Aircraft flexes flaps with safety

Lexington, MA-Willard Crowe knows that safety drives many decisions in the aerospace business. He is a senior design engineer at Raytheon Aircraft Co. and part of the design team that uses flex shafts to extend and retract flap controls on the company's new 19-passenger Beech 1900 aircraft.

There's a lot at stake-if the flaps of an aircraft don't work simultaneously, the aircraft becomes unstable. The 1900 uses four flex shafts from S.S. White (Piscataway, NJ) per plane. Two of the units control the right and left inboard flaps and two units control the right and left outboard flaps.

For accurate movement, Crowe uses a single electric motor with a reduction gearbox that drives all four flex shafts at the same time. The shafts, in turn, rotate jackscrews that raise and lower the flaps, which operate in a track inside each wing. Each pair of flex shaft cables, one long and one short, must operate at equal speeds in forward and reverse operation.

Beech aircraft has used some flex shafts in the past, but never at this length. The original aircraft used a single pair of 63-inch flex shafts in its flap designs. "As the plane's wing got longer with each new design, the flex shafts also grew," explains Crowe. The 1900 now uses a pair of 140-inch flex shafts.

Any instability in the motion of the flexible shafts creates unequal flap movement, making the plane unstable. To eliminate this concern, S.S. White manufactures their flexible shafts by wrapping the wire either left or right. Special part numbers help maintenance crews replacing shafts to assure that the wrap is going in the right direction. Each cable pair is installed in opposite directions-one pair goes left while the other goes right. For flexible shafts, this means that when in operation, one pair turns with the wrap while the other turns against the wrap.

To protect the flap control mechanisms, each flexible shaft is encased so that no bare cables can be accidentally damaged. The cables are inside the airframe, so they are out of the open air. Although the cable casings and cable locations are not entirely sealed from the outside environment, the flap subassembly is reliable on the 1900.

Other changes to the aircraft include the use of a larger electric motor and a slight change in the gearing. "But the basic concept is the same," says Crowe. "It's the history of safety and long life that also makes Beech a preferred aircraft for many customers."

For more information about flexible shafts from S.S. White Technologies: Enter 534