In an attempt to increase the C-337's cruising speed a cantilever wing was installed on a C-337 prototype. Dick Kemper conducted fairly extensive flight tests starting in December 1965. All of the rework was confined to the wing (and fuselage) area between the twin booms. As in the single-engine models with cantilever wings, a massive carry through spar was needed to carry the wing bending loads. Aerodynamicists had estimated a 3 mph speed improvement by removing the wing struts. How- ever, flight tests showed only a 2 mph speed gain Another finding was a disconcerting "boom bounce" in perfectly smooth air which was strong enough t be felt in all seats. This was a random (non-periodic phenomenon, and no explanation could be found. In view of the insignificant speed gain, the boom bounce, the extra weight associated with the spar carry-through forging, and the additional control cable friction from the more circuitous routing the fuselage sides, the project was canceled in 1966.
The C-T337 Super Skymaster Story
As the naturally-aspirated C-337 became a well established part of our production lines, we turned our attention to a turbocharged version to be designated the T337. Like the C-320 project, this turbocharged C-T337 was to be a major exercise in engine cooling. Cowl flap doors were modified both in size and deflection. An additional problem was fuel vapor in this gravity-flow fuel system that made it more difficult for the engine-driven and auxiliary fuel pumps to draw fuel from lines located in the closely packed and sometimes overheated engine compartments. In the C-320 we had installed submerged aux fuel pumps in the tip tanks where they could push the fuel to the engine. Also we specified more frequent boost pump operation in hot weather or high altitude. In contrast, the C-T337's aux pumps, located on the firewall, had to draw vapor-laden fuel from distant tanks, and we were somewhat reluctant to demand their use in many normal operations. The author recalls the many efforts made to shield fuel lines from heat sources, and Dick Kemper's and Paul Leckman's continual testing to measure their effectiveness in hot weather both in Wichita and in the desert southwest.
Since the Continental IO-360 engine had never been turbocharged, we, again, were pioneering this development with the help of Continental's Jim Champion and AiResearch engineers. Although the controller unit kept the manifold pressure fairly constant in climbs, there was no automatic device for controlling fuel flow. Thus the pilot had to attend to this chore by monitoring the fuel flow (pressure) gauges in accordance with markings on the gauge and also on a placard on the panel. The full-power cooling climbs would be conducted with the mixtures set for those schedules. Procedures for engine restarts at 15,000 to 30,000 feet had to be developed to forestall the possibility of engine flooding in the thin air. This newly-designated engine became a TSIO-360-A with an initial time between overhaul recommendation of 1,400 hours as compared to 1,500 hours for the normally-aspirated engines. Finally, Dick prepared a very comprehensive series of paragraphs for the owners manual explaining the reasons for occasional manifold pressure excursions at high altitude (when the waste gate is closed) as the airspeed varies or the power is changed.
Takeoff tests were conducted to simulate an engine failure after lift-off and, thereby, develop performance figures for the Emergency Procedures section of the later owner's manuals. Total take-off distances varied from 4300 feet at sea level to 5410 feet at 5,000 feet altitude at 4700 pounds gross weight with a turbocharged T337. In response to customer inquiries, we were obliged to run some engine-out take-offs from a standing start (at light weight) on long runways. This was to give guidance to FAA field inspectors preparing ferry permits for returning an airplane to a repair facility following an engine failure near a distant airport. Once again we were surprised at the inferior performance of the front engine as compared to the rear engine's acceleration. This operation also required runway lengths at sea level well in excess of 5,000 feet, and its success was highly dependent upon cool outside air temperatures and suitable wind velocities.
During our frequent delivery flights or business trips in the C-T337 our test pilots would seek out unusual atmospheric conditions such as icing layers with clear air above and below. I recall collecting 2-inches of rime ice over Indiana with no serious penalty in speed. As with all our models, we flew the C-T337 to both hot weather and cold weather locations for testing. I recall asking Paul Leckman in one wintry period to fly north in search of -40° F temperatures for the C-T337 prototype. As he flew north he called in periodically, always thinking that the requested temperatures in Canada were only a few hundred miles farther north. To my surprise, he finally found them at Bettles, Alaska, about 25 miles north of the arctic circle! The test objectives were to assess the engine starting capabilities in a coldsoaked (overnight) airplane using appropriate preheating procedures and an external battery cart. In addition, we usually evaluated in-flight winter kit (inlet shields) effectiveness and the performance of anti-congealing oil coolers (also protected by a shield). At the test pilot's option, he could also evaluate in-flight propeller feathering and engine restarts. Fortunately, there was usually a strong temperature inversion where outside air temperatures on the ground would be much colder than temperatures aloft. Even so, restarting an engine with the propeller feathered was often a chore even under the best of conditions much less sub-zero conditions. In later years, Hartzell developed an unfeathering accumulator that instantly unfeathered the propeller to a windmilling position, and this solved the difficulty. Cessna offered this device as optional equipment, and it was popular with fixed-base operators who offered CLT flight training involving frequent engine-out operations.
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[Another Story: CESSNA 337 SKYMASTER: JUST SAY THREE-THREE-SEVEN]