Understanding Tail wheel caster as it effects shimmy

[Bruce Fenstermacher]

Caster is the angle of the king pin in a tailwheel to level in the direction of travel. When the aircraft is loaded to max in a normal configuration the tailwheel should have a slightly positive caster angle. At worst is should be neutral or 90°. To much positive angle of caster is detrimental to steering straight without the wheel wanting to caster to one side or another as this the weight of the aircraft forces the wheel to caster to one side or the other unless it is balanced perfectly in the center. We rarely see tailwheels with too much positive caster.

Negative caster is having less than a 90° angle between the king pin in the direction of travel. If you have lots of friction in your tailwheel swivel from wear, lack of service or over tightening the king pin you can overcome some negative caster. But usually negative caster leads to shimmy because while the wheel wants to roll in the direction of travel, the force of the weight of the aircraft acts to push the wheel from one side to the other. We know this condition as shimmy. We've all experienced the wobbly shopping cart wheel. This is due to negative caster. When you push the elevator down in a 170, weight is lifted off the tailwheel for a time and internal frictions in the tailwheel stop the wheel from swiveling left and right. The cause is likely a negative caster angle.

On our 170s, there is no way to adjust the caster. Some aircraft designs might allow a shim to be added to the leaf springs to change the angle but not the 170. Caster is set largely by the aft bend on the main leaf spring. If your 170 tailwheel has a negative caster angle it is most likely because the main spring is fatigued and bending when it shouldn't be. It needs to be replaced before it breaks causing lots of expensive damage to your rudder and elevator not to mention the tailwheel itself.

[n2582d]

Great synopsis Bruce! I’d recommend including it in the “Landing Gear and Brakes” section of the Mx Library. You say this should be checked, “when the aircraft is loaded to max in a normal configuration”. I wonder how much of a difference C.G. affects the tailwheel caster angle? It would be interesting to compare the angle at full forward vs. full aft C.G. at max. gross weight. If I calculated correctly, at 2200 lbs., max. forward C.G. would have 182 lbs. on the tail and max. rear CG. would have 236.5 lbs. on the tail, a difference of 54.5 lbs.

[Bruce Fenstermacher]

Gary, I can't recall where I heard or saw a reference that the caster angle should be checked at gross, normally loaded. But it makes sense as that is likely the worst case scenario one would encounter. Actually gross weight with the most rear CG possible would be the worse case.

As most of us have seen demonstrated, if you have a shimmy problem, the most forward CG possible at the lightest gross weight would be advantages as it would take less effort with down elevator to lift the weight of the tailwheel reducing the shimmy forces.