Flightdeck Friday – Jetski Edition (Convair XF2Y-1 Sea Dart)

Prologue

The concept of seaplane fighters was neither new nor novel – several examples had been put to desultory effect during WW2 and even combining floats with jets (like the British SR.A.1) still failed to overcome the shortcomings of the type. The problem lay in a combination of weight and drag – ruthless enemies where fighters are concerned as both weighed mightily against maneuverability and speed. Drag usually factored large because of either the fixed floats (configured in either a single centerline with smaller wing floats or twin floats) or the effects of a flying boat hull (see the previous FF account of the P6M SeaMariner).

By the early 1950s though, there was thought that retractable skis, called hydroskis, would solve the drag problem and more powerful, afterburning engines allay, if not overcome the weight issue. The advent of jet engines removed the issue of propeller location/clearance that had plagued previous hydroski designs – unlike a float plane, a hydroski plane rests on its hull until sufficient speed is built up to raise it on the skis. Of note, the use of hydroskis was not scalable. In other words, as aircraft size and weight grew hydroskis were not an option; therefore they remained viable only with small airframes.

In 1950, Convair had embarked on a design study that yielded a series of swept wing, shallow, blended hull, jet-powered seaplanes, one of which was a fighter design (the Skate). While Convair was working on developing a good hydronamic shape, the NACA (predecessor to NASA) was working in the opposite direction – take a good aircraft and adapt it to seaborne operations through the use of hydroskis. .

Parallel to Convair’s studies, BuAer (predecessor to NAVAIR) was investigating the feasibility of long-range strike and fighter aircraft that could be sea-based, complimenting the carrier-based aircraft by expanding the basing options and building on several decades of experience in operating sea-based patrol aircraft. This dovetailed with Convair’s ongoing development efforts through a formal Operational Requirements issuance (OR) on 30 Nov 1949 (OR CA 05501A). The OR called for an advanced seaplane fighter capable of operating from forward bases in all weather conditions. Convair would continue refining the Skate, but also investigate the use of hydroskis. As research results showed increasing promise in the use of hydroskis, the Navy revised its performance requirements upward.

Development

Over in another part of Convair, a radical aircraft was taking shape for the Air Force. Based on the XF-92A, the YF-102 was taking shape – a delta wing, afterburner fighter designed for the interceptor mission. Using the lessons learned from the XF-92 and YF-102 development, the Y2-2 began to emerge as a twin-engine, delta wing fighter that would rely on a 2-hydroski arrangement. On 19 Jan 1951, BuAer issued a Letter of Intent for Contract (51-527) for two Y2-2 airframes for R&D purposes necessary for a seaplane-class of fighters. Westinghouse J-46-WE-2 engines (2) would provide the thrust. The pressurized cockpit used two panes of class that formed a sharp-V, similar to the YF-102. For the prototypes only, there would be no additional canopy – production model aircraft would have a more conventional arrangement.

Designated the XF2Y-1 Sea Dart (BuNo 137634), the first prototype was launched into < ?xml:namespace prefix = st1 />San Diego Bay on 16 Dec 1952. Problems developed with the twin-ski arrangement and a phenomenon called “ski-pounding.” At about 50 KIAS, a thundering vibration set in that was aggravated with greater wave heights. As the skis flexed, the buffeting was resonated throughout the airframe, reaching the point where the pilot was unable to read the instruments. Several minor mods were made and the second prototype was configured with a single ski in an attempt to counter the pounding. It would only be after many high speed taxi runs and several months before the Sea Dart would take to the air on 9 April 1953.

Once in the air, the Sea Dart experienced the same problems so many other jets of that period encountered – engines that underperformed and top speeds that fell well short of the design mark. In the case of the Sea Dart, optimistic engineers had predicted Mach 1.5 in level flight (the OR specified M1.25) – yet it only reached Mach .99. Engine inlet problems with airflow induced by the location of the ducts atop the fuselage aggravated the already poor performing J46’s and unknown to Convair’s engineers at the time; the Sea Dart was suffering from the same aerodynamic issues that would plague the YF-102. Eventually, Convair solved the problem for the YF-102 by implementing an area-rule fuselage and that, along with a more powerful single engine, was planned for a follow-on production model.

CONOPS

Armed with 4 x 20mm cannon and 44 x 2.75 in folding fin rockets in retractable pods in the forward fuselage, guided by an AN/APQ-50 radar tied into the Aero 13E weapons system, the Sea Dart would provide forward air defense from amphibious ships, submarines, or coastal bases. In fact, the Marines conducted a 1954 study on the uses of the Sea Dart as part of what it was calling the “seadrome” (think – “Sea Base”). In essence, this base would be rapidly deployed to support amphibious operations, providing immediate air cover. The seadrome would consist of all the equipment needed for daily operations and required by the Sea Dart – maintenance, fueling, re-armament, etc. The Sea Dart itself was designed with at sea maintenance in mind. Everything was placed to facilitate access from above either directly or through access points.

Twilight

With the end of the Korean War came mounting concerns over whether the capabilities offered by the Sea Dart offset it’s growing costs, support requirements and questions over resolving the ski-pounding and top end speed were justified. The Sea Dart had exceeded Mach 1, but in a shallow dive. Prototype 2 was destroyed in a high speed pass, killing the pilot, C.E. Richbourg. The cause of the break-up was again, a phenomenon common to the early trans-/supersonic aircraft, pitch divergent oscillations. Pitch divergence was encountered at transonic speeds in low level flight and the hydraulically-boosted flight controls of the day were inadequate to cope with the induced oscillations, causing a rapid build-up of aerodynamic pressures on the airframe leading to in-flight breakup. The Sea Dart would not fly any more high speed profiles. As post-war funding levels for conventional forces started to be scaled back, the Navy determined that the costs did not offset perceived benefits and by the end of 1954, the program was terminated after 5 prototypes had been built.

General characteristics< ?xml:namespace prefix = o />

Crew: 1
Length: 52 ft 7 in (16 m)
Wingspan: 33 ft 8 in (10.3 m)
Height: 16 ft 2 in (4.9 m)
Wing area: 568 ft² (53 m²)
Empty weight: 12,625 lb (5,730 kg)
Loaded weight: 16,500 lb (7,480 kg)
Max takeoff weight: 21,500 lb (9,750 kg)
Powerplant: 2× Westinghouse J46-WE-2 turbojets, 12,000 lbf (53 kN) each

Performance (estimated)
Maximum speed: 695 mph (604 knots, 1,120 km/h)
Range: 513 mi (446 nm, 826 km)
Service ceiling: 54,800 ft (16,700 m)
Rate of climb: 17,100 ft/min (86.7 m/s)
Wing loading: 29.0 lb/ft² (142 kg/m²)
Thrust/weight: 1.45

Armament (planned)
Guns: 4× 20 mm (0.787 in) cannon
Rockets: Unguided rockets

Sources

Pioneers & Prototypes: Convair F2Y Sea Dart. International Air Power Review. Vol 12
Gunston, Bill. Fighters of the Fifties. Osceola, WI: Specialty Press. 1981
http://en.wikipedia.org/wiki/F2Y_Sea_Dart