Tail wheel shimmy

[Fresno]

John, thank you for the pics and the advice. Thank you for the heads up!

[Scott c]

Thanks for the info John. My shimmy came back after tightening the bolt. I took it apart today and noticed my bushing was also a little long, maybe a 64th of an inch. I ground it flush and am going to put a new bolt in tomorrow because it is too long too. The nut bottoms out. I have some wear in the saddle, not too much i hope. Wonder if bushwheel knows about and has corrected the problem?

[Clay Hammond]

If your castor angle is off kilter then no amount of tightening, greasing, and tuning will help. If the angles are right then a certain amount (some might even say a great amount) of tolerance/slop is acceptable, because the tire/wheel just falls in line with hardly a chatter. Check your spring angle under your normal fuel and payloads...is it flexing too far? Think then to your landings...are you dropping it in even just a little? This will induce an excessive load on the tailwheel and can induce the shimmy, even though only momentary. If it starts shimmying, what happens if you unload the stick? By that I mean as soon as it starts to shimmy release any back pressure on the stick. Does the shimmy immediately diminish or go away entirely? If so then your tailspring flexing is ultimately your culprit.

This technical write-up by Gilbert Pierce explains everything in much better detail:

http://www.pierceaero.net/tws.php

http://www.pierceaero.net/techdata/tws.jpg

I'm not discounting your overhaul of the tailwheel system, but all of the above in my post is frankly more crucial to the whole equation of eliminating shimmy.

tws.jpg

[Scott c]

Thanks Clay. I will make the measurements.

[Bob Watkins]

ADVICE PLEASE.................
From Bob Watkins

I am looking for Expert Tail Wheel Advice. I am mainly a Learjet pilot Who has been trying to stabilize my 3200B tail wheel since purchased new. We only have 378 hours of total tail wheel time on our 2010 Carbon Cub - that came with 2 initial tail wheel problems. We are East coast with few grass airports. The first problem has been solved for 3 years. The second is still in play.

FIRST PROBLEM GONE for 3 YEARS - TAIL WHEEL WOBBLE: (1) We check and tighten tail wheel bolts every 25 hours, (2) Keep 50lbs of air in the tire. (3) As per Wup's advice at Alaskan Bush Wheel, we are running the 2 longer spring leafs out of a STD 3 leaf PA-18 spring pack. This keeps the vertical pivot axle of the tail wheel (through the clutch assy.) close to 7 degrees counter clockwise from vertical (when viewed from the right side of the plane) at all weights - and as perfectly illustrated in previous thread commentary. The un-steerable loud, tailwheel vibrations are HISTORY.

THE SECOND PROBLEM - YET TO BE SOLVED:

LANDING SITUATION:
A narrow pavement…
Aft CG, (& heavy)
LEFT 15–20Kts 90 degree x-wind
Normal CROSS CONTROLLED slip into the x-wind
Using zero flaps for added airspeed rudder authority
Touch down, on one upwind main wheel
Good parallel runway alignment on touch down
As 2nd main touches down, tail is up with large amounts of rudder
First few seconds everything controlled - aligned and peaceful.

HOWEVER: As I go to put the tail down for better x-wind control; holding (necessary) rudder deflection against the x-wind (and rightfully/wrongly thinking) that the tail wheel will stream line into the aircraft direction against the spring, all hell breaks loose!! The tail wheel forcefully bangs/ricochets off the pavement like a basketball, not once but on 2 lowering attempts! On the third attempt, it sticks but the plane immediately& violently swerves 30 degrees to the right!! It had to be de-clutched to do that. Right?

Full left rudder did nothing. Without stomping on the left brake I would have surely exited the runway borderline.

I do have two guessed solutions. But I want and need expert tail wheel pilot confirmation:
SOLUTIONS:
1. Land, holding the tail up, initially controlling runway alignment with differential breaking?? Then neutralize the rudder first before putting the tail down second??

2. Or, has someone installed the new www.Beringer-aero.com locking tail wheel designed to replace the Scott tail-wheel common to Super Cubs??? Rather than locking straight ahead, this wheel locks to the rudder angle, and yet unlocks for 360 degree rotation on the ground.

[Clay Hammond]

Hi Bob,

Sent you an email. Lets see what we can do to isolate the issue and get it fixed. Check your inbox and we'll set up a visit.

DO NOT use brakes with the tail still up and rolling out. Recipe for disaster.

3200 tailwheel works. Lets see what might be happening with your's first, and then look at possible need for alternative (unlikely).

Clay Hammond
SWT Northeast
East Coast CC dealer

[docron.ski]

ADVICE PLEASE.................
From Bob Watkins

I am looking for Expert Tail Wheel Advice. I am mainly a Learjet pilot Who has been trying to stabilize my 3200B tail wheel since purchased new. We only have 378 hours of total tail wheel time on our 2010 Carbon Cub - that came with 2 initial tail wheel problems. We are East coast with few grass airports. The first problem has been solved for 3 years. The second is still in play.

FIRST PROBLEM GONE for 3 YEARS - TAIL WHEEL WOBBLE: (1) We check and tighten tail wheel bolts every 25 hours, (2) Keep 50lbs of air in the tire. (3) As per Wup's advice at Alaskan Bush Wheel, we are running the 2 longer spring leafs out of a STD 3 leaf PA-18 spring pack. This keeps the vertical pivot axle of the tail wheel (through the clutch assy.) close to 7 degrees counter clockwise from vertical (when viewed from the right side of the plane) at all weights - and as perfectly illustrated in previous thread commentary. The un-steerable loud, tailwheel vibrations are HISTORY.

THE SECOND PROBLEM - YET TO BE SOLVED:

LANDING SITUATION:
A narrow pavement…
Aft CG, (& heavy)
LEFT 15–20Kts 90 degree x-wind
Normal CROSS CONTROLLED slip into the x-wind
Using zero flaps for added airspeed rudder authority
Touch down, on one upwind main wheel
Good parallel runway alignment on touch down
As 2nd main touches down, tail is up with large amounts of rudder
First few seconds everything controlled - aligned and peaceful.

HOWEVER: As I go to put the tail down for better x-wind control; holding (necessary) rudder deflection against the x-wind (and rightfully/wrongly thinking) that the tail wheel will stream line into the aircraft direction against the spring, all hell breaks loose!! The tail wheel forcefully bangs/ricochets off the pavement like a basketball, not once but on 2 lowering attempts! On the third attempt, it sticks but the plane immediately& violently swerves 30 degrees to the right!! It had to be de-clutched to do that. Right?

Full left rudder did nothing. Without stomping on the left brake I would have surely exited the runway borderline.

I do have two guessed solutions. But I want and need expert tail wheel pilot confirmation:
SOLUTIONS:
1. Land, holding the tail up, initially controlling runway alignment with differential breaking?? Then neutralize the rudder first before putting the tail down second??

2. Or, has someone installed the new www.Beringer-aero.com locking tail wheel designed to replace the Scott tail-wheel common to Super Cubs??? Rather than locking straight ahead, this wheel locks to the rudder angle, and yet unlocks for 360 degree rotation on the ground.

From manufactures description:

Double pivot system, lockable from the cockpit:
Unlocked: the tailwheel is free to castor for taxiing. Steering is accomplished using the brakes on the rudder pedals.
Locked: the tailwheel is locked to the rudder for take off and landing. The tailwheel follows the rudder movement.

Note- the tailwheel does not lock straight ahead, it locks to the rudder. Landing with RT rudder (lt X-wind) would angle the tail wheel to make a RT turn. Holding the tail up until the rudder could be place straight ahead, though gentle differential braking would be needed to keep the nose off the pavement, or possibly landing with the tailwheel unlocked and in full castor mode would work. ( I never flown with a free castor tailwheel so I dont know how hard this is in a x-wind. I would think it would be much different than the effect after my tailwheel broke lock in a similar landing)