P-38 Lightning Why Not Merlin Engines?

Why didn't the USAAF ask for Merlin powered P-38s when the Merlin upgrade worked so well on the P-51s?
Why did the P-38 soldier on without paddle props and limited to 60 inches of manifold pressure throughout the war when both the 47 and 51 got paddle props and manifold pressure values of 72" and 75" respectively?

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[jimfarmer]

Non engineers don't understand how hard it is to make design changes in complex systems. You did a very good job identifying the difficulty.

[ericb.]

P-38, my favorite WW II fighter/attack airplane. Long range, fast, highly armed and beautiful. American design at its best, Kelly Johnson strikes again.

[Torby]

Always have an affection for the P38. That was my first plastic model. Dad helped me put it together.

[rogermason]

I wonder if another factor against using Merlins in a P-38 might be better utilization of total US mfg capacity. Adding P-38 2x per air-frame engine requirements would drive a big increase in Packard Merlin production demand vs leaving the total P-51/P-38 engine requirements split between Allison and Packard.

[icewaterslim]

Quality video. The new tech in the Allison's turbo superchargers, was complex and buggy at Northern Europe's cold high-altitude missions and gave newly trained pilots a helluva lot to deal with and almost always during combat when it was least appreciated. It's why they were reconsidered for 8th AAF Bomber escort duty and put to a much more advantageous environment in the Pacific and why their legacy in the Mediterranean was more fortunate as well.

[gort]

Greg, this is your best video yet. I can't thank you enough for addressing the Merlin swap issue. At least your viewers will be better informed than the droves of internet fanboys that don't understand the airplane and think the V-1710 was the problem and that the Merlin would have worked magic on the airplane.
I especially appreciated the detail on the paddle blade props. I love the P-38, but agree that further development would have been a misapplication of resources by that time. Plus, spending a huge amount of money on making an airplane with compressibility issues go really fast at altitude was not the best option in light of the available alternatives. None of this takes away from the original brilliance of the P-38 when one considers the circumstances behind its design.

[carltyson]

Greg, running out of superlatives for your videos. They just keep getting better and better...and they have been the best on YT for some time. Great work. It reveals so much about the issues that faced designers and engineers in the period. Some really amazing folks did some amazing things. Have to feel for the Allison team...with the proper boosting their engine was a beast. By the time they got it right with the King Cobra it was too late. Kudos for such great work.

[AaronStuartHall]

Having watched your earlier videos, I immediately knew the P-38's supercharging system would be at the heart of your answer regarding the suggested Merlin swap. I am in awe at your ability to explain complex technical issues to a lay (but inquisitive) audience of non-engineers.

[jamesrumizen]

Greg, as usual an excellent video. Thanks for straightening out the what ifs on the Merlin powered P-38. One thing I remember reading in Daniel D. Whitney's "Vees for Victory, The Story of the Allison V-1710 Aircraft Engine 1929-1948" was that the Army Air Corps was insisting the Allison use the new GE turbocharger instead of a two stage supercharger. As I remember, this was well before the U.S. entered WWII and may have even been before the 1939 start of the war in Europe. The book also notes, that while the proposed Merlin powered P-38 would have been faster than the Allison powered version, it would also have had shorter range, which was one of the P-38's great advantages. Thanks again for a great video.

[nolanbowen]

Thanks for pointing out how incredibly complicated this whole issue was. Even with these problems the P38 was a great warplane. I heard once it had the highest kill ration of the American aircraft which, of course included the Pacific theater.

[OldGeezer]

Greg, you're incredible with your explanations. One thing I have to confess, I was always taken by the beauty of the P-38 and the P-51-D, but after watching al your videos, I'm all in for the P-47-D, H and J! I always thought of them as big, lumbering slowpokes. Was I EVER wrong! Thanks for helping us get better informed.

[idahorx]

Concerning the paddle props-the 3 bladed paddle props fitted on the "K" prototypes not only was wider, but longer (bigger diameter). The extra diameter was the issue-to keep the tip speed subsonic the prop had to turn slower. To turn slower meant different reduction gearing (2.35 vs 2.0 IIRC). The different reduction gearing meant a different distance between the crankshaft and the prop centerline-which was the real issue. That meant structural changes to keep the thrust axis located properly and new sheet metal to make that all work. So-the issue was the bigger diameter, 3 bladed (paddle) props. The question I've never seen answered is why didn't it get 4 bladed props, with the same OD as the standard, 3 bladed one, like virtually every other fighter did? The extra blade lets the high activity paddle "work" at the same RPM as the standard, 3-bladed one, eliminating the need for different reduction gearing and revising the thrust axis. Seems like it would have been a much simpler way to go.

The other issue, why not a liquid to air intercooler like the Merlin got? Much more streamlined to put a small liquid cooled heat exchanger in the intake air stream, rather than duct all the intake air out to and back from to the air to air one. The original design with the wing leading edge intercoolers was a great concept-intercooling with no added drag. Unfortunately it ran out of cooling capacity at much over 1200hp. The big, chin-mounted intercoolers did the job properly, but added drag-even though they made a lot more power, J & L models were only marginally faster than the G. A liquid cooled intercooler offered more packaging options for the secondary side, potentially utilizing the Meredith effect like the Mustang did to reduce drag. It would also eliminate all that duct work in a crowded engine compartment.

Big issue was still compressibility. At high altitudes the standard P-38s weren't far from compressibility speed even in level flight-more power would have got them there quicker. Granted, the huge boost in clime rate alone would have justified it.

The other big question-why were no other plants built to produce the P-38? Every other fighter was built in multiple facilities, meaning that new designs could be cut in at one plant, without impacting production at the others. The '38 was only built at Lockheed's Burbank plant until very late in the war, when Vultee built ~100, none of which made it into combat IIRC. Until the P-51-B finally saw combat in December 1943, the Lightning was our only high performance, long range, high altitude fighter. Seems like it would have been obvious that more plants needed to be building them.

[rtstephen]

Another excellent video! I'd like to point out that the P-38 had only one production facility which was in Burbank. Another production facility didn't open until very late in the war. A Consolidation-Vultee IIRC. Working up various experimental a/c along with ramping up the P-80 in only one plant probably forestalled any major modifications to the already quite capable P-38J/L as Greg points out. Meanwhile the P-47D and P-51D were being produced in two facilities respectively, allowing modifications to be introduced without completely shutting down production.
I have to say as a fan of the P-38 the "K" would have been a winner with climb and acceleration outweighing compressablity issues IMO.

[russmarasheski]

Your knowledge and ability to explain things in lamens terms regarding aviation are unmatched on YT

[konstantinatanassov]

Great Video! I didn't think, that there were people actually thinking that an engine replacement in the P-38 was needed and possible, even less - beneficial.
Ultimately, this question is resolved by this video.
This has already been resolved in advance, within the video about superchargers, where Turbo- and Mechanical Superchargers were compared with their pro-s and con-s.

[gordoh]

Excellent! You have covered this well here and previously. I believe we have all viewed "P-38 A Personal Story". The second quote in the beginning "Once the P-38 received the aileron boost and dive brakes... ". Also, General Kenney made it abundantly clear, he wanted all available P-38s in the Pacific theater. This was for obvious reasons. The P-38 did better in warmer climates and did not handle the high altitude low temperature in the European theater as well as the P-47 and P-51. Lastly, I believe we all want 2 engines over water with very little landing choices available.

[ahnonymuch]

This one is for Greg. Your videos always discuss very complex technical issues, but even I, complete noob, understand what you are talking about. Great job. It also shows how complex designing an aeroplane had become by then.

[tryscience]

Thorough, high quality engineering analysis of Merlin versus Allison forced induction power plants in the p-38. Really excellent

[Ford_Raptor_R_hp_V]

*in Sept 1943 Lockheed delivered to the AAF at Eglin field, a P-38 fitted with 1, 875 bhp Allisons and Paddle-Bladed Propellers.*

*it had a Top Speed of 432 mph on Military Power, and >450 mph on War Emergency Power. 4, 800 ft/min climb rate and 5 mins to 20, 000 ft*

[Itsjustme-Justme]

Fitting two stage Merlins in a P-38 means a major redesign of the whole engine nacelle if not the whole airplane.
The two stage supercharger is attached directly to the Merlin's back side, making it quite long. It gets even longer because the two supercharger stages are on the same shaft and between the stages there is the intercooler.
Does that fit in front of the P-38's firewall without shifting the center of gravity forward? I don't think so. The big turbos are behind the CG. When they are not installed the CG of the airplane shifts forward.
So, there are two things shifting the CG forward. That is not a good idea because there barely is enough equipment inside the booms that could be repositioned further back to compensate for the CG shift.

An important reason for not switching to paddle blades on the P-38 is, when the paddle blade prop became available there was not that desperate need for more performance anymore. The Japanese were already getting weaker and were already outnumbered by Allied fighters. In Europe the P-51 already had started to replace the P-38. The expensive and time consuming conversion of production lines, time consuming field conversions of existing aircraft and difficult conversion of the supply system for replacement parts simply was not worth it. The P-38 was an expensive airplane. Its main advantage over other fighters was its range. New versions of the P-51 and P-47 already were in developement in 1944 and they were designed to overcome their previous disadvantage in range without getting as expensive as the P-38. No doubt the Air Force already was thinking about phasing out the P-38 in mid 1944.