Why propellers of P-38 Lightning rotate outwards?

[ppopsie]

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]

channel more air over the control surfaces?

.

 

[SoD Stitch]

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]

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]

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]

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]

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]

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]

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

 

[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.

 

[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.

 

[mkloby]

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]

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]

>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]

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.

 

[drgondog]

simply stated - if a rotor or prop turns (top to bottom) one way, the fuselage holding the sucker wants to rotate the other way ... so aileron and rudder (or tail rotor thrust) needed to 'counteract' that nasty 'ol prop.

Of couse in case of Helo, some tail rotors 'pull' into fuse and others 'thrust'. An imbedded tail rotor simplifies the aerodynamics of the tail rotor. 'lose a tail rotor and you can kiss your ass goodbye if you have any altitude at all. Very bad 'thing'. It has been a very long time but I believe the King Cobra at Bell had a rotor system delivering 600,000+ foot pound of torque.

Joe dead on on reducing power on TO for a lost fan on a 38... because as he explained it the asymmetrical thrust for live engine (and lift on live wing) turns everything to dead engine - a very bad thing. Of couse it has to be done slowly..

SoD correct about the actual design reasons for 38 prop rotation scheme due to effect on centerbody/wing area..

 

[drgondog]

simply stated - if a rotor or prop turns (top to bottom) one way, the fuselage holding the sucker wants to rotate the other way ... so aileron and rudder (or tail rotor thrust) needed to 'counteract' that nasty 'ol prop.

Of couse in case of Helo, some tail rotors 'pull' into fuse and others 'thrust'. An imbedded tail rotor simplifies the aerodynamics of the tail rotor. 'lose a tail rotor and you can kiss your ass goodbye if you have any altitude at all. Very bad 'thing'. It has been a very long time but I believe the King Cobra at Bell had a huge (4 bladed -two engine) rotor system delivering 600,000(??) pounds of torque. (Don't hold me to the very big number - my memory is not reliable here)

Joe dead on on reducing power on TO for a lost fan on a 38... because as he explained it the asymmetrical thrust for live engine (and lift on live wing) turns everything to dead engine - a very bad thing. Of couse it has to be done slowly..

SoD correct about the actual design reasons for 38 prop rotation scheme due to effect on centerbody/wing area..

For what is worth I'm trying to research my notes of nearly 40 years afo on the whopping torque number

 

[kool kitty89]

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.

That's what I thought, but GaryMcl seems to be saying that counter-rotating means counter-clockwise-rotating...


I've also heard that, in addition to improving airflow characteristics over the wings/booms, the outward (from the top) rotating props made for a more stable gun platform.

One other interesting thing is that the Allison V-1710 was a big part of what made this counter-rotation practical as:
"Another feature of the V-1710 design was its ability to turn the output shaft either clockwise or counter-clockwise by assembling the engine with the crankshaft turned end-for-end, by installing an idler gear in the drive train to the supercharger and accessories and by installing a starter turning the proper direction. So, there was no need to re-arrange the ignition wiring and firing order, nor the oil and Glycol circuits to accommodate the direction of rotation."
It would be much harder (and more expensive) if the engines didn't offer this feature. (one reason for not using Merlins; the Contenental I-1430 may have been a problem to in the XP-49, unreliabillity aside)

 

[GaryMcL]

That's what I thought, but GaryMcl seems to be saying that counter-rotating means counter-clockwise-rotating...

Your confusion is no doubt my fault because I had the clockwise/counter-clockwise references backwards in my original post. I realized that and posted the error correction later.

A counter-rotating prop, i.e. rotating opposite from normal, will spin counter-clockwise as viewed from behind the engine. A normally rotating prop rotates clockwise as viewed from behind.

Viewed from in front of the aircraft, normal rotation is counter-clockwise.

If you stand in front of the aircraft and look at the prop you'll find the leading edge of the prop blade to the left on a normally rotating engine which means that it will rotate counter-clockwise as viewed from the front.

Sorry about the original error.

 

[kool kitty89]

But what I always thought counter-rotating propellers, as mkloby seems to be saying too, is when you have 2 propellers on 2 separate engines rotating in opposite directions, not a single propeller rotating the opposite from normal.

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.

Wikipedia says likewise too: Counter-rotating propellers - Wikipedia, the free encyclopedia

Thus the XP-38 hat counter-rotating props rotating inward from the top (left-prop CW, right-prop CCW when viewed from behind/from the cockpit) while the production models used the opposite rotation with both outwardly rotating from the top. (which as previously stated was to improve air-flow and platform stability)

Ergo, the Wright flyer also used counter-rotating propellers.