From the start, aircraft designers faced the challenge of overcoming heavy control loads. Large control surfaces require lots of force, and unless the pilots spend their off hours at the gym, they can be a limiting factor. One of the earliest solutions was the aileron horn, a tab that juts off the end of the wing. It was popularized by the Fokker Dr. I in WWI, and is still in use today on the venerable ATR-72 turboprop.
Rooted In WWI
As early airplanes became larger and faster, control loads became extremely heavy. This made aircraft almost impossible to control. To make control load inputs more reasonable for pilots, aircraft designers began using aerodynamic counterbalances.
The idea of adding an aerodynamic counterbalance was actually borrowed from boats. Designers found that by extending a control surface's area forward of the hinge, the force needed to move the control surface was significantly decreased.
Aileron horns made their first appearance in World War I, when engineers began extending ailerons beyond the wingtip to allow a horn ahead of the hinge. One of the most well-known examples of the aileron horn was the Fokker Dr.I triplane. The same design principle was also used on the rudder and horizontal stabilizer of these early aircraft.
Simple, But Effective
The operation of an aileron horn is fairly simple. By extending the horn ahead of the aileron hinge, the horn moves in the opposite direction of the aileron when the aircraft rolls left or right. As the horn extends up (or down) into the relative wind, the air flowing around it creates a force that helps neutralize the force of air pressing against the deflected aileron. By neutralizing the forces, the flight controls become much lighter.
Still In Use On Airliners Today
While more advanced counterbalancing systems are used on most aircraft today, versions of the aileron horn can still be found. One of the best examples is the ATR-72 turboprop. The ATR-72 has mechanically actuated ailerons, meaning that they are controlled by cables, and not by hydraulics. Because of this, aileron horns are necessary for pilots to be able to reasonably roll the aircraft left and right.
Have you ever flown an airplane with aileron horns? If so, tell us about it in the comments below!
