Why propellers of P-38 Lightning rotate outwards?

[GaryMcL]

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]

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.

 

[FLYBOYJ]

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.

 

[kool kitty89]

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]

You mean turbochargers.

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

 

[kool kitty89]

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]

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]

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]

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

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]

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]

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]

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]

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.

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]

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.



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!

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.

 

[pbehn]

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

 

[Token]

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.

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!

 

[tomo pauk]

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]

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.

 

[FLYBOYJ]

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.

 

[Token]

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!