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Tailwheel Question

islandmonkey

islandmonkey

Registered User
I am English, I live in Germany
I have a 1965 Super Cub 150 that was imported into Germany in 1966. I bought the Plane last year from the chief editor of one of Germany's most popular flying magazines. The plane was originally used as a glider tug and has a Tost towing attachment mounted on the rear. The Tost mounting is incorporated with the tailwheel mounting. The tailwheel leaf springs are inch and a quarter (30mm) and there are four leafs. The tail wheel itself is a Scott 3200 with an adapter to accommodate the narrower leaf spring. The leaf spring mounting is fine and firm and there is no sideways movement of the leafs. The Scott tailwheel however does swivel slightly on its attachment to the leaf and there is a noticeable vertical sideways movement of the tailwheel in the Scott swivel housing. Looking at the Scott tailwheel it looks as if it could do with a good degrease.

My question are:

Do the above factors contribute to the squirrelly nature of this plane when landing on pavement (tarmac)? Landings are fine on grass.

Also what type of leaf spring is fitted? Is it off a Pawnee?

I have just ordered a ABI-3224A Standard Tailwheel Assembly (Scott 3200A Replacement) from Alaskan Bushwheel via Spruce. The intention is to fit the new Alaskan tailwheel and to refurbish the old Scott 3200.

The price of new Scott 3200 tailwheels is as we say in Britain taking the xxxx!
 
Here is a good article my Dad wrote on tailwheel shimmy.

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

You can replace the spacer on the old tailwheel if that is where your wear is. I have shimmed between the spacer and the spring or the tailwheel and the spacer in a pinch. 1 1/4" is the stock Super Cub tailwheel spring from Piper.

I remove the tailwheel annually, disassemble, clean, inspect and reassemble. Did away with surprise tailwheel problems.
 
Islandmonkey

A shimmying tailwheel and a squirrelly airplane on pavement are two different things.

The article Steve refers to is THE fix for tailwheel shimmy.

Squirrelly behaviour on pavement probably has more to do with mainwheel toe-in or toe-out. If you do a search on here there have been several good threads about this and how to correct it.

John Scott
 
Hi Steve, As ever, thanks for the quick reply. I have already read your dads excellent article on tailwheel shimmy which I have experienced before on other Super Cubs when landing on tarmac or pavement but not on this one. Thanks also for the tips on shimming and cleaning the tailwheel. That will happen this weekend.

Hi John, Thanks for pointing that out. However I have measured the mains for possible toe out and I have discovered that I do have around about 3 millimetres of toe in. See this thread:

http://www.supercub.org/phpbb2/viewtopic.php?p=149066&highlight=#149066


Tony
 
just to check with your alignment issues you have to really measure each wheel from the centerline of the airplane for a toe in or out condition for each wheel separatly i havent a clue how much 3 millimeters of toe in is when you convert it to degrees so what you have to do is project the centerline of the airplane and each wheel forward to a known distance and figure out how many degrees you have i dont think it gives alot of meaningful information to just measure the tires [they may be toed in compared to each other and not aligned to the airplane] but i would think if you had three mm differance from the front of the tire to the back of the tire you would have several degrees of toe in
 
We find that the problem starts at around a half inch or more toe in, measured at the tire ridges. That's around 12 mm?

Never really thought about a cub with perfect toe in, but not aligned with the fuselage. I would imagine it could be landed without too much problem - the toe-in tends to make the shock struts walk a bit, which may be why the toe-ins get squirrelly.

I completely agree that squirrelly has very little to do with tailwheels - but I had one break on a Stinson, and the darn thing just left the runway. We welded it back together and continued on with it in the full swivel mode.
 
Here's what I've been told by one IA about toe-in / toe-out, that makes sense to me.

Consider a plane with toe-in, and assume the plane is traveling in a straight line, with equal weight on each tire. Assume that each tire toes in an equal amount as measured from the airplane's center line. Because the tires are not parallel, each tire is skidding sideways some. Because there is equal weight and the the geometry is symmetrical each tire skids equally.

Now consider the same aircraft, in a left turn, and with the same toe-in. As it turns, it tends to lean to the outside of the turn, i.e. to the right. As it leans to the right the right-hand tire will now bear more of the plane's weight than the left-hand tire. Now, because the right-hand tire is bearing more weight it will skid sideways less than the left-hand tire. Because it skids less, it's toe-in acts with more authority on the plane, tending to steer it to the left, tightening the turn. Toe-in therefore results in decreased directional stability, tending to increase the taildragger's inherent instability in a turn.

Now consider the same aircraft in the same left turn, but now with the tires toed out. Again, the right-hand tire is skidding less and has more toe-out authority than the left-hand tire, tending to steer the plane to the right. Toe-out therefore results in increased directional stability, in other words, tending to reduce the taildragger's inherent instability in a turn.

Bottom line, toe-out tends to increase directional stability, while toe-in tends to reduce directional stability.
 
tailwheel question

We find that the problem starts at around a half inch or more toe in, measured at the tire ridges. That's around 12 mm?


the problem with measuring at the tires for me is i dont know if you have a perfect 12 mm of toe in or 6 on each wheel or if you have 3o toe out on one and 18 toe in on the other and unfortunatly if you didnt measure from the centerline of the fusalage neither do you!!

Now consider the same aircraft in the same left turn, but now with the tires toed out. Again, the right-hand tire is skidding less and has more toe-out authority than the left-hand tire, tending to steer the plane to the right. Toe-out therefore results in increased directional stability, in other words, tending to reduce the taildragger's inherent instability in a turn.

Bottom line, toe-out tends to increase directional stability, while toe-in tends to reduce directional stability.

ive heard that one before i do not believe it
i did car and truck front end alignments in vocational school and with very few exceptions all vehicles from cars trucks fourwheelers snowmobiles have toe in and its not because the toe in or out is to give you stability its because you are setting it at a specified spec at a static load when you are actually using it whether is taking off or landing or diving down the road in your car there is a certain load and its acting to pull back on the tire and the desired effect is when its not static you have zero toe in or out and anything while in load thats not zero is going to be somewhat less stable the stabilzing specification to wheels skiis etc is the angle called castor and thats what makes things want to go straight and is designed into things [it used to be adjustable on cars probably isnt anymore ]and the reason toe out is bad is because when the particular vehicle is being ued or under load the toe out gets more or farther away from zero
_________________
 
tail wheel question

ok i did some figuring and [took these measurements off my pacer so might not be exact for a SC] a set of 8.50s is 22 inches diameter the stance is approx 82 inches if you measure say the front of the tire 1/2 inches toe in from the other tire meaning what? the back is 82-1/4 and the front is 81-3/4 ? you have a total toe in of 2.6 degrees or if both sides are equal you have 1.3 degrees per side toe in which is a bunch out of spec
 
Jay, I've often wondered why toe-in could be better on a car, if toe-out is better on a tailwheel airplane, and never came up with a real satisfactory understanding. So reading your post got me thinking about it again.

I've thought that in a car, a little bit of toe, either way, would take up any 'slop' in the steering linkage. But why in instead of out? It seems that part of the answer would be the caster and camber, which are designed to be compatible with the specified amount of toe-in. With a different amount of toe, say toe-out, it would seem that the the tire would be tipped during straight ahead travel, and cause wear one side of the tread.

Another factor I just read on this site http://www.ozebiz.com.au/racetech/theory/align.html is that as a tire encounters a bump is is pushed back against the steering linkage, causing an incremental amount of toe-out. If there is already a bit of toe-in then it's explained that no net toe-out results. However that if the wheels are set with toe out that excessive toe out and steering instability result. The author of the above link says that toe-in tends to cause under-steer, while toe-out tends to cause over-steer, and for that reason street cars are typically set with toe-in, while race cars are set with toe-out.

So again, why could it be different for an airplane? I'm thinking that because the airplane's wheels are rigidly aligned with the airframe, while the car's wheels are relatively loose, and also deliberately angled as we steer.

Check out the link and let us know what you think.
 
When checking MLG wheel alignment due to squirrelliness, it's not a bad idea to measure forward to a point on each MLG from a point aft (the t/w attach bolt isn't a bad point to use), triangulating to be sure that one MLG wheel isn't ahead of it's mate. Probably a major deal to fix if it was, but that could otherwise explain squirrelliness if the mains are good re: toe-in/out (castor).

Rooster
 
Great thread. But my question is, assuming we can figure out what is correct, how do you adjust it?
 
tailwheel question

ok i read it and i think we agree toe in increases stability while toe out decreases it?
on cars all of these angles are or at least used to be adjustable on an airplane they are not the camber its built into the gear it kinda averages out depending on the load and the bounce hehehe and the bungees the castor is built into the gear i know some have modified their gear but i dont think most people want to do it its adjustable with some cutting and welding but the toe [also built into the gear] while not really adjustable it is the eastiest to bend or even get welded on wrong so in order to adjust your toe you have to bend the gear and you cant just shove a pipe in there and bend it [ i dont think ] without a chance of bending the longerons
ive heard many times the proper toe in is about 1/2 degree or less just as long as you have no toe out
 
Seems like I remember Clyde Smith telling me Piper did it with a pipe. I have done it and it worked. A little discretion is in order.
 
I am with Steve here. There should be no toe in or toe out on a cub. You can get as sophisticated as you'd like when you measure this stuff, but we have found that those aircraft that measure ok on the tire ridges are stable, and those that measure more than a half inch off are not. The Cub still does OK, but do not try to do wheel landings on a badly toed in Stinson.

I have seen the ultra precise angle iron jig process - once the owner figured it all out to within a tenth of a degree or so, he heated the offending leg up with a rosebud, and bent it so that it was in line with the other. Then it flew just as good as the ones we measured using only the tire ridges. Be advised, these Cub fuselages are all over the map - when you finally get the lasers out, it will be time to order a new fuselage.
 
Hi Guys. Thanks very much for all the replies. This weekend I jacked (lifted by hand) the rear end on to a couple of pallets and a beer crate with some padding on top to inspect the tailwheel. As I said in my original post, I have a Tost banner/glider tow hook mounted which has the leaf springs running through the Tost mounting bracket. I found that with a good heave and a sideways pull on the leaf springs that these could be moved about about three or four inches measured at the tailwheel mount. QED, I now know why its so squirrelly on tarmac or pavement. It seems that a previous owner or maintenance organisation has missed off the saddle that clamps the springs to the tailpost. The Tost housing is such that the saddle is not visible. I will keep you all posted as I am calling Tost today to see what they can do.


Tony
 
Steve:

I just had my Super Cub annualled and took your Dad's article to the FBO and told them to follow it and correct the tailwheel shimmy.

For the first time ever, the tailwheel stays right where it is supposed to. A great fix.

Thanks.

375HandH
 
My compliments to Mr. Pierce, too, great advice.

And ummmm, Island Monkey, how'd your Tost hitch work out?

Thanks. cubscout
 
One "small" addition to all of the above to check while your at it. The bracket will make a BIG difference and most likely solve the problem. But, many times lack of "arch" in the tail wheel springs can make for lots of "footwork". Since you have checked everything else, see if your tail wheel bolt is parallel to the ground, or at least close. Insufficient arch will tilt the bottom of the bolt back, making your Scott not pivot, turn or hold direction well. We have found that many times just basic ground handling improve also. (This may have been covered in Mr Pierce's father's post, I don't remember. If so, I apologize.) Larry C.
 
tws.jpg


Yes, that is the key.
 
And ummmm, Island Monkey, how'd your Tost hitch work out?
Click to expand...

Hi cubscout,

The original tow hook bracket fitted on my Cub was not a Tost product. I forked out a lot of money to Tost to buy an STC'd Tost original bracket, fitted that along with an Alaskan Bushwheel (Scott) replacement. I also fitted a new rudder to tail wheel control horn and new control springs. Result is that the tail wheel and tail wheel leaf spring has now no movement what so ever inside the new Tost bracket. In addition the old tail wheel was breaking out with much less side force. Just a gentle push and it would break out. The new Alaskan Bushwheel needs the back end to be lifted slightly first before it will break out. The plane is now much more controllable in slow taxi and at fast taxi as at the later stage of the landing run.

I next have to purchase a new Tost tow release as I did not know the history of the original and to be honest it looked a bit manky. Tost Tow releases are limited to 2000 operations in Germany. Tost are a good company to deal with. I work near Munich so I paid Tost a visit at their HQ in downtown Munich. They were very helpful but their products are very expensive. I will post a photo of the whole installation at the weekend.

A question for Steve Pierce, could you take a look at this thread please:

http://www.supercub.org/phpbb2/viewtopic.php?t=12891

I need some advice on Southco fastener sizes for Super Cub cowlings. Thanks in advance.

Tony
 
Appears to be answered..

Must add, that while doing all sorts of Tailwheel endorsements, I fly various planes all during the year. I often see loose spring attach points or sagging springs which cause poor geometry. These usually end up causing a wild ride all over the place once the tail-wheel has weight placed upon it.

If you add a poor main gear alignment, tundra tires, and an under-inflated tail tire, all sorts of weird things happen once you land on dry pavement.
 
After maintaining airplanes for the past 18 years I think the tail wheel is the most abused and neglected part on an airplane. Tail wheel castings loose on the spring, wrong spring, bent or bowed springs, spring loose to fuselage, under inflated tires and just worn out tail wheel assemblies, wrong hardware installed and the list goes on.
 
Plus there is that one part in the 3200s that you can install upside-down and it wont work so swell....
 
A new question for this old thread - - -

My 3200 suddenly will not steer to the right. Steering to the left is fine. Springs and chains are fine. Before I take it apart, what am I likely to find / parts needed?
 
I'm familiar with that, thanks. I'm hoping for someone with experience to say "oh yes, for that symptom you will need such and so part(s)", before I take it apart.
 
There is a spring in there that looks a bit like a trapezoid without a base. One ear of that could be broken, or the little notch the ends fit in might be rounded. Easy to replace, except there is a pin in there that gets pressed in/out.
 
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