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Old 12-19-2003, 01:39 PM
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Default RE: Tricycle gear question.

Triggerinva’s analysis is correct as far as it goes, but only for one gear geometry, that is where the wheel axle is behind the gear axis of rotation.

To be stable, the wheel must castor, and this can be achieved by several means. All bicycles, motorcycles, old type three wheel ATV’s, and most full scale airplanes utilize a steering axis that is canted forward. As long as the wheel axle is on or ahead of the steering axis, the forward cant is a stable, castoring configuration. There is an added advantage to this forward cant, in that it greatly reduces the bending load on the strut, and on the firewall in a hard landing.

The geometry where the wheel axle is behind the steering axis is like a castor on a rolling chair or tool box, etc. This is used on the Grumman American Yankee (the only full scale aircraft that comes to mind). While this configuration is stable it often has a tendency to shimmy which is why it’s seldom used on any full size vehicles. This configuration results in high bending loads in the strut and at the firewall. That is why a springy “knee action” is usually incorporated along with this geometry to absorb some of this load.

By far the simplest installation is to attach the mount directly to the firewall. If the firewall is canted, the steering axis can be changed by simply adding a plywood wedge beneath it as necessary. The good news is that if the angles are small it really doesn’t make much difference. The airplane is on the ground only briefly during takeoff and control is not usually a problem even with a bent, out of place nosegear.