H in t s F or H o m e b uil d e r s
A Tail Wheel Shimmy
No, it's not a dance
By Cy Galley
Do you fly a taildragger? Does your tail wheel shimmy,
shake, or bang from stop to stop? Most tail wheels will
shimmy quite easily if the design geometry is wrong (and
most are), if the friction device (Scott No. 3234, thrust plate
No. 4 of diagram) is misadjusted or gets grease on it, if
there is looseness in the various parts, or if the mount or
spring is not secure.
The first item, geometry, is easy to check. The pivot should
be vertical under load. The top should never be slanted
to the front of the aircraft. If the pivot top is slanted under
load a little toward the back of the plane, that's okay.
As your tail spring weakens with age and less than spectacular landings, the pivot will slant toward the front of
the plane. You can replace the spring or have it re-arched.
The re-arching has to be done cold. No heat! Some spring
companies will do it, but our local truck spring company
will not have anything to do with aircraft springs due to the
company's worry of liability.
So if you have a large hydraulic arbor press at your disposal, press away. I have also seen it done with a large
sledgehammer and a piece of channel iron. This gets a
little noisy but works very well. While you are at it, make
sure someone hasn't removed a leaf in the misguided attempt to make the plane "ride" better.
Also included in the geometry check is the lean of the
wheel. It should stand vertical from side to side. If it leans,
then the spring or mount is twisted, or the axle has been
bent. Check your hard rubber tire for "coning." A hard
rubber wheel can be re-trued on a lathe by grinding or
sanding. The pneumatic tire should not have flats or cups,
and the pressure should be around 45 to 50 pounds per
square inch.
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Vol.2 N o.8 /August 201 3