Cheyenne Engine Cowlings,
Cheyenne Engine Cowlings,
Reprint from Papa Alpha 31 Magazine
The engine cowlings that enclose an aircraft turbo prop engine serve several purposes. The most obvious is to provide an aerodynamic enclosure for the engine. They also supply air pressure to the engine inlet. In addition, the cowlings incorporate a mechanism to prevent large chunks of ice from being ingested by the engine, i.e. engine ice protection. All Cheyenne cowlings use the particle separator method of ice protection. This involves an ice door inside the cowling moving down to deflect any ice chunks away from the intake. In addition, a lower ice exit door opens to let the deflected ice out of the cowling. The intake air then goes around the ice deflector door and up into the engine intake. The idea here is that an object of any appreciable mass can not make the turn around the forward ice deflector door and up into the engine intake.
In 1984, the Cheyenne IA was introduced incorporating a new cowling that promised ten to twelve knots more cruising speed. This was because additional ram air pressure was delivered to the engine. The new cowling featured a different inlet location and shape, and the internal mechanism was redesigned to improve air pressure build up at the engine intake. To improve the performance in icing conditions, the Cheyenne IA’s cowling incorporated a “turning vane” that smoothed the air flow around the forward ice door when it was down. The turning vane prevented the extreme turbulence and separation of air behind the forward ice door, as experienced with the original cowling, and, therefore, reduced the pressure loss at the engine inlet. A separate external inlet for the oil cooler was also added. Additional ram air pressure delivered by the Cheyenne IA cowling resulted in more aircraft performance.
Also in 1984, the Beechcraft C90 A was introduced with a new cowling that resulted in increased ram air pressure to the engines and improved aircraft performance.
Both Beech and Piper advertised their cowling improvements as reasons to buy the new 1984 model aircraft and neither system was offered as a retrofit to update older models. In all fairness, neither system was easily retrofitable as both would require extensive modification work in the field. In 1985 American Aviation, Inc. was formed with the intent of bringing new technology to the existing fleet of business aircraft. One of the first projects considered was to retrofit the existing Cheyenne I’s, II’s, and IIXL’s with high technology cowlings. Before settling on an all new design, adaptation of the IA cowling was considered. A review of the IA design was accomplished and maintenance facilities were contacted for their input.
In researching the IA design, feedback was received from maintenance shops that the IA’s ice door actuators had a short life, were hard to obtain and were very expensive. The consensus was that we utilize the existing Cheyenne I, II, and IIXL actuators if at all possible. Several reports were also received that the inlet deicing element was a different size and somewhat hard to obtain. Again maintenance shops encouraged us to use the same deicing element or parting strip as used on the I, II, and IIXL. In addition, the separate oil cooler inlet was considered by some to be less effective and required extra attention by the pilot for on ground operation. It soon became obvious that although the IA design did offer improved performance, it would be a mistake to adapt it for use without major changes.
Taking recommendations from maintenance shops into consideration, American Aviation began to design a new Cheyenne cowling with the help of the latest CAD CAM, solid modeling computer equipment. Several test flights were made to document the effectiveness of the basic cowling and obtain base line performance data for all of the Cheyenne models.
A pressure survey of the prop blast area was performed and this information fed into the computer so that the cowling inlet location and shape could be optimized and final performance improvements predicted.
With a prototype cowling system installed, numerous flight tests were performed to verify and fine tune the cowling for maximum efficiency with ice protection off and on. Ultimately a final cowling configuration was developed that offered all of the IA cowling performance advantages, with none of the disadvantages.
Utilizing a relocated and improved inlet, an improved internal ducting shape and mechanism, and a unique movable aft door, the new cowling provided that dramatic and predicted performance improvements we were looking for.
In addition, because of the unique movable aft door, the existing door actuator loads were actually reduced, further extending (rather than reducing), the door actuator’s life. The inlet shape was also changed and relocated for maximum pressure recovery, without changing the boot size or part number. Lastly the oil cooler inlet was not changed so its effectiveness was not diminished.
Paramount in American Aviation’s cowling development program was the requirement that the finished product be easily installed by the average maintenance facility and be cost effective. To date, more than 10% of the Cheyenne’s have already been upgraded and 90% of those were installed by local maintenance facilities.
Cheyenne operators report faster cruising speeds at the same ITT limits and better climb performance as a result of more torque.
There are many other high technology advancements that can be made to the Cheyenne models and American Aviation is working to make them available soon.