Problem with right turns when tail comes down

[RogerP]

I have been having a random problem with my RV8 (taildragger) where the aircraft tries to exit the runway in a right turn when the tail comes down on landings. This occurs about 10% of the landings, and is a mechanical initiation of some sort in the tailwheel assembly. The Brakes have been eliminated as a source of the initiation at this point. The cause of why the tailwheel initiates the right turn is a mystery. I make all my landings as wheel landings due to this unpredictable tendency. Most landings are uneventful but about 10% of the time as soon as the tail wheel is down I get a very strong right turn energy and most of the time I can over come it with left brake. Full left rudder only exaggerates the problem as it dissconnects the tailwheel and rudder as per design and the tailwheel is on its own. I have tried neutralizing the rudder which improves my braking leverage (foot angle). Recently this problem has become much worse and when it occurs and has resulted in two recent events where the aircraft ran off the runway with left brake applied. In both cases when I exited the runway and had decelerated to well under 20 mph I released the left brake and both time the plane swapped ends (ground loop) to the right. I have found the tail wheel wheel spring loose at the weldment receiver becasue the attach bolt had no nut on the bolt, and the bolt was loose. This was allowing rotation of the spring and the tailwheel. Rotation was approximately some 10 degrees or so transversly to the aircraft direction. The wheel fork additionally was also slightly loose but was more an issue as it caused additional rotation in the same arc as the loose spring bolt. I have replaced and tightened up all the connections and not yet flown it. Any thoughts on what is wrong in the tail wheel. My thoughts are that if the wheel is allowed to rotate on its edge that will result in an induced deflection and turn in one direction or the other, much like a bicycle or motorcycle. The continuity of right turns must mean to rotation is always one diirection. I would like techinical advice from anyone before I fly again.

[p51dplt]

Are you pulling the tail down or letting it fall on its own? Pulling will cause the gyroscopic precession to cause a swerve to the right, but you should be able to overcome that with rudder. Have you tried a no flap landing yet? I find that mine has a mind of its own at time is the transition from on the mains to back on all three. I tried retracting the flaps before the tail comes down and it seems to help as well as landing flaps up. There have been theories of the flaps and canopy blanking out the tail in the transition from on the mains to 3 point. I'm sure you have made sure your tailwheel stinger is aligned correctly, but if it isn't that could cause a tracking issue. For whatever reason the spring gear in the 8's make it the most challenging of the series.

Good luck, I hope you find something soon.

[cfiidon]

Other than making certain the tail wheel is aligned correctly, you might investigate your toe-in/toe-out condition. Much has been written, but my belief is that a neutral or small amount of toe-out is more stabilizing.

I confess that I, too, had an "off road excursion". I pulled the tail down too early. Probably precession coupled with a lightening of weight on the mains followed by a return of weight asymmetrically. At least that's my best theory so far. It turned out it was also toed in.

It's a sick feeling to suddenly be a passenger where no amount of opposite rudder does a thing. I did manage to miss all the hard and expensive stuff, like a big old runway sign.

After investigating mechanical issues, try increasing taxi speeds, but don't get carried away. My initial tendency was to be overly aggressive with rudder rather than small fast inputs early, usually returning to neutral in between. It is a dance.

[chris mitchell]

If you always turn right and have ruled out significant problems with brakes and main gear alignment (both of which seem unlikely anyway as you report the problem only occurs about 10% of the time) that suggests to me that the problem lies in the tailwheel swivel arrangement and that the tailwheel is not tracking/steering correctly. Suggest you take it all apart, clean and grease, and make sure that the little spring loaded-pin is free of any small burrs and nicks that might make it less laible to engage/more likely to disengage.

I have had a similar problem, as have a couple of others, cured by the above maintenance. I suspect what happens is that the pin pops out of its detent too easily and you then have a freely castoring tailwheel. Full left rudder will have no effect except via airflow over the rudder, but asymmetric braking will work fine.

Hope that helps.

Chris

[newt]

I suspect what happens is that the pin pops out of its detent too easily and you then have a freely castoring tailwheel.

Some taildraggers are designed with freely castoring tailwheels, and they remain controllable on rollout, so that can't be it.

I'd be checking that the tailwheel isn't "twisted" on the stinger. If it is out of alignment, it's possible that taking up the weight will ease it out to the side. Looking from the rear, see if it has an anti-clockwise twist on it.

It sounds like what you were describing in your message, so maybe you've already found the problem.


Another thing to look at is whether your starboard brake is "sticking" slightly. It might not be noticeable at high speeds because you'd have enough rudder authority to overcome it. And it might not be noticeable at high power because p-factor will be trying to swing you out to the left anyway. But at low speed and low power (e.g., when the tail is coming down after a wheeler!) perhaps a "grabby" brake can dominate?


- mark

[schristo@mac.com]

These small video cameras are incredible... You can get a great view of the tailwheel with a gopro mounted on the wingtip looking back. Works well on top, or under, the wing. Grab some video during your touchdown and roll out to see what is happening.

[rv8ch]

Here is a video showing a "crazy tailwheel". Had something like this happen to me once. Was not fun!

http://www.youtube.com/watch?v=HkDQBXH02fE

[skylor]

Roger,

You definitely need to get a handle on this problem and fix it before wrecking your plane in a ground loop!

Here are some areas that I would check:

1. Free-rolling: Does your airplane roll free and straight on the ground, when you hand pull it? How about when someone is sitting in the front seat? If you have any problems here, you need to find out why and correct them.

2. Wheel pant clearance: Are you flying with wheel pants installed? If so, check the tire clearance on the right rear wheel pant, and look for signs if tire rub. It's possible that in the tail low attitude the tire deformation around the contact-patch is causing a rub against the wheel pant. Low tire pressures can also contribute to this.

3. Landing gear alignment: Check the alignment of the main landing gear to verify that they are set up per plans. In particular, verify that the gear is aligned square with the fuselage center-line. Also verify that the main gear mounting bolts are correctly torqued ant that you don't have any fore-aft movement of the gear legs. Finally, check to make sure (sounds like you already have) that the tail wheel strut is correctly aligned.

4. Main axle alignment: Take measurements to verify that the main axles are properly aligned to zero toe with the fuselage set in the level reference attitude*.

* On my own -8, I found it to be a little unpredictable and squirrelly on landing roll-out, particularly while transitioning from wheel landing attitude to 3-point. Nothing scary, just uncomfortable. After a hand full of flights this way, I went ahead and rechecked the wheel-toe. I found that I did have it set up pretty much to plans, with just perhaps a hint of toe-in (0.1 to 0.2 degrees). I decided to install 1/2 degree wedge shims (sold by Van's) between the axles and the gear-legs to give a slight amount of toe-out. This slight change really made a tremendous amount of difference in stability on landing roll-out and now the plane is extremely stable and predictable.

5. Tail wheel configuration: You haven't stated what you are using for a tail wheel fork and steering linkage, but definitely look for problems in this area. For my own tastes, I found that I like the feel of the stock fork the best, and I have the tail-lynx steering cable set up with a fair amount of slack. This ensures that the tail-wheel is essentially free-castering through a small range of rudder movement.

6. Technique: If you can't find any real issues with the above items, perhaps you need to get a second opinion from an experienced -8 pilot. Also check you technique. Are you inadvertently applying uneven braking during landing roll-outs? Are you over-controlling the rudder? Are you pinning the tail wheel with full elevator immediately upon setting it down, or are you letting it straighten itself out before you bring the stick back? Techniques in these areas tend to vary a bit from person to person, but it's well worth a look.

Skylor
RV-8
N808SJ

[SHIPCHIEF]

An additional thing to check:
Our RV-4 was a bit difficult to steer, but it was caused by a sticky tailweel fork bearing. Not much effort is involved with disassembling, cleaning and re-greasing the unit, then reassemble and test for freedom of steering movement and check for exessive slop.
Ours was just getting stuck with dried up grease. The zerk fitting does not get grease in where it matters. You have to take it apart to lube it up.
I hope you find your problem soon...

[RogerP]

I replaced the Cleveland Brakes with Grove Double Piston brakes and I have far more control now than with the Clevelands. While I continue to work the problem with the tail managing the excursions is so much easier with the Groves.

Originally Posted by p51dplt

Are you pulling the tail down or letting it fall on its own? Pulling will cause the gyroscopic precession to cause a swerve to the right, but you should be able to overcome that with rudder. Have you tried a no flap landing yet? I find that mine has a mind of its own at time is the transition from on the mains to back on all three. I tried retracting the flaps before the tail comes down and it seems to help as well as landing flaps up. There have been theories of the flaps and canopy blanking out the tail in the transition from on the mains to 3 point. I'm sure you have made sure your tailwheel stinger is aligned correctly, but if it isn't that could cause a tracking issue. For whatever reason the spring gear in the 8's make it the most challenging of the series.

Good luck, I hope you find something soon.