Cheyenne I, II, & IIXL
When American Aviation first became interested in improving the Cheyenne's performance, American focused their attention on the cowlings. First, American's aeronautical engineers analyzed the existing cowling system and identified the areas that needed improvement. Then American Aviation designed a new multi-component air pressure recovery system combining;
1.) a newly designed cowling inlet repositioned to maximize the recovery of the ram air velocity from the propeller.
2.) computer designed aerodynamic inlet ducting to minimize air separation and flow losses.
3.) fore and aft variable air flow deflectors and a fixed inlet turning vane which functions as a unit to direct and minimize flow losses in icing conditions.
4.) a positive air pressure seal system designed to prevent leakage. This maximizes ram air pressure build-up around the engine inlet.
As a result of this new design, the amount of ram recovery was increased from 78% to 98% during normal operation and increased from 0% to 78% during icing conditions. See Efficiency Chart
These dramatic increases in ram recovery resulted in TAS increases of 8 to 12 knots without increasing engine ITTs. At the pilot's discretion, normal speeds can be maintained while lowering engine ITTs approximately 25ºC, resulting in longer engine life and minimizing hot section overhaul costs. The aircraft's critical altitude is also increased approximately 2,000 feet with additional torque maintained throughout the remaining climb to altitude, resulting in reduced time to climb. In addition, the Cheyenne's performance is significantly increased during flight into visible moisture with ice doors deployed. Cruising speeds in a American Aviation modified Cheyenne with ice doors deployed compare favorably with those of a basic Cheyenne in non-icing conditions.
If you have ever looked at the Cheyenne's exhaust pipes, you may have viewed them as a source of excessive drag. American Aviation's analysis did show that the standard pipe shape is aerodynamically dirty and the pipes are not properly aligned with the airflow. Using modern technology, American corrected the problem by developing a low drag exhaust system known as Aerodynamic Speed Stacks.
The Speed Stacks were designed to reduce drag and keep the aircraft cleaner. During engineering flight test, several possible shapes, cut-off lengths, and exit profiles were evaluated before completing the design. In addition, American tested several positions of the stacks in relation to the longitudinal axis of the aircraft. This proved to be very important and required that four distinct stacks be manufactured, as different flange hole locations are required to clock them to their most aerodynamically correct positions. The result were well worth the effort.
Aerodynamic Speed Stacks not only look sleek and fast, they are! As a result of the new shape, airspeeds are increased from 4 to 8 knots and the boundary layer relief is improved, which means more exhaust residue will be blown clear of the airframe.
With maximum recovery and minimum drag the American Aviation Ram Air Recovery System and Aerodynamic Speed Stacks are a winning combination for the Cheyenne I, II, IIXL.
Reprint Papa Alpha 31T Article
Cheyenne Engine Cowlings,
Development History