Page 26 - Volume 13 Number 9

I’d like to tell you a few interesting things that I have experienced over the years involving flaps. I hope you will find them interesting and educational.
Let me begin by reviewing the basic flap system design in King Airs and, with minor changes, in most other Beechcraft airplanes. The semi-fowler flaps – ones that extend aft to increase the wing’s chord as they go down – are driven by jackscrews and ride on tracks connected to the wing’s rear spar structure ... two tracks and one jackscrew per flap segment or flap panel. As my colleague, Dean Benedict, has written in this magazine, there are rollers, bushings and Teflon washers that connect each segment to its two tracks and it is unfortunately common to find these installed incorrectly, leading to track damage that can be time-consuming and expensive to repair. The jackscrews are driven by flex-drive cables that pass into the fuselage where they connect to a transmission or gearbox assembly that
is mounted under the cabin aisle floorboards on the forward side of the rear spar.
Mounted beneath the gearbox is the drive motor – a 28-volt DC, reversing, electric motor. In this case, “reversing” means that it can run in two directions for up and down flap travel, depending on which of its dual field windings is energized. When the motor is running in the “up” direction, the “down” winding is acting as a generator, and vice versa. However, with no demand, no “load,” placed on that generator it is providing insignificant resistance to motor rotation. Unlike in Bonanzas and other models with a single flap panel per side, the two panels per side on the King Air require more power to operate and, in turn, there is more momentum to keep them coasting after the motor is no longer receiving power. This coasting momentum can drive all flap panels to the absolute limit of track travel, putting undesirable strain on the components.
Ask the Expert
Flap Stories
by Tom Clements
The flap motor, gearbox, drive cables and Dynamic Brake Relay.
24 • KING AIR MAGAZINE
SEPTEMBER 2019