Issue 14 - P-38 Lightning: The ultimate Lightnings
By: Tim Callaway and Julian Humphries
The P-38K, L and M - The last three variants of Lockheed’s big fighter included the model produced in the largest numbers, a two-seat radar-equipped night fighter and the fastest and highest climbing Lightning, of which only two were built.
P-38K Lockheed Model 422-85-22
Only two of the P-38K model were produced, the first of which was an experiment by Lockheed – a hybrid based on an old RP-38E two-seat test aircraft.
This test ‘mule’ had previously been used to try out the new intercooler system as fitted to the P-38J so it already had the deep engine nacelles and intakes of that version. In an effort to further improve the type’s performance at high altitude, the aircraft was fitted with a pair of the new paddle-bladed Hamilton Standard Hydromatic propellers.
This propeller had already been fitted to the Republic P-47C Thunderbolt and had given that aircraft a significant improvement in performance, particularly in its rate of climb and acceleration. The new propellers required a larger diameter spinner, so a set of handmade cowlings were fitted to blend them into the nacelles, giving the aircraft a somewhat rough and ready look.
Despite this, the modified aircraft, with a Lockheed test observer on board in the second seat, climbed easily to 45,000 feet (13,716m). This hybrid aircraft was known unofficially as the ‘K-mule’ or the XP-38K, and since only one photograph exists of the type, this aircraft is often mistaken for the next aircraft built.
Encouraged by this performance increase, Lockheed decided to construct a second aircraft to take advantage of the performance offered by the propellers when coupled with the new version of the Allison V-1710, the 75/77 (F15R/L). The aircraft chosen to be modified was the 12th service test P-38G, serial number 42-13558, which had originally been earmarked to become one of the P-38J prototypes.
The new engines could produce 1875hp in War Emergency Power, some 450hp more than those in the P-38J then under development. The new propellers also required a change in the reduction gearbox. The standard P-38 ratio with Curtiss Electric propellers was two to one, the new propellers required 2.36 to one. Again, the bigger spinners required modified cowlings, and again, these were not of the best fit, being handmade.
The engines were paired with General Electric B-14 turbosuperchargers, making this the only P-38 to be fitted with that model. Lastly, the entire engine and propeller combination resulted in a higher thrust line than had previously been the case. The much modified aircraft was redesignated as the P-38K-1 and was used in an extensive flight test programme between February and April 1943.
The results of the flight tests showed the P-38K to have a performance increase above all expectations. The maximum speed of the K at 29,600 feet (9022m) was 432mph (695kph), some 40mph faster than the P-38J at the same altitude, and that was only using military power. Under War Emergency Power, it was estimated that the maximum speed would be in excess of 450mph (720kph).
The second K was also flown to an altitude of 45,000 feet (13,716m), but this was on a very hot and humid day. The test engineers estimated that on a standard day the aircraft could reach 48,000 feet (14,630m). With the improved efficiency of the new propellers, it was expected that the aircraft’s range would increase by between 10 and 15% on internal fuel.
While these changes were impressive enough, the real change was in the rate of climb. Fully loaded, the K could climb at 4800 feet per minute in military power, reaching 20,000 feet (6100m) in just five minutes from a standing start on the runway. The new aircraft achieved all this with a full coat of camouflage paint and with the ill fitting handmade cowlings. What additional improvements could have been made by a clean aircraft can only be guessed at.
After these highly successful trials, the K was delivered to the USAAF at Eglin Field in September 1943. Here, it was flown in comparative trials against the North American P-51B Mustang and the latest version of the Thunderbolt, the P-47D.
The P-38K proved superior to all the other fighters then in production, particularly in rate of climb and maximum speed. Yet despite these performance advances, two major problems were foreseen with the P-38K programme. The first of these was that to re-tool and re-jig the Burbank production lines would cause a two to three week hiatus in production of the P-38, which the US War Production Board deemed unacceptable at this stage of the war.
Secondly, there were concerns that the F15 version of the Allison engine could not be produced in sufficient quantity to quickly meet demand. Despite its promise, the P-38K was to remain a trials aircraft, and the project was abandoned.
P-38L Lockheed Model 422-87-23
With the P-38J established on the production line and the P-38K cancelled, development of the fighter continued with the introduction of the Allison V1710-111/113 engine to produce the next, and last, production variant of the Lightning, the P-38L.
Externally, the P-38L was almost indistinguishable from the P-38J with most of the changes being in detail or internal. More P-38Ls were to be built than any other version of the Lightning, with a total of 3923 being produced, the last of which was completed in August 1945.
The first production batch of 1290 aircraft, designated as the P-38L-1, was fitted with the new engines which were rated to produce 1475hp, although a war emergency rating of 1600hp was available at high altitude. These finally solved all of the overheating and power loss problems encountered by some of the earlier models of the P-38, and gave the P-38L a maximum speed of 414mph (666kph) at 25,000 feet (7620m).
These engines were paired with General Electric B-33 turbosuperchargers, as had been fitted to the H and J models. The gun camera, which had been mounted in the nose of the P-38, was moved to an extension on the front of the port pylon to reduce the vibration it suffered when the guns were fired. New radio antennae were also fitted, as was a tail warning radar.
The landing lights, which had been retractable units under each wing, were replaced with a pair of fixed lights mounted behind a plexiglass cover on the leading edge of the port wing. Lastly, four electrical booster pumps from the outer and inner wing fuel tanks were added, causing a small blister and access door to be added to the wing just outboard and inboard of the engines.
This first batch was followed by 2520 of the L-5 version built at Burbank by Lockheed, and an additional 113 L-5s built under licence at the Nashville, Tennessee, plant of the Consolidated-Vultee Aircraft Corporation. The full designation of the P-38 types listed here should, strictly speaking, include the final letters –LO for Lockheed and –VN for Consolidated-Vultee, but these have been omitted for clarity.
The two types, the P-38L-5-LO and P-38L-5-VN were identical in all respects and the only way to tell them apart was through identification of the individual aircraft serial numbers. One of the most welcome changes to the L-5 model for the pilots was the addition of an electrical socket in the cockpit, into which an electrically heated flying suit could be plugged. The cockpit heating problem had finally been solved.
The L-5 model had a number of armament improvements. The inboard pylons were further strengthened to accept either a pair of 2000lb (907kg) bombs or two 300 gallon (1364 litre) drop tanks. The L could still carry the tube launched M10 4.5in (114mm) rocket projectiles in their triple tube packages, but several aircraft were fitted with zero-length rocket launchers to mount seven 5in (127mm) High Velocity Aircraft Rockets (HVARs) under each wing.
This trial, while successful, was dropped in favour of a single mount under each wing that resembled an inverted Christmas tree. These mounts could carry five of the big HVARs, which were 72in (182.9cm) long, weighed 140lb (63.5kg) each and had an effective range of about 400yd (365m).
The five point mountings were also supplied as kits and were retro-fitted to earlier J aircraft already in service. The addition of the rockets gave the P-38 a tremendous increase in firepower in its ground attack role, but were not widely used in combat. The L version had an empty weight of 12,800lb (5810kg) and a maximum overload weight of 21,600lb (9800kg), so its wing loading was correspondingly higher than any other version of the P-38 at 63.1lb per square foot (308.1kg per sq m).
Unusually, there were no photo reconnaissance versions of the P-38L constructed on the production lines. Instead, 500 P-38L-1s were converted into F-5E-4s. These were followed by an unknown number of F-5Fs and 63 F-5Gs, each carrying a variety of cameras. A number of P-38Ls were also converted into two-seat aircraft designated TP-38Ls and used as trainers to introduce pilots new to the Lightning.
As already mentioned, the ‘Pathfinder’ aircraft equipped with Bombing Through Overcast radar, that led bombing formations of standard P-38s, were also mostly converted from P-38Ls. With the production lines in full swing, and several thousand more P-38s on the order books from both companies, the sudden end of the war against the last remaining Axis power, Japan, brought about the cancellation of these contracts.
P-38M Model 522-87-23
There was one more model of the P-38, despite the P-38L being the last version in production.
The twin engine layout and excellent performance of the P-38 made it an obvious choice for conversion into a night fighter. Several unofficial field modifications had already taken place for trials in the Pacific theatre, before the radar-equipped P-38M made its maiden flight on February 5, 1945.
The new version was based on the highly successful L model, and 75 were converted in total. A second crew member was perched above and behind the pilot, under a very cramped blown canopy, to operate the radar. The AN/APS-6 radar was suspended under the forward fuselage immediately ahead of the nose wheel in a cigar-shaped glass fibre pod.
The standard fighter armament was retained, but extended flash hiders were fitted to the muzzles to preserve the pilot’s night vision. Although it possessed a better performance than the early versions of the Northrop P-61 Black Widow night fighter, the purpose built aircraft had already entered operational use in both the European and Pacific theatres, so the use of the P-38M in combat was both limited and short lived.
Dimensionally identical to P38-L, the M had a slightly lower maximum speed of 391mph (629kph) at 27,700ft (8440m) owing to its higher airframe drag.