NA-73X

How it all begun

Radical design

The first of many: NA-73X

Specifications

Serial numbers

 

 

How it all begun

By the end of 1937, the increasing threat of German rearmament was alarming the French and British. They increased their own production of aircraft, but at the same time were also looking outside the continent to American sources as well.

By 1939, both Governments were at war with the Nazis and were struggling to get their hands on combat aircraft. America 's entry into WWII was still nearly two years away. On the European continent however, nothing seemed to be able to slow down the uprising of the German military as one country after another fell under Nazi control. It would only be a matter of time before the British would be facing the German war machine alone.

The British needed weapons in large quantity very fast and nothing was given higher priority then fighter aircraft. The only aircraft the US had at that time that suited this purpose were the Bell P-39 Airacobra and Curtiss P-40 Warhawk. However, both aircraft were already obsolete designs and were certainly no match for the Luftwaffe's Bf-109 Messerschmitt.
The British were on the lookout for a new long range fighter aircraft, but Curtiss was heavily committed building the P-40 for the USAAC (United States Army Air Corps) and thus had no production capacity to spare.

In 1938, the British had already purchased a large order from North American Aviation (resulting in substantial business for NAA), but they were all trainers (Harvards) and not the fighter aircraft they so desperately needed.
Since Curtiss was unable to meet the demand, a British delegation led by Sir Henry Self (head of the British Purchasing Commission, Aircraft Division) paid a visit to James H. “Dutch” Kindleberger (president of North American Aviation) and proposed that North American Aviation build P-40's under license from Curtiss for the RAF.

The people at North American however weren't very keen on building the somewhat obsolete P-40's, so they proposed to the British to build a new fighter aircraft, from scratch. They offered to design and deliver the first prototype within 120 days (this was equal to the time otherwise needed to prepare and convert the NAA plant for the construction of P-40's)!

The British were eventually sold to the idea and Wilfrid Freeman, head of the British Ministry of Aircraft Production, ordered 320 aircraft in March, 1940. On April 10th, 1940, the proposal was accepted and work on the prototype (assigned North American's model number NA-73) begun. The contract specified initial prototype delivery in January of 1941 and completion by September of 1941.

NAA mailed the British delegation drawings of a design concept in early May of 1940 and on the 29th of May the British awarded a contract to NA for development of the NA-73X.

At that time, the USAAC reserved for itself the right to block any foreign aircraft sales that it regarded as not in the Army's interest, for whatever reason. The USAAC had no interest in the aircraft at that time, so on May 4th , 1940, the US Army reluctantly agreed not to block the British sale. It was thanks to First Lieutenant Benjamin S. Kelsey, head of the Army Air Corps Pursuit Project Office, that a condition was added to the contract: two examples of the initial production batch for Britain were to be turned over to the USAAC for testing free of charge. In other words, this meant that the British would buy the US Army two aircraft for which at that time it did not have the funds available to purchase them themselves.

The NA-73X prototype contract was finally signed on May 23rd , 1940 and thus the design team at NAA started the task at hand.

 

Radical design

NAA had actually been quietly working on such a fighter project since the summer of 1939, and by that date they had actually already completed much of the detail design. On May 29th , a provisional RAF approval was issued for 320 aircraft, depending on satisfactory testing of the prototype. RAF serial numbers were to be AG345 through AG664.

The successful design of the new fighter was the work of a dedicated team:

From left to right: Louis Walt, Raymond Rice and Edgar Schmued (NAA)

  • Chief-designer: Edgar Schmued (native-born German who immigrated to the US in 1930) (standing on the right side)
  • Aerodynamics specialist: Edward Horkey
  • NAA chief Engineer: Raymond H. Rice (standing in the middle)
  • First Lieutenant Benjamin S. Kelsey, head of the Army Air Corps Pursuit Projects Office at Wright Field

 

 


The design proved to be very radical at that time and the team worked day and night to get the first prototype ready in the agreed 120 days. The new aircraft was designed to be built around the Allison V-1710 12 cylinder engine. An in-line, water cooled engine was chosen in order to make the front of the aircraft as sleek as possible.

Major design features were as follows:

  • The use of a laminar flow wing:

    With a degree in aeronautical engineering from MIT, Kelsey was qualified in aircraft design and performance. He also knew of the National Advisory Committee on Aeronautics (forerunner to NASA) studies on the laminar flow airfoil and he understood its importance to new aircraft design and performance (as a result NACA's Eastman Jacobs was assigned to NAA's team).
    A laminar flow wing reduces drag considerably because the wing is thinner then the previously used conventional wings. The leading edge of the wing is more pointy and the upper and lower half of the wing are almost identical to each other. The thickest part of a laminar wing occurs at 50% chord while in the conventional design maximum thickness is at about a fifth of the way across the wing from the leading edge. As a result, drag is significantly reduced because the wing takes considerably less effort to cut through the air. This was a very bold step especially considering the timeline promised.
  • An underbelly radiator scoop:

    The BPC knew that NAA had never built a first-line fighter and required that it purchase flight test and wind tunnel reports of the aborted Curtiss XP-46 project. The XP-46 was roughly similar to the P-51 with its underbelly radiator scoop. At the request of the British, NAA Vice-President Leland Atwood consulted with Curtiss and obtained info on the XP-46. NAA paid $56,000 dollars for all of the data.

    To date there is still much debate about just how much of the data from Curtiss was used. Curtiss engineers state that it was almost total while those at NAA claim that little information was used. Chief aerodynamicist Ed Horkey examined the data and claimed it to be “obsolete and very amateurisch”. Also NAA had presented their concept to the British before blueprints were obtained from Curtiss.

    The truth is probably somewhere in between. The prototype could not have been built in just 120 days without considerable use of the Curtiss data. But equally as clear is the innovative thinking added by the NAA design team.

    One element which was taken from the XP-46 was the use of a radiator scoop.
    Having a liquid cooled engine meant installing a coolant radiator. On the Spitfire, coolant radiators were installed underneath the wings. This design had a few drawbacks: firstly, when on the ground, the intake of the radiator was partially blocked by the undercarriage which caused temperature problems while running the engine on the ground. Secondly, if a stray bullet went through the radiator, the engine would overheat which lead to a forced landing.

    NA-73X was designed to have the coolant radiator installed underneath the fuselage, behind and below the cockpit. Putting the scoop (which provided cooling air for glycol and oil cooling) under the fuselage was mainly done to preserve the streamlined design of the fighter.

    The drawback of such an arrangement was the extra weight and combat vulnerability of the long pipes that led to and from the engine. Having its own intake and exhaust system however proved to be a welcome bonus to the design. The hot air from the coolant radiator actually created additional thrust as the hot air exited at a greater velocity than it entered. As a result, a controllable flap was made at the rear of the housing, so this thrust could be regulated.
  • Metal skin:

    NA-73X had an all metal skin except for the rudder and elevators which were fabric-covered. The stressed aluminium skin also had all flush riveting or screws for speed.
  • Square cut wings and tailplane
  • 2 large self-sealing fuel tanks, one 90 US gallon tank in each wing, that could hold about double the fuel a Spitfire carried.
  • A wide track landing gear that would retract inward. This resulted in far better ground handling then a Me-109 or Spitfire had. When the gear was down, the inner gear doors would also retract to keep drag to a minimum.
  • A fully retractable and steerable tail wheel.
  • 8mm thick armor behind the pilot's seat
  • The front of the Razorback canopy was armored glass and had a hinged panel on the left and hinged panel on top. The panel could be jettisoned in flight in case of emergency.
  • A 10-foot-6-inch (3,2 meter) diameter Curtiss Electric 3-blade propeller

Special attention was paid to features which would make the aircraft easy and inexpensive to manufacture and at the same time to get the highest performance possible. The two wing spars had to be far enough apart to accommodate the length of a 0.50-in machine gun, with only the barrel protruding ahead of the main spar.

The design also allowed a bomb or other store to be carried on a single pylon under each wing.

As a result of some of the data taken from the design of the XP-46 and also because of the radical and innovative thinking of NAA's design team, the new fighter contained many of the latest developments in aerodynamics.

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