The North American F-100 Super Sabre was the first fighter in the USAF active service inventory capable of attaining supersonic performance in level flight. Although it was originally designed strictly for the air superiority role, it came into its own as a fighter-bomber, and flew many of the early combat missions during the Vietnam War.
The quest for a supersonic version of the Sabre began in February of 1949, when Raymond Rice and Edgar Schmued of North American Aviation began work on a company-financed design study for an advanced version of the Sabre capable of achieving supersonic performance in level flight. This study came up with a proposal for obtaining this performance improvement by increasing the wing sweepback of the standard F-86 to 45 degrees. However, wind tunnel studies indicated that only a relatively small increase in maximum speed would be attained by this change, due to the steep rise in aerodynamic drag that takes place as near-sonic speeds are attained.
Clearly, aerodynamic changes alone would not be sufficient to achieve supersonic speeds in level flight. More power would be needed--a lot more power. To this end, the Allison Division of General Motors offered an upgraded J35 turbojet rated at 9000 lb.st, which nearly doubled the thrust of the J47 engine of the F-86. General Electric proposed an advanced version of the J47 rated at 9400 lb.st. dry and 13,000 lb.st with afterburning.
On Sept 14, 1949, NAA decided that they would concentrate their efforts on the development of an improved version of the F-86D all-weather interceptor with the goal of achieving supersonic performance. It was to be powered by the Advanced J47 proposed by General Electric. A maximum speed of Mach 1.03 at 35,000 feet was envisaged. The "Advanced F-86D" (as the aircraft came to be known) had a 45 degree wing sweep and an area-ruled fuselage. However, the USAF rejected this idea, proposing that NAA design a day fighter instead.
North American decided to try and give the USAF what it asked for, and responded with a proposal for an "Advanced F-86E", which was to be a high-performance day-fighter. The Advanced F-86E that emerged in January of 1951 was similar in many respects to the Advanced F-86D, but had a slimmer fuselage and featured a nose air intake without a radome on the top. The USAF rejected this idea as well, but indicated that the idea might be reworked into an air-superiority fighter.
The next proposal was known by the company as the Sabre 45, the "45" indicating the angle of wing sweepback. It combined features of the Advanced F-86D and the Advanced F-86E, but was to be powered by the new Pratt & Whitney J57-P-1 turbojet, rated at 15,000 lb.st. with afterburner. Maximum speed was estimated at Mach 1.3 (860 mph) at 35,000 feet and combat radius was 670 miles. Estimated combat gross weight was 23,750 pounds. Armament was to be a quartet of 20-mm T-130 cannon.
The Sabre 45 finally succeeded in attracting the interest of the USAF. NAA asked that the USAF purchase two prototypes of the Sabre 45, one to be used for aerodynamic testing and the other to be used for armament trials.
In October of 1951, the USAF Council decided to press for the development of the Sabre 45 proposal, in spite of some misgivings of key development personnel who claimed that the design would be too costly and complex for a basic day fighter. With the Korean War as a reminder of the perilous state of USSR/US relations, the Air Force Council wanted the Sabre 45 in service in a hurry. They thought that two prototypes were not enough to achieve early operational status and recommended that the plane be purchased in quantity and full production be initiated even before initial flight testing was completed. This is a high-risk philosophy, one which would give the Air Force a new plane in a hurry if everything went as planned, but one which would risk the high costs and long delays of a lot of in-service modifications should unexpected problems turn up during flight testing. However, considering the pressures of the hot war in Korea and the cold war with the Soviet Union, the risk was considered worth taking. On November 1, 1951, the Air Force issued a Letter Contract for two Sabre 45 prototypes, plus 110 production aircraft.
On November 9, the Sabre 45 mockup was inspected. The Mockup Board received more than a hundred configuration change requests, and they identified several armament deficiencies and requested a number of modifications that would improve the capabilities of the aircraft.
Work on an F-100A production version was begun under the company designation of NA-192 on November 20, 1951. On December 7, 1951 the USAF officially designated the Sabre 45 as the F-100, the first of the Century Series of fighters. The two prototypes were designated YF-100 by the USAF and NA-180 by the company.
A Letter Contract for two YF-100 prototypes was issued on January 3, 1952. On February 11, 1952, the Air Force issued another Letter Contract for 23 production F-100As with Fiscal Year 1952 funds.
Following the mockup inspection, North American reshaped the fuselage with an even higher fineness ratio and provided an extended clamshell-type cockpit canopy. The horizontal tail was moved to a position below the chord plane of the wing, in the hope that it would keep the tail clear of the wing wake at high angles of attack and would help to prevent the dangerous tendency of a swept-wing aircraft to pitch up suddenly and violently following a stall. YF-86D serial number 50-577 was experimentally modified to check out this new low-set horizontal tail arrangement.
On June 23, 1952, the USAF approved the proposed changes, but asked NAA to consider the installation of external weapons racks and to substitute non-self-sealing tanks for existing bladder tanks to save 400 pounds of weight.
During the final design stage, NAA engineers made several changes to reduce drag and increase engine thrust. They changed the air intake lip so that it had a sharp edge in order to improve the air flow and to deliver higher-energy air to the engine at supersonic speeds. The nose was made nine inches longer. The thickness/chord ratio of the horizontal and vertical tails was decreased to 0.035. This last change was scheduled for production aircraft, but was too late to appear on the two prototypes. These changes were approved on August 26, 1952. At that time, the USAF authorized the procurement of 250 additional F-100As.
The YF-100 that finally emerged from these design changes was sufficiently different from that originally planned that it was redesignated YF-100A. The YF-100A had a wing thickness/chord (t/c) ratio of 0.082, as compared to the F-86's 0.10 t/c ratio. A glove airfoil section was added to the wing leading edge which reduced the wing t/c ratio still further to 0.07. The ailerons were mounted inboard on the wing rather than near the tips in order to reduce the tendency of the wing to twist during aileron deflection at high speeds. The YF-100A was somewhat unusual for its time in that no wing flaps were provided. However, the wing leading edge did have five-segment automatic slats that were actuated by aerodynamic forces. These slats were intended to increase lift at takeoff, to delay wing buffet, to improve lateral control near the stall, and to permit tighter turns.
A retractable speed brake was mounted on the fuselage belly. The tall and narrow vertical fin was swept back, and was provided with a thin rudder.
No fuel tanks were mounted in the wing, all of the internal fuel being accommodated inside the fuselage in five non-self-sealing bladder tanks having a total capacity of 750 US gallons. Two underwing drop tanks each carrying 275 US gallons could be fitted.
The mainwheels retracted inwards into the fuselage, and the twin nose wheels retracted backwards into the fuselage.
Heat-resistant titanium was used in the initial design stage, marking the first time that this metal had been used in large quantities in an airplane. In fact, North American actually used 80 percent of all the titanium produced in the United States until 1954 in the manufacture of the Super Sabre. Titanium is an extremely strong and light metal and is more resistant to heat than is aluminum. However, it is rather brittle and difficult to machine, and the extensive use of titanium sent the cost of Super Sabre manufacture through the roof.
The armament was to consist of four T-160 (later M39) cannon, two on each side of the fuselage below the cockpit. Ammunition capacity was 275 rpg. An A-4 ranging gunsight was to be fitted which computed the lead automatically, aided by ranging information from a radar antenna mounted inside the upper nose inlet lip and covered by a flush-mounted dielectric panel.
The engine for the YF-100A was the Pratt & Whitney J57-P-7, rated at 9220 lb.st. dry and 14,800 lb.st with afterburner. This engine was, however, derated from these numbers and was not tuned up to give its full power. On August 26, 1952, the USAF increased its order to 273 aircraft, plus one static test example.
As the first YF-100A was nearing completion, the Air Force recognized that two prototypes would not be sufficient for the test program, and they decided that the first ten F-100As off the line would be used as test aircraft as well.
The first YF-100A (52-5754) was completed on April 24, 1953. It was moved in high secrecy from the Los Angeles factory out to Edwards AFB. Company test pilot George S. Welch made the maiden flight on May 25, 1953. The YF-100A exceeded the speed of sound on its first flight.
On July 6 the YF-100A achieved a speed of Mach 1.44 in a long dive from 51,000 feet. However, on the early test flights, some problems were encountered with rudder flutter. The installation of hydraulic rudder dampers solved this problem.
USAF test pilots found that the YF-100A outperformed any other production fighter in the Air Force, but they also found that there were some serious shortcomings that might cause problems upon introduction into service. The visibility over the nose was poor during takeoff and landing. The longitudinal stability in high-speed level flight was considered inadequate. The low-speed handling was rather poor, and there was a tendency of the YF-100A to yaw and pitch near the stall, the left wing dropping uncontrollably. The climbing rate was too slow without afterburner--it took 16 minutes to get to 40,000 feet. In addition, the landing was difficult and the touchdown speed was high.
Phase II tests were completed by September 15, 1953, at which time the YF-100A had made 39 flights tasting a total of 19 hours, 42 minutes.
The second YF-100A (52-5755) flew on October 14, 1953.
In order to show off the Super Sabre, the USAF decided to use it to break the world's speed record. At that time, the world speed record was held by the Douglas XF4D-1 Skyray which had averaged 753.4 mph for two runs in each direction over a three-km straight course. In order to set a new record, the Super Sabre had to top the previous mark by at least one percent, which meant that the aircraft had to average at least 760.9 mph over the 3-km course. Lt.Col. Frank K. (Pete) Everest was chosen to make the attempt. On his first try over a 3-kilometer course laid out over the Salton Sea, Lt.Col. Everest flying YF-100A 54-5754 averaged 757.75 mph on the required four runs, faster than the Skyray but not one percent faster, so it did not count. It was decided to make another try over a different course, a 15-kilometer course where the one-percent margin rule would not apply. Under the rules of the Federation Aeronautique Internationale (FAI), the fastest speed over either course counted as the official world speed record. On October 29, 1953, Lt.Col. Everest averaged 755.149 mph to set a new official world speed record. Throughout the entire record-breaking run, Lt.Col. Everest kept his YF-100A within 100 feet of the ground. This was the last world speed record set at low altitude.
52-5754/5755 North American YF-100A Super Sabre
Engine: One Pratt & Whitney XJ57-P-7 turbojet, 9500 lb.st. dry and 13,200 lb.st. with afterburning. Dimensions: Wingspan 36 feet 7 inches, length 47 feet 1 1/4 inches, height 16 feet 3 inches, wing area 385 square feet. Performance: Maximum speed 660 mph at 43,350 feet. Landing speed 160 mph. Initial climb rate 12,500 feet/minute. Service ceiling 52,600 feet. Normal range 422 miles, maximum range 1410 miles. Fuel capacity 1307 US gallons. Weights were 18,135 pounds empty, 24,789 pounds gross.