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aviation resources
Tail Wheel Shimmy
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 didn’t 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 1950’s 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 don’t 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 it’s 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- it’s
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.
Tail Wheel with Positive and Negative Castor Angles – exaggerated The terms positive and negative
are simply the naming the convention I choose to use as they agree with my text book references.
Gilbert Pierce
Technical Counselor
EAA Chapter 182
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