P38 a super plane?

[Angus]

This still boggles me a bit, so forgive me....
"same aircraft that has the better climb at any given speed also out-accelerates the other aircraft at that given speed by the same facto"

Does that apply at any speed or just at exactly that climb speed and upwards for a bit?

Say a slow steep climber meets a faster speed climber.
The slow guy will then outaccelerate the fast guy initially, but eventually the curves cross??

[Captain Virgil Hilts]

Originally posted by HoHun
Hi Hilts,

So much has been made of the lackluster performance of the P-38 with the 8th AF, I thought it might be nice to have an inside look at what went wrong.

Quite interesting, but it really touches only one aspect of performance: Range.

("Sort of engine trouble" doesn't seem to be very serious - or maybe the pilots just had a lot of confidence due to engine redundancy :-)

Actually, if you read it more closely, Levier touches briefly on the reliability issues as well, speaking of pilots who were running too much RPM and too little manifold pressure blowing up engines. Remember that much of the 8th AF's complaint with the P-38 and the Allison was that they "burned valves, threw rods, and blew up". That was a direct result of the improper engine power settings. Most all of the ills the 8th AF experienced were directly related to those power settings.

The low manifold pressure caused the engines to run too cold, which dropped cockpit temperatures, congealed the oil in the oil coolers, caused malfunctions in the turbo regulators, caused serious problems in the intake tract (backfires routinely wrecked intakes, intercoolers, and ductwork), and caused the engines to fail when full power was suddenly called for in combat (think about starting your car at -40 degrees and immediately pulling out into high speed traffic, and what happens when you stomp the throttle). The problems caused and damage done by those settings would take far too long to adequately cover here.



With regard to the turbocharger RPM: The British test of the P-38F mentioned the turbos spun up from idle to full take-off boost in about 10 s. That spin-up time was considered a serious issue tells us a lot about how fast-paced the fights must have been ...

Regards,

Henning (HoHun)

The P-38F had VERY long and complex ductwork, and going from idle with no load to take off run up would certainly take a long time. Turbocharged drag cars take twice as long to stage because they have to build boost as they stage to get ready for the light. The P-38J and later models were vastly improved. However, running at low boost means running at low load. The engine had to be loaded hard before it would build boost, and loading a cold engine that hard that fast was an open invitation to disaster. No doubt the combination of those incorrect settings and the complexity of the controls cost many pilots their plane, and their lives. Lockheed did their best to solve the problem, but the USAAC just couldn't get their feces cohesive.

[HoHun]

Hi Hilts,

>Actually, if you read it more closely, Levier touches briefly on the reliability issues as well

Not that I missed it, but I was looking for hints on performance issues.

Regards,

Henning (HoHun)

[MANDO]

Originally posted by Angus
Say a slow steep climber meets a faster speed climber.
The slow guy will then outaccelerate the fast guy initially, but eventually the curves cross??

That is. Think that usual speeds for substained climbs go from 150 to 180 mph, quite slow (with the exception of Me 262). Past the "cross" point, the faster plane will outaccelerate and outclimb to the slow climber.

As an example, 190A8 vs SpitV, Spit will have the advantage below 250 mph, while 190A8 will start gaining from that point.

[Captain Virgil Hilts]

Originally posted by HoHun
Hi Hilts,

>THE USAAC downrated the P-38L by disallowing WEP (see the report from Levier above for their reasons) engines

Hm, you're not referring to the same report in which Levier solves the range issues, I guess? That one seems to be about boosts in the cruise range entirely.

>The Merlin was also not designed to be turbocharged, meaning that it would have given up considerable power above 20K.

Well, the Merlin 61 was a pretty good high-altitude engine anyway. I've not looked at the weights, but I'd be surprised if the Merlin outweighed the Allison if you feature in the latter's turbo-supercharger. However the Merlin's weight distribution would have been all wrong since it was all in front of the firewall, while the turbo-superchargers brought back the centre of gravity nicely. In my opinion, it would have required a major redesign of the P-38 to fit the Merlin.

Regards,

Henning (HoHun)

Levier wasn't just solving range issues (even lower RPM with higher manifold pressure would have yielded even greater range) he was solving reliability issues as well when he spoke of "pilots blowing their engines with too much RPM and too little manifold pressure". That was an extremely common thing with the 8th AF due to their improper use of power settings. Those problems are what drove the USAAC to downrate the P-38L engines.

Regarding the Merlin and altitude, please refer to Greg Shaw's post above.

I'll expand on it here. As he stated, the Merlin had a two speed two stage crankdriven centrifugal supercharger. Supercharger speed was tied directly to crank speed. As such, the Merlin models were each design for a specific maximum altitude at which peak performance was to be achieved. The two speeds created two bands of peak performance, each with an altitude range of about 4K feet. The choice of the high altitude determined BOTH gear ratios, and as such, the location of both ranges. If the high altitude desired was for example 27K feet, you'd get your peak there, and decent performance between 25K and 29K. After that, you only lose, in either direction. The further outside that range, the worse performance is degraded. So you had two good performance ranges of 4K feet each, with as much as 10K to 15K in between that had relatively poor performance. Finally, once the centrifugal (actually ANY crank driven supercharger) supercharger reaches a certain speed, the faster you spin it, the less power it makes, and the more it takes to spin it. It also overheats the air causing detonation and mixture problems.

For performance at a wide range of altitudes, especially if you need sea level performance at very high altitudes, the crank driven supercharger is VERY inferior to the turbocharger, in piston engines.

[Captain Virgil Hilts]

Originally posted by HoHun
Hi Hilts,

>Actually, if you read it more closely, Levier touches briefly on the reliability issues as well

Not that I missed it, but I was looking for hints on performance issues.

Regards,

Henning (HoHun)

With the Allisons, performance and reliability were directly related. That is quite common with complex compound supercharged engines. What they were doing was causing the engine to use its power to destroy itself, rather than to propel the plane.

[HoHun]

Hi Hilts,

>Those problems are what drove the USAAC to downrate the P-38L engines.

Interesting - what were the relative USAAF and Lockheed WEP ratings?

>For performance at a wide range of altitudes, especially if you need sea level performance at very high altitudes, the crank driven supercharger is VERY inferior to the turbocharger, in piston engines.

Well, the two-stage Merlin gave the Spitfire IX and the P-51B excellent high-altitude performance. However, you might be right that they wouldn't have sufficed for the P-38 - it seems to have been a bit more power-hungry than the P-51 at least.

By the way, do you happen to have a power-over-altitude chart for the turbocharged Allison? I believe the Vought diagrams F4UDOA posted are a bit generic when it comes to turbocharged engines.

Regards,

Henning (HoHun)

[Captain Virgil Hilts]

Originally posted by HoHun
Hi Hilts,

>Those problems are what drove the USAAC to downrate the P-38L engines.

Interesting - what were the relative USAAF and Lockheed WEP ratings?

>For performance at a wide range of altitudes, especially if you need sea level performance at very high altitudes, the crank driven supercharger is VERY inferior to the turbocharger, in piston engines.

Well, the two-stage Merlin gave the Spitfire IX and the P-51B excellent high-altitude performance. However, you might be right that they wouldn't have sufficed for the P-38 - it seems to have been a bit more power-hungry than the P-51 at least.

By the way, do you happen to have a power-over-altitude chart for the turbocharged Allison? I believe the Vought diagrams F4UDOA posted are a bit generic when it comes to turbocharged engines.

Regards,

Henning (HoHun)

The USAAC does not give WEP ratings for the P-38L-1-Lo or the P-38L-5-Lo, that's why the P-38L is SUPPOSEDLY several MPH SLOWER than the P-38J. Lockheed and Allison rate the Allisons in the P-38L series at 1475HP Military and 1725HP WEP. Lockheed logs show a top speed of 442MPH at WEP. Note that the P-38J is rated at 1600HP at WEP, with a top speed of 421MPH. The P-38L is rated at 1475HP at Military Power at 414MPH.

The Spitfire and the P-51 performed well at least as much due to weight and aerodynamics as due to the Merlin. If you look closely, their area of advantage is much narrower than the P-47 or the P-38. Neither could maintain sea level power at high altitudes.

I do not have a chart handy, and I have not yet acquired a scanner any way. The J and L models were however able to maintain sea level power up to around 25K, and did not lose power nearly so quickly as Merlin powered planes above that.

[Naudet]

Henning,

the effects of the position of the cooling flaps on airspeed was also tested for the FW190D9. Dunno if you have the complete Rechlin flight test for Wk.-Nr. 210 006, but it contains a small graph illustrating this effect at 3000rpm.
Interestingly the best position for the D9 was not fully closed, but "slightly open" at Position 2. (The cooling flap positions very equaly devided into 10 Steps, with 0 being fully closed and 10 being fully opened).
All speed/climb charts of this test also have a note on the cooling flap position used during the measurement flights.

[gwshaw]

I'm working on a more detailed engine posting, but this is a rough differences from earlier Allison engines in the P-38J.

The P-38L engines were basically the same as the P-38J, just incorporating all the improvements Allison had made up to that time.

They both still retained the 5.5 in bore x 6.0 in stroke, the same 6.65:1 compression ratio and the same 8.1:1 blower ratio. For all important particulars they were the same engine, and would produce the same amount of power at the same rpm/map settings.

They did have the new venturi intake manifold, that prevented the fuel from separating out of the charge due to too efficient aftercoolers. Most importantly they had the new 12 counterweight crankshaft, vice the 6 counterweight one standard in earlier engines.

Right there is the reason for the higher power ratings. The new crankshaft allowed for 3200 rpm vice 3000 rpm as the max sustained rpm allowed. The earlier 3000 x 8.1:1 x 9.5 inch blower provided enough compression for just over 60 in Hg & 1600 hp when fed slightly over SL static pressure by the turbosuperchargers. The new 3200 x 8.1:1 x 9.5 in blower generated compression for approx 65 in Hg instead.

Since hp = rpm * torque (manifold pressure is easiest approximation for torque) the increased map and rpm generated more power for the P-38L.

1425 * 3200/3000 * 54/54 = 1520 hp mil

1600 * 3200/3000 * 65/60 = 1850 hp wep

But you are going to lose a chunk of that additional power to the blower and other accessories, they requires more energy at 3200 rpm than they did at 3000. I'll see if I can calculate the mass flow and get the blower power requirements. But it was likely about 50 hp, putting it at around 1475 hp mil, and about 1800 hp wep. My guess is that the 1725 hp figure actually comes from 62.5 in Hg & 3200 rpm, giving 1777 - 50 = 1725 hp. I have seen both figures listed, although the 1725 hp figure is most common.

That was the big difference between the L-5 and the earlier models. But, as has been mentioned, the USAAF didn't authorize the 3200 rpm settings, keeping the 3000 rpm * 60 in Hg max from the earlier models as the wep figure, and 3000 rpm * 54 in Hg as the mil power. Lockheed and Allison did, and thats why their figures are higher than the 415 mph mil/ 425 mph wep that that the J was capable of.

Greg Shaw