P-38 Lightning-why no 4-blade paddle prop?

[IdahoRenegade]

My take on the P-38 is that the Allison was rated at 1,425 HP (in the P-38J and later, anyway) with 1,600 HP at what would be WER. The propellers could handle that much HP and the turbocharger allowed the Allison to maintain more of its HP to a much higher level than simple single-stage supercharging did, but they never did run too much HP to the props at any altitude. The P-38 never really got a big shot of extra HP. Yes, it went from early Allison levels (1,050 HP) up to 1,425 HP / 1,600 HP, but the prop could handle that. Had the P-38 gotten an engine of maybe 2,250 HP, I think it would have needed some propeller attention, but since it never really did take a big HP jump, it wasn't necessary. Also, had the P-38 gotten a big HP jump, it would have been even easier to get to critical Mach number and have compressibility issues. That's the probable main reason it never DID get a lot more HP ... it was already pushing the critical Mach limit somewhat too often.

...Overspeed to maybe 3,150 engine rpm would not be a big deal because you'd be in a dive and not really wanting more thrust. Hitting critical Mach number in a dive would be a MUCH larger concern than propeller efficiency ... at least to the pilot.

In terms of HP of the '38, each engine wasn't far behind the Merlin in the '51. I have seen various source quoting mil power at 1425 or 1475 with WEP (depending on fuel and boost) from 1600 to 1725. Not a lot off the 1490/1720 of the '51, which obviously got the 4 blade prop. And at high altitude the HP on the Lightning didn't fall off as quickly as the '51s did with a mechanical supercharger. Your comment about how quickly it could already hit critical mach does make a lot of sense though. There might simply not have been the perceived need.

The other issue perhaps comes down to the single-source production of the '38 until the very end of the war (when Vultee built 100 or so). WPB was hesitant to allow anything to shut down the Lockheed production line. That at least was the claimed reason the P-38K never went into production. I need to study up a bit more on prop mechanisms to appreciate how big a change that would have been and how the mechanism actually works.

 

[GregP]

The Allison started right at 1,000 HP and sort of ran out of steam at about 15,000 - 16,000 feet unless it had a turbo. It wound up at 1,600 HP in P-38 trim, but HAD the turbocharger, so it maintained a lot more HP as it went up. The prop was a Curtiss Electric and it can easily absorb 1,600 HP at 1,500 rpm in the pitch range it has. Simply, they didn't need any more propeller blades.

In operation, what happened was that as the HP went up, the HP available at height got better due to the turbos. Earlier P-38s (F, G, H) could generate 1,325 HP but only had intercooler capacity (the exhaust went out to the wingtip and came back through a hollow leading edge) for about 1,050 HP. If they used more than 1,050 HP, it was OK but the temperature started climbing and it could only go so high until failure. The P-38J and later variants used a deeper radiator and enlarged cooling capacity and could USE the extra HP for longer than the earlier variants could. In fact, it could cool 1,600 HP for as long as they were authorized to use it.

The Planes of Fame Museum operates a P-38J. The turbos are still in place but non functional. They operate as an exhaust system. The little side intakes just under and to the outside of the turbos are plugged and the former intercooler intakes in the middle of the front air scoop are now carburetor air inlets. Each inlet currently has three passages. 2 are oil cooler air and the middle one is carburetor intake air. Our plane DOES sometimes see 57" MAP at an airshow, but not often. I posted a video sometime back of the Horsemen when the Horsemen flew two P-51s and our P-38 as the 3-plane team. It was certainly operating at high power for that airshow and all the practice flights. It didn't need that power level to CLIMB with the P-51s, but did to maintain the speed they flew at in the act. I'm sure Steve did a bit of throttle-jockey work during the vertical maneuvers to stay together.

Here is a re-post of that video:



I don't really get tired of seeing it.

 

[Shortround6]

In terms of HP of the '38, each engine wasn't far behind the Merlin in the '51. I have seen various source quoting mil power at 1425 or 1475 with WEP (depending on fuel and boost) from 1600 to 1725. Not a lot off the 1490/1720 of the '51, which obviously got the 4 blade prop. And at high altitude the HP on the Lightning didn't fall off as quickly as the '51s did with a mechanical supercharger.

TANSTAAFL.
P-38s (at least "J"s) were using 11'6" props while Mustangs with Merlins used 11'2" props (Allison Mustangs used 10'9" props). 4 inches doesn't sound like a lot but the P-38 had about 6% more disk area than the Mustang did per engine. the "J"s had different props than earlier P-38s. They went from 661lbs per pair on the prototypes/early production to 827lbs per pair on the "J"s. or 413.5lbs each assuming left and right hand weighed the same. Mustang 4 blade prop went 483lbs.
The 4 blade props might very well have more "bite" under certain conditions but the plane gained 140lbs even if nothing else had to be changed (ballast or moved equipment to keep the CG the same?)

Maybe 4 blade props would have improved performance, but not by as much as it appears at first glance.

 

[Milosh]

Were not the P-38Ks to have 4 bladed props?

 

[Shortround6]

I believe wide cord 3 blade props. Maybe larger diameter which required a different gear ratio to keep tip speed down???

 

[GrauGeist]

Were not the P-38Ks to have 4 bladed props?

You may be thinking of the XP-58, as it had four bladed props.

 

[FLYBOYJ]

Flyboy, I wasnt thinking in any particular direction but I was thinking in particular about two aircraft, firstly the Spitfire, without a complete re design of wings and landing gear the maximum prop diameter couldn't really be increased so it sprouted more blades. The second is the Corsair which was laid out as far as the wing shape and landing gear was concerned to have the biggest prop available yet still be able to land on a carrier.

There are a lot of well read people here, the Spitfire and Bf109 were designed in an era when 1000BHP was massive, in a short time designers were considering 2000BHP and the issues around what that meant. One, I believe, must have been how you transmit that power to the air, basically you must increase diameter or increase the number of blades, or both. I dont know if any readers have any info on the subject.

There's no perfect solution, no matter what you do there will always be a compromise. That article explains some of the "where and whys" but in the end it's all about what you want to gain. Climb performance? Speed? Efficiency? Pulling power?

 

[IdahoRenegade]

Were not the P-38Ks to have 4 bladed props?

Wider 3 bladed props (according to Bodie) and IIRC a slightly bigger diameter as well. The larger diameter dictated a higher gear reduction to keep the tip speed within limits. Which in turn changed the trust line, necessitating structure and cowling changes. I'm not sure, but even 4 bladed props might have required a larger spinner, dictating cowling changes and production delays.

 

[IdahoRenegade]

TANSTAAFL.
P-38s (at least "J"s) were using 11'6" props while Mustangs with Merlins used 11'2" props (Allison Mustangs used 10'9" props). 4 inches doesn't sound like a lot but the P-38 had about 6% more disk area than the Mustang did per engine. the "J"s had different props than earlier P-38s. They went from 661lbs per pair on the prototypes/early production to 827lbs per pair on the "J"s. or 413.5lbs each assuming left and right hand weighed the same. Mustang 4 blade prop went 483lbs.
The 4 blade props might very well have more "bite" under certain conditions but the plane gained 140lbs even if nothing else had to be changed (ballast or moved equipment to keep the CG the same?)

Maybe 4 blade props would have improved performance, but not by as much as it appears at first glance.

Thanks for the detailed reply. I hadn't realized that the Js got a different prop than the earlier models.

 

[drgondog]

I'm pretty sure the prop diameters were the same for P-38E though P-38L

 

[GrauGeist]

The P-38K (K-1-LO) was fitted with the Hamilton-Standard "high activity" prop, though it was still a three blade configuration.

It did have the larger spinners and the standard ratio of the Curtiss props was changed from 2.00:1 to 2.36:1

I don't have the diameter of the H-S props handy, but they were noticably larger than the Curtiss props' diameter.

 

[tomo pauk]

(X)P-38K was with 12 ft 6 in diameter prop. Pdf here.

 

[drgondog]

You are both correct - I was thinking strictly production P-38 so the discussion regarding the 38K was kinda off the horizon.

 

[GrauGeist]

You are both correct - I was thinking strictly production P-38 so the discussion regarding the 38K was kinda off the horizon.

Well, the K-1-LO proved that a three bladed propellor could improve the P-38's performance without an additional blade on each hub but I find it a little baffling as to why the AAF didn't follow through with it as the tests were done early enough in the war (1943), that it could have easily been introduced before war's end.

 

[IdahoRenegade]

Article on the K:

Whatever Happened To The P-38K ?

Mostly taken from Bodie's book. Note that the test cycle was completed in FE-AP '43. Would have been nice to have this available in the ETO by say September. Almost a crime that another production facility for the '38 wasn't developed in '42.

 

[tomo pauk]

Well, the K-1-LO proved that a three bladed propellor could improve the P-38's performance without an additional blade on each hub but I find it a little baffling as to why the AAF didn't follow through with it as the tests were done early enough in the war (1943), that it could have easily been introduced before war's end.

Article on the K:
Whatever Happened To The P-38K ?
Mostly taken from Bodie's book. Note that the test cycle was completed in FE-AP '43. Would have been nice to have this available in the ETO by say September. Almost a crime that another production facility for the '38 wasn't developed in '42.

The bigger prop is at disadvantage vs. 4-bladed prop that 'retains' diameter, since it requires a new reduction gear in order for the prop tip to not go faster than local speed of sound. The new reduction gear needs revised cowling, that interferes with current production in the factory. The USAF was in position to order a P-38H/J with both better prop and water injection, a far less of a hussle for the production line, yet unfortuntely they did not considered it, aparently.
When all is said and done, the shortcoming of the P-38 in 1943 was not propulsive power, but other issues - low critical Mach number, only one generator, faulty cockpit heating, issues with engine intake; all of this required the immediate attention. The situation was not helped with only one production source, indeed.

 

[GrauGeist]

The bigger prop is at disadvantage vs. 4-bladed prop that 'retains' diameter, since it requires a new reduction gear in order for the prop tip to not go faster than local speed of sound. The new reduction gear needs revised cowling, that interferes with current production in the factory. The USAF was in position to order a P-38H/J with both better prop and water injection, a far less of a hussle for the production line, yet unfortuntely they did not considered it, aparently.
When all is said and done, the shortcoming of the P-38 in 1943 was not propulsive power, but other issues - low critical Mach number, only one generator, faulty cockpit heating, issues with engine intake; all of this required the immediate attention. The situation was not helped with only one production source, indeed.

Yet the change from the Curtiss prop to the H-S prop (and change in gearing) led to substantial improvements in flight performance. Making the nessecary changes would not have been that much of a setback as they weren't as complex as changes made to the P-47 (B/C to D) or P-51 (B/C to D), for example.

 

[mikewint]

There was a thread a while back on Propellers in general and I did a bit of research at the time. So for what it's worth:
First off there is a large drag penalty associated with any local flow going supersonic due to the energy needed to generate the shockwaves in the flow. Additionally trans-sonic flow (Mach 0.8-1.2 roughly) creates a lot of instability in the overall aerodynamics. The shockwaves, which are actually huge changes in pressure over a very small distance, change the overall pressure distribution on the surface which can mean you aren't nearly as aerodynamically efficient or effective. In trans-sonic flow the locations and strength of these shock waves is dynamically shifting. On a propeller this can cause oscillations which obviously load up all of the associated structure in ways it wasn't designed for.
A propeller is essentially a spinning disc; the tangential velocity depends on the radius outwards from the axis. So, the very center always has a tangential velocity of about zero. If the tip of the prop is super-sonic, then somewhere along the radius you are transitioning between, sub- and super-sonic.
In that setup, the shockwave is just hanging out in the atmosphere between your prop blades. Its location is unstable and can slosh around all over your prop; you don't really have any control over anything. This is opposed to the nicely ordered and well defined shockwaves on super-sonic jets or turbines or rockets. Therefore there is no physics reasoning fundamentally stopping you from running a prop faster than the speed of sound, it is just a bad engineering idea.
Along those lines there was a test bed aircraft, the Republic XF-84H. Which as designed to test the effect of contra-rotating, supersonic, turbine driven, props. They found there were too many other issues for it to work well (top speed Mach 0.83). Namely, you have a prop that's creating a shock wave every time a blade passes. It made for an incredibly loud noise (audible to 25 miles away, giving the aircraft the nickname "Thunderscreech") plus the aircraft was getting hit over and over by the prop pressure waves which disoriented both the aircraft and pilot, in fact, it even gave one guy a seizure.

 

[IdahoRenegade]

Yet the change from the Curtiss prop to the H-S prop (and change in gearing) led to substantial improvements in flight performance. Making the nessecary changes would not have been that much of a setback as they weren't as complex as changes made to the P-47 (B/C to D) or P-51 (B/C to D), for example.

Both of those aircraft had multiple sources of production from a fairly early period of the war. That would allow them to convert one line/facility over while maintaining production at the other. Just speculation on my part but that might be a reason the WPB was more inclined to allow major changes to those aircraft.

 

[GrauGeist]

Both of those aircraft had multiple sources of production from a fairly early period of the war. That would allow them to convert one line/facility over while maintaining production at the other. Just speculation on my part but that might be a reason the WPB was more inclined to allow major changes to those aircraft.

The P-47 Started out production at Farmingdale and by 1942, a new plant was opened at Evansville under the authority of the Army. Around this same time, Curtiss was requested to manufacture P-47s at their Buffalo facility.

The P-51 was manufactured in Los Angeles and as the war progressed: Columbus, Ohio, Dallas, Texas, and Kansas City, Kansas.

Lockheed was manufacturing the P-38 in Burbank (Los Angeles) and in 1943, revamped their assembly line to increase the P-38 output and during the 8 day factory refit, the P-38 production continued - outside.
Lockheed's facilities (and Vega's) were such that they could manufacture P-38s as well as Venturas, Hudsons and licensed B-17s. *IF* the Army decided that the P-38K modifications warranted a change, it could have easily been done without breaking stride in production output.