F-100 Super Sabre Vietnam

[Chanelle Kirksey]

TheNorth American Aviation F-100 Super Sabre was the bridge between the subsonic and supersonic ages. It set the last subsonic speed record and the first supersonic speed record [Gunston]. It became the first supersonic production aircraft, and it was the first aircraft from which a pilot ejected at supersonic speed [Chant and Taylor]. Like the other early 100 series fighters, it pioneered the aerodynamics of supersonic flight and the complex control limits of thin supersonic wings, tails, and rudders. Also like all of the early 100 series fighters, it was only marginally supersonic. In military power, it often had difficulty pulling up rapidly when it had to, and the powerful afterburner was off or completely on. Test pilot George Welch described the effect of turning on the afterburner as being like a kick from a well-fed mule [Winchester]

Although the Hun was limited, it pioneered the design of operational supersonic aircraft. It made extensive use of heat-resistant titanium, its fuselage was ultra-streamlined, it had very thin control surfaces, and it had a thin intake duct in its nose [Boeing]. It was also the first U.S. aircraft with the horizontal tail positioned at the bottom of the fuselage [Boeing]. The layout of its cockpit controls was ergonomically and functionally sophisticated and yet simple [Boeing].

The Super Sabre design began as a successor to the legendary F-86 Sabre. Like the F-86, the F-100 would be a day fighter; it would not have the long-range radar an interceptor needed, and it could not fight at night or in bad weather. However, unlike the F-86, the F-100 would be supersonic in level flight. This heritage led to its official name, Super Sabre. North American initially called the F-100 the Sabre 45 because the new aircraft would need a 45 degree wing sweep to fly supersonically [MilitaryFactory.com].

While the Sabre models before the Hotel version had six 50 caliber machine guns firing through the sides of the fuselage, the F-100 had four 20 mm cannons firing from the bottom of the nose. This gun position prevented the flash from blinding the pilot. The four General Electric M39 revolver cannons each had five chambers but a single barrel [Hill Air Force Base]. While one chamber was firing, another would have its shell ejected, and another would have its shell loaded. Each gun could fire 1,500 twenty-millimeter cannon shells per minute. This was a 50% higher rate of fire than the 50 caliber machine gun the M-39 replaced [Air Force Armament Museum]. Each cannon had 200 rounds [Janes], although it could carry up to 275 [Gardner]. Its high rate of fire gave pilots only a few seconds of firing before their cannons ran dry. The M39 was effective against both aircraft and ground targets. For aiming the gun in a dogfight, the pilot had small ranging radar under the top of the oval air intake. A lead-computing computer put a pip on his heads up display to indicate where he could fire

Unfortunately, the Hun was designed before the F-86 began to reach combat in Korea. This meant that it could not benefit from the lessons of the Korean War. In particular, it would not be a simple, rapidly climbing, high-altitude, and nimble fighter [Davies and Menard, Holmes]. Instead, making the aircraft supersonic required a doubling in empty weight compared the F-86 [Sharpe]. Nimbleness was not a critical design goal, and although the Super Sabrehandled well at altitude, it could never be called agile. The Hun was also very sophisticated and complex. It was the first aircraft to make extensive use of titanium [Gardner]. That and other technological advances made it the first fighter to cost a million dollars a copy [Davies and Menard]. Even with three times the thrust of the F-86 [Sharpe], the F-100 was only modestly supersonic in full afterburner.

The Hun used the new Pratt and Whitney J57 engine, which had a powerful but thirsty afterburner. The next two Century series aircraft also used the J57, with the F-101 using two. This engine also powered the B-52, and its civilian version, the JT-3, powered the Boeing 707 and DC-8 commercial passenger liners. By the time later Century series fighters appeared, newer and more powerful engines would be available, bringing Mach 2 performance to fighters and bombers produced only two or three years later.

Comparing the F-86 and the F-100 directly [Hanson], the Hun was seven feet longer, but its wingspan was only a foot longer. With 16,000 pounds of thrust in afterburner, the F-100 had three times the power of the F-86 while being only 3,000 pounds heavier.

A more subtle problem in the overall design was that because ailerons had to be move inboard to prevent them from operating improperly when the swept wing flexed, the aircraft had no flaps to reduce its landing speed. Flaps would not appear until the Delta model [Historynet.com]. Although the F-100 had automatic leading edge slats to help reduce landing speed, sometimes they only deployed on one wing [Davies and Menard]. If this happened during landing, the aircraft was likely to snap roll without warning.

During take-offs and landings, was a danger that the pilot would pitch the nose too high [Davies and Menard]. If that happened, the aircraft might begin the deadly Super Sabre. First, the tips of the wings would stall. This would move the center of lift forward, forcing the nose further up. This would stall more of the wing and increase the pitch up. As the Hun pitched up almost vertically, inertial coupling would cause it to yaw left and right in a way almost impossible to control. The sabre dance was not recoverable, and F-100A ejection seats did not work at ground level. The pilot could only hope to survive the crash. In one case, a crash by 1LT Barty Brooks on January 10, 1956 was caught on film [Cockrell].

To improve horizontal stability and low-speed control, North American Aviation refitted all F-100As with a 27 percent larger vertical fin and wider wings [Baugher]. Although these changes increased directional stability, the Hun remained a tricky plane to fly. (The Brooks accident was a year after the changes were made.) One pilot with 2,000 hours of experience described a Super Sabre landing as a controlled crash [Winchester]. A quarter of all F-100s were lost in accidents [Davies and Menard].

The larger the object, the larger its rotational inertia will be. Aircraft, of course, are very large. If the pilot pulls up on the stick, causing the nose to rise, the aircraft may yaw. This is called inertial coupling. Changing direction also means that the aircraft is no longer flying directly into the wind. This creates additional unplanned movement.

For World War II aircraft, inertial coupling occurred but was not a problem because aircraft had large control surfaces that stabilized the airplane against small coupling and aerodynamic changes. Wings were large and thick, and the empennage had a large vertical fin and a large horizontal tail. For supersonic aircraft, wings had to be small and thin. The vertical and horizontal empennage surfaces also had to be very thin. There was now a large heavy fuselage and small control surfaces. The traditional stabilizing forces against inertial coupling and small aerodynamic changes were ineffective.

The F-100 was not the first aircraft to experience inertial coupling problems. However, prior supersonic aircraft were experimental planes that broke the sound barrier but spent little time in supersonic flight. In contrast, the F-100 was the first aircraft to experience extended supersonic flight. The crisis only came when Welch conducted a long high-speed dive with a high-G pull-up.

Even with improvements, the Air Force was disappointed when it evaluated the F-100A as a fighter in Project Hot Rod. This late-1955 series of test found that the F-100A was superior to other fighters but still had limited tactical effectiveness [Gardner]. The USAF did what it tended to do with other disappointing fighters; it turned it into a fighter/bomber.

The next model was not really an F-100. Its goal was to meet requirements for an aircraft competition that was eventually won by the Republic F-105 design [JoeBaugher.com]. As the figure below shows, the F-107A Ultra Sabre, as the final North American design was finally, called, had an intake at its top to leave room for a larger radar dish in the nose.

In September 1956, North American shifted production to the F-100D. While the F-100C was designed hastily, the F-100D design was a more evolved aircraft. The Delta model had a larger fin, a bigger horizontal stabilizer, and more hard points for stores [Holmes, Gunston]. It had a stronger engine and could deliver 7,500 pounds of bombs [Gunston]. It was given inboard flaps, which considerably slowed landing speeds [Gunston]. The ailerons were moved to the outward portion of the wing, giving greater control authority [Gunston]. Its refueling apparatus was modified so that the pilot to accept fuel for both its internal tanks and its external drop tanks [Taylor]. It even had an autopilot that functioned at supersonic speed [Gunston]. This became the definitive model of the Super Sabre, and 1,264 were built [Holmes]. Even with larger control surfaces, however, Deltas were tricky to fly. At least 100 were lost in crashes before the termination of their use in the 1970s [Holmes]. Both F-100Cs and F-100Ds would be used heavily in Vietnam as bomb trucks.

The USAF Thunderbirds flew F-100Cs and later F-100Ds from 1956 through 1969 [afthunderbirds.com]. This thirteen-year tenure was interrupted briefly in 1964, when the Thunderbirds made a transition to F-105Bs. However, Thuds lacked the necessary maneuverability for airshows. After an accident, the Thunderbirds returned to their F-100Ds for the rest of the season [afthunderbirds.com]. Although it was never part of the official air show, if the sponsor of a show requested it, one of the Thunderbirds would make a supersonic pass [afthunderbirds.com]. In general, the shows of that era involved loud, low, fast passes close to the crowd. The F-100 was perfect for that type of performance.

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