A lawn dart on the water

by Dr. Marshall Michel
86th Airlift Wing historian

The post World War II late 1940s and early 1950s were a period of considerable innovation in military aviation in the United States.

The jet engine, while still limited, offered vast potential for the development of high-performance aircraft. At the same time, driven by the beginning of the Cold War, the United States placed a greater emphasis on developing the Strategic Air Command’s bombers as America’s lead offensive strike force and a fleet of short-range interceptors for continental defense. These developments were detrimental to the U.S. Navy’s carrier force, whose mobile conventional forces seemed obsolete.

To maintain its role in national defense and lessen the role of its aircraft carriers, the Navy developed an innovative series of jet waterborne aircraft to fill these roles. The Martin P6M Seamaster attack bomber has already been described, but an even more innovative waterborne aircraft was the Convair XF2Y Sea Dart fighter.

Propeller-driven seaplane fighters had never achieved high performance because they needed to be mounted on high-drag floats for the mundane but important requirement of keeping the propeller out of the water.

This requirement disappeared with the jet engine and in late 1948, Convair offered the Navy a unique proposal: a jet “floating fighter” with V-shaped watertight hull that rode on the surface of the sea when stationary or moving slowly in the water.

 On takeoff and landing, the aircraft, dubbed the XFY2-1 Sea Dart, was to use two retractable water skis and ride into the sky as the speed increased and it lifted out of the water. When power was applied, the leading edge of the skis broke the water, then the skis were extended to an intermediate position until about 50 mph and to their fully extended position as the aircraft accelerated to takeoff speed.

When the aircraft was on dry land, it had a limited amount of ground maneuverability from small fixed wheels at the rear end of the extended skis.
The proposal was attractive for another reason – it allowed the XF2Y to use a high-performance delta wing, which offered the promise of supersonic performance. Because of its long takeoff roll and poor low-speed characteristics, such a wing was impossible to use on a carrier.

As built, the XF2Y-1 had both a delta wing and delta tail, like a lawn dart, and was intended to have two afterburning J-46 engines with 6,100 pounds of thrust each, with intakes above the wing to prevent them from ingesting sea water. The Navy accepted the design and ordered two prototypes with low-powered J-34 engines in late 1951, then ordered 12 production aircraft before a prototype had even flown.

The first flight tests revealed that, with the small engines, the Sea Dart was severely underpowered, but more seriously, the skis vibrated continuously during takeoff and landing making the aircraft extremely difficult to control.

In 1953, the more powerful J-46 engines were installed in the prototype. However, the engines failed to produce the projected thrust, and since the XF2Y-1’s fuselage was not area- ruled (more on that next week), the Sea Dart had high transonic drag and could not exceed the speed of sound in level flight. Nevertheless, on Aug. 3, 1954, the prototype went through the sound barrier while in a shallow dive – the first and only seaplane to go supersonic.

Unfortunately, on Nov. 4, 1954, the prototype disintegrated in midair because of pilot-induced pitch oscillations when it was pushed past its safety margin during a low-altitude, high-speed fly by. Since the accident occurred over San Diego Bay as part of a display to Navy officers and press representatives, the event was extremely well documented as the plane and bits and pieces of flaming debris fell into the bay.

When the highly publicized fatal crash with the surrounding bad publicity was added to the problems with the vibrating water skis that seemed to be insoluble, the program was doomed. The Navy decided to introduce supersonic carrier-based fighters, and the SeaDart program was relegated to test status.

An advanced version had been built and tested, the XF2Y-2, with a single water ski, an area-ruled fuselage, and a single afterburning Pratt and Whitney J-75 turbojet of 15,000 pounds of thrust, but it was also cancelled. Tests had shown the single ski could handle crosswind takeoffs and landings and operate in waves of up to 6 to 10 feet in height.

But the Navy was not quite finished with the XF2Y. In 1962, all the services were ordered to redesignate their aircraft to conform to the new tri-service unified aircraft designation scheme. For some reason, though it never entered service and the program was by then long dead, the SeaDart was assigned the designation F-7.

(For questions or comments, e-mail Dr. Michel at marshall.michel@ramstein.af.mil)