Expert: Ray Wilkinson - 12/28/2006

Question
Hi Ray,
Since a few months I found airplanes and aerodynamics really interesting. I have learned a lot about these things recently. One question keeps bugging me, and I could not find an answer on the net or from people I have access to: On many commercial airplanes, such as the Boeing B737, there are boxes attached to the trailing wing edge. The things that are attached to the bottom of the wing with no outlets or other visible attachment. The only thing I can imagine is that they are housings for motors which control the flaps, spoilers and ailerons. However, this is just my guess. Can you tell me what they are?
I'm looking forward to hearing from you and offer a computer science answer in exchange!
Greetz from Berlin
Dennis


Answer
Hi Dennis

Sorry for the delay in responding, as I'm on leave and don't have much time to attend to emails.

You are mostly correct - the fairings are called canoe fairings (because of their shape) and cover the flap tracks and drives. The flap motors are inside the fuselage on most aircraft, and the spoiler actuators are underneath the spoilers themselves.

Taking the Boeing 747 as an example, the flaps are driven by a single hydraulic motor each side, backed up by an electric motor in case the hydraulics fail. The two sides are linked by a synchronising rod to ensure both sides move together - this is very important as asymmetric flaps could overpower the ailerons and cause the aircraft to roll into the ground. On each side, the hydraulic and electric motors drive a common gearbox, which rotates a torque tube running along the rear spar, just forward of the flap. At intervals, a small gearbox turns the motion through 90 degrees, turning a long threaded rod which runs inside the canoe fairing. On this screw is a small runner called a ballscrew, which behaves exactly like a low-friction nut, so as the torque tube rotates the ballscrew moves forward or aft. The ballscrew is connected to the flap, so each flap is driven at several points exactly in line, so it doesn't jam.

Also under the canoe fairing is a flap track, which controls the position and angle of the flap, so it moves aft and down as it deploys. A flap carriage, much like a rollercoaster car, runs on the track and carries the flap.

I hope you can visualise this arrangement.

Regards

Ray