P-38 Ailerons

[Zipper730]

Did they use any form of aerodynamic assist (i.e having the leading edge slide up and out to help move the surface)?

 

[ThomasP]

This is the only video of the aileron movement I have seen off-hand:

The hinge type was of the piano variety, where (if I understand it correctly) the entire surface moves around a simple arc centered on the forward hinge pivot. +25°/-20° total movement up/down for the P-38. From the drawings I have seen the leading edge of the aileron is not shaped like an airfoil, rather more of a concave shape. No forward movement or lift/lower of the leading edge is present (that I can see), other than the slight protrusion of the upper/lower leading edges of the aileron when moved up/down through the simple arc.

Also, I could be wrong.

 

[Zipper730]

I was curious because the P-38 supposedly had a fair rate of roll at intermediate speed, poor at low and high, but I could be wrong.

 

[ThomasP]

*You are correct for the early models until the late-production Js(?) when boosted aileron controls started to be fitted. Too much rolling inertia/wing area at low speeds, too much stick force(?) at high speeds. The boosted aileron controls allowed full(?) deflection at any speed below compressibility effects. I read they strengthened the ailerons also at this point, but I can not say for sure this is true.

early

http://www.wwiiaircraftperformance.org/p-38/p-38e-rolling-velocities.jpg

late

http://www.wwiiaircraftperformance.org/p-38/p-38j-roll.jpg

IMO: It (*low roll rate) makes sense for the early models, as it was designed/intended as an interceptor of bombers. As far as I know, even late-war the early models could out-roll most (all?) 2-4 engine propeller driven bombers at pretty much any speed they could fly at. It was only when it began to be used as a general purpose fighter that higher roll rates became important.

*edited a couple sentences to hopefully make what I tried to say clearer.

 

[Zipper730]

T ThomasP

Why would aileron control be better with the powered system at low speeds? I figure at high speeds, you'd see a clear advantage, but at low speeds: Unless the plane has some form of aerodynamic boost, you'd figure the controls would be lightest at low-speed, and heaviest at high-speed.

 

[ThomasP]

The boosted aileron system would not normally help with roll rate at low speeds, unless the stick forces were unusually high. I do not think that was the case with the P-38. At least I have not read anywhere that they were.

The boost system on the P-38 was selectable on/off. At low speeds the aileron boost was normally not used. At some higher speed (about 200 mph on the chart) the pilot would switch the boost system on to help increase the roll rate.

The chart for the unboosted vs boosted ailerons is somewhat deceptive (unintentionally). To compare the unboosted vs the boosted roll rates you have to take the unboosted curve (upper) and overlay it on the boosted curve (lower). If you do this you will see that at low speed the chart curves are basically the same from ~125 mph to ~250 mph. Ignore the time to bank to 90° numbers to the left of the unboosted curve (upper) as they do not apply in this case.

If you do the math I believe you will find that the 150 mph to 200 mph roll rates from the P-38E chart and P-38J chart (overlayed on the lower curve) are about the same at 60°/sec to 75°/sec. It is only above the 250 mph point that the P-38J boosted roll rates diverge significantly.

 

[Zipper730]

The chart for the unboosted vs boosted ailerons is somewhat deceptive (unintentionally). To compare the unboosted vs the boosted roll rates you have to take the unboosted curve (upper) and overlay it on the boosted curve (lower). If you do this you will see that at low speed the chart curves are basically the same from ~125 mph to ~250 mph. Ignore the time to bank to 90° numbers to the left of the unboosted curve (upper) as they do not apply in this case.

I assume control loads would be a bit lighter...

 

[ThomasP]

If the boost was used at low speed then yes the stick forces would be lighter, possibly too light for safe operation (ie possible over-control, unintentional 'flick' maneuver, etc) due to the lack of 'feel'. Post-war aileron boost used progressive systems that increased boost gradually based on various parameters and built in limits. The P-38 system was (I think) pure all or nothing. This is why it was switched on by the pilot when/if he felt it was needed. The chart uses 200 mph as the activation point, below 200 mph the boost would not normally be used.

 

[chuter]

This is the only video of the aileron movement I have seen off-hand:

The hinge type was of the piano variety, where (if I understand it correctly) the entire surface moves around a simple arc centered on the forward hinge pivot. +25°/-20° total movement up/down for the P-38. From the drawings I have seen the leading edge of the aileron is not shaped like an airfoil, rather more of a concave shape. No forward movement or lift/lower of the leading edge is present (that I can see), other than the slight protrusion of the upper/lower leading edges of the aileron when moved up/down through the simple arc.

Also, I could be wrong.

Yes, the unboosted ailerons moved from +25° to -20°. This differential aileron travel reduced or eliminated adverse yaw (ex: right aileron deflection swinging the nose to the left). When the boost system was installed the aileron travel was squared up to 25° + AND -. This improved roll across the board including a bit at the low end, albeit with adverse yaw which could be countered with rudder. The Flight Manual says the boost system can be turned off in flight if something goes amiss, but says that once the system is turned off it shouldn't be turned back on until back on the ground, probably something to do with the pump.

 

[MIflyer]

In the book Jungle Ace it is described how some P-38 pilots in the PI went to pick up some new P-38L's. Once in flight they found them so sensitive as to be almost unflyable. They had real concerns about being able to even land and one pilot said he was going to point it over a big lake near the base and then bail out. But one said he had found an "Aileron Boost" control and when he switched it off, things went back to normal. No one had told them of the boost feature and it apparently had been turned on during ground testing and then not turned off.

Later fighters like the F-106 had a "Q Feel" system that varied the feedback pressure given to the pilot according to speed and altitude so that it did not take a radically different control movement in the landing pattern as compared to high speed and high altitude. If the Q Feel had a failure you could end up with so much pressure on the system that the pilot could not move the controls, such as was known to happen with the rudder when the rudder force feel regulator ate one of its seals and locked up.

 

[FLYBOYJ]

In the book Jungle Ace it is described how some P-38 pilots in the PI went to pick up some new P-38L's. Once in flight they found them so sensitive as to be almost unflyable. They had real concerns about being able to even land and one pilot said he was going to point it over a big lake near the base and then bail out. But one said he had found an "Aileron Boost" control and when he switched it off, things went back to normal. No one had told them of the boost feature and it apparently had been turned on during ground testing and then not turned off.

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