Tailwheel shimmy is a complex problem with many contributing factors, and small changes that you make to your tailwheel assembly can put you into a shimmy-prone condition. Changing to a taller, longer tailwheel fork like our Condor2 or the reduced friction of JDAir’s popular tailwheel mounting yoke can sometimes shift an already shimmy-prone tailwheel geometry into a shimmy-generating one.
Tailwheel shimmy has been an issue with airplanes for as long as tailwheels have been around. Fortunately, there is a reliable fix that’s relatively easy to accomplish (click here to skip to the solution). Let’s talk a little about tailwheel geometry.
As discussed in this article from the EAA, the biggest contributing factor to a shimmying tailwheel is the geometry of the pivoting axis of the tailwheel steering in relation to the ground. Envision a line that runs directly through the center of the pivoting axis and extends to the ground (marked in red below). We’ll call this the steering axis line. Then consider a second line, which is perpendicular to the ground and intersects the first line at the midpoint of the pivoting joint of the tailwheel (marked below in blue). In ideal tailwheel geometry (ideal at least for the purposes of shimmy and stability), the steering axis line should hit the ground in front of the perpendicular line as shown below… we will refer to this as “positive steering angle.” Obviously, there is a limit in how far we should go with this. Dramatically increasing the steering angle will adversely impact the handling. Another way to think of steering angle is to think of the nut on the top of the assembly leaning forward or back. If the nut is leaning back, this is positive steering angle.
If you’ve got a brand new tail spring and an unladen airplane, what you probably have is a neutral steering angle… the angle of Van’s mounting socket is set so that the pivoting shaft of the tail spring sits straight up and down when the plane is on the ground.
The problem is that your airplane is only brand new once, and it’s never operated without a little extra weight in it. The tail spring is — you guessed it — a SPRING. And like any spring, it will start to deform when weight is applied to it. Put weight in the airplane, especially aft of CG, and your tail spring will start to bend downwards to some degree, which takes the formerly neutral steering angle and tilts the steering axis forward, moving us into NEGATIVE steering angle. This effect will be magnified over time as wear and hard landings exacerbate the bend in the spring. Negative steering angle is bad, and among other issues that it presents, it opens the door for shimmy to creep in and ruin your day. This is also the condition that I see 95% of taildragger RVs in.
That’s not to say that your RV definitely is experiencing shimmy… it probably isn’t. But your negative steering angle will make it more susceptible to shimmying if the other variables that contribute to shimmy are changed.
But my tail spring doesn’t look bent!
Most of the time when I tell people to look at their tail spring, they do and conclude that it’s not bent. The straightness of the rod spring is difficult to assess by eye or even with tools due to the taper, and there are other factors that affect steering angle like tailwheel height. The thing to look at is the steering angle at the mounting socket. The stinger below looks straight, until you look at the steering angle specifically.
Let’s talk solutions.
If you’ve got a negative steering angle and/or you’re experiencing a shimmying tailwheel, there are a number of possible solutions to fix the geometry. You could replace the mounting socket with one that has a shallower angle. Trouble is, those sockets don’t exist, and even if they did, replacing the mounting socket is a headache. A new tailspring may temporarily fix the problem, but it’s still a huge headache to replace that part.
The easiest fix that we’ve found comes from a Van’s Air Force forum post here, in which a user suggests simply putting a bend in the tail spring to get the steering angle into the correct configuration. This was done by using a fairly simple hydraulic press tubing bender to put a 9-degree bend in the spring (the required bend angle will vary depending on your specific situation). This operation was done without adding heat. NOTE: if you go this route, make sure to observe appropriate safety measures.
The result took the original negative steering angle and corrected to a positive angle. The pilot reports improved steering as a result of this modification.