I noticed when flying my Piper Clipper heavily loaded I experience tail
wheel shimmy on my Scott 3200 when landing on a hard surface such as
concrete or asphalt. Several years ago I was parked at the landing end of
runway 36L at Oshkosh. I always get tail wheel shimmy when landing there.
This afforded me the opportunity to observe many landings as I lounged under
my wing. It was here that I made the discovery that about 50% of the tail
wheel airplanes landing on 36L experienced tail wheel shimmy. I believe the
grooved runway exacerbates the problem. Anyway those tail wheels were not
just shaking side to side, they were rotating around their pivot axis 360
degrees and doing so violently. On my recent trip to Alaska I had tail wheel
shimmy on almost every landing unless I really greased it on. On my return I
vowed to solve the problem.
I started the quest for a solution on the Internet. I was told that if you
raised your tail wheel tire air pressure it would assure the tail wheel
would shimmy no more. I was told to reduce the tail wheel air pressure. I
was told I had too much grease in the tail wheel. I was told that if the
tail wheel didnít spit grease at you when walked by it, it did not have
enough grease hence the shimmy. I was told to loosen my steering springs, I
was told to tighten my springs. I was told that the pivot axis must be
absolutely vertical so that the surface the tail wheel swivels on is
parallel with the ground hence the pivot bolt would be vertical. Mine was. I
was told the pivot bolt must face forward at the top, I was told the pivot
bolt must face aft at the top. So what did I do? I took the tail wheel apart
and made sure it was mechanically in top-notch condition and adjusted to the
manufactures specifications-again. It was. Then I tried each and every
remedy listed above except changing the angle of the pivot bolt, no help.
The Scott 2000 tail wheel does require some tension on the steering springs
to control the unlock tension and hence shimmy. The Scott 3200 installation
instructions say that chain tension is not required or recommended.
Next I got out an old 1950ís auto repair manual that explained king pin
front wheel suspension systems and steering castor angle. If you have ever
pushed a grocery cart through the supermarket with one of the front wheels
shaking side to side you have experienced wheel shimmy and improper castor
angle. What I learned from the chapter on steering alignment was basic
steering geometry. To measure your steering geometry, drop a line drawn
parallel to and through the pivot axis and extend it to the floor and make a
mark on the floor where this line hits or use a straight edge parallel to
the steering axis shaft. Next drop a line vertically from your tail wheel
axle to the floor or again use a straight edge and make a mark on the floor.
This will also be where your tail wheel contacts the floor. Now move this
line or straight edge that passed from the axle to the wheel/floor contact
point horizontally until intersects the pivot axis line at the pivot axis
midpoint The line that is parallel to the steering axis must hit the floor
ahead of the line dropped vertically from the wheel axle. The angle formed
by these two lines is your castor angle. The larger the castor angle the
better as far as tail wheel shimmy is concerned. In other words, the farther
ahead of the tail wheel that the steering axis line hits the floor the
greater the castor angle and the less likely that will have shimmy. To put
it another way, the steering axis pin or bolt must be vertical or tilted
with the top pointing behind or to the rear of the airplane when the
airplane is fully loaded. Emphasis on fully loaded.
When my airplane was empty the steering axis bolt was vertical. When I
loaded the airplane, the tail wheel spring compressed and the top of the
steering axis bolt was pointing to the front of the airplane. This would put
the extension of a line drawn through the steering axis behind the tail
wheel contact point. Bad news-it will now shimmy. You donít want the castor
angle to be too large because it will make steering on the ground more
difficult. The large castor angle will tend to lift the rear of the airplane
slightly as you turn the aircraft. This is called self-centering effect.
Having the steering axis bolt vertical or inclined slightly with the top
pointing back when fully loaded should be sufficient.
So how do you correct this angle? There are two easy solutions. If your
airplane is like most, the spring is bolted at the front to the airframe
with a bolt that passes through the spring leaves. The spring then rests on
a pad several inches behind the point through which the through bolt passes.
Usually the spring is clamped to the pad at this point. You can add a shim
between the pad and the spring to increase your steering angle. Or you can
take the route I took. I took the spring off and laid it on a piece of
poster board and traced out itís arch. Then I took the spring to a spring
shop and had them re-bend the spring until the tail wheel end of it was
about 1-1/2 inches below the original. In other words I increased the arch
slightly. Walla-no more shimmy when loaded.
One other point. You should carry sufficient air pressure in your tail wheel
to keep the tire firmly attached to the rim when it hits the pavement on
landing. Due to the small diameter of the tail wheel, it accelerates very
rapidly on contact. If you have insufficient pressure in the tire it will
slip on the rim and cut the valve stem. You now have a flat tail wheel tire.
I know- itís happened to me twice. I now carry a minimum of 45 pounds of
pressure in my Scott tail wheel. The same thing can happen to your main
tires but with more surface contact area around the rim it is less likely to
happen unless your plane lands at very high speeds.
Click here for Tail Wheel Geometry Drawing
Tail Wheel with Positive and Negative Castor Angles Ė exaggerated The terms
positive and negative are simply the naming the convention I choose to use
as they agree with my text book references.
EAA Chapter 182