THE 456th FIGHTER INTERCEPTOR SQUADRON

THE PROTECTORS OF  S. A. C.

 

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TheYP-38 "Lighting"

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Despite of the loss of XP-38, the Lightning had shown its true potential. On Apr 27, 1939 a Limited Procurement Order for 13 YP-38 service test aircraft was issued [39-689/701]. Lockheed's designation was Model 122-62-02.

    The YP-38 was upgraded for production with a pair of 1150hp Allison V-1710-27/-29 (F2R/F2L) engines with B-2 turbos-uperchargers and spur reduction gearing rather than the former epicyclic type of gearing, which caused the engine's thrust line to be raised. Propellers changed to outward-rotating rather than inward-rotating as on the XP-38. The chin-mounted lip intake under the prop spinner was replaced by a pair of cooling intakes. Enlarged coolant radiators were adopted on both sides of the tail booms.

    Armament was revised to two .30 and two .50 machine guns, and a 37mm Browning M9 cannon with 15 rounds. The .50 guns each carried 200 rpg, and the .30 guns 500 rpg each. One or two YP-38s were seen with prominent gun enclosure tubes protecting the two .50s and flush plates covering the other gun ports, but in reality most YPs were flown without guns installed. At 14,348#, its structural redesign made it lighter than the overweight XP-38.

The first YP-38 flew on Sep 16, 1940 with Marshall Headle at the controls. In Mar 1940 the Army received its first one for service trials, but production lagged seriously behind schedule and all 13 were not completed until June 1941. Maximum speed was 405 mph at 10,000', climb to 20,000' took only six minutes. Normal range was 650 miles, and weights were 11,171# empty, 13,500# gross, and 14,348# maximum take-off.

    During trials the YP-38s suffered severe tail buffeting in high-speed dives, making pull-out difficult. On Nov 4, 1941 the tail booms of [39-689] came off during a dive, and test pilot Ralph Virden was killed. It was initially falsely diagnosed as elevator flutter, and a set of external mass balances were added above and below the elevator. The problem was later solved by adding large wing-root fillets at the points where the wings joined the fuselage, which had to be done very carefully—a loose fit would severely impair flight characteristics.

 

 

TheYP-38

 

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The YP-38

In spite of the XP-38 crash, Kelly Johnson had some enhancements planned for the next prototype. The order for additional P-38 prototypes allowed Johnson to able to incorporate many improvements into the next design. The first YP-38 was rolled out nineteen months later.

Lockheed was struggling to fill many types of orders and was rapidly growing. The main focus was to produce the Hudson. There were not many available engineers and designers to work on the fledging P-38 project. Lockheed looked to the 1939 contract with Curtis to produce the P-40 as an example. In 1939, there were not many orders for the P-40, and it was basically a break-even proposition for Curtis. Lockheed was informed not to expect many orders for the P-38. Lockheed also had put up most of the money for the first prototype, and the proposition for profit was limited. So as any company would do, they focused on making money. The country was not at war, so there was no immediate need to produce something that would at best break even. In June 1939, Lockheed took over a local distillery building and began to use it for YP-38 production.

Lockheed engineers were following Allison improvements in their V-1710 engine and planned to incorporate the new V-1710-F engine in the YP-38 models. Lockheed started production for the thirteen YP-38s soon after taking over the distillery building. During this time, Bob Gross thought that no more than sixty models would ever be produced. He based his belief on the overwhelming favoritism being placed on bombers than fighters in the military. The military believed a bomber with massive armor and machine guns would not encounter problems with enemy fighters.

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 A Lockheed YP-38 Lightning (the second prototype of the Lightning series) undergoes drag clean-up in the NACA Langley 30 x 60-ft. Full-Scale Tunnel.

The YP-38 design was also improved for production ease. The XP-38 was not designed with this mindset, and would be extremely hard to produce in any significant numbers. The new Alison engine was rated at 1150 hp at 20,000 ft. Propeller rotation was changed and reduced downwash onto the wing/center section/fuselage juncture. This solved the problem of disturbed airflow over the horizontal stabilizer (tail flutter & buffeting). The YP-38 had a designed empty weight of 11,171 lbs., and a designed gross weight of 13,500 lbs. This rose to 14,348 when additional space was allotted for fuel tanks. Lockheed engineers guaranteed a high speed on 405 mph at 20,000 ft.

The YP-38 models began trickling out of the factory and immediate testing was conducted. Marshall Headle, Milo Burcham, Ralph Virden, Jimmy Mattern, and Swede Parker performed initial Lockheed testing. Headle and Burcham teamed up with Dr. F. E. Poole in an attempt to anticipate the many "unknowns" that would be encountered. Working with the Mayo Clinic, procedures were developed to hopefully prevent any problems due to excessive altitudes. Lockheed constructed a special altitude chamber to test new equipment. During this time period, many of the standard pilot equipment were very primitive. Oxygen systems were unreliable, there were no ejection seats, and data recording was only beginning to move from the "knee pad" methods were only a few of the developing techniques. Soon after testing began, Marshall Headle was seriously injured in an altitude chamber accident, which permanently ended his flying career, and led to a premature death.

The last YP-38 trickled out of the factory in May 1941. By this point of production, Lockheed released some of the YP-38s over to the military for additional testing. Pilots from the First Pursuit Group at Selfridge Field, Michigan, were able to perform additional testing. These pilots would form the initial cadre of P-38 pilots during the war. Major Signa Gilkey was one of these pilots who flew the YP-38. During one flight, he decided to perform a limited test dive. Gilkey underestimated the potential speed buildup of the aircraft, and soon built up excessive speeds. He was one of the first military pilots to experience firsthand the problems of compressibility. He was able to recover the aircraft and land safely.

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Marshal Headle and The YP-38

By September 1941, the YP-38s were in a committed program to test compressibility. Test engineers wanted the test pilots to go past 300 mph starting above 30,000 ft. This was not normally done, and many of the test pilots thought the test dives were too ambitious at this early stage. Ralph Virden was committed to fly the tests and took off on November 4, 1941 for a series of test dives. Partially through the testing, an object broke off from the aircraft. The aircraft entered an inverted spin and crashed. Virden was killed. Kelly Johnson would later say, "I was back in my office when I heard Virden's plane screaming towards the plant. That most unusual sound probably resulted from the propellers striking the air at an angle abnormal to the line of flight." Johnson concluded that a spring tab like broke, which caused full deflection (Virden's aircraft was observed to rise sharply prior to the part breaking off). At a speed of 300 mph at 3,000 ft. of altitude, this deflection would cause the airframe to exceed design criteria. Designers were pushing the limits of aerodynamic knowledge and material strength in the quest for maximum performance. Often these limits were exceeded leading to unexpected or tragic events.

The YP-38 was destined to spend the rest of its operational life with dive testing. If the problems with compressibility were not figured out, much of the aircraft potential as a fighter would be removed. The YP-38 proved to be a great step towards operational P-38s during the war. Without the hard work and sacrifice of the Lockheed engineers and test pilots, the P-38 may have never developed into the aircraft it was. It opened the door for many other aircraft which experienced compressibility and other related phenomena, and allowed the engineers and designers to immediately know what exactly was happening and were able to overcome these obstacles much easier.

 

 

The Lockheed YP-38 "Lightning"

 

Joe Baugher
 

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The Lockheed YP-38 "Lightning"

In spite of the loss of the XP-38, the Lightning had shown its true potential. On April 27, 1939 a Limited Procurement Order for thirteen YP-38 service test aircraft was issued. Army serials were 39-689/701. The company designation for the planes was Model 122-62-02.

The YP-38 was redesigned for production and had a pair of 1150 hp Allison V-1710-27 and -29 (F2R and F2L) engines equipped with B-2 turbosuperchargers. These engines were equipped with spur reduction gearing rather than the former epicyclic type of gearing. This caused the engine's thrust line to be raised upward. The propellers were outward-rotating rather than inward-rotating as on the XP-38 (that is, the port propeller turned counterclockwise when seen from the rear and the starboard propeller turned clockwise).

The chin-mounted lip intake under the propeller spinner was replaced by a pair of cooling intakes. Enlarged coolant radiators were adopted on both sides of the tail booms.

Armament was revised to substitute two 0.30-in machine guns for two of the four 0.50-in machine guns, and a 37-mm Browning M9 cannon with 15 rounds was substituted for the 20-mm weapon. The 0.50 inch guns carried 200 rounds per gun and the 0.30 inch guns carried 500 rounds per gun. All the guns were mounted in the nose, with the 0.50 inch guns mounted above the 0.30-inch guns. One or two YP-38s were seen with prominent gun enclosure tubes protecting the two 0.50-inch machine guns, with flush plates covering the other gun ports. In reality, most YP-38s were flown without guns installed. At 14,348 lbs, the YP-38 was lighter than the overweight XP-38 due to structural redesign.

The first YP-38 flew on September 16, 1940 with Marshall Headle at the controls. In March 1940, the Army received its first YP-38 for service trials. Production lagged seriously behind schedule, and all thirteen YP-38s had not been completed until June of 1941. Maximum speed was 405 mph at 10,000 feet, and an altitude of 20,000 feet could be reached in six minutes. Normal range was 650 miles. Empty weight was 11,171 lbs, gross weight was 13,500 lbs, and maximum takeoff weight was 14,348 lbs.

During trials, the YP-38s ran into a problem in which the tail began to buffet severely during high speed dives, making it difficult to pull out. On November 4, 1941, the tail booms of YP-38 39-689 came off during a high speed dive over Glendale, California. Test pilot Ralph Virden was killed. This was initially falsely diagnosed as elevator flutter, and a set of external mass balances were added above and below the elevator. This problem was later solved by adding large wing-root fillets at the points where the wings joined the fuselage. This filleting had to be done very carefully, since failure to ensure a tight fit could severely impair the flight characteristics

References:

  1. Lockheed Aircraft Since 1913, Rene J. Francillon, Naval Institute Press, 1987
     
  2. The P-38J-M Lockheed Lightning, Profile Publications, Le Roy Weber Profile Publications, Ltd, 1965.
     
  3. War Planes of the Second World War, Fighters, Volume Four, William Green, Doubleday, 1964.
     
  4. Famous Fighters of the Second World War, William Green, Doubleday, 1967.
     
  5. The American Fighter, Enzo Anguluci and Peter Bowers, Orion Books, 1987.
     
  6. Wings of the Weird and Wonderful, Captain Eric Brown, Airlife, 1985.
     
  7. United States Military Aircraft since 1909, Gordon Swanborough and Peter M. Bowers, Smithsonian Institution Press, 1989.

 

 

 

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