Tail Wags Dog

“JUST GIVE ME A PLACE TO STAND,” ARCHIMEDESS AID

- Peter Garrison

“Hinge moment” is the technical name for the force required to deflect a control surface. In small, relatively slow airplanes, hinge moments are not very large; pilots move the controls with ease. But as control surfaces get larger and speeds get higher, hinge moments grow rapidly. They increase with the chord length of the control surface, with the square of speed and with the cube of the linear dimensions. Thus, if you double the speed, the hinge moments grow four times greater; if you double the size of an airplane, keeping all of its proportions unchanged, they become eight times greater. If you double both size and speed, hinge moments increase by a factor of 32.

From Kitty Hawk onward, airplanes got bigger and faster, but pilots didn’t get any stronger. Obviously, this couldn’t go on forever. There is, furthermore, only so much you can accomplish with leverage between cockpit and control surface. In principle, a wheel-type control could have a large mechanical advantage, if you didn’t mind having to turn it 10 or 20 times to roll into a bank, but that would not make for a very agile airplane. A stick was at an even worse handicap, its movement limited by the space between the pilot’s knees.

Now, it was well-known from the world of boats that hinge moments could be reduced by putting a portion of a surface, like a rudder, ahead of the axis of rotation, or “hinge line.” Some early airplanes had completely balanced rudders and no fixed fin at all. Many also had a balancing area projecting ahead of the ailerons at the wingtips. But for really large airplanes, like the multiengine bombers that came into being during the First World War, keeping control forces within comfortable limits with aerodynamic balance was very difficult.