Aircraft has right stuff

Aug. 1, 2003
Flexible shafts assist aircraft’s flap actuator system.

Pilatus, the aircraft manufacturer located in Stans, Switzerland, produces a uniquely versatile, multipurpose aircraft, the PC-12. As a business aircraft, it offers multiple configurations ranging from executive-level transportation to long-distance delivery of cargo and oversized equipment.

Eaton Aerospace Electromechanical Systems provides the subsystem used in the PC-12’s bidirectional flap actuation system. According to Peter Foote, senior project engineer at the time of its design, “We had only two choices for the job. Those choices included the use of torque tubes or of flexible shafts.”

The use of torque tubes leaves the design with limited connection ratios, meaning the U-joints can be expensive and more complicated to install. Also, when the lube seal becomes worn, leakage occurs. Flex shafts transfer torque via a wrapped wire system with nearly unlimited connection ratios, and no U-joint connection, and don’t use a sealed lubrication system.

The PC-12 had already been designed to incorporate a power drive unit in the fuselage, in the belly of the aircraft. To power the flap actuation system, a 28- V dc motor with reduction gearbox was designed by Eaton. Because the company designed and manufactured the power drive unit, it was also able to design in the interconnection necessary to transfer the motion from the motor to the wing.

Linear worm gear and ball screw assemblies were incorporated as the final link to the flap actuation subsystem. This assembly would be used, in a bidirectional manner. If for any reason the flaps did not operate symmetrically, the unit would be shut down immediately. Asymmetrical operation of the aircraft flaps causes the angular position of the aircraft to become unstable. It is better to fly or land the aircraft with the flaps inoperable rather than unstable. Sensor feedback assures that the actuation system operates symmetrically.

Once it came down to designing the power transmission system from the power drive unit to the flap worm gear actuators, flex shafts became the only viable option. To transfer power from the fuselage to the wing, there would be no direct shot. Torque tubes are stiff and heavy when compared to flex shafts, which provide lightweight and complete flexibility. The PC- 12 incorporates four flap actuators, two in the left wing and two in the right wing. Of the manufacturers on the market, S.S. White turned out to be Eaton’s choice of suppliers. According to Foote, “It offered superior technical support during all aspects of design.”

This included initial specifications as well as the willingness of S.S. White to support all of Eaton’s qualification efforts. Pricing never became an issue once the product proved itself as the right component. The flex shafts are able to fit into a variety of difficult power transmission situations. Because of their unique technical design, they perform well in bidirectional applications. Units can be installed using a single power source, decreasing the need for additional hardware.

The flex shafts for the PC-12 are manufactured on high-end winding machines designed and built specifically for quality flex shaft production. Complete testing of each unit is performed at the S.S. White facility and is shipped along with the test data paperwork to the customer. The final shaft designed for the PC-12 application performed as modeled. Since installation began not a single customer complaint has been associated with shaft problems on the aircraft.

Additional test data performed by Eaton indicates that the flex shafts will last up to half of the life of the aircraft, well above many of the other components in the final design. The flex shafts are easy to swap out because the hassle of alignment specifications has been eliminated.

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