Page 20 - May 23

ASK THE EXPERT
Stories About Flaps
by Tom Clements
I’d like to tell you a few interesting things that I have 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.
To prevent this coasting travel, Beech uses a Dynamic Brake Relay. It does the following: Whenever power is
18 • KING AIR MAGAZINE
experienced over the years involving flaps. I hope you
removed from one field winding, the other winding is now shorted to the airframe. This puts an “infinite” load on the winding that is acting as a generator. With huge resistance to rotation being provided by that winding and no longer any driving force being received by the other winding, the assembly comes to a screeching halt.
It acts as if some strong mechanical brake were suddenly applied to the motor’s output shaft but it is all done by a magnetic field, not by a physical brake. Cool!
The Dynamic Brake Relay operates whenever the flaps hit a limit switch – Up, Down or Approach. It also activates in earlier King Airs – the ones in which the flaps may be stopped in any position between Approach and Down – when the flap handle is moved from Down to Approach while the flaps are extending between Approach and Down. The fact that the flaps will stop immediately means that when we want to put them at 60%, we can wait to move the handle from Down to Approach until we see the indicator pointing right at 60%. They won’t coast on down to 65% or more, not with our Dynamic Brake friend!
What if they do coast a bit? If this is happening, you will likely also find no free play on the flaps when you move their trailing edges up and down on the preflight inspection since they have coasted to the ends of the tracks. The likely cause of this is a bad Dynamic Brake Relay or highly worn flap motor brushes.
Back to the motor/gearbox connection: The output shaft of the motor acts as the “worm” that rotates two shafts, “worm gears,” one rotating clockwise and the other counterclockwise, or vice versa, depending on whether Up or Down is selected. Both left and right outboard flap segments are connected to one of these shafts and the inboard segments are connected to the other shaft. Although extremely rare, if one of these
MAY 2023