Convair Model 48 "Charger"
The first flying prototype COIN (for Counter INsurgency), the Convair Model 48 "Charger," was designed according to a Marine Corps specific operating requirement (SOR) that specified a take-off and landing distance of 500 feet over a 50-foot obstacle and included a requirement for "single-engine survivability." Designed for reconnaissance, close support and paratroop ferrying missions, the counter-insurgency (COIN) Charger lost its Marine contract bid to the similar, but more powerful, OV-10 Bronco.
Requests tor COIN proposals were issued to 22 manufacturers in October 1963. Responses were received from nine companies: Beech, Douglas, Convair. Goodyear, Helio, Hiller, Lockheed, Martin and North American. The Beach design was designated PD-183 and was to be equipped with the same two engines as the Charger, Canadian T74/PT-6A Turbo-props. The Douglas design designated D-855 was a single fuselage design characterized by a T-shaped tail unit, and a rear loading door. It was drafted to utilize either United Aircraft of Canada T74/PT6-As or Garret AiResearch T76 turboprops. The most interesting, yet somewhat similar design to the Convair Charger and North American OV-10 Bronco was the Martin entrant. It was designed around two T74/PT6-As in twin booms, but instead of a T-tail as used by Convair and North American, the Martin design utilized an inverted V empennage with boundary layer controls.
The Convair Model 48 "Charger" COIN type STOL aircraft had two propellers driven by turbine engines, and double-hinged, single-slotted flaps to deflect the slipstream on the largely immersed wing. It was capable of good low-speed performance and had acceptable handling qualities in the STOL regime (with landing and take-off distances consistently less than 800 feet over a 50-foot obstacle), provided the possibility of engine failure was ignored. This performance was achieved, despite flaps with only medium effectiveness, because the aircraft had a low aspect ratio, a high power loading, and a "no-flare" landing gear design. The performance of the aircraft compared favorably with that of a large four-engined STOL aircraft, which was much more sophisticated (it included a fail-safe propulsion system). If flown above above the minimum single-engine control speed, however, in compliance with the normal safety restrictions for twin-engine airplanes, major aspects of the performance of the aircraft were no better than that obtainable with many small "twins" then in current production and most of the original objectives of the COIN concept are compromised.
The aircraft had two 650 SHP engines driving opposite rotation, 9-foot-diameter propellers; the tips rotated upward in the center. Retractable Krueger flaps were used on the inboard leading edges of the wing which was largely immersed in the propeller slipstream. The 44-percent chord trailing-edge flap was single slotted and double hinged, and was deflected 60°/30 for the landing approach and 2Oo/O0 for take-off. The control system was entirely mechanical; lateral control was obtained with circular-arc spoilers only and longitudinal control with a free-floating, single-hinged (geared, camber-changing) horizontal tail (called a stabilator). The twin rudders were conventional.
To meet the "single-engine survivability" requirement, Convair engineers incorporated a "torque-equalizer" device to reduce the power on one engine automatically in the event the other failed, thereby allowing the pilot to hold the wings near-level long enough to eject safely; this device was operable during all the NASA flight tests in the STOL regime (with the exception, of course, of the single engine investigations). The high-speed performance of the airplane did not meet the original expectations, but comparison with other aircraft indicates that careful attention to detailed improvements would probably net a reasonable cruise performance with no appreciable penalty in the low-speed regime.
For more than two years prior to the Navy issuing RFP (Request for Proposal) in 1964, Convair engineers had been evaluating designs for use in the limited war and counter-insurgency arenas. These designs were evaluated by many military and civilian representatives of the armed forces and Department of Defense. Shortly after the Navy issued a procurement invitation on 28 October 1963, the design was formalized. Once approved, it only took 40 weeks from go-ahead until first flight on 29 November 1964. Even though a prototype was flying shortly after the requirement was published, North American won the contract with a paper airplane, the OV-10 Bronco.
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