Strong Crosswind, not a great outcome

[gereed75]

Here is one reference. I am sure there are others

http://tailwheelersjournal.com/2012/...heel-steering/

[David-aviator]

Thanks for the report Steve and glad it worked out ok with no damage to airplane.

The discussion about the tail wheel unlocking sure is interesting, have not had it happen but know its been close.

I am a total neophyte with 140 hours tail wheel time, most in the 8, so the envelope is adjusted accordingly. I try to not fly on gusty windy days. But somedays it just blows up and there you are, you have to land. The most I've survived is about 18 knots at 45 degrees to runway.

The approach and landing is all about technique. We try to figure out what works and stick with it. You've got a lot more experience in the 8 than I, so I pay attention.

Sounds like you did everything just right and in the end lost it anyhow, how come? Probably skidding tail wheel or it unlocked.

An unlocked tail wheel is a challenge. A few years ago I was getting a RV tail wheel check out in an RV-6. The IP signed me off but I though I did terrible. Later I found out why, the darn tail wheel steering was disconnected the entire check out. It was busted. No wonder I had such trouble keeping the thing aligned on take off and landing. Felt like I was wearing out the brakes.

So tail wheel steering is important.

The only thing about your flight that I do different and won't change is the approach. I don't slip but fly a crab and drop a wing and kick it straight to land. That's pure technique, carry over from other airplanes where a slip was out of the question.

Again, thanks for the report.

[gereed75]

Originally Posted by DanH

+1 for sure. My original tailwheel setup (with cables and springs, not a single-sided link) had the pin very near disengagement at full rudder deflection, and in fact it did go full swivel unexpectedly while landing in a good crosswind at OSH.

Today, a close look will find that the tailwheel steering attach points on the rudder horn have been moved inboard about 1/2" on each side. It allows full rudder deflection with less tailwheel deflection.

Side note; the ball bearing JD Air tailwheel I'm flying now seems to have a very firm lock pin. Can't say that about all tailwheels.

Dan, I think you are wrong on this one. The same linear displacement (the rudder pulling on the steering arm) will produce more angular rotation of the tail wheel with a shorter lever arm.

Think of a derailleur on a bike. A bigger rear gear is a lower gear ratio producing less wheel rotation for a given pedal travel.

NOPE. Re-read. You are right. Moving the attach point in on THE RUDDER HORN will produce less angular deflection of the tail wheel.

I think I will take a look at this on my steering link equipped 6.

Thanks Steve for prompting this thoughtful discussion.

[scsmith]

Originally Posted by rvbuilder2002

I don't know if it was a factor or not, but from your description it sounds like it could be.....

I have been cautioning builders for many years (ever since there was a rocket link on the RV-7 prototype) that they need to confirm that when the rudder is hard over at the left and right stop, that it isn't rotating the arm on the tail wheel beyond the disengage point.
I believe a lot of the ground loops that have occurred over the years were caused by this. Chains and springs have enough slack that this isn't a problem.

This is one of the reasons I don't like the rocket style links, and why the RV-7 prototype was switched back to springs.

It is also possible for the tail wheel to break out under load if the arm and/or lock pin has gotten worn.

These are exactly the two things I am thinking about. Will inspect today.
A third factor that I will check today is that with full rudder deflection, and pushing pretty hard on it, I wonder if the pedal might touch the firewall with less than full braking. I'm sure I would have checked that when I built it, but maybe not including the flexing of the system under hard pushing.

For those that have brought up differential braking as part of the control process, the answer is 'yes, of course'. Honestly, after 30+ years of tailwheel flying, I don't think I can describe exactly what I do with rudder and brake -- its pretty subconscious. I think I start mixing in some brake at about half rudder, and add more brake pressure as I add more rudder beyond that. You are basically doing whatever is needed to fix any heading error. The startling thing about this event is how instantaneous it was -- everything was fine, rolling out nicely, I'm sure I had some mix of brake and rudder. And then it wasnt.

It was kind of a spooky thing, but I remember as it was sliding how quiet it was -- I could not hear any tire noise at all. I will be really furious if I check and find that with a hard push on the pedal, there isn't any braking available because the pedal is touching the firewall!

[scsmith]

Originally Posted by gereed75

I have a different theory on this I would like to throw out for discussion.....

In a xwind, as the the aircraft slows, forward stick used to hold the tail up. As speed further decreases, more rudder will be required to hold alinement. Now when the tail finally drops, the rudder and more importantly, tail wheel (still locked) will be very displaced. When it touches down, it will swerve the aircraft until it unlocks.

This is somewhat opposite to what I think Steve is saying. To avoid this, one of two techniques could be used:

1) get the tail wheel on the deck sooner, rather than holding it off, while the rudder has more authority and therefore it and the tai wheel would be straighter.
2) If holding the tai wheel off as long as possible, neutralize the rudder just as the tail wheel touches down, so that the tail wheel is straight when it touches. Once on the deck, a straight tail wheel will hold the aircraft straight more effectively.

Is this (a severely cocked tail wheel at touchdown) a more likely explanation of what happened here and in other ground loops.

Isn't a less cocked tail wheel at higher rudder deflections what is going on in a "loose chains" rigged airplane??

Gary, I've experienced this behavior with tight chains, but less so with the rocket link. The link has enough stretch with an internal spring that when the wheel comes down, it straightens out some to minimize swerve. Loose chains would no doubt be better in this respect, but I don't like the lag time. I might think about going back to my Tail Lynx system.
I agree with you that doing this earlier in the roll-out also minimizes it.

Note also that the swerve, if any, is away from the wind, since that's what you are doing with the rudder.
In this particular case, I specifically noticed that there was no swerve when the wheel touched. An interesting thought though - what if it was turned so much that when it came down, it just started skidding, and then never regained traction. Hmmm. Again, I wish I could take a look at the marks on the runway. But now, in addition to it being a towered airport, it is also 300 miles away.

[scsmith]

One question for Scott and Dan, about setting up the wheel steering so that the cam is not close to releasing the pin to unlock the wheel at full rudder.
If it isn't, how would you ever get the wheel to unlock when you want it to?

Mine is set so that at full rudder, plus a bit of spring deflection in the link, it releases.
During roll out, as I mentioned, at full rudder, the link spring is deflected the other way, letting the wheel straighten some, so it should not unlock.

A bit of wear on the arm or pin, though, and it might. I will inspect today.

[sblack]

I think you only want it to unlock when you stand on one brake and drive the tw to a very large angle, like >70 deg.

For the victim, if you were holding everything in with a large rudder deflection and TW input during the rollout and the TW suddenly disengaged, you were doomed. One would have to be some mix of Chuck Yeager, Bob Hoover and Superman to react to that. Once the rotation started and the yaw inertia got wound up you were nothing more than a very interested passenger. There was nothing you could have done. There but for the grace of a well rigged TW go all of us.

Check your TW as suggested (I'm gonna do that too) and be very happy that you aren't doing an engine teardown or worse. I think you were damned lucky. I wouldn't worry about your pride. Better to have that dented than your wallet!

[scsmith]

Originally Posted by sblack

I think you only want it to unlock when you stand on one brake and drive the tw to a very large angle, like >70 deg.

But it can never get to a large angle if it doesn't unlock. If you go full rudder, and stretch the springs, and it doesn't move far enough to cam out the pin, that's it. Its locked. Its not going anywhere. (assuming its not worn enough to release with force)

its not supposed to release unless the cam action retracts the pin, and the cam action can't retract the pin unless the wheel can turn to that angle, and it can't turn to that angle unless the rudder deflection plus spring extension allows it to.

[sblack]

Originally Posted by scsmith

But it can never get to a large angle if it doesn't unlock. If you go full rudder, and stretch the springs, and it doesn't move far enough to cam out the pin, that's it. Its locked. Its not going anywhere. (assuming its not worn enough to release with force)

its not supposed to release unless the cam action retracts the pin, and the cam action can't retract the pin unless the wheel can turn to that angle, and it can't turn to that angle unless the rudder deflection plus spring extension allows it to.

No, you don't need a large rudder deflection. What you need is a large difference between the angle of the arm connected to the rudder and the wheel pivot (kingpost?). When you stomp on a brake, or you push the tail with your hand when pulling the airplane out of the hangar, it puts a sideload on the tire, which rotates the kingpost. If the kingpost angle gets beyond a certain limit the pin pops out. In fact I think that is the only scenario in which you want the pin to disengage. YOu don't want to be able to disengage it with rudder inputs - for the reasons discussed above. centered. I have never had mine disengage due to rudder inputs. That would not be good.

[gereed75]

Originally Posted by DanH

+1 for sure. My original tailwheel setup (with cables and springs, not a single-sided link) had the pin very near disengagement at full rudder deflection, and in fact it did go full swivel unexpectedly while landing in a good crosswind at OSH.

Today, a close look will find that the tailwheel steering attach points on the rudder horn have been moved inboard about 1/2" on each side. It allows full rudder deflection with less tailwheel deflection.

Side note; the ball bearing JD Air tailwheel I'm flying now seems to have a very firm lock pin. Can't say that about all tailwheels.

Originally Posted by scsmith

One question for Scott and Dan, about setting up the wheel steering so that the cam is not close to releasing the pin to unlock the wheel at full rudder.
If it isn't, how would you ever get the wheel to unlock when you want it to?

Mine is set so that at full rudder, plus a bit of spring deflection in the link, it releases.
During roll out, as I mentioned, at full rudder, the link spring is deflected the other way, letting the wheel straighten some, so it should not unlock.

A bit of wear on the arm or pin, though, and it might. I will inspect today.

Steve. I pondered this question also, specifically today as I sat astride the jacked up tail wheel, drill and 3/8" wrenches in hand. After doing some yanking on the tail wheel and rudder (gently ) I was able to convince myself that when turning an arc with full rudder, I think the internal link spring will be tension, limiting the tail wheel rotation. Stomping a brake should tighten the turn and put the spring in compression, allowing enough rotation to cam out the lock pin.

At least that is my theory I have moved the link mount point on the rudder post in about .6". Flying Thursday and will see if the theory is correct