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



## IdahoRenegade (Jan 27, 2016)

Every "front line" US WWII fighter from about '43 on that I can think of got a 4 bladed "paddle" or high activity prop. My understanding is that this was for a couple of reasons, someone please correct me if I'm wrong. First, engine power increased dramatically throughout the war. Increasing the surface area of the blade made it possible to "bite" more air and better utilize the increased available power. In addition, high altitude, low air density operation made these props even more important at high altitudes. Bob Johnson's book Thunderbolt told of the performance boost this made on that plane. Swept area of the prop can be increased by increasing it's diameter (which has an obvious practical limit) or by increasing the number of blades (3 to 4). 

The P-47, 51, F4U and Hellcat (I believe) were all refitted to 4 blade paddle props. I don't recall the 39 or 40 ever getting them, but they are a different animal. Why was not the same change made to the '38? I know the K model (2 protos) was fitted with a 3-blade high activity prop, and saw a significant performance increase. However, this was larger in diameter than the standard windmill, requiring different reduction gearing, which in turn drove changes to the structure and cowling. It seems like it would have been much more direct just to fit a 4 blade prop (perhaps off the Mustang, but adding a LH version) to it, with the same diameter, reduction gearing and engine. Was there any particular reason that wasn't possible or practical?


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## gumbyk (Jan 28, 2016)

Besides that fact that you're suddenly producing a prop for one aircraft (the LH version that you speak of). I think that having two engines may have something to do with it, there might simply have been no need for it when each engine is running at a lower power setting for the cruise.


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## GrauGeist (Jan 28, 2016)

IdahoRenegade said:


> I don't recall the 39 or 40 ever getting them, but they are a different animal.


The P-39F/J/K and P-39Q-21 through Q-25 used a hydraulic Aeroproducts four blade prop.

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## tomo pauk (Jan 28, 2016)

Even the 3-blade wide-chord would've improved the already good RoC on the P-38, the diameter was already generous to start with. Perhaps also add a few mph to the top speed.
Why no wide-blade prop on the P-38? Guess we'd never know.


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## grampi (Jan 28, 2016)

I had always wondered the same thing about the F7F Tigercat too...


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## FLYBOYJ (Jan 28, 2016)

Aside from developing maximum efficiency based on torque and engine HP (something calculated by engineers) a 4 blade prop also adds weight. Ask someone who has picked up a blade removed a prop hub of a WW2 fighter

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## Shortround6 (Jan 28, 2016)

In the case of the Tigercat it was using 13'2" props to begin with. Prop blade design was always evolving and since the engines in the Tigercat were two speed engines but not 2 stage and were rated at 1700hp at 16,000ft military power compared to the 1800hp at 23,000ft that the engine in the F4U-4 was rated at perhaps the engineers got it right and an extra blade or wider blades were not needed. Please note that at altitudes of around 20,000ft and above a Late model P-38 could make 1600hp in combat power settings.
Using oversize props at low altitude can actually cut speed/performance (not to mention weight penalty )so it is always a bit of a balancing act.

Please note that prop blade dimensions are rather hard to estimate from photographs. Also the width of a prop blade (cord) is going to appear different on different diameter propellers. A small diameter propeller that looks like it has wide (long cord) blades may actually have the same or similar width blades (cord) to larger diameter propeller even though they look different due to the different aspect ratio of the blades.

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## pbehn (Jan 28, 2016)

Isnt having a tricycle/tail dragger landing gear a factor in deciding which prop to use?


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## FLYBOYJ (Jan 28, 2016)

pbehn said:


> Isnt having a tricycle/tail dragger landing gear a factor in deciding which prop to use?



This depends on a whole lot of factors. Aircraft use, hp, engine RPM, torque, I could go on and on.

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## pbehn (Jan 28, 2016)

FLYBOYJ said:


> This depends on a whole lot of factors. Aircraft use, hp, engine RPM, torque, I could go on and on.


I can see that FB but in simple terms the fewer blades the better however with a tail dragger increasing prop diameter starts to increase the chances of it hitting the ground.


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## FLYBOYJ (Jan 28, 2016)

pbehn said:


> I can see that FB but in simple terms the fewer blades the better however with a tail dragger increasing prop diameter starts to increase the chances of it hitting the ground.


By increasing prop diameter, it doesn't mean you're increasing efficiency. Propeller Performance: An introduction, by EPI Inc.


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## Shortround6 (Jan 28, 2016)

In the interest of _maximum efficiency_, some 37-40hp engines in 1930s were fitted with single blade props (with counter weight) on Early Piper/Taylorcraft/Areocna airplanes. 
No prop is going to be the best prop 100% of the time so the designer/users tried to pick the best prop for the intended use of the aircraft averaged over multiple conditions or biased toward one aspect or another, better take-off vs high altitude or other trade-off.

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## FLYBOYJ (Jan 28, 2016)



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## Shortround6 (Jan 28, 2016)

Thank you. 

We have another thread going about long range fighters. 
A large prop with many blades or broad cord blades _may _be the best for climb or high altitude work. 
However while cruising (even high speed cruise) it may be transmitting 50-70% of the "peak" power of the engine and the larger surface area prop will have more drag and lower efficiency in cruise. 
DO you want that last few percent in climb/rate or top speed at 25,000ft and _give up _a few dozen miles in operational radius? or go for the range/radius and give up a bit on peak performance. 

Think of propellers a bit like tires on a car. What works best on pavement (low altitude air) doesn't work so good on sand/mud (high altitude air) and vise versa.

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## mikewint (Jan 28, 2016)

Learn something every day!! Would NEVER have imagined a one-blade prop

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## GrauGeist (Jan 28, 2016)

pbehn said:


> I can see that FB but in simple terms the fewer blades the better however with a tail dragger increasing prop diameter starts to increase the chances of it hitting the ground.


The Curtiss SC-1 had a small diameter prop (10' 2") but still had good performance in spite of the drag imposed by it's float and outriders. It was powered by a Wright R-1820-62

Here's a photo of the SC-1 with it's fixed gear mounted to show just how small it's prop actually was:


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## gumbyk (Jan 28, 2016)

GrauGeist said:


> The Curtiss SC-1 had a small diameter prop (10' 2") but still had good performance in spite of the drag imposed by it's float and outriders. It was powered by a Wright R-1820-62
> 
> Here's a photo of the SC-1 with it's fixed gear mounted to show just how small it's prop actually was:
> View attachment 310232


But with the float fitted, there was much less clearance.





Also, its not uncommon for floatplanes to have a shorter prop - it helps take-off performance and reduces erosion of the blade due to the water spray.


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## GrauGeist (Jan 28, 2016)

I considered showing a photo of the SC-1 with it's float, but the fixed gear photo is far better to illustrate just how small the props were.


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## pbehn (Jan 28, 2016)

FLYBOYJ said:


> By increasing prop diameter, it doesn't mean you're increasing efficiency. Propeller Performance: An introduction, by EPI Inc.


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.


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## GregP (Jan 28, 2016)

For propellers, you can increase 1) diameter, 2) number of blades, 3) blade chord, and 4) change the propeller airfoil. There isn't much else to do until you start looking at Q-tips and strange blade geometry, unless you look at the maybe cuffs around the blade hub. The max RPM is set when the tip goes supersonic (or close to it) and you start losing efficiency rapidly.

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.

The Allison was rated at 3,000 rpm and had a 2 : 1 gear reduction. So prop rpm was 1,500 at 3,000 engine rpm. The P-38 had a prop diameter of 11 feet 6 inches. At 1,500 rpm, the tip was travelling at 615 mph or 903 feet per second. So at sea level the prop tip was already going Mach 0.808 on a standard day and the speed of sound drops with altitude. At 35,000 feet on a standard day, 3,000 engine rpm results in a tip speed of Mach 0.92. I'd think your want a prop tip redline of Mach 0.88 - 0.92 or so, and the P-38 was right there. For cruise, you'd probably want a tip velocity of some 315 - 350 mph or so, which translates into some 1,700 rpm or so at cruise. 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.

The calculations above are tangential velocity and do not take into account forward velocity. At 350 mph true airspeed at sea level and 3000 rpm, the tip speed counting forward velocity comes out to 1,039 feet per second, or Mach 0.928. At 35,000 feet on a standard day the tip speed at 3,000 rpm and 350 mph true airspeed would be Mach 1.067. I don't think they needed any more diameter. Maybe less.

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## IdahoRenegade (Jan 29, 2016)

GregP said:


> 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.


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## GregP (Jan 29, 2016)

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.

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## Shortround6 (Jan 29, 2016)

IdahoRenegade said:


> 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.

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## Milosh (Jan 29, 2016)

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


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## Shortround6 (Jan 29, 2016)

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


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## GrauGeist (Jan 29, 2016)

Milosh said:


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


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


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## FLYBOYJ (Jan 29, 2016)

pbehn said:


> 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?


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## IdahoRenegade (Jan 29, 2016)

Milosh said:


> 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.

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## IdahoRenegade (Jan 29, 2016)

Shortround6 said:


> 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.


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## drgondog (Jan 29, 2016)

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


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## GrauGeist (Jan 29, 2016)

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.

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## tomo pauk (Jan 29, 2016)

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


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## drgondog (Jan 29, 2016)

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


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## GrauGeist (Jan 29, 2016)

drgondog said:


> 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.


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## IdahoRenegade (Jan 29, 2016)

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.


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## tomo pauk (Jan 29, 2016)

GrauGeist said:


> 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 said:


> 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.

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## GrauGeist (Jan 29, 2016)

tomo pauk said:


> 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.


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## mikewint (Jan 29, 2016)

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.


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## IdahoRenegade (Jan 29, 2016)

GrauGeist said:


> 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.


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## GrauGeist (Jan 30, 2016)

IdahoRenegade said:


> 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.


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## FLYBOYJ (Jan 30, 2016)

IdahoRenegade said:


> 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.


Production changes (even major mods) were usually pretty transparent.


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## IdahoRenegade (Feb 4, 2016)

FLYBOYJ said:


> Production changes (even major mods) were usually pretty transparent.



Lockheed estimated 2-3 weeks to incorporate the K model changes (Bodie), and that assumes Allison could have delivered the engines as promised. The War Production Board wasn't willing to accept that loss of production. Given the timeframe (Q2 of 1943) that makes some degree of sense. AT THAT TIME the Lightning was our only high performance, long range fighter. The Thunderbolts were still very range limited, and it would be another 6+ months before a P-51B would even fly a combat mission. Disappointing that it was never in production, but given the priority at the time, it makes some degree of sense. Still, once the 51B was available, it doesn't seem like it would have been a major sacrifice to cut it in.



tomo pauk said:


> 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.



I suspect that was the bigger issue. Addressing those issues was a much bigger priority, and they weren't addressed until the J-25-LO and L models in early '44.

From the link:
*There were still other modifications that were necessary. The Hamilton Standard props required a spinner of greater diameter, and the thrust line was slightly higher as well. This in turn, required that new cowlings be manufactured to properly blend the spinners into the engine nacelles. These were hand made and the fit was less than perfect. The new propellers necessitated a change to the reduction gear ratio. The Curtiss Electric props had a normal ratio of 2.00 to 1. The ratio was changed to 2.36 to 1.

Flight tests were conducted from late February through the end of April 1943. Performance was better than hoped for. Maximum speed at critical altitude (29,600 ft) was 432 mph (Military Power). At 40,000 feet, the "K" zipped along at a speed that was 40 mph faster than the current production P-38J could attain at this same height. Maximum speed in War Emergency Power, at critical altitude, was expected to exceed 450 mph. The increase in ceiling was just as remarkable. Flown to 45,000 ft on an extremely hot and humid day, Lockheed engineers predicted a "standard day" service ceiling in excess of 48,000 ft! Improvement of the cowling fit and the elimination of the heavy coat of paint would have gained even more performance. Due to the added efficiency of the new propellers, range was expected to increase by 10 to 15 %. Lockheed appeared to have a world-beater on their hands.

The plane, now designated the P-38K-1-LO was flown to Elgin Field for evaluation by the USAAF. Flown against the P-51B and the P-47D, this Lightning proved to be vastly superior to both in every category of measured performance. What astounded the evaluation team was the incredible rate of climb demonstrated by the P-38K. From a standing start on the runway, the aircraft could take off and climb to 20,000 feet in 5 minutes flat! The "K", fully loaded, had an initial rate of climb of 4,800 fpm in Military Power. In War Emergency Power, over 5,000 fpm was predicted.​*
Maybe some wishful thinking as far as performance was concerned, but sure would have been nice to see.

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## FLYBOYJ (Feb 4, 2016)

IdahoRenegade said:


> Lockheed estimated 2-3 weeks to incorporate the K model changes (Bodie), and that assumes Allison could have delivered the engines as promised. The War Production Board wasn't willing to accept that loss of production. Given the timeframe (Q2 of 1943) that makes some degree of sense. AT THAT TIME the Lightning was our only high performance, long range fighter. The Thunderbolts were still very range limited, and it would be another 6+ months before a P-51B would even fly a combat mission. Disappointing that it was never in production, but given the priority at the time, it makes some degree of sense. Still, once the 51B was available, it doesn't seem like it would have been a major sacrifice to cut it in.


 This decision has always been arguable. 2 or 3 weeks worth of a production stoppage during that period would have meant about 80 aircraft not being delivered. In hindsight what would have been gained by the P-38K would have possibly eclipsed the production stoppage. I know people who were working at Lockheed during this period, it was always felt the reasoning for not building the P-38K was political

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## IdahoRenegade (Feb 4, 2016)

FLYBOYJ said:


> This decision has always been arguable. 2 or 3 weeks worth of a production stoppage during that period would have meant about 80 aircraft not being delivered. In hindsight what would have been gained by the P-38K would have possibly eclipsed the production stoppage. I know people who were working at Lockheed during this period, it was always felt the reasoning for not building the P-38K was political



Tough call no doubt. General Kenney was screaming for every P-38 he could get in the SWPA as were MTO commanders. Through mid-43 it's questionable that either theater ever had over 80 flyable Lightnings at one time.


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## drgondog (Feb 5, 2016)

Lockheed claimed, and Bodie repeated the performance claims above, but the flight tests at Eglin showed that the P-38K-1 actual performance at 500 pounds under combat load clean was 'marginally' better than a stock P-38L-5 and nowhere near the claims for climb which were slightly above the P-38J at the same weight.

It should be noted that the tests had concluded on the XP-51F and G which had better than the 'projected' P-38K and far better than the actual flight test results. And far cheaper for the P-51H production price agreement.

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## GregP (Feb 5, 2016)

Also, the standard formulas for predicting performance gains with a change in HP and drag don't support the claimed performance gains either. And they are very close to being spot-on for most changes in HP/drag when the actual values are known.

We don't, of course, know the dag change for the P-38K, but we DO know the change in HP and the drag can be estimated quite well. The change in HP coupled with a reasonable estimate of the drag indicate a slight increase in performance, but nothing like the claimed performance changes.

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## FLYBOYJ (May 14, 2017)

RebelAirForce.com said:


> BECAUSE KELLY JOHNSON SAID SO. THAT'S WHY.

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## drgondog (May 14, 2017)

That is why the P-38K engine (-F15) had a modified gear ration of 2.36 to 1 ---> so the 13'6" Curtis Prop would work.


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## pbehn (May 14, 2017)

drgondog said:


> That is why the P-38K engine (-F15) had a modified gear ration of 2.36 to 1 ---> so the 13'6" Curtis Prop would work.


Easy to focus on 4 blade props versus 3 blade, the P38 prop at 13'6" was bigger than the P51 4 blade paddle and not concerned with prop strikes.


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## Shortround6 (May 14, 2017)

RebelAirForce.com said:


> BECAUSE KELLY JOHNSON SAID SO. THAT'S WHY.


See post #23 in this thread.


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## pbehn (May 14, 2017)

Shortround6 said:


> See post #23 in this thread.




With the P51 was the 4 blades paddle prop to increase maximum performance or to improve economy at cruise speed (or both)? For an escort fighter covering miles and staying aloft with less fuel was just as valid as maximum speed.


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## FLYBOYJ (May 15, 2017)

RebelAirForce.com said:


> I agree.
> 
> Bigger prop more air is caught. Its the last inches that do the most.



Bigger prop means more weight and more mass for the engine or gear box to move. It's not that simple.


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## Shortround6 (May 15, 2017)

Probably both.

I would note that the P-38 with it's 11.6ft props had about 14% more disc area than a P-51 with a 3 bladed prop on an Allison engine.

And to add to what FlyboyJ said, the 4 bladed prop on a Mustang weighed about 70lbs more than the 3 blade on the P-38.


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## tomo pauk (May 15, 2017)

Everyone was attaching ever better/heavier/more efficient props to their aircraft as the engine power grew, so I don't think it was much of a problem for the P-38 to get a 4-blade prop, the blades being preferably of wide chord.
With that said, P-38 have had more acute problems than choice of the props - just 1 generator per A/C, faulty heating, low mach limit, messy cockpit, low rate of roll. Granted, some of those problems were adressed, if a bit too late.
The biggest of it's problems (not tied to the design) was lack of another source for mass production.


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## FLYBOYJ (May 15, 2017)

tomo pauk said:


> Everyone was attaching ever better/heavier/more efficient props to their aircraft as the engine power grew, so I don't think it was much of a problem for the P-38 to get a 4-blade prop, the blades being preferably of wide chord.
> With that said, P-38 have had more acute problems than choice of the props - just 1 generator per A/C, faulty heating, low mach limit, messy cockpit, low rate of roll. Granted, some of those problems were adressed, if a bit too late.
> The biggest of it's problems (not tied to the design) was lack of another source for mass production.



The P-38's cockpit was no more messy than any other twin engine aircraft of the day.


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## tomo pauk (May 15, 2017)

FLYBOYJ said:


> The P-38's cockpit was no more messy than any other twin engine aircraft of the day.



Many features were not related to the number of engines - I'll just post stuff from 'America's hundred thousand', pg. 164:
"One pilot with considerable P-38 experience said: The cockpit had gotten lousier and lousier (with succesive models), and you could see fewer gages"
"At 1944 gathering of fighter pilots 55 percent rated the P-38 cockpit worst in arangement of many fighter types present, and noted it (P-38) had the least convinient landing gear and flap controls of all."

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## FLYBOYJ (May 15, 2017)

tomo pauk said:


> Many features were not related to the number of engines - I'll just post stuff from 'America's hundred thousand', pg. 164:
> "One pilot with considerable P-38 experience said: The cockpit had gotten lousier and lousier (with succesive models), and you could see fewer gages"
> "At 1944 gathering of fighter pilots 55 percent rated the P-38 cockpit worst in arangement of many fighter types present, and noted it (P-38) had the least convinient landing gear and flap controls of all."



Yep - the old Col Rau letter - fully aware of all this but if you really look into this you'll find that most of the negatives written about the P-38 cockpit was done so by pilots with little or no multi engine experience, very swayed and biased towards their easier to fly single engine machines (and I can't blame them). Look at photos of the P-38 cockpit and compare them to the cockpit of the A-20, B-25 and even the Beaufighter.


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## FLYBOYJ (May 15, 2017)

Landing gear levers on the P-38 and P-51 were on the same side of the cockpit;


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## tomo pauk (May 15, 2017)

FLYBOYJ said:


> Yep - the old Col Rau letter - fully aware of all this but if you really look into this you'll find that most of the negatives written about the P-38 cockpit was done so by pilots with little or no multi engine experience, very swayed and biased towards their easier to fly single engine machines (and I can't blame them). Look at photos of the P-38 cockpit and compare them to the cockpit of the A-20, B-25 and even the Beaufighter.
> ...



Thanks for the feedback, Joe.
Still - none of this was written by Col. Rau. Granted, having two engines to cater for will add compexity in operation vs. P-51s and 47s.


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## FLYBOYJ (May 15, 2017)

tomo pauk said:


> Thanks for the feedback, Joe.
> Still - none of this was written by Col. Rau. Granted, having two engines to cater for will add compexity in operation vs. P-51s and 47s.



You're right but he was one of the first ones to complain about the P-38 and many of his sentiments were reflected in that report.


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## Shortround6 (May 15, 2017)

I would note that a P-38J with 300rpg for the .50 cal guns and 300 gallons of fuel ( 16,597lbs) was rated at climbing to 23,450ft in 6.49 minutes and was climbing at 2665fpm at 25,000ft. using WER power

A P-51D at 9760lbs took 6.4 (6.6?) minutes to climb to 20,000ft and was climbing at 2375fpm at 25,000ft. 
The P-38 was over a minute quicker to 25,000ft using WER power on 100/130 fuel, 100/150 might change things a bit.

The P-38 might well have done better with 4 bladed props but since it could out climb both the P-51D and the P-47D with paddle blades and water injection the actual _need _for four bladed propellers doesn't seem to anywhere near as marked as for the P-47.

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## drgondog (May 15, 2017)

The combat weight of the P-38J is 17,360 with 420 gallons of fuel. The Test report (IIRC) you are referencing SR, did not have fuel in LE tanks.

That said the P-38J/L out climbed the 51B/D below 20K at respective full combat weights, after that the P-51 ROC was higher. Moral of the comparison is that the P-38 was superior from SL to 10K, then steadily lost advantage until the P-51 overtook it near FTH based on the rated RPM of the Turbos'. 

When 75" could be pulled with 150 octane to the 70" of the P-38J, the Mustang matched comparable combat weight P-38J/L to 20 and 25 and 30 K.

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## Shortround6 (May 15, 2017)

It may have been a bit light on ammo too. Performance for the K showed no difference in speed until over 30,000ft although a large difference in climb was at all altitudes. 

However the K used a 12ft 6in three bladed prop.







I have no idea how a 12ft 6in 3 blade compares to a 11ft 3in four blade (or any 4 blade of approximately the same dimension.)

The P-38K prop had 18% more disk area than a normal P-38 and obviously much more blade area. How a smaller 4 blade would have done is the question. Better than the stock props but not as good as the big 3 blade?

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## FLYBOYJ (May 16, 2017)

RebelAirForce.com said:


> If Kelly Johnson said it flew better with three blades rather than four I'm inclined to trust his judgement.



The decision in this matter would not be Kelly Johnson's anyway. After the design was bought by the government any major design changes had to be approved by the government.


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## tomo pauk (May 16, 2017)

RebelAirForce.com said:


> If Kelly Johnson said it flew better with three blades rather than four I'm inclined to trust his judgement.



I'll politely ask for source to back up that staement.



> Chuck Yeager told me in person he thought the P-38 was better.



Better than what, and based on what?

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## GregP (May 16, 2017)

Chuck likes the P-38, for sure. Said so himself at many airshows. Down low, he liked the P-39, too.


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## FLYBOYJ (May 17, 2017)

RebelAirForce.com said:


> Than the P-51D
> 
> My source for the Kelly Johnson fact is The Skunk Works by Ben Rich.



OK - I'm throwing up the BULLSHIT flag. In the book "Skunk Works" by Ben Rich, he (or his collaborator) mentions that the P-38 "was the most maneuverable propeller driven fighter of the war" (Page 111 and nothing is mentioned about Kelly Johnson except that he designed the P-38) which was proven to be not true; this was just the opinion of Mr. Rich (I briefly worked with him, knew his step-daughter) and although the man was brilliant and I will always have tons of respect for this man, he was obviously wrong on this one.

With that said, if you wish to be a useful contributing member of this site, please do some research. Three syllable responses don't go over well here.

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## wuzak (May 17, 2017)

Can I ask, was the P-38 ever flown with 4 bladed props, even if only experimentally?


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## FLYBOYJ (May 18, 2017)

wuzak said:


> Can I ask, was the P-38 ever flown with 4 bladed props, even if only experimentally?


AFAIK no


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## Peter Gunn (May 18, 2017)

Just curious and maybe I missed it in the thread somewhere, (gotta splittin' headache this morning), but is there any data to compare the P-38 with the 12ft 6in three bladed prop and the stock thin three blade? It sure looks like the props in SR6's photo would take a heck of a bite out of the air.


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## Shortround6 (May 18, 2017)

http://www.wwiiaircraftperformance.org/p-38/P-38J_performance_11march44.pdf

take two aspirin and enjoy the nice weather until about 10;30 am, then find the air conditioning.

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## Peter Gunn (May 18, 2017)

Thanks SR, funny you should mention 10:30, have a meeting I really don't want to go to then, well, at least the A/C part is covered. I already took the aspirin.

Looking over the chart you posted, I can see why, based off just that and my laymans logic, they felt the standard props were fine. Unless you're low down or over 30K there doesn't seem to be much difference.


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## tomo pauk (May 18, 2017)

RebelAirForce.com said:


> Than the P-51D
> 
> My source for the Kelly Johnson fact is The Skunk Works by Ben Rich.



Unfortunately, you didn't posted a single picture showing a real P-38 with 4-baded props in the 1st place. Further, no specific quote on Lockheed testing the alleged P-38 with 4-bladed prop.
Re. Yaeger saying he liked P-38 more than P-51 - fact has it that eg. P-51s were thashing Luftwaffe big time during the Big Week, vs. a similar number of similarly long ranged P-38 that barely claimed an aircraft in same time and area. (me loving the P-38 for around 40 years) Not that P-38 didn't have had it's qualities.

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## soulezoo (May 18, 2017)

Is this where one is supposed to throw in a "fork-tailed devil" comment? Just kidding...

Well, I wasn't around for any of these purported conversations so I am in no position to refute them, but as members on this forum know I live close to Gen Yeager and have had a couple of brief conversations. (Although, out of respect for him, I don't pry any-- just let him say what he wants to. I figured he's been grilled enough times by enough people over the years on his opinions that I didn't want to add to that)

Anyway, other than having a lot of respect for the Dora, he's indicated that the Mustang was his favorite to fly. (I write it that way because I have not heard him directly say to me that xxxx aircraft was "best")

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## drgondog (May 18, 2017)

Peter Gunn said:


> Just curious and maybe I missed it in the thread somewhere, (gotta splittin' headache this morning), but is there any data to compare the P-38 with the 12ft 6in three bladed prop and the stock thin three blade? It sure looks like the props in SR6's photo would take a heck of a bite out of the air.


Yes - the P-38K (Allison 75/77 with different reduction gear ratio than 2:1) had 12-6 Curtis props. In the only published Flight test I have seen it offered nearly 500 fpm advantage at low altitude climb rate compared to P-38J. Top speed at 24 or 25 K was 414mph IIRC at 3000 Rpm and 60" Hg

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## Peter Gunn (May 19, 2017)

drgondog said:


> The combat weight of the P-38J is 17,360 with 420 gallons of fuel. The Test report (IIRC) you are referencing SR, did not have fuel in LE tanks.
> 
> That said the P-38J/L out climbed the 51B/D below 20K at respective full combat weights, after that the P-51 ROC was higher. Moral of the comparison is that the P-38 was superior from SL to 10K, then steadily lost advantage until the P-51 overtook it near FTH based on the rated RPM of the Turbos'.
> 
> When 75" could be pulled with 150 octane to the 70" of the P-38J, the Mustang matched comparable combat weight P-38J/L to 20 and 25 and 30 K.



Just need clarification (my bad, sorry) on your last sentence, with the Mustang pulling 75" and 150 octane and the 38J pulling 70", were they equal in ROC through all altitudes then? I have no doubt the Mustang was faster considering how slippery it is.


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## wuzak (May 21, 2017)

Peter Gunn said:


> Just need clarification (my bad, sorry) on your last sentence, with the Mustang pulling 75" and 150 octane and the 38J pulling 70", were they equal in ROC through all altitudes then? I have no doubt the Mustang was faster considering how slippery it is.



The P-51 would only be able to hold the 75" MAP to a certain altitude before it fell off (critical or full throttle altitude). The P-38 should be able to maintain the 70" to a higher altitude.

So it may be that at the higher altitudes the P-38 gains the upper hand in ROC.

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## drgondog (May 22, 2017)

Wuzak - Interestingly, the P-38 had the advantage during the turbo supercharged full power delivery which tapered off around between 20 and 22,000 feet to fall to the equivalent Allison non-supercharged power rating as it climbed further. Depending on the engine and boost the Mustang ROC crossed over and exceeded the P-38 thrust in the area of FTH for both engines.

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