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Very squirrelly tail wheel controllability

redhawk

redhawk

Well Known Member
Hello out there,
In the operation of my new -8 I recently purchased (I'm a former -4 owner),
It is noticeably "squirrly" on taxi out and especially on landing after the tail comes down (almost ground looped it the other day - thought my rocket steering link broke or went over center!!). I have checked my toe in and out and have determined its slightly toed in.
I am wondering if my steering link really did lock over center as I felt I had no control for a few seconds when it made a hard right !
Has anyone ever thought of using dual rocket steering links - one for each side?
I would think this might solve the erratic steering problem?
My rocket link rod ends are as far out as they could be to fit the steering arm on my new Aviation Products tailwheel.
Maybe I should go back to the chain set up !?
Thanks for your comments....
 
Are you sure the tailwheel locking pin is engaged?

If not, remove it, clean it and relube it.

Good luck and congrats on the new 8!
 
Replace the link with springs until you get used to the -8 and/or figure out what is going on with your landing gear.
 
I know everybody has their opinion, but if you have any toe in it WILL be squirrly. I have experienced it on a cub and on snowmobiles. The cure is 0 toe or slight toe out. Speaking from experience.
 
One rocket link is plenty enough as long as your toe-in/toe-out is correct and the rocket link is adjusted correctly. I had some issues with the tailwheel pin not locking on the RV-8 but it really wasn't that squirrely. Turned out I needed to carve the locking pin's slot in the control arm a bit deeper per Vince Frazier's instructions and that fixed the missed-locking issue.
 
Mike has it right. I discovered this while designing the landing gear for my Moni Motorglider.

Got most of my design information from Pazmany's book on landing gear design.

Think of it this way; if you have toe-in and you begin to swerve left, centrifugal force places more weight on the right wheel. If this wheel is toed-in, it will cause the aircraft to swerve MORE to the left. On the other hand if the wheel is toed-out slightly, it will cause the aircraft to be pulled straight.

I know there is a lot of controversy on this subject, but when I built my gear originally, I had a small amount of toe-in. The airplane was quite unstable on the ground. After research, I shimmed for a small amount of toe-out and the airplane became a "pussy-cat"!

Also if you look at the standard RV gear attached to the engine mount, the standard set-up is for straight ahead without weight on the gear. Now, because of the gear design, when you apply weight to the gear, the wheels will actually toe-out slightly as the gear spreads.

BTW, before you "A" drivers jump in, a tricycle gear configuration is entirely a different matter.
 
No springs at all

When I had my RV 4, I removed both springs on the tail wheel and it worked perfectly except for one thing. Taxiing in a X wind contributed a lot to brake wear. To save my brakes the springs are back on.

Steve
 
Mel said:
Think of it this way; if you have toe-in and you begin to swerve left, centrifugal force places more weight on the right wheel. If this wheel is toed-in, it will cause the aircraft to swerve MORE to the left. On the other hand if the wheel is toed-out slightly, it will cause the aircraft to be pulled straight.

I know there is a lot of controversy on this subject, but when I built my gear originally, I had a small amount of toe-in. The airplane was quite unstable on the ground. After research, I shimmed for a small amount of toe-out and the airplane became a "pussy-cat"!

Also if you look at the standard RV gear attached to the engine mount, the standard set-up is for straight ahead without weight on the gear. Now, because of the gear design, when you apply weight to the gear, the wheels will actually toe-out slightly as the gear spreads.
Click to expand...

Mel,
I've not measured my toe-out with the gear hanging but it appears to be toed out quite a bit. I've only done taxi testing right now but there appears to be some scuffing on my right tire.
How did you shim out to correct to achieve a toe-out? From the engine mount? I would like to remove some toe-out.
 
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A second rocket link won't help you because you can probably already overpower the locking pin on the tailwheel with one link. A second one would just add weight.
 
Adjusting toe in out

Hi Adam,
After many tries at getting consistent measurement results, Ive found the best way to measure toe in or out is to first roll your plane on to greased metal pans. By this I mean get 2 metal drip pans, pour a 1/4 cup of oil in their centers, then place a smaller square of flat " cookie sheet" on the area of oil. When you roll the plane on to these cookie sheets grab your prop and just pull down a few times and you'll see the wheels slide to their natural resting position. Make sure both tires are inflated identically, then simply take the longest level (straight edge) you can find (3 ft is good), place one end over the top of the axle nut holding it flat against the sidewalls, and let the other end rest on the ground with a piece of tape under it. Make sure the level is held flat against the tire sidewalls, and make a mark on the tape where the level touches the ground. Repeat this forward and aft for each tire, then measure the distance between the rear 2 marks and the front 2 marks and compare the difference !
 
DanLandry said:
Mel,
How did you shim out to correct to achieve a toe-out? From the engine mount? I would like to remove some toe-out.
Click to expand...

The landing gear I shimmed was a flat aluminum gear of my own design.

I see that you have an RV -7. Not much way to shim the round tapered RV gear. Once it is drilled in place, it's pretty much "fixed".
 
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I'd note that since this is the "RV-8" forum, the OP has the simple option of unbolting the axles from the flat gear legs and adding shims as required to get the toe-in just where he wants it.

Mel is, of course, absolutely correct that with the round gear legs, it's set when you drill them to the engine mount!
 
it's all about the BRAKES

At first I too had a heck of a time keeping my 8 going the way I wanted. I blamed it on one thing and another until I finally took the forward baggage floor out, sat in the cockpit and pushed the pedals to see if I was applying brakes.
I had a helper watch my foot action on the pedals and observe the fluid in the brake return lines. Sure enough I was getting brakes with rudder pedal movement despite keeping my toes well away.
I believe the 8's brake setup is unique to the fleet and requires those pedal extensions to mitigate any tendency to apply brakes inadvertently during rudder throws.
Looking back through the archives I found Randy Leverold had warned us of this but I must have missed it. Anyway, pedal extensions were my salvation.

Ralph
Dues paid 2012
 
Redhawk, you should come to Rocky Mountain Metro's Wednesday lunch (11:30 ish at the cafe). Jim Gray is typically there I believe and I imagine he may be helpful. He has a gorgeous RV-8 based there.

If you need an extra pair of hands...just give a shout. Always up for wrenching on someone else's plane. I'll bring my own hammer. :)

Don't worry, the only thing I lack more than experience.... is skill.
 
Steve Barnes said:
When I had my RV 4, I removed both springs on the tail wheel and it worked perfectly except for one thing. Taxiing in a X wind contributed a lot to brake wear. To save my brakes the springs are back on.

Steve
Click to expand...

Amen! I have been operating this way for over 100 hours and my plane is the nicest ground handling taildragger I have ever flown. I removed both springs and the locking pin. I have decided to change the pads every 100 hours (annually) although they were only half worn last time. Taxing in a x-wind can be a slight pain but no more so than a fully castoring nose wheel.
 
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Taxiing?

Remag said:
Taxing in a x-wing can be a slight pain...
Click to expand...

I've never had the opportunity of taxiing in a X-Wing, or a Tie Fighter for that matter, but maybe it depends on whether or not you have the "S" foils locked int he attack position?
 
Are not all tail wheel aircraft a bit squirrly? :)

A friend who recently completed and is flying his RV-7 inadvertently touched the mike switch during landing and was heard to say ....keep it straight, keep it straight, keep it straight.... and straight he kept it. I hope I do as well with the 8, he is doing very well with the 7.

I had a check out in a RV-6 several years ago and had a devil of a time steering it during take off and landing but managed not to wreck the machine and was signed off. Later I learned the tail wheel center lock mechanism was broken on that airplane that day, the tail wheel was free wheeling as could be.
It was an interesting ride to say the least and was part of the reason I decided to forgo a tail wheel airplane at that time. I couldn't figure why anyone would want to put up with that challenge on every flight.
 
two silver bullets

We put two on our RV7 cause we have had them fail three times. The other one of course saves the day. Personally I like the tail wheel springs tighter than the silver bullet provides but the springs get into the rudder henc the other link.
 
Chains or a link are both good. If you use chains make sure you have the 1/2inch droop that is called out for in the manual. Make sure your brakes are not dragging. It sounds like you may be introducing brakes during take off's and landings. The 8 is very smooth and docile and as far as taildraggers go. It has plenty of rudder authority. Make sure you are not pushing on the brakes and it should not be squirrely at all.

bird
 
Toe in Toe out

I checked Toe in by laying a straight 2x4 across
the front of wheels and using a large set square.
Just sight down the tire (or brake disk if tires are out of round)
I checked my RV8 this way and the wheels were 2% toe in.
After I shimmed to slightly toe out the plane was much less
squirly on roll-out.
 
Several years ago, we had a spirited discussion about toe on another forum. That discussion led to the email quoted below.

Here is the email correspondence between me and Ladislao Pazmany:

Dear Mr. Pazmany:

For a conventional gear lightplane with tapered rod spring main gear, should the main wheels have any toe-in or toe-out?
Vince

Dear Vince:

Unfortunately your question doesn't have a straight answer. There is a big controversy on the subject. Please review the attached document in order to understand it better. (the attached document was scanned from my book Landing Gear Design For Light Aircraft )
Regards Paz

toe-in vs toe-out
toe-in.jpg


I don't think that anyone can argue that ideally zero toe is the best. Personally, I shoot for a slight toe-in or zero toe, preferably zero if possible.

I think if you surveyed 10,000 mechanics, you'd likely find a 50/50 split on stories about how toe-in or toe-out was reset to zero and the plane magically became a pussycat.
 
redhawk said:
Hello out there,
In the operation of my new -8 I recently purchased (I'm a former -4 owner),
It is noticeably "squirrly" on taxi out and especially on landing after the tail comes down (almost ground looped it the other day - thought my rocket steering link broke or went over center!!). I have checked my toe in and out and have determined its slightly toed in.
Click to expand...

Your toe-in is the problem, not the steering link. Did you check your toe with the airplane set up with a typical flight condition, ie 3/4 fuel, pilot in the front seat?
 
Adam said:
Mel,
What is the best way to check for toe in toe out?
Click to expand...

I do not have the rocket link, just the springs, and the tail controlability is a bit more squirrely after the tailwheel comes down, but not bad.. when the tail is up its no work at all, when the tail comes down you have to work a tad to keep straight.

I have read that toe in will definitely give your plane some squirreliness if that is a word. How about this, on taxi, if you take your feet off of the pedals how stable is it? I can roll straight all the way down a taxi way without having to move my feet much if at all. It seems if you have a toe in, once the plane starts to turn, the toe in will make it turn harder and harder.

I would maybe set the plane to longerons level, clamp a long straight angle or something to each gear leg on the flat part where the axles bolt in, and measure the distance between the angle pieces in front and behind the gear. If it toes in, buy the axle shims that Van's sells and try to even it up and see what that does for you. Our gear legs turned out pretty straight, probably by luck, and didn't require any shimming.
 
Side question, does the rocket link make the tailwheel noticeably more touchy? Because I really want one..
 
flyenforfun said:
I would maybe set the plane to longerons level,
Click to expand...

Why do that. The airplane never taxi's or is on the ground at that attitude. If you check the toe you check it in the attitude the airplane will spend 99.999~% of the time and thats the three point attitude.
 
rocketbob said:
Why do that. The airplane never taxi's or is on the ground at that attitude. If you check the toe you check it in the attitude the airplane will spend 99.999~% of the time and thats the three point attitude.
Click to expand...

Because I'm pretty sure the Van's instruction manual tells you to check this with the airplane longerons level.

Plus when I touch down in a wheel landing with the tail in the air and the longerons near level, I'd like my wheels to not toe in when im moving fast. If that means I'll be toe out (more stable) when my tailwheel comes down, I am fine with that.

Just basing this off of my airplane that is in this config and has very stable ground handling characteristics.
 
flyenforfun said:
Side question, does the rocket link make the tail wheel noticeably more touchy? Because I really want one..
Click to expand...

I'd have to say yes - when you touch the rudder pedals, you get a response from the tail wheel immediately - not after you get the slack out of the chains (if you had chains). Now personally, I like tight chains, so I like the links...but for many, it is an acquired taste, and many never acquire it. It's a trade between sensitivity and "stability", and you have to pick where you live on that spectrum.

Fortunately, it is not hard to switch back if you don't like it - and you can probably even find someone willing to buy it from you if you go back to chains.
 
Check a few things

I think it is important for you to check a few things that are simple to do before going crazy checking toe in/toe out details.

We at JDair have sold hundreds of our tail links and have learned a few things in the course of interacting with pilots. (I'm sure Vince will echo some of the same things)

1) It is important to regularly service your tail wheel assembly. I do mine every 25 hours during my oil changes. I have a small saw horse and lift the tail during oil changes. I take my tail wheel assembly off, clean, inspect, regrease and reassemble. Pay attention to the locking pin. Check for burrs in both the pin and the notch. File out as necessary. After reassembly, make sure the tailwheel will break free easily before putting the link back on.

This is the most common problem we have discovered. In many cases pilots do not know when the last service was performed. Do this regularly. It is a 20 minute process.

2) While you have it apart, check that the tail wheel assembly is true with the rudder. Adjust the link as needed to get it centered with the rudder. WE have heard some have pre loaded some right tailwheel to counter act torque. Well, when you land, you don't have that torque anymore. So if you've preloaded right, guess where it is going to go?:eek:

3) Yes, the steering link will add considerable sensitivity to steering, (regardless of brand) especially if your are coming from a plane with chains. Once you become accustomed to this you'll be fine. It is easy to get behind when you are not accustomed to this increased control authority. Minimize unnecessary control inputs. Avoid "happy feet."

4) Finally, if all of the above have failed you could analyze the toe in/toe out integrity. I seriously doubt this is the problem. With thousands of RV's flying out there I think this would have reared its ugly head more often. Possible, but unlikely.

Good luck and I hope this helps. Please keep us posted so everyone can glean some knowledge from your experience.
 
Darwin is correct. Keep the stuff back there occasionally maintained and you'll wonder what all the fuss is about since you'll never have any of the problems mentioned.

We have a tailwheel and steering link tech sheet that I will gladly email anyone who asks for it. No charge. It has everything you'll ever want to know about these things and more. :)

Please email me directly at [email protected] if you want a copy.
 
Oh gawd.... here we go again..... I think there are still some misconceptions out there about this topic.

The toe in/toe out question was thrashed mercilessly on the F1 forum a couple years ago.

IMHO, there are a few things that apply to RV, Wittman trailing rod type gear:
1) toe in and toe out are equally squirrelly. Think of it this way: both wheels are crooked regardless of whether they are toed in or out. Imagine if you could swap the wheels, they'd still be crooked!
2) applying the brakes will cause the wheels to TOE OUT. This is the reason that I say you should set the tires to toe in slightly if they can't be made perfectly zero toe.
3) it is impossible to get the tires to be perfect at all attitudes because when the tail comes up, the tires will TOE OUT. Another reason for slight toe in on this type of gear.

RocketBob is correct. Set the toe (if you can set it!) in the 3-point attitude, at typical flying weight, i.e. one pilot, full fuel, etc. Use sandbags if you're doing this during construction.

Most of the above also applies to any other type of landing gear, except #3 may not be valid for RV-8 gear (and others) where the wheels/lower gear legs are effectively (mostly) parallel to each other regardless of attitude.

An easy way to tell if you've got an alignment problem is to roll your airplane across a SMOOTH, SHINY hangar floor. If the tires squeal and squeak continually it is because they are misaligned. Fix the alignment and the squeaking magically disappears.

Also, crooked tires wear out MUCH more quickly. If your tires are shot in less than 100 hours, you might have a problem. Your choice is to fix it OR simply buy tires more often!

YMMV. What I've said above requires some thoughtful reflection so PLEASE don't go cutting off your gear legs without complete understanding of the issues involved. :eek:
 
The -8 is unique among "thousands of RV's flying....".

All other tailwheel RV's have tapered rod legs swept rearward. When loaded, the gear legs spread outward. They also twist torsionally.

A few years back I did some design work to put tapered legs on a custom biplane....truss, sockets, and new legs, which I had Langair machine. Here's a screen grab approximating the load deflection case. Note how much the contact patch moves in relation to the socket axis (long red line). It is inboard of the torsional axis in flight, but moves outboard at 1G and way out at the 3.7G bounce limit. The distance from the contact patch to the torsional axis (blue line) is an arm:

35avsjc.jpg


If the plane of the gear leg is perpendicular to the earth while deflecting as above, there will be no torsional twist. If the leg is swept rearward (like the RV4, 6, 7), the torsional twist will result in toe out. If the leg is swept forward the torsional twist will result in toe in.

A swerve is similar to a drop, with a subtle difference. Consider an RV7 beginning a swerve to the right. Inertia rolls the aircraft to the outside; the left leg is loaded (toe out) while the right leg is unloaded and remains more or less neutral toe. This is an anti-swerve reaction.

I have not examined RV8 leg/tire geometry in detail, but we can safely assume it is similar to taper leg geometry in the way the torsional arm increases with load deflection. The primary difference is sweep; with the tail on the ground the gear sweeps forward. With load the torsional deflection results in toe-in...pro-swerve geometry.

Or so it looks to me :rolleyes:
 
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DanH said:
A swerve is similar to a drop, with a subtle difference. Consider an RV7 beginning a swerve to the right. Inertia rolls the aircraft to the outside; the left leg is loaded (toe out) while the right leg is unloaded and remains more or less neutral toe. This is an anti-swerve reaction.
Click to expand...

Exactly right and why I set my Rocket up with a small amount of toe-out when loaded for a typical flight condition. In that condition the outboard wheel will begin to skip/bounce and so toe out in that scenario helps stabilize the airplane. All one has to do is observe the active runway from the flightline for a couple of hours at OSH and watch all the saved ground loops that happen.
 
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rocketbob said:
Exactly right and why I set my Rocket up with a small amount of toe-out when loaded for a typical flight condition. In that condition the outboard wheel will begin to skip/bounce and so toe out in that scenario helps stabilize the airplane. All one has to do is observe the active runway from the flightline for a couple of hours at OSH and watch all the saved ground loops that happen.
Click to expand...

I've learned alot of stuff while building this plane and I've read alot of opinions. One thing I learned through all of it is it always worked out best for me when I just did what Van's told me to do. And thats what I did and I haven't had a problem.
 
flyenforfun said:
I've learned alot of stuff while building this plane and I've read alot of opinions. One thing I learned through all of it is it always worked out best for me when I just did what Van's told me to do. And thats what I did and I haven't had a problem.
Click to expand...

I've learned over nearly 20 years of being around Van's airplanes that not everything they come up with is optimal and there's always room for improvement. I can name a few things that were face-palm fails in my book of things Van's designed. Like the revisions of the alternate air door on the FAB box.

In this case if Van's used a socket at the lower axle on the tapered-gear airplanes like the Rocket does tire wear can be reduced to zero and the airplane can be made more stable by being able to fine-tune toe.
 
Bob, perhaps I'm reading Dan H's post incorrectly, but according to how I understand it, and in sync with my observations, there aren't any likely scenarios that will cause the RV trailing rod gear to suddenly toe in. OTOH, there are several factors that will cause additional toe-out.

So, explain to me again using small words, why would anyone want to intentionally build toe-out into this system? In other words, if a swerve, brake application, extra weight, or raising the tail all cause additional toe-out, why add any to begin with?
 
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Because in a ground loop or where the gear is misaligned the outside leg is loaded, the loads are applied to that wheel in a direction far off perpendicular to the wheel. If one straightened that wheel towards the vector of the load (toed it out) the force disappears, hence the skipping disappears, hence the airplane will tend to steer itself out of misalignment. You ever see the crosswind gear on a Cessna 195? Thats exactly what it does. Toeing it in in this situation will only make it worse.

If you watch an airplane ground loop (I've seen three up close) and if said ground loop fails one of the gear its always the outside gear that fails. Forget the gear on the inside of the turn because there's no load on it for it to do anything.

About three years ago I witnessed a known poor taildragger pilot ground loop and total a L-19 on a paved runway, in fact we were watching the landing because we knew how this guy lack some fundamental skills (he was taught to let go of the controls if something went wrong, no kidding...and he's damaged three airplanes). The gear failed exactly as I described.

In any situation where the airplane is misaligned on the runway, the wheel with the most load on it is going to be stabilized with toe-out.
 
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Bob, you're hardly a bad pilot and unlikely to ground loop anything. I'm still incredulous that you're building in an error when you know that the only loads that YOUR plane is likely to see are the ones mentioned above that cause the gear to toe out. Regardless, I'm sure you're plane will perform well... just eat a few more tires during its life. Get it done!
 
That was an extreme case I used as an example. I used 1/2 deg. toe-out when I set my gear up. I think in the real world this will work out quite well.
 
data point

For the sake of one data point. Our -8 has the grove, non airfoil gear, 0.91 degrees of toe-in and about 2 degrees of positive camber.

It tracks very straight and is very stable on roll out. A complete sweetheart.
 
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