Speed Cowl Development History.

Since its introduction in 1989, the Pilatus PC-12 design has become the best selling single engine turboprop.   It naturally attracted the attention of American Aviation Inc. a company with four decades of success in making performance enhancing improvements for turbine powered aircraft such as the King Air and Cheyenne airframes.  American’s engineering team was tasked to look at the Pilatus design and see if any improvements could be made to that aircraft.

They identified the PC-12 cowling inlet system as an older design, and that the airplane would most likely benefit from a more modern and efficient inlet.  Numerous instrumented flight tests of the stock PC-12 confirmed that a new cowling / inlet system could make major improvements.

A new prototype design incorporating cutting-edge technology was developed and flight tested confirming optimum performance could be achieved.  The performance improvements were so substantial that the costs to manufacture an all new cowling / inlet system from carbon fiber and obtain FAA and EASA approval could be justified.   This lead to the manufacture and FAA certification of the Speed Cowl which combined a cowling inlet with maximum recovery of high-velocity ram air, and internal ducting that was aerodynamically designed to minimize flow losses of high-velocity air to the engine’s plenum.  The results were higher available torque at the same ITT settings, which significantly improved the performance of the Pratt and Whitney PT6A-67 turbine engine.

Speed Cowl™ has been flight tested in multiple flight configurations, climb, cruise at various altitudes, descent, with the inertial separator door open and closed—all at varied torque and ITT settings.

The results showed the cruise true airspeeds and climb rates increased significantly in the flight levels, without exceeding any aircraft or engine limitations.  The performance improvements naturally depend on the altitude, outside air temperature and ITT settings used.  New performance charts are provided due to the increased efficiency of the cowling / inlet system.


Improved Performance

Ram air recovery percentage is the ratio of the measured static pressure around the engine’s plenum compared to the dynamic pressure of the aircraft’s velocity through the air. Higher ram air recovery percentage results in optimum engine performance.

Here is the comparison:

Factory Cowling Ram Air Recovery %
Climb 45.2%
Cruise 72.2%
Cruise with inertial separator open 22.2%
Speed Cowl™ Ram Air Recovery %
Climb 99.9% (Increased by 54.7%)
Cruise 96.7% (Increased by 24.5%)
Cruise with inertial separator open 55.6% (Increased by 33.4%)

The improved ram air recovery produced by this new design dramatically increases aircraft performance. During climb, maximum torque (PSI) can be maintained longer. Transitional altitude, (the altitude where the PSI begins to drop off when maximum ITT is reached), is increased, allowing additional torque to be available throughout the remaining climb—resulting in reduced time to reach cruise altitude. At cruise altitude FL180 and above, a higher PSI can be maintained at normal ITT settings increasing the airspeed. Performance is also notably increased during flight into icing conditions with the inertial separator door open.


Internal Aerodynamic Design

Internal Aerodynamic Design is a key factor in achieving optimal engine performance. The aerodynamic efficiency of the cowl inlet and ducting are measured on how well dynamic air pressure, created by the aircraft’s velocity through the air, is captured at the cowl inlet and converted into static pressure around the engine’s plenum. This static pressure is generally expressed as a percentage of what is known as ram air recovery. For instance, 100% ram air recovery would indicate that 100% of the dynamic air pressure is captured by cowl inlet and 100% of that is converted to static pressure at the engine’s plenum. The higher the ram recovery percentage, the better the engine will perform.

At a given interstage turbine temperature (ITT), there is a finite amount of energy that can be produced by a turboprop engine. This energy is shared by the two power absorbing sections of the engine—the compressor section, which compresses air for combustion, and the power section, which turns the propeller. By increasing the ram air recovery to the engine’s plenum, less energy is used by the compressor section leaving more energy to drive the propeller.


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