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Tailwheel free-swivel breakout force

T

Tango Mike

Well Known Member
A recent thread on ground loops reminded me of an issue I'm dealing with in my '97 RV-4 and a question I've been wanting to pose on the forum.

Unlike any other RV I've flown, my -4 has a tailwheel that seems to be anxiously awaiting the next opportunity to suddenly freewheel when taxiing in strong, gusty crosswinds and make me look like a fool.

I've seen a suggestion on how to make the transition into freewheeling smoother by increasing the radius slightly on the pin. It occurred to me that a worn pin, and possibly a pin spring that is worn out, might be the problem.

With the exception of tight turns on the ramp or positioning the airplane by hand, a freewheeling tailwheel is my enemy, and to make the transition harder and more positive, I'm wondering if the first step might be to replace the pin and spring and assess the effect on ground handling.

Any comments or suggestions would be appreciated. Thanks.

Tosh
 
Tango Mike said:
With the exception of tight turns on the ramp or positioning the airplane by hand, a freewheeling tailwheel is my enemy

Tosh
Click to expand...

I've been running my RV-7 for 230 hours now, with no tailwheel steering and no pin. It just fully castors (free wheels). It's the way I love it and it WON'T cause a ground loop. Crosswind taxi can be challenging, but no more than a fully castoring nose wheel.
 
Thank you for the inputs.

My original post may have muddied the waters a bit by using the term free-swivel, because my -4 has an old style partially swiveling tailwheel.

In 2007 I replaced the tailwheel assembly on a -6 with one of Vince Frazier's and tried to do the same on the -4, but we couldn't get it done. I sent him the fork dimensions, to which he responded that they matched nothing like he had on hand, that it might be an off-brand tailwheel, but he could make me one with these dimensions. I elected to do that, shipped him the existing fork, and he sent me a custom one. From that point on, we ran into a series of issues with getting the new fork to fit on the spring and line up the holes. The existing spring is lightly tapered, and when we finally stopped trying, he noted that I had "a truly custom set up if he ever saw one."

The objective of that effort had nothing to do with the amount of force required to release the tailwheel, only to obtain the advantages of the SEGA design, so I didn't want to spend any more time and money on it. Rather than open up that box of trouble again, I'd like to first try replacing only the pin and spring and use the existing fork to assess the effect. If I can find them in "the list," that is, always a challenge for a non-builder.

Mark, I appreciate your position on using a tailwheel with no steering and no pin, but I ran into a situation last week that has convinced me to fix this. And it's not an issue of avoiding a ground loop, per se, but maintaining directional control of the airplane at all times.

I landed on Rwy 35 with winds reported as 330-340 at 25 gust to 35. The runway is on the east side of the field, my hangar is on the far west side, so I cleared Rwy 35 onto Rwy 26 and had a long way to taxi in a crosswind that was causing me fits.

An RV-8 had landed ahead of me, and the pilot elected to stay on Rwy 26 all the way to the end. He told me later that he had to add power 3 or 4 times with left rudder to maintain directional control. I was doing that and tapping a little left brake at times to help, but with the low breakout force of the tailwheel on this airplane, it might be that the winds simply exceeded the practical crosswind limits for even taxiing it safely.

In retrospect, I'm not sure why I elected to clear Rwy 26 about halfway down and transition to the parallel taxiway. But that decision effectively reduced the amount of hard surface I had to my right to deal with an excursion for the remainder of the taxi. And at one point, a gust weathervaned the airplane 90 degrees right and into the grass, literally in a few seconds.

I lost control of the airplane, an obviously helpless feeling. No harm done except to my pride, and maybe under those conditions I should have just parked it. But it taught me a lesson to remember, which is that staying on Rwy 26 would have given me far more hard surface to the right to get the airplane back under control. But if the tower hadn't offered that option and I had to use the taxiway, ground traffic permitting, I should have been offset to the left side to give me as much buffer zone of concrete as possible.

As we often say, "I learned about flying from that . . ."
 
The term "breakout force" is probably not quite accurate, breakout "angle" would be more correct. The wheel should release when it reaches a certain angle, and the force required to reach that angle is set by the steering springs, so stronger steering springs should keep it from breaking out so soon.
 
Tosh,

I wish we'd known that you were having problems. It sounds like you may have a spring and custom tailwheel laying around in your parts pile. If you'd like to send them to us, I'll gladly see what can be done to make them right.


All of the commonly found bent, non-full swiveling, RV springs (aka stingers) have a taper to them that must be turned down in a lathe to make these things work. It's not a difficult job, but does require a lathe.

The straight stinger, for the full-swiveling tailwheels are typically 0.635" OD at the end of the spring. No mods are typically required to the spring, although matching the existing holes might need to be done if changing the mounting socket.

If you'd like to send them to us, send them to:
Vince Frazier
Flyboy Accessories
3963 Caborn Road North
Mount Vernon, IN 47620

For those that want to view our tech tips for installing, maintaining, converting your tailwheel, please just drop me an email to [email protected] and I'll send you a copy. No charge.
 
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dbris said:
The term "breakout force" is probably not quite accurate, breakout "angle" would be more correct. The wheel should release when it reaches a certain angle, and the force required to reach that angle is set by the steering springs, so stronger steering springs should keep it from breaking out so soon.
Click to expand...

Thanks for that, but it confuses me.

I understand and agree with the "angle" as being a critical element, but I don't understand how spring tension controls it.

In the tailwheel steering mode, one spring is under more tension than the other when the rudder is displaced, and how soon that tension is applied depends on the slack in the tailwheel chains. I've seen RV's with the chains hanging down with a significant curve, which I've found makes steering a lot less precise than I like. Mine are snugged up, and the springs are barely under tension in a static condition. When I try to put them under more tension by hand, they are tough. I can't imagine that they need to be even stronger.

With the rudder displaced way beyond the point at which the pin retracts and the tailwheel is no longer directly in sync with the rudder, there is no tension in the springs. The rudder and the tailwheel assembly are effectively in the same condition relative to each other as when the rudder is streamlined with the longitudinal axis of the airplane.

That said, I freely admit an amateurish understanding of mechanics and am more than willing to be convinced otherwise.
 
Vince,

You went out of your way to help me solve this problem in 2007 and we agreed to give up.

Part of that decision had to do with the time and expense of trying again after you sent me the custom fork and I had to return it. We had issues with the holes lining up and the fork assembly not fitting well on the spring.

I kept our email correspondence and reviewed it just this morning. You offered to keep trying, and I was reluctant to do that based on anticipating greater expense, time, and effort to solve a problem that at the time wasn't that big a deal.

And I'd like to repeat that my experience working with you was absolutely nothing but positive.

Tosh
 
Consider this picture. In the hangar, the tail is off the ground and the rudder pedals are locked in the neutral position. Push the wheel to the right to try to unlock it (right crosswind) and you'll see that the left spring stretches until it unlocks. If you were to replace the spring with a weaker one, you'll use less pressure with your hands to unlock it, and if you make it stronger it will take more pressure. So it will take less of a crosswind to unlock it with weak springs and more crosswind with stronger springs. I hope that's a little clearer. I've been messing with tailwheels since 69' and I know they can be confusing.

By the way, I agree with you about slack springs, they need to snugged up.
 
dbris said:
Consider this picture. In the hangar, the tail is off the ground and the rudder pedals are locked in the neutral position. Push the wheel to the right to try to unlock it (right crosswind) and you'll see that the left spring stretches until it unlocks. If you were to replace the spring with a weaker one, you'll use less pressure with your hands to unlock it, and if you make it stronger it will take more pressure. So it will take less of a crosswind to unlock it with weak springs and more crosswind with stronger springs. I hope that's a little clearer. I've been messing with tailwheels since 69' and I know they can be confusing.

By the way, I agree with you about slack springs, they need to snugged up.
Click to expand...
Thanks for the clarification, and I'll try that. But let's picture the same scenario with the tailwheel chains disconnected so that the springs are not in play.

As we push the tailwheel to one side it will rotate with little resistance through the angle permitted by the movement of the pin in the groove until it reaches the "stop."

From that point, we have to push harder to overcome the resistance provided by the spring pushing the pin into the groove. With sharper (tighter radius) shoulders on a new pin that isn't worn, and a fresh spring that hasn't lost any of its ability to resist compression, the force required to break the tailwheel loose into free swivel has to be in play as well. The springs have no bearing on that because they aren't connected.

Based on this scenario, I'm having a hard time believing that the condition of the pin and spring don't count for something in determining how "eager" the tailwheel is to free swivel.
 
Tango Mike said:
Based on this scenario, I'm having a hard time believing that the condition of the pin and spring don't count for something in determining how "eager" the tailwheel is to free swivel.
Click to expand...

A worn out locking mechanism could certainly have something to do with it kicking out early, and that should be checked and repaired if necessary.
 
bring 'er up here for the Doctor to have a look

Hey Tosh:

Sounds like you need some new parts. Bring 'er up to 41XS and let me have a look - could be a worn pin, or a worn arm, or both - I have both in stock.

Then again, it could simply be a loose nut in the cockpit! :-0

I can show ya some nifty stuff we are working on here - but ya gotta keep it secret. Email me to sync up: f1boss at gmail.

BTW here is another important tailwheel tip: Loose fasteners on your tailwheel assy to spring attach? Don't fall for the expensive tapered pin nonsense - go to your local tractor supply and get a couple 3/16" roll pins. Pound 'em in and trim 'em off flush. Use a backer weight while you pound 'em in, ya know.

Carry on!
Mark
 
Tosh,

Thanks for clarifying. Sometimes (usually) my memory of events is the first thing that fails me.

Anyway, if we can help with anything, just let us know.
 
Mark,

I'm OK with the roll pins too. Way cheaper than a tapered pins, but......

.... be careful to get good quality pins. I've seen some that were soft and some that are brittle. Ugh. I suggest buying an extra one and beating the stuffing out if it on an anvil to see how it holds up!

Tapered pins are pricey, but if the holes are bad, they are often the best way to save the day. If the holes are egg shaped, a roll pin won't help.

YMMV.
 
Vince:

I just got back from a Macho Grande visit with Mark. We verified that the original tailwheel setup on this airplane is non-standard, especially since the assembly is made of aluminum, and it's obvious that over the years, the steel pin has won in the fight with the aluminum groove. The resistance felt as the pin exits the worn edges of the groove is way too light.

When we worked together before, I sent you only the fork. I'd like to send you the fork and spring so you can build me a custom SEGA assembly that we know will fit. I'll contact you through your site to arrange the details.

Tosh
 
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