# Why propellers of P-38 Lightning rotate outwards?



## ppopsie (Nov 26, 2007)

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 (Nov 26, 2007)

channel more air over the control surfaces?

.


----------



## SoD Stitch (Nov 26, 2007)

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 (Nov 26, 2007)

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 (Nov 27, 2007)

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 (Nov 27, 2007)

GaryMcL said:


> 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.
> 
> ...



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


----------



## ppopsie (Nov 27, 2007)

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 (Nov 27, 2007)

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 (Nov 27, 2007)

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


----------



## ppopsie (Nov 27, 2007)

>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 (Nov 27, 2007)

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 (Nov 27, 2007)

Hakenkreuz said:


> 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 (Nov 27, 2007)

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 (Nov 27, 2007)

ppopsie said:


> >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 (Nov 27, 2007)

Hakenkreuz said:


> 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 (Dec 4, 2007)

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 (Dec 5, 2007)

drgondog said:


> 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)
> 
> ...



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


----------



## kool kitty89 (Dec 13, 2007)

mkloby said:


> 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)

Reactions: Like Like:
1 | Like List reactions


----------



## GaryMcL (Dec 14, 2007)

kool kitty89 said:


> 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 (Dec 14, 2007)

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.


mkloby said:


> 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.


----------



## GaryMcL (Dec 16, 2007)

kool kitty89 said:


> 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.



That is right. What I was trying to do was describe the two engines rotating in opposite directions individually because to accomplish this you have one engine rotating normally and one rotating in the opposite direction and each will have differing torque and p-factor effects.

The confusion is probably my use of the word counter to describe the engine in the pair that is operating the reverse of normal, i.e. counter to normal. I should have use something like reverse or opposite but I used counter the same as would be used to describe something running the reverse direction of a clock, i.e. counter-clockwise. And the clock references probably confused things more by throwing in more counters. Just too many counters.

Made the whole discussion kinda counter-intuitive.

Does this help any?


----------



## ThunderThud (Dec 16, 2007)

Did you also Know that the initial batch of P-38's Destin for the RAF Had no superchargers installed as the U.S. Deemed them to be a secret part of the aircraft.This made the P-38 a slug compared to its US counterpart.

Reactions: Informative Informative:
1 | Like List reactions


----------



## FLYBOYJ (Dec 16, 2007)

ThunderThud said:


> Did you also Know that the initial batch of P-38's Destin for the RAF Had no superchargers installed as the U.S. Deemed them to be a secret part of the aircraft.This made the P-38 a slug compared to its US counterpart.


That's incorrect.

It was on the British order specification to omit the superchargers. The British P-38s were slugs and as a result deemed not suitable for RAF. That situation did help Lockheed and the USAAF because at the time of that order there was an upcoming production gap. That situation allowed the production line to remain in tact with no interruptions.

I believe Martin Cadin mentioned that in his book "Forked Tailed Devil." I also heard this from a number of old Lockheed folks when I worked there in the 1980s.

Reactions: Like Like:
1 | Like List reactions


----------



## kool kitty89 (Dec 17, 2007)

You mean turbochargers, the integral superchargers of the allisons were retained iirc. The british planes also didn't use counter-rotation so the trops would be interchangable with their P-40s'. 

Another note is that the P-38 w/out turbos performed even worse than a P-39 w/out a turbo! Though performance with both craft w/turbos goes to the 38, except possibly in medium-high speed maneuvering and terminal dive control.


----------



## FLYBOYJ (Dec 17, 2007)

kool kitty89 said:


> You mean turbochargers.


Yep - to a maintainer we know they're different but in the end we also know the end result.


----------



## kool kitty89 (Dec 17, 2007)

I wonder how a Brit version would've done adapted to merlins; though it wouldn't have been easy to set up counter-rotation, they didn't use that anyway. And assuming the merlin has the improved variable supercharger for altitude performance. Plus they'd have the advantage of using an indigenous engine.

Also those British P-38s performed so badly that they were returned to the US and used as trainers. (with counter-props, but still no turbos)


The P-38 certainly did get one significant thing from this trade though, its name. It was the Brits that came up with Lightning and it stuck, though the birds they got sure weren't fit for the name...

On the coment on the ban on exporting turbos, I think this did occur, just not until after this trade took place. I think the ban was lifted (at least for the British) after we entered the war. Though I don't think the Russians ever got any turbocharged a/c, or did the get some P-47s. I know Britain got some 'Bolts, especialy the P-47M for V-1 interceptions, though it arrived too late to see much use aganst Buzz-Bombs, it was quite effective aganst jets. (Assigned to the 56th Fighter Group, and were responsible for all four of that group's jet shoot-downs.)


----------



## dairwin (Dec 23, 2007)

kool kitty89 said:


> It would be much harder (and more expensive) if the engines didn't offer this feature. (one reason for not using Merlins);
> 
> .... I wonder how a Brit version would've done adapted to merlins; though it wouldn't have been easy to set up counter-rotation, they didn't use that anyway.




Merlin 131.


----------



## FLYBOYJ (Dec 23, 2007)

kool kitty89 said:


> On the coment on the ban on exporting turbos, I think this did occur, just not until after this trade took place. I think the ban was lifted (at least for the British) after we entered the war. Though I don't think the Russians ever got any turbocharged a/c, or did the get some P-47s.


The British got their castrated P-38s in 1941. At the same time they received 20 B-17Cs. Guess that was on those aircraft????

"Four *supercharged* (turbocharged) 1200 hp Wright GR-1820-65 (G-205A) Cyclones."​
They ordered the aircraft in late 1940, they were delivered in the spring of 1941. here's a clip showing their use. In the middle of the clip there's a scene showing the "Supercharger" on the bottom of the engines!!!


_View: https://www.youtube.com/watch?v=OhpJv1eTsLs_


I don't know where you're getting this turbocharger ban - I did see it mentioned on a P-38 website - I think that writer was a bit delusional or had a great imagination. I worked with guys who were there - building and flying P-38s and I never once heard them talk about a "turbocharger ban."

BTW the Russians received 203 P-47s

3 P-47D-10-RE serials 42-75201 to 42-75203
100 P-47D-22-RE serials 42-2553975201 to 42-25638
50 P-47D-27-RE serials 42-27015 to 42-27064
50 P-47D-27-RE serials 42-27115 to 42-27164


----------



## SoD Stitch (Dec 23, 2007)

FLYBOYJ said:


> The British got their castrated P-38s in 1941. At the same time they received 20 B-17Cs. Guess that was on those aircraft????
> 
> "Four *supercharged* (turbocharged) 1200 hp Wright GR-1820-65 (G-205A) Cyclones."​
> They ordered the aircraft in late 1940, they were delivered in the spring of 1941. here's a clip showing their use. In the middle of the clip there's a scene showing the "Supercharger" on the bottom of the engines!!!
> ...




Flyboy: I've never heard anything about a "turbocharger ban" either; I know that turbochargers were difficult to manufacture, which is why Lockheed approached General Electric to build the turbochargers for the YP-38 in '38 (GE had a lot of experience bulding high-temp steam turbines at that time), but it's not too hard to figure out how a turbocharger works. The only limitations are materials; the theory behind turbo-superchargers was easy, it was actually manufacturing the high-temp alloys that was the hard (and expensive!) part.


----------



## FLYBOYJ (Dec 24, 2007)

Turbochargers were made through-out the 1930s by several nations. I do know that during the war when Lockheed was building the P-38 the turbochargers were "GFE" - Government Furnished Equipment.


----------



## kool kitty89 (Dec 26, 2007)

I saw the thing about the ban a couple times (both on the P-38 and P-39 iirc) but it always did seem a bit odd. Thanks for the correction guys.

And he merlins probably weren't practical to fit to the P-38 due to the extensive changes needed (mainly structural changes to the nacelles). I wonder though, how much of the performance loss was due to the lack of turbos and how much was due to the elimination of the counter-rotation. (down to ~300 mph is a pretty big drop from ~400 mph...)


----------



## VonDeeBz (Dec 6, 2019)

ppopsie said:


> 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.





I am late to this thread.

Originally the plane had engines rotating the same way.

The Allison engines were changed to contra rotating props outwards to reduce the effects of torque and make the plane more stable in flight

Here is a doc on the plane

this HTML class. Value is


----------



## Token (Dec 6, 2019)

VonDeeBz said:


> I am late to this thread.
> 
> 
> 
> Originally the plane had engines *rotating the same way*.



No, I don't think so.

I am pretty sure the P-38, from the earliest concepts and before metal started being bent, was designed with counter rotating props. The original aircraft, the first flying copy,the XP-38, had counter rotating props, but they rotated inwards (the tops of the props moving towards the pilot). This was changed and all other US models rotated the other direction, outwards.

The reason often stated for the change to outward rotation was to increase stability.

The model 322's (export P-38's) built for the British and French had props that rotated in the same direction, all rotating right handed. But those were not ordered until more than 15 months after the XP-38 flew and about 6 months after the first YP-38s were delivered in the US and after the first batches of P-38D's and E's had been ordered.

The Brits and French ordered them that way to reduce the parts tail for the aircraft, requiring them to only order / stock one type of engine for spares.



VonDeeBz said:


> The Allison engines were changed to contra rotating props outwards to reduce the effects of torque and make the plane more stable in flight



Counter rotating props, be they inward or outward, reduce the effects of torque pretty much equally. The change from inward to outward was, supposedly, done to increase stability.

T!


----------



## VonDeeBz (Dec 6, 2019)

Token said:


> No, I don't think so.
> 
> I am pretty sure the P-38, from the earliest concepts and before metal started being bent, was designed with counter rotating props. The original aircraft, the first flying copy,the XP-38, had counter rotating props, but they rotated inwards (the tops of the props moving towards the pilot). This was changed and all other US models rotated the other direction, outwards.
> 
> ...




The 1st batch of P38s were meant for the British who ordered them for some reason without superchargers and the props both rotated the same way. Lockheed called this version the castrated P38.

Lockheed had a lot of issues with the plane and the contra rotating props pointing outward as I stated helped to reduce the effects of torque and make the P38 more stable in flight.The advantage of such designs is that counter-rotating propellers balance the effects of torque and P-factor, meaning that such aircraft do not have a critical engine in the case of engine failure.

In designing the P-38, the decision was made to reverse the counter-rotation such that the tops of the propeller arcs move outwards, away from each other. Tests on the initial XP-38 prototype demonstrated greater accuracy in gunnery with the unusual configuration.

There you have it. It also says as such in the documentary I linked.

Reactions: Like Like:
1 | Like List reactions


----------



## pbehn (Dec 6, 2019)

I thought a twin flying on one engine was helped by the props turning with tips toward the pilot at top.


----------



## Token (Dec 7, 2019)

VonDeeBz said:


> The 1st batch of P38s were meant for the British who ordered them for some reason without superchargers and the props both rotated the same way. Lockheed called this version the castrated P38.



The first "batch" of P-38's were not meant for the British. The P-38 was an American design, for the USAAC, before the British considered it. The British did get on board early as the P-38 shared some parts commonality with other US aircraft the British were already flying.

The very first P-38, the XP-38 (37-457), had counter rotating props, from day one, and had them from well before the aircraft was ready to fly. The counter rotating props on the XP-38 rotated inwards. This is well documented.

(edit to add picture and related text)
Below is a picture of the XP-38. You can clearly see, by their pitch, the props are counter rotating, and rotating inwards.






The first P-38's ordered by the Army Air Corp were the YP-38, and they had counter rotating props, rotating outwards, to improve stability in comparison to the inwards rotating props of the XP-38. All American P-38's from that time on had the props rotating outward.

(edit to add picture and related text)
Below is a picture of a YP-38. You can see by their pitch that the props are counter rotating and rotating outwards.






The YP-38 was being built for the US and the P-38, P-38D, and the P-38E were ordered by the US before the British and French ordered the 322B and 322F. The 322B (the British model) was the first model with props that rotated in the same direction.

The P-38 was designed with counter rotating props, and the US took delivery of several aircraft before the British ordered the aircraft with props rotating in the same direction. The British ordered the aircraft with one motor type only, and props rotating right handed, to simplify the supply chain as this matched their existing requirements and parts supply for P-40 Tomahawk aircraft (using the same engine). They were ordered without turbosuperchargers as they were envisioned to be used in a different role, primarily at lower altitudes.

As I said, the British and French versions were not using counter rotating props, but all others did, and the aircraft was designed with counter rotating props from very early in the project. The British and the French changed the design themselves (not a Lockheed change) to simplify the supply chain.

For timelines (roughly, from memory so there may be some minor errors in date but not general timing):

The P-38 specification was let in 1937 (some say initially late 1936). Lockheed won and was awarded the contract to build the XP-38 in June of 1937. The XP-38 (already with counter rotating props) flew in late January, 1939. The XP-38 crashed in early February, 1939. The USAAC ordered YP-38's (with counter rotating props) in April, 1939. The YP-38 first flew in September of 1940. In September of 1939, a year before the first YP-38 flew, the USAAC ordered 66 production P-38's (all with counter rotating props), about 30 were to become "P-38's" and about 36 were to become "P-38D's". Orders were placed for the P-38E (with counter rotating props) before delivery of the YP-38 was complete. The first P-38E rolled off in October, 1941.

The British ordered the model 322 Lightning Mk 1 (a modified P-38E without turbocharger and with motors rotating in the same direction) in March 1940. The P-38E would have been the third batch of P-38's (after the YP-38, P-38, and several experimentals), and the 322 would have been the 4th batch or it might be considered the third batch as it was concurrent to the P-38E. The first Lightning I's arrived in England in March of 1942. So, before the British ordered the 322B with right handed props, the XP-38, YP-38, P-38, P-38D, and P-38E had all been ordered or planned, and several flown, with counter rotating props.



VonDeeBz said:


> Lockheed had a lot of issues with the plane and the contra rotating props pointing outward as I stated helped to reduce the effects of torque and make the P38 more stable in flight. The advantage of such designs is that counter-rotating propellers balance the effects of torque and P-factor, meaning that such aircraft do not have a critical engine in the case of engine failure.
> 
> 
> 
> In designing the P-38, the decision was made to reverse the counter-rotation such that the tops of the propeller arcs move outwards, away from each other. Tests on the initial XP-38 prototype demonstrated greater accuracy in gunnery with the unusual configuration.



Errr....no, the XP-38 did not demonstrate greater accuracy with the "unusual configuration" of props rotating outwards. The single XP-38 had counter rotating props (from day one) that rotated inwards, not outwards. The XP-38 never had props rotating in the same direction, and never had props rotating outwards. It only flew with counter rotating props rotating inwards. The XP-38 crashed (16 days after it first flew) before any changes to rotation were tried. The very first YP-38 had props that rotated outwards.

There were several "issues" in designing the P-38, there almost always are in major new aircraft designs. But in regards to T and P factors it makes little difference if the rotation is inwards or outwards, both rotations are roughly the same with regards to P and T factors. The flow towards the outboard wing panels, vs the inboard panels, did make for greater stability at lower speeds, but no real change in P or T factors.

T!

Reactions: Like Like:
2 | Like List reactions


----------



## tomo pauk (Dec 7, 2019)

VonDeeBz said:


> The 1st batch of P38s were meant for the British who ordered them for some reason without superchargers ...



Source, please.



> Lockheed called this version the castrated P38.



Did they?


----------



## Joe Broady (Dec 26, 2019)

Token said:


> There were several "issues" in designing the P-38, there almost always are in major new aircraft designs. But in regards to T and P factors it makes little difference if the rotation is inwards or outwards, both rotations are roughly the same with regards to P and T factors. The flow towards the outboard wing panels, vs the inboard panels, did make for greater stability at lower speeds, but no real change in P or T factors.



Maybe I have a mental blind spot, but it seems to me inward vs. outward would make a difference. As I understand P factor, it causes one half of the propeller disc to make more thrust than the other, when the propeller axis is not aligned with the relative wind. In the case where that axis is pitched up, it's the down going half of the disc that makes more thrust. That's why a conventional twin (both props clockwise as seen from behind) is harder to fly with the left engine out. Due to P factor, the center of thrust from the right engine shifts a little outboard, thereby increasing the asymmetric thrust problem. On the other hand, with the right engine out, P factor shifts the left engine center of thrust inboard, a favorable effect.

In the P-38 both props rotate outward like the right hand engine of a conventional twin, so whichever engine fails, P factor is unfavorable. But if the props both rotated inward, P factor would assist the pilot regardless of which engine fails. Is that right?

Regarding engine failure on takeoff, the Pilot's Operating Instructions for the H/J/L models says if it happens below 120 mph, close both throttles and land. If above that speed, reduce power enough to maintain control, then gradually add more power, hold rudder and aileron to maintain straight and level, jettison stores, trim rudder, feather dead engine.

Reactions: Agree Agree:
2 | Like List reactions


----------



## FLYBOYJ (Dec 26, 2019)

Joe Broady said:


> Maybe I have a mental blind spot, but it seems to me inward vs. outward would make a difference. As I understand P factor, it causes one half of the propeller disc to make more thrust than the other, when the propeller axis is not aligned with the relative wind. In the case where that axis is pitched up, it's the down going half of the disc that makes more thrust. That's why a conventional twin (both props clockwise as seen from behind) is harder to fly with the left engine out. Due to P factor, the center of thrust from the right engine shifts a little outboard, thereby increasing the asymmetric thrust problem. On the other hand, with the right engine out, P factor shifts the left engine center of thrust inboard, a favorable effect.
> 
> In the P-38 both props rotate outward like the right hand engine of a conventional twin, so whichever engine fails, P factor is unfavorable. But if the props both rotated inward, P factor would assist the pilot regardless of which engine fails. Is that right?
> 
> Regarding engine failure on takeoff, the Pilot's Operating Instructions for the H/J/L models says if it happens below 120 mph, close both throttles and land. If above that speed, reduce power enough to maintain control, then gradually add more power, hold rudder and aileron to maintain straight and level, jettison stores, trim rudder, feather dead engine.



Or in other terms, the P-38 has no critical engine *or* both engines on the P-38 are critical engines.

Reactions: Agree Agree:
1 | Like List reactions


----------



## Token (Dec 27, 2019)

Joe Broady said:


> Maybe I have a mental blind spot, but it seems to me inward vs. outward would make a difference. As I understand P factor, it causes one half of the propeller disc to make more thrust than the other, when the propeller axis is not aligned with the relative wind. In the case where that axis is pitched up, it's the down going half of the disc that makes more thrust. That's why a conventional twin (both props clockwise as seen from behind) is harder to fly with the left engine out. Due to P factor, the center of thrust from the right engine shifts a little outboard, thereby increasing the asymmetric thrust problem. On the other hand, with the right engine out, P factor shifts the left engine center of thrust inboard, a favorable effect.
> 
> In the P-38 both props rotate outward like the right hand engine of a conventional twin, so whichever engine fails, P factor is unfavorable. But if the props both rotated inward, P factor would assist the pilot regardless of which engine fails. Is that right?
> 
> Regarding engine failure on takeoff, the Pilot's Operating Instructions for the H/J/L models says if it happens below 120 mph, close both throttles and land. If above that speed, reduce power enough to maintain control, then gradually add more power, hold rudder and aileron to maintain straight and level, jettison stores, trim rudder, feather dead engine.



I was speaking more in regards to why the change from inwards to outwards, and correspondingly I worded it poorly with regards to P factor, especially since I was thinking mostly of T and said both T and P. Yes, P factor was increased by changing to outwards rotation, and might have been why they tried inwards rotation first with the XP-38. As FLYBOYJ said, with counter rotating props there is no critical engine, or both engines are critical, depending on how you view it. Everything I have come across seems to indicate that the change to outward was to increase stability, and that P factor was not a decisive factor. I said "no real change in P or T factors" and I should have said something like "not changed necessarily to change P or T factors".

If you look at what I was responding to you can see how I got narrow sighted in my response. For the purposes of my response it fit, if poorly, but as a stand alone statement it did not.

T!


----------



## Sid327 (Dec 27, 2019)

drgondog said:


> 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...



.....Not so.
....also 600,000lb/ft from a tail rotor? Not so.

But off topic. Sorry to dump this on the thread.


----------



## drgondog (Dec 28, 2019)

Sid327 said:


> .....Not so.
> ....also 600,000lb/ft from a tail rotor? Not so.
> 
> But off topic. Sorry to dump this on the thread.


Main rotor. Unrestrained when tail rotor lost. Ship rotates violently in the opposite direction from main rotor.


----------



## XBe02Drvr (Dec 28, 2019)

Token said:


> As FLYBOYJ said, with counter rotating props there is no critical engine, or both engines are critical, depending on how you view it. Everything I have come across seems to indicate that the change to outward was to increase stability, and that P factor was not a decisive factor.


IIRC, in another thread that touched this topic, consensus was reached that improved performance in high AOA/high G turning and pull up situations was deemed more important than single engine handling.
Cheers,
Wes

Reactions: Agree Agree:
1 | Like List reactions


----------



## DerAdlerIstGelandet (Dec 28, 2019)

drgondog said:


> Main rotor. Unrestrained when tail rotor lost. Ship rotates violently in the opposite direction from main rotor.



Depends on your airspeed and condition of flight. For instance the loss of tail rotor emergency procedure in a Blackhawk was to remain above 80 kts and perform a roll on landing. I’ll have to look at my checklist when I get back home from my Germany vacation in January to verify, but that is what I recall it being.

I used to work in the off-shore helicopter industry as well, and we had helos lose their tail rotor control and effectivity over the Gulf, fly one hour back to the airport and safely perform run on landings. The S-92 had a design flaw with the tail rotor bearing, and it tail rotor loss occurred to several of them. All returned and landed safely using run on landings.

Reactions: Like Like:
1 | Like List reactions


----------



## XBe02Drvr (Dec 28, 2019)

DerAdlerIstGelandet said:


> All returned and landed safely using run on landings.


I've had two hours stick time in a Jetranger, mostly cruise, but with a few transitions and a couple scary attempts at hover, and the idea of an 80 knot roll on landing with no tail rotor is downright terrifying. The IP, our company's RW chief pilot was calm, collected, and encouraging throughout, but my boss, the FW chief pilot riding in back, was a nervous wreck and finally ordered me to give the ship back to the IP for the rest of the trip. He was kind of a PITA type, and I have to admit I was relishing his discomfort.
Cheers,
Wes

Reactions: Funny Funny:
1 | Like List reactions


----------



## Tieleader (Dec 28, 2019)

I asked our chief pilot, who flies our P-38L amongst others, and he said that the XP-38 did have inward turning blades but these caused handling issues. All later versions had the outward turning types.


----------



## DerAdlerIstGelandet (Dec 29, 2019)

XBe02Drvr said:


> I've had two hours stick time in a Jetranger, mostly cruise, but with a few transitions and a couple scary attempts at hover, and the idea of an 80 knot roll on landing with no tail rotor is downright terrifying. The IP, our company's RW chief pilot was calm, collected, and encouraging throughout, but my boss, the FW chief pilot riding in back, was a nervous wreck and finally ordered me to give the ship back to the IP for the rest of the trip. He was kind of a PITA type, and I have to admit I was relishing his discomfort.
> Cheers,
> Wes



I could not tell you if tail rotor loss was recoverable in a 206 or 407.


----------



## XBe02Drvr (Dec 29, 2019)

DerAdlerIstGelandet said:


> I could not tell you if tail rotor loss was recoverable in a 206 or 407.


It's hard to imagine, with its relatively short tail boom, skimpy vertical stabilizer, and fragile looking skids. OTOH, one of my RW-to-FW conversion students reputedly pulled it off in a Charlie gunship on the Marston at Chu Lai back in the day. (I didn't learn this from him. Some of the Huey jocks in our local Army Guard say he was a legend who got a Silver Star for some of his exploits in 'Nam.)
Cheers,
Wes


----------



## drgondog (Dec 29, 2019)

DerAdlerIstGelandet said:


> Depends on your airspeed and condition of flight. For instance the loss of tail rotor emergency procedure in a Blackhawk was to remain above 80 kts and perform a roll on landing. I’ll have to look at my checklist when I get back home from my Germany vacation in January to verify, but that is what I recall it being.
> 
> I used to work in the off-shore helicopter industry as well, and we had helos lose their tail rotor control and effectivity over the Gulf, fly one hour back to the airport and safely perform run on landings. The S-92 had a design flaw with the tail rotor bearing, and it tail rotor loss occurred to several of them. All returned and landed safely using run on landings.


Chris I should have been more specific and pointed to Bell w/two blade articulated rotor system and no rudder. That said, just about any helicopter with single rotor system at low airspeed is in deep trouble.

Reactions: Agree Agree:
3 | Like List reactions


----------



## Sid327 (Dec 29, 2019)

drgondog said:


> 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...



A Bell 212,

With an 1800shp twin pack loses roughly 200hp through the drive-train (transmission loss) and has a 1294shp txmsn limit. But assuming theoretically it was delivering 1600shp *to the main rotor* that works out to roughly 26,000 ft/lbs of torque one foot from the mast (though the blade tips would be making considerably more).
A conventional two bladed tail rotor which all the civilian Bell helicopters utilise, that were developed from the UH-1 (including the B412 with the four blade MR Head) would only use (again roughly) less than 20% of the total power (at low speeds and in the hover). A fenestron type of tail rotor can absorb up to 25% of total power in the hover but next to zero above about 40knots.

If you lost tail rotor drive in flight with any conventional helicopter type (single main rotor, conventional t/rotor) in straight & level cruise there would be a slight yaw and mild pitch change but there wouldn't be any need for a change of underwear.
In a fenestron type heli, you wouldn't know about it in a stabilised cruise until you made a power, or attitude change.

The old Bell two bladed main rotor system was known as a semi-rigid head (U.S.) or semi-teetering type (UK, Europe).

Depending on type of heli and assuming a long enough hard surface, safe run-ons or rolling landings can be made at ground speeds as low as 35-40 knots, particularly if you have a decent wind from about the 10 o' clock direction (for CCW MRH) or from around the 2 o' clock direction if you prefer to fly French types (opposite turning MRH).


This really doesn't belong here. Maybe a mod could consider moving these rotary type posts?


----------



## DerAdlerIstGelandet (Dec 29, 2019)

Sid327 said:


> A Bell 212,
> 
> With an 1800shp twin pack loses roughly 200hp through the drive-train (transmission loss) and has a 1294shp txmsn limit. But assuming theoretically it was delivering 1600shp *to the main rotor* that works out to roughly 26,000 ft/lbs of torque one foot from the mast (though the blade tips would be making considerably more).
> A conventional two bladed tail rotor which all the civilian Bell helicopters utilise, that were developed from the UH-1 (including the B412 with the four blade MR Head) would only use (again roughly) less than 20% of the total power (at low speeds and in the hover). A fenestron type of tail rotor can absorb up to 25% of total power in the hover but next to zero above about 40knots.
> ...



Thats true. Now that I think about it, we had a 212 lose its TR in the gulf and return for a run on as well. Well when I say we, I don’t mean we, it was well before I started working there. I only read the report.

And I agree. I will move them all when I am not on my phone.


----------



## GrauGeist (Dec 29, 2019)

The reason Lockheed went with the "outer" rotation versus "inner" rotation, was due to lift issues. 
Inner rotation was robbing the necessary lift needed from the inner mainwing, reversing the rotation corrected that issue.

Reactions: Like Like:
1 | Winner Winner:
1 | Like List reactions


----------



## Zipper730 (Dec 31, 2019)

This question might seem silly, but I was trying to figure this out for some time: Did the XP/YP-38 had the same propeller diameter than the P-38's?


----------



## Brent (Jan 2, 2020)

kool kitty89 said:


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


----------



## Brent (Jan 2, 2020)

I seem to recall an article that was authored by Corky Meyer, who recalled that the XP-38 had an overly long take off run, until they discovered that the original prop rotation was creating enough turbulence that the wing center section was not producing lift. The engine direction swap solved the problem, as has been said.


----------



## haitipro (Jan 2, 2020)

ppopsie said:


> 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.




I was lucky enough to fly a P-38 and, ignoring all of the technical possibilities, it made the aircraft a "torque free" handling situation, unlike other twins I have flown, including the Mosquito, which required your attention. The P-38 had zero torque reaction when rapid, full-power was applied on take-off.

Reactions: Like Like:
2 | Like List reactions


----------



## chuter (Jan 3, 2020)

Regarding propeller direction and its effect, we can look at the Corsair. We'll confine ourselves to the carrier approach and go around regime. The prop's right hand rotation swirled the air (propwash) around the fuselage to the right, with the prop, causing the inboard left wing to have a higher relative Angle of Attack than the inboard right wing. If the aircraft got "slow enough" and power was increased "high enough" the relative AoA on the left inboard wing (upward blade) would exceed the wing's critical AoA and stall while the right inboard wing (downward blade) would be stall "resistant" due to its much lower relative AoA and, with the left inboard wing stall being pulled rapidly outboard by the now (safe to assume) down left aileron the plane rolls left. This was "fixed" by the right hand wing's subsequent stall strip mounted outboard of the propwash to induce a matching stall on the right side. (This hurt turn performance, however, but who cared?)

This same effect is what they were alleviating on the P-38. With inboard turning props (XP-38) you have better single engine performance particularly at higher AoA's due to a resulting thrustline (P factor) closer to the aircraft centerline (less corrective rudder) BUT at higher power the wing outboard of the boom (upward moving prop blade) is where your stall will begin (the relative AoA thing). If you are engaged in a tight turn at high power you could easily (if you're fitter than I am) start riding the edge of the stall. As the stall starts/stops/can't decide the effect on the plane is rather dramatic as the wing is losing-a-smidge/gaining-it-back lift outboard of one/both/alternately the booms requiring rapid and substantial aileron correction to maintain roll angle. With outboard turning props this higher power, higher AoA stall effect is much less as the fluctuating lift loss is inboard of the booms much nearer the centerline. 

Sssooooo ... Lockheed went with two "critical" engines sacrificing improved single engine performance for improved both-engine, or we should say, combat, performance.

Reactions: Like Like:
1 | Like List reactions


----------

