Ground Looping Wots Wot tail dragger

[John Muir]

Doesn't matter if it's full size or model size, aeroplanes is aeroplanes and physics is physics. We can't even blame aerodynamic differences because we're talking about effects on the ground. So I'll stick with what makes most logical sense to me and which seems to support my own observations.

 

But hey, each to their own   

 

John.

[Ultymate]

I always find ground handling more difficult on tarmac, sometimes when slightly crosswind even with the model (taildragger) idling and stationary on the runway they'll weathercock into the breeze by dragging the taiwheel sideways. Another thing I've found that if you run Toe out on the main wheels forward motion spreads the legs and can reduce prop clearance never good if you're running marginal prop clearance

[John Muir]

Oops, I messed up that link I put in a few posts back, and forgot to put in a description. So now the text is disappearing under the adverts up the right hand side. Sorry 'bout that. Won't happen again.

[Former Member]

[This posting has been removed]

[Former Member]

[This posting has been removed]

[John Muir]

Thanks for fixing that Phil.

 

[Biggles' Elder Brother - Moderator]

Hi All,

 

OK - I'll declare an interest up front - I'm a professional engineer and this problem falls inside my sphere! Its a problem in what we call "dynamics" and that's part of my specialist field.

 

First point, I think you are discussing toe-in and toe-out from the wrong perspective if you will allow me to say so. The point of building in toe in is not to deal with torque reactions or any other relatively large scale effects which force deviations from straight running. Once you have a swing established no amount of toe in (or toe out) is going to help you. So I'm afraid all this talk of inertia and changing weight distributions is just a complete red herring!

 

So, what is toe-in/out for? The basic fact is that in a real world situation you cannot possibly build the wheels exactly in line. Assembly and manufacturing errors mean that the wheels will always either toe in or toe out. The "smack in line" situation is an infinitesially small point - you cannot build any real system to that accuracy. So given you have to have toe-in or toe-out which is best?

 

The answer is toe-in. The reason is this. Suppose our plane is running dead straight and everything is fine. Now it deviates - just a very tiny amount - to starboard. Now if this swing is small, then the plane nose is rotating slightly starboard but the at this point the direction of travel is still basically straight ahead - the plane has not started to turn yet. If the wheels are set to toe-in then the starboard wheel is now more accurately aligned with the direction of motion - this means it is "side-slipping" less and so what we call its rolling resistance (sort of like its drag if you like) decreases. But on the port wheel, it is now even further out of line with the direction of travel and so its rolling resistance (its drag) increases. So now we have more drag on the port wheel than on the starboard wheel. This drag difference sets up a couple (or moment) which acts to pull the plane back straight again - in line with its direction of motion. The process is inherently stable because once the plane is "straight ahead" the "out of lineness" of the two wheels is the same (they are both toed-in) and so the drag forces balance and the correction couple is zero.

 

Now, the real point, I almost want to write this in caps - but that would be rude - so imagine it in caps cos its really important. This only works for very small deviations from straight ahead. But that's not a problem because toe-in is only intended to deal with this sort of small random wandering - not significant big rotations caused by externally applied large forces. Its like when you drive your car down the motorway with the steering wheel set straight - its the toe-in which makes the car track straight despite small undulations in the road. But once the deviation is bigger - like when you turn the steering wheel - all this goes out the window - if it didn't you wouldn't be able to change direction easily at speed!

 

So concluding, toe in is good, because it helps your plane run straight in the absence of any other major forces. But it wont stop torque swing or put your plane back on line when its hit a big tussock of grass. And its not what its designed to do - that's what a rudder is for!

 

BEB

[Tim Mackey]

Excellent post BEB - clever chaps you lot down on the Wirral

Say hi to Graham Roscoe next time he's down the patch - nice chap.... and his dad.

[David Turner]

BEB's post highlights  the traditional explanation of the mechanism by which toe-in is supposed to enhance directional stablility.  It was a view to which I subscribed until a couple of years' ago, when I read PDR's explanation." Aha" I thought, "here is this aeronautical engineer who has been good enough to pass along his wisdom."  Naturally, I accepted his view...that toe-out was the thing to have. 

 

However, both views are proposed by professional engineers...so people like me are still in limbo.

 

And each explanation sounds very plausible.

 

I wonder whether there has ever been any objective evaluation into this problem? You know, real-world trials as opposed to theorising; however plausible that latter may sound.

[kc]

Would a non steerable tailwheel or tailskid help the problem?

[Biggles' Elder Brother - Moderator]

Hi,

 

your point is a good one David. I have access to on-line engineering research journals - I'll take a quick look to see if there ever has been an experimental study. One problem is of course that on-line journals only tend to go back to about the early 1970's - but I can look

 

In a sense its all pretty academic - if you think about it any toe-in we put into models is going to be pretty "rough and ready" and compared to the bumpness of say grass relative to the size of a model's wheels pretty ineffectual! Scaling up the bumps on our strip would mean its like a car going over ground with 8" bumps in it - the amount of toe-in or toe-out then becomes meaningless! But its an interesting debating point as someone said earlier.

 

BEB

[Martin Harris - Moderator]

I think BEB's post supports my previous amateurish attempt to explain stability from toe-in but the more I read and think about this subject the more difficulty I have in weighing the evidence!  I think it's time to sit back, watch and learn but I do think BEB has hit the nail on the head with the difference between stability and dealing with developing swings.

[David Turner]

True enough. A tailwheel setup is inherently unstable and cannot be made stable  by "tweaks", such as toe-in or toe-out.  It may, at best, be made a little less unstable.

 

Whereas a tricycle setup is inherently stable and cannot be made unstable by  "tweaks", such as toe-in or toe-out.. It may, at worst, be made a little less stable.

 

How's that sound?

Edited By David Turner on 02/11/2009 18:19:08

[Ultymate]

As it says on another thread "practice makes perfect" fly it more often

[Ernie]

Hi Guys, What's wrong with my axle with the wheels fixed to it?

ernie 

[Biggles' Elder Brother - Moderator]

Hi Ernie,

 

the problem with the axle solution is that then the plane wouldn't want to turn at all! Even when you wanted it to when taxying.

 

When the plane turns, the inside wheel follows a smaller circle than the outside one - so it travels a shorter distance. If the two wheels were fixed so they had to turn the same amount it could not do that. So one or both of the wheel would be forced to slip along the ground resulting in lots of friction and some very jerky turns! This is why cars have differentials - to allow the inside wheel to turn less than the outside wheel when going round corners.

 

I've had a look at the literature. All I can find are papers on how to calculate the amount of toe-in needed on different vehicles with different suspension geometries and different wheel sizes. No one even discusses toe-out in the technical literature except one guy...and that was a discussion on how to avoid a wheel going into unintentional toe-out when undergoing large suspension displacements. If I come across anything I'll let you all know.

 

BEB

[Biggles' Elder Brother - Moderator]

OK Guys - an update.

 

If you go to;

 

http://www.aviataircraft.com/pix/eproductletters/e364.pdf

 

You can read all about the proceedure for an undercarriage rework on a Christian Eagle II. Its an official technical proceedure note from Christian detailing a rework which should be carried out because of evidence of some cracking on longerons following heavy landings. It actually makes fairily interesting reading.

 

The real point is, it includes the proceedure for realingning the main undercarriage wheels after the work - and describes them being st up (with full fuel load) at 1/16" toe-in.

 

If its good enough for them - it'll do me!

 

BEB

[Martin Harris - Moderator]

I think I agree in principle but can we really set 1/64" of toe-in on a 1/4 scale Eagle with plastic hubs running on piano wire ?

 

There's got to be something more to the subject - my Airsail Chippie behaved perfectly with no deliberate toe-in (or out)  with a fully castering tailwheel before I modified it to allow me to taxi crosswind in anything more than the lightest breeze... 

Edited By Martin Harris on 02/11/2009 20:53:22

[Biggles' Elder Brother - Moderator]

Hi Martin,

 

unless its gross I don't think toe-in or toe-out are relavant to models at all! Its simply too small an effect. It might matter if your building really big jets taking off from billard table smooth tarmac - but for us mere mortals the effects of the bumps in the grass, side winds and the slop in the plastic wheel on the piano wire are much greater!

 

As I say, its an interesting debating point - but I don't belive it makes any real difference on a typical model. We'd be better off practicing using our left thumb better

 

BEB

[Martin Harris - Moderator]

[Ernie]

Hi BEB 

This is great stuff, it never ceases to amaze me how much onfo the forum can throw up

ernie

[Biggles' Elder Brother - Moderator]

aRGGhhhh!

 

Latest info...reading on in the Christien service document it appears that they fill the plane with fuel, set 1/16" toe-in, then....with a pilot and passenger the suspension geometry is such that this changes to 1/16" toe-out!

 

So, fully laden on take-off, the plane would seem to be running slightly toe-out. But what the state is on landing is uncertain. It will be close to neutral I guess - with either a tiny amount of toe-in or toe-out depending on the landing weight!

 

So, David, it seems that everyone is right! A great ending!

 

BEB

 

PS I still believe it makes no difference for 99.9% of models though!

[Mick Cayton]

I have an Elf bi-plane vintage model, with the undercarriage well forward as was the style of the day. This model would not track straight - I reckon I had a 10% chance of a take-off before bottleing and shutting down when it looked like rolling across the patch. This was so bad I considered it dangerous with others in the vicinity and stopped flying the damned thing in company.

 

That was until someone mentioned a gyro. I bought a cheapy gyro (~£20) from Ebay and stuck it on the rudder servo. Job done! The model stracks straight every time; I just have to remember to turn it off when airborne as the rudder is the primary control.

 

This approach might be treating the symptoms and not the cause with the WotsWot, but it works very well on my bi-plane and if it get the model safely in the air... Just don't forget to turn it off as the rudder behaves rather oddly sometimes if you use the control with the gyro on.

 

Cheers,

 

Mick.

[Clifton Park]

The gyro sounds a good idea if all else fails.

 

toolman59

[Clifton Park]

Followiing up on Mick's suggestion to use a gyro, I have looked around for a suitable A/C gyro but most seem to be Heli types. I have no experience with model gyros, so I really am a bit at  a loss as to what type of gyro would be suitable for controlling the rudder during take off and then being able to switch it off once airborne.  The gyros I have seen are mostly pizo with heading lock or rate settings, the cheaper ones seem to be in series between the RX and servo with an extra lead for a gain channel.  Any suggestions would be welcome, as it would be an interesting exercise providing it can be done at reasonable cost.

 

I am using a Futaba FF9 C Tx with an eight channel DC Rx

 

  

toolman59