EL TAIL WAGS DOG

‘Hinge moment’ is the technical name for the force required to deflect a control surface. In small, relatively slow aeroplanes 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.

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HINGE MOMENTS INCREASE with the chord length of the control surface, with the square of speed and with the cube of the linear dimensions of the aeroplane. Thus, if you double the speed the hinge moments grow four times greater; if you double the size of an aeroplane, 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, aeroplanes 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 wheeltype control could have a large mechanical advantage, if you didn’t mind having to turn it ten or twenty times to roll into a bank; but that would not make for a very agile aeroplane. 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, such as a boat’s a rudder, ahead of the axis of rotation, or ‘hinge line’. Some early aeroplanes had completely balanced rudders and no fixed fin at all. Many also had balancing area projecting ahead of the ailerons at the wingtips. These expedients worked for small aeroplanes, but for large ones, like the multi-engine bombers that came into being during the First World War, keeping control forces within comfortable limits with aerodynamic balance was very difficult.

During that war, however, a German engineer named Anton Flettner had an ingeniously simple idea for applying leverage in a new way.