Remove WEP limitation from the P-38L

[Captain Virgil Hilts]

Fair enough, but I thought maybe it applied to the reasoning behind the USAAC's decision.

The pre J model P-38 had problems with the leading edge mounted intercoolers, moving the intercoolers from the wings to the chin solved that, so more boost could be run, for longer periods. They also upgraded the Prestone radiators and the scoops, that solved any overheating problems.

[hitech]

No, the RPM had nothing to do with the manifold pressure. On an Allison with a turbocharger (it already has a crank driven centrifugal supercharger) the MAP (manifold absolute pressure) is regulated by throttle opening and the waste gate. It did not need to turn 200 RPM in order to get 10" more boost. All you need to do is open the throttle further and/or adjust the waste gate. Also, with a turbocharged engine, boost is also at least partially regulated by load. Of course, if you change the prop pitch to allow the engine to turn more RPM, you remove load from the engine, when you remove the load, the turbocharger boost often drops. My turbocharged V6 TType Regal is a prime example. If you rev the engine  in neutral, it barely builds boost. If you rev it in gear, against the torque converter, loading it, it will build all the boost you can stand. The normal procedure for taking off in a P-38 is to hold it against the brakes (creating the load) until you make 50"+ of boost (a P-38 is supposed to have good enough brakes to sit still while you build that boost, although it won't do it in AH II).

This evaluation is not exactly correct. Holding the brakes has nothing to do with building load. Holding brakes or not, the load on the prop will be exactly the same as the rpm increases with open throttle.

2nd the increased pressure is not do to the load on the car as you state, but it is do to the throttle being open  more at the same RPM when the car is under load.

HiTech

[j500ss]

No, the RPM had nothing to do with the manifold pressure. On an Allison with a turbocharger (it already has a crank driven centrifugal supercharger) the MAP (manifold absolute pressure) is regulated by throttle opening and the waste gate. It did not need to turn 200 RPM in order to get 10" more boost. All you need to do is open the throttle further and/or adjust the waste gate. Also, with a turbocharged engine, boost is also at least partially regulated by load. Of course, if you change the prop pitch to allow the engine to turn more RPM, you remove load from the engine, when you remove the load, the turbocharger boost often drops. My turbocharged V6 TType Regal is a prime example. If you rev the engine  in neutral, it barely builds boost. If you rev it in gear, against the torque converter, loading it, it will build all the boost you can stand. The normal procedure for taking off in a P-38 is to hold it against the brakes (creating the load) until you make 50"+ of boost (a P-38 is supposed to have good enough brakes to sit still while you build that boost, although it won't do it in AH II).

Your really close about how a turbo works, you just need to add 1 thing.  The heat that the load generates is what actually makes the turbo spin faster, thus creating more boost.  Boost is controlled by quality of the turbo itself, or a wastegate.   Not all engines that are turboed have a wastegate,  but most high performance ones do. 

[Captain Virgil Hilts]

This evaluation is not exactly correct. Holding the brakes has nothing to do with building load. Holding brakes or not, the load on the prop will be exactly the same as the rpm increases with open throttle.

2nd the increased pressure is not do to the load on the car as you state, but it is do to the throttle being open  more at the same RPM when the car is under load.

HiTech

If the plane is not moving, there is a greater load on the engine. It takes a certain amount of energy to make the plane move. If that energy is produced by the engine at a given RPM at a given throttle opening, if the plane does not move at that speed, where does that energy go? In other words, at wide open throttle and 3000 RPM, an engine produces 1600HP, and the plane flies at 360MPH at sea level. If the same plane is sitting on the runway at wide open throttle and at 3000 RPM, where does the energy go?

No, on my car, and most other turbocharged cars, you cannot build the same amount of boost at wide open throttle in neutral as you can in gear and loaded against the brakes and the torque converter. If you simply open the throttle in neutral, the engine will rev with little or no load and create much less boost. Mine might make 5 psi even as high as 5000 RPM in neutral sitting still, but it will make 15 psi at 2500 in low gear sitting still.

[hitech]

If the plane is not moving, there is a greater load on the engine. It takes a certain amount of energy to make the plane move. If that energy is produced by the engine at a given RPM at a given throttle opening, if the plane does not move at that speed, where does that energy go? In other words, at wide open throttle and 3000 RPM, an engine produces 1600HP, and the plane flies at 360MPH at sea level. If the same plane is sitting on the runway at wide open throttle and at 3000 RPM, where does the energy go?

The energy goes into drag on the prop. On any constant speed prop, the torque will remain the same for any set MP/RPM and Alt. With the throttle wide open holding breaks the prop using all the torque the engine can produce, or it would be gaining RPM. As the plane starts moving the engine torque will not increase the prop rpm will not change, but the pitch of the blades will change slightly to use the exact same torque as when the plane is standing still. Hence there is no difference.

No, on my car, and most other turbocharged cars, you cannot build the same amount of boost at wide open throttle in neutral as you can in gear and loaded against the brakes and the torque converter. If you simply open the throttle in neutral, the engine will rev with little or no load and create much less boost. Mine might make 5 psi even as high as 5000 RPM in neutral sitting still, but it will make 15 psi at 2500 in low gear sitting still.

One question, if you are running at 5000 RPM in neutral is the gas pedal (i.e. throttle) in the same position as when you are 5000 RPM and under load?

In a sense it is the load that is creating the extra psi, but really it is the throttle having to be opened more (hence more air moving threw engine) to hold the same RPM with the extra load. But on an airplane with a constant speed prop the load/torque does not change if you are moving or standing still.


HiTech

[Ack-Ack]

The energy goes into drag on the prop. On any constant speed prop, the torque will remain the same for any set MP/RPM and Alt. With the throttle wide open holding breaks the prop using all the torque the engine can produce, or it would be gaining RPM. As the plane starts moving the engine torque will not increase the prop rpm will not change, but the pitch of the blades will change slightly to use the exact same torque as when the plane is standing still. Hence there is no difference.

One question, if you are running at 5000 RPM in neutral is the gas pedal (i.e. throttle) in the same position as when you are 5000 RPM and under load?

In a sense it is the load that is creating the extra psi, but really it is the throttle having to be opened more (hence more air moving threw engine) to hold the same RPM with the extra load. But on an airplane with a constant speed prop the load/torque does not change if you are moving or standing still.


HiTech

The easiest way to resolve this debate and for all to save face is just to give us the P-38H.


ack-ack

[texastc316]

I agree with ack ack. As usual.

[Soulyss]

and here I was going to tell everyone to just fly the G and then you don't have to worry about WEP.  But I like AKAK's idea more.

[TwinBoom]

P-38H

[EskimoJoe]

SAPP tards