On big American fighters

Discussion in 'Aviation' started by tomo pauk, Feb 26, 2015.

  1. tomo pauk

    tomo pauk Creator of Interesting Threads

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    In order not to clog up the current RAF bombing thread, I'd reckon it is better to discuss the merits and shortcomings of big US fighters in a separate thread.

    The P-38 was drop-tank ready before the 1st production F4U made 1st flight - Feb. vs. June 1942. That, and numbers available means that, until at least mid 1943, the P-38 is a more readily available long range fighter than F4U.
    The F4U, however, does have less problems than P-38. Some of whom were not eradicated for too long.

    The cockpit heating issue was solved once the generator was added to the second engine, that meant there was enough of electricity to heat the guns, so the part of the engine heat previously used to heat the guns was re-routed to the cockpit.
    With that said, I'm all for non-turbo P-38, should work well up to 15000 ft before 1942, and up to 20000 ft once the 9.60:1 S/C gearing is used.

    As I've noted in another thread, the P-47 was ferrying via Island while using drop tanks installed under wing, already in August 1943. In the same time, the 5th AF was receiving Australian-made 'shallow' drop belly-tanks, 200 gals - indeed a quick fix, thanks to Gen. Kenney.

    The wing racks were not very much liked by P-47 drivers, or at least that is what I've read. There seem to be a re-inventing of the wheel - produce the racks of Lockheed's design, as used on P-38 and you're set.
    The F4U received 1st one drop tank, than two drop tank facility, while the AU-1 (post-war attack version with 1-stage engine) was managing up to 3 x 150 gal tanks.
    Two locations for the internal fuel tankage might be also be of interest. One is where the un-protected tanks on the XF4U were installed (besides at outer wing panel) - behind the spar, between the wheel well and fuselage. Another location is under the pilot - the F4U was one of rare aircraft without 'proper' cockpit floor. Too many tanks would make managing the fuel flow a place to easily to make a mistake, though: swithcing the fuel selector to an emptied tank.
     
  2. varsity07840

    varsity07840 Member

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    If I remember correctly, the P-38F was the first drop tank ready model. P-38Es that went to Alaska in August of '42 were retro fitted for tanks.

    Duane
     
  3. Shortround6

    Shortround6 Well-Known Member

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    I would note that deleting the turbo does not mean more power low down. Early P-38 engines used lower supercharger gears than the contemporary P-39 and P-40. 7.48 gears? This is what allowed early P-38 engines to be rated at 1225-1325 hp military instead of 1150 or so.

    Building low altitude P-38s is a luxury that only comes with hindsight as we know that opponent's didn't develop high altitude fighters as fast as expected. It also rather over looks the fact that for all of 1942 and in early 1943 the U.S. Army only had the P-38 and Merlin powered P-40s for fighters that were any good much above 15,000ft. It could take weeks (if not a couple of months) to get a U.S. fighter from factory door to combat service. Playing jiggery-pokkery with production and trying to guess which specialty P-38 would be needed at a particular spot on the globe 2-6 months in the future requires a very good crystal ball.
     
  4. tomo pauk

    tomo pauk Creator of Interesting Threads

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    For the non-turbo P-38 to be of any use, we must have the second source of them, early enough.

    The engine power of the non-turbo P-38 need to be upped by how much at medium altitudes (10% ?) because of exhaust thrust via 'plain vanilla' exhausts, not the collector-system exhausts as used on ill-fated Lightning Is intended for the RAF. That gives some 1250+ HP, up until 12000-14500 ft, no ram.
    The air intake also will need to be extended, looking like the one at P-40 or P-51, to take full advantage of ram effect. Such a non-turbo P-38 would not be that far away from the P-38F/G, will be lighter than those, and a far more useful aircraft than P-40 or P-39.
    Of course, no C15 engines of same rotation like at Lighting I, but F series of opposite rotation.
     
  5. Shortround6

    Shortround6 Well-Known Member

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    Exhaust thrust doesn't do a lot for climb and it does darn little for take-off.

    Is one non-turbo P-38 worth two P-39s or P-40s?
     
  6. tomo pauk

    tomo pauk Creator of Interesting Threads

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    Exhaust thrust is present on an engine that stands still or that moves at moderate speed, granted that it is greater as speed and altitude are greater. Eg. the BMW 801C was generating 104 kg of thrust at 900 m and at zero speed (engine was making 1600 PS); it was 108 kg at 5.4 km km at 500 km/h (engine making 1390 PS).

    If one wants to have fighters with combat radius beyond 300-350 miles, it certainly is.
     
  7. Shortround6

    Shortround6 Well-Known Member

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    Thrust is not horsepower.
    The better the match between exhaust gas escape velocity and speed of the aircraft the more power the same thrust was actually worth.
    During the 1950s some planes were fitted with exhaust augmentation devices to help take-off or cruise performance. This usually involved an outer tube around the actual exhaust pipe/s that drew in air that was mixed with and accelerated by the exhaust gases. More total mass at lower velocity to suit the speed range desired. Same basic theory as pure jet engines vs turbo-fans. Pure jet engines aren't all that efficient even at jet airliner cruising speeds.
     
  8. tomo pauk

    tomo pauk Creator of Interesting Threads

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    Of course. Thrust pushes the aircraft through the air, horsepower does not. At least not until it starts to spin a prop.

    FWIW: again the BMW 801C as an example - at zero speed, 4.6 km of altitude and 1380 PS prop power, it was generating 106 kg of thrust. At 500 km/h, 5.4 km and 1390 PS, it was generating 108 kg of thrust.
     
  9. Shortround6

    Shortround6 Well-Known Member

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    Great, now what was the power of that thrust?
    Again zero speed it was zero power.
    At 500kph we can figure the power out IF we knew the the exhaust gas velocity. The power at 500kph is a lot more than at 250kph.

    And think about propellers. Bombers used larger, slower turning propellers than fighters using the same engines to move a higher mass of air at a lower velocity to suit the bombers speed. The actual "thrust" may have been close to equal but the actual power used at given speed was not.

    I am on a kindle right now and don't know how to cut and paste or link very well.
     
  10. GregP

    GregP Well-Known Member

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    The P-38F used the -49 (right hand) and -53 (left hand) engines (F6R and L) with a 9.5-inch impeller at 7.48 : 1 ratio. Both wee rated at 1,325 HP @ 3,000 rpm and 25,000 feet. The prop reduction gear ratio was 2.01 : 1.

    Later F's used the F10-R and L variants.
     
  11. kool kitty89

    kool kitty89 Well-Known Member

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    #11 kool kitty89, Feb 27, 2015
    Last edited: Feb 27, 2015
    I remember reading that the initial wing pylons used on the P-47 were significantly more harmful to performance than revised, streamlined replacements. I'm not sure if the latter were as clean as the P-38's racks though. (visually speaking, the pylons on the F4U seem closer in design to the P-38, but I'm not sure on how they directly compare)

    And given how general this has gotten in terms of comparing the merits of the basic P-47 vs F4U airframe design (and potential for alternate development) it's probably worth noting the lack of non-turbo P-47 experiments by Republic. Removing the weight and bulk of the turbocharger and ducting and adopting a 2-stage supercharged R-2800 (with upgraded models later on similar to the F4U-1 to F4U-4 and F4U-5) is noteworthy in as far as hypothetically more versatile or generally useful. (perhaps more significant than the drag/weight reduction would have been added clearange beneath te fuselage for more centerline external storage along with potential space for more fuel tankage where the turbo would have gone)

    Admittedly, the turbo/engine arrangement in the P-47 had other advantages like low cockpit noise/vibration levels and ducting acting as crumple zones in belly landings.

    That and the issue of fan cooled engine configurations comes to mind. On either plane, with or without turbo, a configuration as seen on the XP-47J seems like it had a lot of merit (as did such arrangements with Bristol and BMW radials) Employing such on the F4U likely would have been a much more practical and useful performance boost than adaptations made for the Super Corsar's massive R-4360. (then again, the XP-72 similarly fared worse than the XP-47J)



    Yes, ram air intakes and left/right handed F series engines. You also avoid the intercooler problems on pre-J models and (with overboosting or once official WER was cleared by allison) significantly more power below 10,000 ft on top of exhaust thrust boost, lower weight, and easier maintenence. (or perhaps not lower weight but expanded fuel capacity)
    9.6 supercharger engines would allow that a bit higher still, but not quite as much pwoer low down. (and no potential for overboosting)

    8.8 supercharged P-38s probably would have made good V-1 interceptors too if stripped down for interceptor duty.


    On the whole it could do a lot of things the P-39 and P-40 couldn't with better range, bombload, speed, climb rate, much better stall characteristics than the P-39, and possibly performance advantages over similarly engined P-51s.



    I'd suggest merlin powered P-38s, but even with the V-1650-1 available early enough to be interesting, the limited units available and greater retooling issues would make it less attractive. (plus poorer cruise performance and reduced maximum range) I'm not sure counter rotating models would have been available either, and torq of similar rotation would degrade performance or handling advantages too.









    I'd expect 8.8 (and later 9.6) supercharger ratios to be used similar to contemporary P-40s and P-39s. With the exception of the early, long-nose C series engines, the 8.8 supercharged types were uprated to 60" Hg boost for emergency power, more for later models with strengthened crankshafts but I believe limited to 60" and 1570 HP for the likes of the V-1710-39 as on the P-40D, E, and similar engines on early P-39 models. 9.6:1 engines were limited to 57" Hg (1480 HP) due to charge heating, though those engines became available in number not too long before the P-38J was available. (the cost/weight savings compared to the all-around performance gains on the J model might not be worthwhile compared to concentrated production of the J/L)

    In addition to the 60" boost emergency power rating, 8.8 supercharged V-1710s were also up-rated to 1325 HP for take-off.



    Effective relative HP gain through exhaust thrust depends on the speed, propeller efficiency, and other factors too. There's some rough guides to it, but at low speeds the gain is relatively modest given the high thrust output from the prop. As speed goes up and prop efficiency goes down, exhaust thrust remains relatively constant (or increases due to ram pressure) and makes up a larger and larger percentage of net thrust.

    For take-off and climb the gains would be pretty limited. Level and dive (and zoom-climb) would be the most relevant situations. (generally in the 300+ MPH range)
     
  12. Shortround6

    Shortround6 Well-Known Member

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    A big trouble with a lot of these "throw the Turbo out" suggestions is that you wind up with limited aircraft instead of general purpose ones.

    What job do you want your P-38Js do do on Monday? what do you want them to do on Wednesday? How about Friday?

    What job do you want the turbo-less P-38 with the F4R (-73) engines do on on Monday? On Wednesday? oops, that 1580hp was only good at 2500ft or below, at 4300ft you are down to 1490hp and at 12,000ft you are down to 1150hp. Guess you won't be flying any mid-altitude missions let alone high altitude.

    Depending on WER power for "routine" missions is also a mistake. The throttles had "tell tales" (wires) that would tell the crew chief if WER power was used on a particular mission and it had to be noted in the log book/s, discussions on extra maintenance made and deductions made from the suggested overhaul life of the engine. In a combat zone you sometimes have to do what you have to do but using an engine set up that calls for more frequent engine replacement (not having to work on turbos may balance out the more frequent spark plug and oil changes needed by WER settings) may not be the best idea either.

    The "idea" of yanking the Turbo out of the P-47 and using the Navy 2 stage engines needs a little looking at too.

    c3b1b9f7b58c.jpg
    f4ucutaway.jpg

    Please notice the "bay" behind the engine on the F4U and in line with the leading edge of the wing. This is were a lot of the ducting and intercooler "stuff" was on the F4U, If you rip the belly and ducts off the P-47 you are going to have to put them somewhere. Maybe move, make smaller the fuselage fuel tank?
     
  13. tomo pauk

    tomo pauk Creator of Interesting Threads

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    The P-47 have had far less issues related to the hi-alt operations (cockpit heating, notable compressibility problems, inter-cooler capacity), so IMO stripping down the turbo does not seem to be as attractive as with P-38? As for weight savings: the R-2800 + accessories on the P-47 weighted 3200-3250 lbs; for the 2-stage R-2800 the combined weight is 2750-2800 lbs. Would the difference of 400-450 lbs be felt on an aircraft that clean went easily above 14000 lbs when loaded with fuel and ammo?
    Then we have the issue of balancing out the turbo-less P-47. Turbo itself was located halfway to the rear of the aircraft, not so close to the CoG as it was the case for the P-38. ~240 lbs of turbo, plus inter-cooler and ducts weights.
    The 2-stage engine is heavier by some 170 lbs, again rather away from the CoG; the inter-coolers will have to go in the 'tunnel', in front of the CoG - all of that worsens the problem with the A/C that is now already nose-heavy. The nose of the aircraft will still have the recognizable P-47 look, that means we don't save anything in drag?

    IMO the main thing the P-47 lacked in 1943 was the proper drop tank facility. Next comes the better prop.

    It took a while for the Packard production to catch up. So I'd rater suggest mating the V-1650-1 with Mustang - need only half of the engines, the CoG issues are as good as non-existing since we will be deleting the nose MGs to compensate for Merlin's greater weight. Merlin does not solve the compressibility issues, nor cockpit heating ones, and Mustang is already problem-free there. The Allison-engined P-51A was good for 408 mph at 17500 ft and 395 mph at 25000 ft (no wing racks for both values), we can safely assume that Mustang with V-1650-1 should out-pace it by some 15 mph. The early racks were taking out 12 mph on the Mustangs.
     
  14. GregP

    GregP Well-Known Member

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    #14 GregP, Feb 27, 2015
    Last edited: Mar 1, 2015
    I would definitely NOT remove the P-38 turbos but WOULD create a thinner wing of the same planform. Of course, that is with aerodynamics that were known 50+ years later ... so maybe I would NOT have done so at the time and would have scratched my head trying to solve the issues like they did.

    But I CERTAINLY COULD have solved the cockpit heater problem EARLY, before production.

    The intake manifold issues SOULD have been a quicker solution than it was, too.

    I would NOT be able to fix a fuel issue I was unaware of, but I CAN recognize an engine with a detonation failure .... assuming it didnlt just grenade. Maybe they all did ... most failures at high power result in schrapnel from the crankshaft / rods / psiton areas, leaving not a whole lot to troubleshoot.

    I certainly don't think I am any smarter than the guys at the time, but I also have NO IDEA what data and evidence they were working with.

    So, all in all, maybe the timeline would NOT have changed unless the evidence or the funds and resources to fix it changed. I don't believe they EVER fixed the P-38's thick airfoil, but the same basic wing design ... albeit a bit thinner, showed up on the P-80 / T-33 / F-94 / CT-133 / Skyfox series.
     
  15. FLYBOYJ

    FLYBOYJ "THE GREAT GAZOO"
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    What needed fixing??????
     
  16. tomo pauk

    tomo pauk Creator of Interesting Threads

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    For the thinner wing to appear on the P-38, or maybe a low-drag wing, several things need to happen IMO: wholesale testing of the XP-38 prior attempting the record flight; USAAC ordering 13 pre-series examples they have had a right to do without having a green light by 'high politics'; introduction of second source.
    Having the generator on both engines would help, not just because of redundancy, but it will provide enough of electricity to heat the guns, hence the engine-generated heat will go 100% to heat the cockpit.

    The 'troublesome British fuel' is a myth.
    Allies standardized on 100/130 grade fuel maybe 6-9 months before the P-38J experienced issues. We can remember that B-17, B-24 and P-47 have had no problems with 'British fuel' in 1943, the bombers even earlier, 1942, despite operating at high altitudes.
    The quick fix to the problem was to cruise on low rpm/high boost, not at high rpm/low boost. Charge is heated enough for fuel not to get separated from the air. A more permanent cure is the improved intake manifold.

    But I guess blaming someone else fuel sounded more benign than admitting 'yes, we are to be blamed for the good deal of problems'.
     
  17. Shortround6

    Shortround6 Well-Known Member

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    The "myth" is compounded by changes in fuel specification. There were several different successive specifications for 100/130 fuel which allowed for different amounts of lead and different amounts of other compounds in order to increase production.
    It was the 3rd specification for 100/130 fuel that caused the most trouble. It was also known months before the P-38s ran into trouble in Europe and Allison had been working on a solution (which resulted in the modified intake manifolds) before the trouble ever showed up in Europe.
    There were several un-related problems which added to the "myth".
     
  18. BiffF15

    BiffF15 Well-Known Member

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    Gents,

    Would someone do a quick cliff notes version of the real P-38 engine problems. I've read too many disjointed things and am unclear on the truth or fiction parts.

    Cheers,
    Biff
     
  19. kool kitty89

    kool kitty89 Well-Known Member

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    #19 kool kitty89, Feb 28, 2015
    Last edited: Feb 28, 2015
    If we're talking single stage R-2800 than yes, definitely a bad idea, but the fact that P&W had fairly good high-alt capable supercharged engines early-war is a big deal. (had Allison been fielding hydraulically driven auxiliary superchargers like that in 1941/42 it'd be a big deal, even with no intercooling -water injection would be significant though ... more or less like what they'd finally managed some 2 years later)


    Rearranging fuel placement could have addressed the CoG issues and possibly expanded internal fuel tankage too.

    The nose geometry might not have changed much, but the fuselage should have slimmed down a good bit, including providing more ground clearance and a fairly straight line of wing/belly space between the landing gear for multiple pylons. (or just a single pylon capable of carrying a bulkier heavier tank -perhaps even a 250 imp gallon P-38 tank)

    Still yes, with as well as the turbo worked on the P-47 this isn't all that attractive of an argument, more academic curiosity. (the P-47J's cowling+fan configruation is a more compelling topic on that aircraft anyway -including reduced drag increasing range -especially at high speed cruise -a fan cooled F4U would have been interesting for similar reasons)

    Plus, while turbo cuts exahust thrust, the gains in power at low speed are far more significant for climb. (so even with any potential weight reduction, P-47 climb performance might not have improved even at low altitude)


    This too. Worth noting that the P-51A could manage 415 MPH at 10,400 ft at 1480 HP WER, though the Merlin 20 series in high gear should make similar power a bit higher up. (not sure when they raised boost limits on the V-1650-1, though)




    P-38 Development, Testing, Training
    They did experiment with different airfoil sections with a modified P-38E, but I'm not sure if any of those actually addressed the center of lift problems at critical mach. (there was also the buffeting issue cause by the wing/body interface, but that was solved with smooth wing fillets)

    It was more than just the thick airfoil too, since the nose-down pitch issue was rather specific to the P-38. (and not blanked controlls like most high speed controll loss, but super heavy ones due to airflow and center of lift shifting and boosted elevators technically being able to overcome this, but not without easily overstressing the airframe -namely ripping off the tail -same risk you'd run if using the trim tabs to pull out)


    Still, the crash of the XP-38 delayed discovery of those compressibility problems. The P-47B was still having serious terminal dive control issues too and might have had that delayed a bit longer if things hadn't gone as favorably.

    That said, the dive-flap solution seems like it could have been reasonably applied as a stop-gap measure early on even with the assumption that a proper solution was later possible. (dive flaps/breaks capable of operating at high speeds might have still taken time to properly engineer, granted, but still a fairly straightforward concept ... ideal placement so they both slowed the plane down AND affected center of lift positively would be more significant though -I'm not sure those flaps DID the latter, but I'd gotten the impression they were designed to do so)

    Besides that, having dive breaks is good for fighter-bomber capabilities and potentially in air to air combat.




    This is better than what I was thinking with the P-47's case in addressing early dive problems. Serious problems encountered on the initial prototype before any series production would be really significant. (more similar to what the P-39 experienced -for better or worse)

    I've also never heard precisely why they switched the direction of counter-rotation on the XP-38 to the YP and production models.


    Though another thing on the turbo-less P-38, even aside from recognizing the intercooler problems, allocating resources to properly engineer the export Lighting models (with or without counter-rotation) and thus showing the genuine advantages of that configuration would have been really significant. Plus, the British may have actually accepted them for service. (should have been a better plane than the P-39 and P-40 in most regards) If counter-rotating engines really did give a noticeable performance boost, that might have sold the British over the parts commonality with P-40s. (still, half the engines could have been compatible, though that would mean switching from C series to F series engines to follow P-40 trends)
     
  20. tomo pauk

    tomo pauk Creator of Interesting Threads

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    Most of engine problems were related to the inter-coolers. The initial power of the engines was to be 1150 HP up until 25000 ft, and chosen inter-cooler type worked okay for that. Problems arose once more boost was used, in quest for more power that engine was capable for. Higher boost means greater temperature of the fuel/air mix (called 'charge') entering the intake manifold and cylinders, too high temperature of the charge would wreck the engine once ignited, due to detonation. The issues were mostly acute at high altitudes, where the 'thin' air would not remove enough of heat from the 'covered' inter-coolers as used on P-38 up until H model.
    The engines on the P-38D and P-38D were never rated above 1150 HP, to the best of my knowledge. New type of engines used on the P-38F were also rated for 1150 HP initially, but from March of 1942 they were up-rated. Miltary power (limited to 5 minutes of use back then) went to 1325 HP on 47 in Hg of manifold pressure, up to 15000 ft. In case of emergency, that power setting was allowed to use on take off. Otherwise, the take off power was 1240 HP on 44.5 in Hg, 5 minutes of duration. Need be, same setting was allowed up to 21000 ft. This might be of interest, the trials of the P-38 (scroll down for tables): link.
    The people at 8th AF were successfully testing the P-38F at more aggressive settings, up to 52 in Hg (making 1450 HP) under 11000 ft and up to 45 in Hg (= 1240 HP) up from 15000 to 25000 ft - no engine failures were reported during 2 months of tests.

    The P-38G received modified engines, that were able to produce up to 1425 (boost of 51 in Hg), military power of 5 minutes duration, but still up to 15000 ft. The take off rating was same as with the P-38F, military power was allowed for take off in emergency. Again the power at high altitudes was limited by low capacity of the inter-coolers.

    All the listed ratings are for 100 oct fuel and 3000 rpm, and are taken from the manual for the P-38D, E, F and G; 8th AF experiments are taken from 'Vee's for victory'

    The P-38H received again newer engines, the inter-cooler was the same, unfortunately. The engines were rated for 1425 HP (boost of 54 in Hg) up to 22000 ft, both for take off and military power of now 15 minutes of duration, when using 100/130 grade fuel. It is my understanding that fuel of higher octane rating allowed for greater boost and hence power, since the charge will less likely to detonate than with lower octane fuel. The 'war emergency rating' was established, 1600 HP up to 7000 ft, for 5 minutes.

    There was certaily a feedback, either from stateside tests, or from theaters, or from both, that engine is having problems, and manifold intake (tubing connecting supercharger with cylinders) is to blame. The fuel was condensing from spray in to droplets. Allison was testing a new intake manifold already in 1943, it was used from early 1944 on.

    The intercoolers of greater capacity were introduced with P-38J, before the new intake is introduced. The new inter-coolers aggravated the problems, if the 'low boost/high rpm' setting was used for cruise - the charge will now cool too much, much more fuel droplets will form, the TEL (fuel additive that upped the octane number) will separate from the fuel, and the engine will be wrecked by detonation.
    Apart from the intake issue, the cure was to use the 'high boost/low rpm setting on cruise, so the charge will be more heated, and the charge will be less prone to forming the droplets.
    Engines on the P-38J were doing up to 1600 HP up to 25000 ft, and were successfully tested for up to 2000 HP on 100/150 grade fuel.

    Not much of a 'short cliff' though :)
     
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