Why propellers of P-38 Lightning rotate outwards?

ppopsie · 11-26-2007, 04:37 PM

What was the rationale that the propellers of P-38s rotate outwards had been a long unanswered question of my own.

If it was to make the control easier on a twin engine aircraft, the propellers should rotate inwards to bring the thrust line inwards and closer each other for the less effect of the thrust difference if one engine got troubled.

Only the reasonable explanation I got so far was that it makes the airflow over the wings stable from one of the present operator of a P-38 but was not in detail and I am not fully satisfied with that.

comiso90 · 11-26-2007, 04:41 PM

channel more air over the control surfaces?

.

SoD Stitch · 11-26-2007, 10:49 PM

Interestingly, the "prototye" P-38, the XP-38, had inwardly-rotating engines (before it crashed). However, all subsequent P-38's (including the first batch of YP-38's) had outwardly rotating propellers; Warren M. Bodie, in his book The Lockheed P-38 Lightning: The Definitive Story Of Lockheed's P-38 Fighter, states that, "Engine rotation was changed so that the propellers rotated outboard (at the top), thereby eliminating or at least reducing the downwash onto the wing centersection/fuselage juncture. There was, by then, no doubt that the disturbed airflow, trapped between the two booms, was having an adverse effect on the horizontal stabilizer. No problem was encountered in reversing propeller rotation direction; they merely had to interchange the left and right engines."

GaryMcL · 11-26-2007, 10:59 PM

I'll see if I can explain this well enough to be understood. I'm sure FBJ'll probably be able to make it clearer.

Simply stated, the outward rotating props help to mitigate the effects of torque and p-factor during an engine-out.

A normally rotating prop, counterclockwise as viewed from behind it, generates torque and p-factor effects that want to pull the aircraft to the left, thus the need for right rudder on a single-engine aircraft during takeoff and other low-speed, high power situations. Think of the P-51's torque rolls when low and slow with a lot of power on.

A counter-rotating prop, i.e. clockwise rotation as viewed from behind, is just the opposite as far as torque and p-factor. Thus in a homebuilt with a Rotax engine with a PSRU, you need LEFT rudder during a takeoff because the prop rotates opposite to the norm.

On a twin with an engine out the remaining engine will try to yaw the aircraft into the dead engine. Think of a P-38 with a right engine out. The left engine, rotating normally, will be powered up creating a right yaw (into the dead engine) due to the offset thrust line. But the torque and p-factor will create a left yaw effect partially countering the yaw from the unbalanced power.

By making the right engine rotate the opposite, when the left engine is out and power is up on the right engine, which creates a left yaw into the dead left engine, the torque and p-factor creates a right yaw to partially offset the power difference. If the right engine rotated normally, the torque and p-factor would make the left yaw tendency even worse.

There was a civilian twin, a Piper I think (maybe the Seminole) that had a counter-rotating right engine for just this reason.

It's kinda the same as the aircraft with two props on the same shaft or the helos that had two coaxial main rotors that rotate counter to each other. It's a torque mitigation thing.

I hope that was understandable. If not I'll try again or let FBJ sort out my mess.

Gary

GaryMcL · 11-26-2007, 11:06 PM

Guess I took ten minutes too long writing the above and should've waited for SoD to get done.

My explanation, if they were having turbulence problems due to the center section, would then just be an unintended or at the least collateral benefit.

Ah well, I should be in bed anyway.

Gary

SoD Stitch · 11-26-2007, 11:09 PM

Quote:

Originally Posted by GaryMcL

Guess I took ten minutes too long writing the above and should've waited for SoD to get done.

My explanation, if they were having turbulence problems due to the center section, would then just be an unintended or at the least collateral benefit.

Ah well, I should be in bed anyway.

Gary

Yeah, I think you & I were typing away at the same time; I just didn't have as much to type!

ppopsie · 11-26-2007, 11:32 PM

Thanks guys. Airflow generated by inwardly counter rotating props adversary affected the center section the wing on the P-38 is quite opposit to the case of the P-82 where outward rotating props had affected badly on taking off by making the wing center section stalled at three point positions. That quite makes sense to me, and my ignorance of the latter.

Mu?? Clockwise as viewed from cockpit yaw the plane to the left and counterclockwise to the right as what I understand and saw on the aircraft. Please check with that.

Was that so two engines and propellers sat at relatively close separation on a smaller airframe like Grumman F5F had could've caused similar problems, couldn't it?

GaryMcL · 11-27-2007, 12:18 AM

Man, I shoulda just kept my mouth shut and went to bed. Sheesh!

Please reverse every mention of clockwise and counterclockwise in what I wrote. You're right. Clockwise as viewed from behind is normal rotation. That would make your recollection of the left engine being normal and the right being counter correct and makes my discussion make sense.

It's a sad, sad thing when you can't even remember which way a clock runs.

Sorry 'bout the confusion. It's all on me.

I quit. I'm outta here for today. Maybe tomorrow I can at least pretend to be semi-intelligent.

Gary

FLYBOYJ · 11-27-2007, 06:29 AM

Close enough Gary - the P-38 had no "Critical Engine." ppopsie - do you fly twins?

ppopsie · 11-27-2007, 06:57 AM

>Fly twins?

Quite regrettably I don't have multi engine ratings but I have flown in one, a DC-3 from the right seat. You can see my left shoulder. It was fun, of course.

Hakenkreuz · 11-27-2007, 07:19 AM

A counter-rotating prop, i.e. clockwise rotation as viewed from behind, is just the opposite as far as torque and p-factor. Thus in a homebuilt with a Rotax engine with a PSRU, you need LEFT rudder during a takeoff because the prop rotates opposite to the norm.

As a Brit i`m not used to american terminology. To me a counter rotating prop is two props one behind the other eg:An 70,Spitfire F.Mk 21 or the Fairey Gannet.

mkloby · 11-27-2007, 08:49 AM

Quote:

Originally Posted by Hakenkreuz

A counter-rotating prop, i.e. clockwise rotation as viewed from behind, is just the opposite as far as torque and p-factor. Thus in a homebuilt with a Rotax engine with a PSRU, you need LEFT rudder during a takeoff because the prop rotates opposite to the norm.

As a Brit i`m not used to american terminology. To me a counter rotating prop is two props one behind the other eg:An 70,Spitfire F.Mk 21 or the Fairey Gannet.

Counter rotating props is termed when you have props turning opposite directions from each other - it is not related to the direction of rotation of a single propeller. We term contra-rotating props two props rotating in opposite directions on the same axis.

In the US, at least for mil aircraft, props generally rotate clockwise. In our helos the rotors generally spin CCW.

FLYBOYJ · 11-27-2007, 10:35 AM

Engine out on a P-38 on takeoff, you actually had to REDUCE power on the good engine prior to trimming and feathering the bad engine.

FLYBOYJ · 11-27-2007, 10:36 AM

Quote:

Originally Posted by ppopsie

>Fly twins?

Quite regrettably I don't have multi engine ratings but I have flown in one, a DC-3 from the right seat. You can see my left shoulder. It was fun, of course.

VERY COOL!

Hakenkreuz · 11-27-2007, 11:36 AM

[quote=Hakenkreuz;294535]A counter-rotating prop, i.e. clockwise rotation as viewed from behind, is just the opposite as far as torque and p-factor. Thus in a homebuilt with a Rotax engine with a PSRU, you need LEFT rudder during a takeoff because the prop rotates opposite to the norm.

What isGary McCL referring to. Two engines turning in different directions.