predictable and just like the Allison and Merlin, not easy to change in midstream.
 
sure they did with the GM-1 / MW-50 and on the rare occasion, both at the same time. DB 605DAM = 2000hp would have been much higher with the N2o.

Kindest regards.
 
Ratsel said:
sure they did with the GM-1 / MW-50 and on the rare occasion, both at the same time. DB 605DAM = 2000hp would have been much higher with the N2o.

Kindest regards.
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Add how was the German use of MW-50 any more outside the box than the allied use of water injection? Tested by the USAAC in the mid/late 30s on a P&W wasp engine.

How much would a DB 605DAM that used N2O to get more than 2000hp have weighed? these aren't drag car engines, the pilots expect not only to be able to use the power but actually get back to base after doing so. The US would not approve a WER power level until a single engine had racked up 7 1/2 hours at that rating (5 min at time) I imagine that most other countries did something the same.
 
Shortround6 said:
How much would a DB 605DAM that used N2O to get more than 2000hp have weighed?
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The exact same weight as the MW-50 setup as the GM-1 used the same delivery system as the MW-50. Tanks/lines/electrical/ everything more or less. Instead of methenol-water mix, it would use the n2o and would be in the 2150hp range. The cooling effects of the n2o has the exact same effect as the methenol-water mix.

As far as the weight goes for using them combined, as the Germans found out it was to much weight.. hence its rarity.
 

A. the cooling effect is not exactly the same.
B. If you use N2O to boost power below the FTL of the engine you are using instead of extra supercharging to burn more fuel to create more heat. Over the FTL is is being used to maintain a certain level of power to a high altitude.
C. Increasing power may require heavier crankcase, crankshaft, rods, pistons etc.

TANSTAAFL

For any side or country.
 
I'd be more worried about overboosting then a nitrous hit. The Germans used a wet system, ie: the fuel pressure goes up when the nitrous is injected. If an engine can handle a blower it will handle the n2o without any problems.

whats a TANSTAAFL?
 
Ratsel said:
I'd be more worried about overboosting then a nitrous hit. The Germans used a wet system, ie: the fuel pressure goes up when the nitrous is injected. If an engine can handle a blower it will handle the n2o without any problems.
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The question is can it handle both at the same time, you are claiming they could have gone over 2000hp. The only way to get more than 2000hp is to increase the BMEP (Brake Mean Effective Pressure, or the mean pressure inside the cylinders) or to increase rpm. Mean pressure is sort of the average pressure. To raise the mean pressure you have to raise the peak pressure. Supercharged aircraft engines were already highly stressed at their limits. They are much more lightly built for their power than car engines. They do have a reserve strength but it is nowhere near what car engines have and on the DB605 you have to ask, after they boosted it from 1475hp to 2000hp, how much reserve was left? And BMEP is not IMEP that blower at 1.80 ata or 1.98 ata was taking more power to drive than a blower supplying 1.42 ata. IMEP (Indicated Mean Effective Pressure is the mean pressure needed to give the total power of the engine, propshaft power, internal friction, pumps and the supercharger) is more the actual limit but is harder to figure than BMEP.
 
a 325 shot on a 2000hp engine should handle it no problem. the Allison V-1710 were handling a 30"hg increase without disasterous results. The internals were relativly the same. the DB 605A -> DB 605D had a cam timing change to handle the extra boost from the DB 603 supercharger.
 
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