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Caster |
R. W. Stuart Caster is a mechanical proposition which should be applied when appropriate to your R C airplane. Do not confuse with Castor which is a superb lubricant for the methanol burning engines. Properly taking advantage of caster will improve the ground steering of the tricycle gear equipment and may slightly improve tail dragger ground handling even though a tail dragger inherently has caster. The dictionary says caster refers to a swivel wheel assembly as used on chairs; the whole thing being called a caster. They are designed so that the vertical swivel shaft is permanently offset from the horizontal axle which carries the wheel. This allows the wheel to always trail the swivel which is a mechanically stable configuration-- a leading wheel would be unstable causing the wheel to dig, dive, or refuse to run true. The rudder of a boat has the same problem when the boat is backing. Slop in the steering system amplifies the instability of a backing steering wheel. Caster at the engineering or design level refers to the proposition itself- not to the wheel. If a swiveling wheel is ahead of its swivel shaft it will be unstable and if the wheel trails its swivel shaft stability is improved. Since our tricycle steering systems are at best very loose it is essential to build in a bit of caster. Caster may be put into a new or existing R C plane by bending (be careful) the vertical part of the shaft below the point at which it leaves the hull so that the wheel is behind the center of rotation of the vertical shaft. A few degrees or 3/8 inch displacement is fine. When all three wheels are on the ground imagine a line down through the wheel mounting bracket and the proper wheel position is slightly behind the point at which this line intersects the ground. In my case a couple of my famous "power" landings will automatically back the wheel up a bit. |
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