P-51D Engine AH and Pilot Handbook

[artik]

I've found this pilot handbook: http://www.scribd.com/doc/34811808/North-American-P-51-Mustang-Pilot-Training-Manual

I've noticed several things:

1st Military power 61' man./3000rpm that is used for takeoff/landing and was limited to 5 minutes of use. Is this because of engine wear of a damage that can be caused to the engine (overheating and so on). If it is due to potential engine failure shouldn't 61'/3000rpm be limited to 5 min. as WEP?
2nd WEP (67' man./3000rpm rating) was not effective under 5K and shouldn't be used (and of course limit of 5 min). According to the AH the WEB kicks in from 0 altitude.

What do I miss?

[Chalenge]

1st: You got your facts wrong. There is no limit on 61" and 3,000 rpm and the training manual does not say that.
2nd: In war you do what you need to do. In AH there are no equipment failures unless you get shot or run into something.

[GScholz]

UNLIMITED WEP FOR EVERYONE!!!

[TheCrazyOrange]

Me 163 WEP

[FLS]

I've found this pilot handbook: http://www.scribd.com/doc/34811808/North-American-P-51-Mustang-Pilot-Training-Manual

I've noticed several things:

1st Military power 61' man./3000rpm that is used for takeoff/landing and was limited to 5 minutes of use. Is this because of engine wear of a damage that can be caused to the engine (overheating and so on). If it is due to potential engine failure shouldn't 61'/3000rpm be limited to 5 min. as WEP?
2nd WEP (67' man./3000rpm rating) was not effective under 5K and shouldn't be used (and of course limit of 5 min). According to the AH the WEB kicks in from 0 altitude.

What do I miss?

It actually says that there is no advantage to WEP below 5k and the 5 minutes at a time limit is for WEP. This is the same in AH.

[artik]

Ok put an updated link: http://www.avialogs.com/en/aircraft/usa/northamericanaviation/p-51mustang/aaf-51-127-5-pilot-training-manual-for-the-p-51-mustang.html

Actually P-51D has WEP under 5K in AH but it looks like according to the manual that you don't need to engage WEP to push over the operating limits (61'/3000rpm) you can do it with the throttle alone.

And I can't currently find a page were the 5 min Mil power... it looks like I really got it wrong.

[GScholz]

Yes, you're misunderstanding. War emergency power (WEP) is not something you "engage", it is not a device or gadget. The Merlin engine does not use any injection systems to boost the engine at WEP; it is just a matter of increasing manifold pressure beyond safe limits by opening the throttle. The last part on page 15 is only relevant for the early aircraft equipped with a separate boost control lever for over-boosting the engine.

[bozon]

Yes, you're misunderstanding. War emergency power (WEP) is not something you "engage", it is not a device or gadget. The Merlin engine does not use any injection systems to boost the engine at WEP; it is just a matter of increasing manifold pressure beyond safe limits by opening the throttle.

Yet somehow in AH the P51D merlin has a 5/10 WEP/cool cycle while the Spits and mossies merlins have a 5/15 cycle. Some real mossies (night fighters only?) had N2O injection to their Merlins that gave a nice boost to speed and was an independent system from the regular Merlin overboost.

[GScholz]

Nitrous oxide was used by the British and the Germans in a few specialized high-altitude aircraft to increase performance above full pressure height. It didn't boost the engine above what was normal. It gave the engine the oxygen it needed when the supercharger no longer could deliver enough pressure.

[FLS]

Yet somehow in AH the P51D merlin has a 5/10 WEP/cool cycle while the Spits and mossies merlins have a 5/15 cycle. Some real mossies (night fighters only?) had N2O injection to their Merlins that gave a nice boost to speed and was an independent system from the regular Merlin overboost.

So the P-51D had better cooling?

[GScholz]

So the P-51D had better cooling?

The system in AH is arbitrary. The engines didn't overheat as such, it's just a "WEP timer" in the game, modeled (somewhat) according to the operational restrictions places on the aircraft. You could WEP a Merlin for far longer than 5 minutes without failure or overheating, but the engine would have to be pulled after that flight and overhauled.

[FLS]

The system in AH is arbitrary. The engines didn't overheat as such, it's just a "WEP timer" in the game, modeled (somewhat) according to the operational restrictions places on the aircraft. You could WEP a Merlin for far longer than 5 minutes without failure or overheating, but the engine would have to be pulled after that flight and overhauled.

If the faster restore time is based on a better cooling system then it's not arbitrary because it's based on differences in the aircraft.

[GScholz]

It is arbitrary when WEP is not related to cooling in R/L. Overheating wasn't normally an issue with using WEP at combat speeds. Most WWII fighters had excess cooling capacity at speed. WEP could overheat internal components in the engine, like piston heads and valves, but that's different from overheating the engine itself and no changes to the cooling system would make a difference.

[FLS]

The 5 minute WEP  limit is the suggested time from the manual. AH forces you to be cautious instead of having to model failure from abuse.

Interesting point about engine heat.  I thought all the heat came from combustion in the cylinders and it was reduced by the cooling system.

[GScholz]

It does, but even if you keep the coolant temperature constant (thats all a cooling system does), internal components can overheat at high boost levels. Piston heads, valves and spark plugs are common failures since they are right at the source of combustion. However, the 5 minute restriction on the Merlin was a more of a maintenance limitation, than a failure limit.

On the DB 600 series of engines, oil was used for additional cooling of the piston heads, which is why you see larger and larger oil coolers on DB 605 powered aircraft like the 109G/K as higher boost levels were introduced. The main coolant radiators remained unchanged however.