Tailwheel Shimmy, Aircraft 

When we built the prototype FW 190 we had a high speed ground loop from a poorly designed tail wheel. Good thing we had Mira Slovak as our test pilot.

I visited the Lear Jet Factory (in 1965) in Wichita, KS. There, I talked to an engineer who explained the five causes of shimmy for both nose wheel and tail wheel. He said (Number one) is ANY looseness between the structure and the wheel will cause shimmy. Boy, do I see it in the video. No 2 is wheel/tire out of balance or out of round. No 3 defective shimmy dampener. (On tail wheels it sometimes is a friction plate within the vertical shaft) No 4 is loose or worn wheel bearings. No 5 is wear or looseness in the steering system. As an aircraft mechanic, I have been correcting wheel shimmy since I got my A&P in 1966. Haven’t lost one yet 😅

I love your corsair Don! You've done an amazing job building her!

Thanks for this comprehensive explanation on tail wheels. It was always a guess for myself and most people as to troubleshooting. All I knew was when they didn’t work it was hard to ignore.

The only Tailwheel shimmy you’ll be doing is listening to music in your garage.

Had that happen once on our Kitfox while doing a high speed taxi. Thought the rudder peddles were attacking me....then back to the hanger to fix the issue....way no fun to experience

here's my take; there is very little to no tension on rudder to tail wheel spring after tail is down , the rudder was slightly crabbed to one side on contact ,as weight is applied to tail spring the spring are no longer in play and results in shimmy. I had this on my Skybolt but would stop if you applied a little fwd stick. make sure you have some tension under load .I also had a small safety cable thru spring in case of spring failure. ( in my case one side came off ). this was with a larger tailwheel. great workmanship on your plane, have fun.

This in undamped, or poorly damped, motion. The initial loading up (torsion) on the leaf spring caused the spring to (as it is designed to do, albeit not in torsion) resist and (hopefully) damp out the motion. Problem is, it is being excited by more input. It isn’t phugoid motion, but it looks close. You are right. The problem is that the leaf spring in the video isn’t constrained in torsion - which it needs to be to work. As for stoping shimmy (and phugoid motion) there are two issues. The first, you are sorting by constraining. The other issue is to make sure you don’t excite a natural frequency. This second issue requires weight placement to change the natural frequency. This is what motorbike steering dampers,do. I am sure you will sort it, you always do.

The issue is that the plane of the swivel "disc" has a negative angle causing the shimmy. The swivel disc or plane must have a positive angle relative to the ground. It's in the Tony Bingelis books and worked on our Champ and the Pietenpol I built.

This is the only channel that I don’t care if I get blocked.

Flew a Stearman for 30 yrs. Never had shimmy. If the lateral springs are too weak, the resonant frequency is low. Stiffer springs raise this frequency and provide a dampening effect. If your fix isn't adequate, add a damper.

1:00 although you may be playing that video at normal speed, it appears that that original video is half speed or slower. I’ve had wheel shimmy plenty of times, the only way to stop it is to lift the tail again and set it down at a slower speed.

Thanks Don!

Maybe the flexibility inherent in the video wheel is their problem which you won't have because your design is stiffer overall.Your yoke and rigid mounting seem more in line with the real F4U which has a very solid shock absorbed rear strut more like yours. Wow the little things one gets into when building a plane.

That spring does not appear to be secure in it's mounting. The spring should not have the ability to roll from side to side, inducing shimmy. Maybe the mounting structure on the video airplane needs to be beefier. I have a Stinson 108-3. The tail weight is over 100 pounds. The spring is bolted into the steel tube frame at the tail. If that spring has the ability to roll or move laterally, then you've got a problem with the mounting. As other tailwheel videos explain, stagger will definitely cause shimmy. The vertical axis of the pivot should lean slightly aft from the vertical. If it leans forward, that will induce shimmy. That's the shopping cart phenomena we're all familiar with. Finally, the tailwheel in the video appears to be a Scott 3200 or the Alaska Bush version of that tailwheel with the bush tire. After rebuilding my tailwheel I called Alaska Bush to ask what the torque of the nut on the spindle bolt should be. They said to tighten all the way, and then back off about 3/4 turn to get desired resistance to turning. In the case of the heavy tail Stinson, you don't want that to be real loosey goosey. You want some resistance to turning back there which will help prevent the thing from starting to shimmy.

It looks to me that the spring is being moved by the wheel. Not the other way around.

shimmy shimmy ya shimmy yam shimmy yay. no one wants a loose tail

Very interesting 👍 Good to know

G'day Don, Yay Team ! I must be weird, because I "Got it" in the last Video when you mentioned the Tailwheel-set inducing a One-Wing Low condition... The way I visualised it, Weight on the 3 Wheels "should" be a Triangle with the Tailwheel on the Perpendicular which bisects the "Baseline" running from Mainwheel to Mainwheel... Whenever the Tailwheel, at the far end of it's Fork, mounted on the end of the Leafspring at the back of the rear Fuselage is deflected Left or Right ; the Rear Contact Point on the Ground is displaced Left or Right from the Centreline, Causing more weight (55% ?) to bear on one Mainwheel or the other - inducing that One-Wing Low "set" which you mentioned... As I see it, the Leaf-Spring is behaving as a Torsional Pendulum, whereby any Deflection left or right of Centreline by the Tailwheel Fork displaces the Contact-point of the Tailwheel from the Centreline, inducing a Twisting moment in the whole Fuselage - which then causes the Tailwheel's Castoring Axle to Lead the Wheel into attempting to chase the Centreline, In the Yaw Axis... But coupled Inertia leads to the Tyre overshooting the mark, & Heterodyning with the Elastic Modulus of the Fuselage..., Plus the Torsional Twistiness of the Leafspring, to result in the Offset Axis of the Out of phase Compound (double) Pendulum resulting from the Leafspring swinging in Roll as well as bending in Pitch oscillations, energising the Tailwheel Forks to Oscillate Violently, as it attempts to use the Leaf-Spring as a Lever with which to attempt to Twist off the entire Tail... My prescription would be to do up the Friction Locknut on that Tailwheel Castoring Axle, to attempt to damp the Tailwheel's Response-Times - when it's invited to go into the WigWag Mode...; and/or to add Stiffness to that Leafspring in the Torsional Mode. Either a thicker, stiffer Leaf-Spring setup...; or come up with one of the Leaves having an "U-Section" rather than being flat - so that the other Leaves "nest" inside the "Fences" and those "fences" or "webs" along the edge of the "Main Leaf" should (?) render the whole thing too stiff to be able to Twist the Wheel any further off-centre than the degree induced by the Yaw-displacement of the Castoring Forks. I amputated all such worries, by using a Bungee-Sprung, Steel-shod, Wooden Tailskid which was locked to the Fin-Post so stayingCentral in Yaw - but allowing 60° of up/down movement in the Skid, against the Bungee..., and the forward end of the Skid featured a "Sprag" which by using Full-Back Stick and Propeller-Blast, or passing Slipstream on the Elevators, the resulting Downforce stretched the Bungee, pushing the Skid's "leading Edge Sprag" down into the Turf, As a "Brake". Such was my Hypothesis. In fact, I never noticed any great ability to control the Braking with application of Full Back Stick ; but it failed to fail, kinda thing - it did the job and it never ever shimmied (!). Keep on keeping on... Stay safe. ;-p Ciao !

It seems easy enough to stiffen it up, but the problem would be adding stiffness without greater weight.

It'll be interesting to see what yours does, when you pick up the tail and set it back down, for the first time.

But your locking tail wheel will stop that? or would parallel arms and a spring shock be better?

I think the problem is there is a lateral bend that makes the contact point of wheel be displaced of centerline of leafspring creating a rotational moment but once wheel rotates the contact point changes side creating the opposite and so on.

How about at tighter assembly and a harder thinner tire.

Are you going to put a camera on yours to check for shimmy?

Learning new things about a tail dragger airplane I didn't know it was a big problem. Why can't someone invent a tail dragger wheel design like a car's A arm with a nitrogen gas strut? I think that would solve some issues. I have seen it done on the Drago Light Sport aircraft using nitrogen gas struts on the main landing gear wheels.

what plane was that? a tiger moth? i was waiting for that tire to burst !

Tail wheel geometry: drive.google.com/file/d/1iD27sbOxZgCXBIZ8KfUigvl8VkT9PAnQ/view?usp=sharing

Given what I have seen so far, I am sure this is nothing Don can’t sort out.

I'm ready to take it up. Want me to fly it for you?

Multi leaf spring and beef up the attaching point so it doesnt stress a small area. That stuff you have on there now is rinky dink trash throw that in the garbage.

Maybe you should get someone that knows what they’re doing.