Wheel Alignment - Toe-in or Toe-out?

[Vev]

All,

 

I'm interested in learning about main wheel alignment on both tail draggers and nose wheel aircraft.

 

I am working on two aircraft and I'm really not happy with how they are tracking and want to improve things.

 

Is there a technical convention re set up for toe-in or toe-out and how many degrees or anything else one should consider?

 

Keen to hear what others know or if I can be directed to any technical papers on the subject suitable for this novice? 

 

Cheers

 

Vev

 

 

[djpacro]

Some general comments from https://www.pprune.org/archive/index.php/t-221613.html here:

 

"As for toe-in/out; a small amount of toe-in will cause the drag to be reduced slightly on the inside wheel if a swing starts, and the yaw moment that this creates is stabilising i.e. it wants to swing you back straight again. For toe-out the converse is true, and any deviation from straight ahead is exaggerated. The elasticity of spring-bar type undercarriages means that drag on the wheel wants to turn the wheels outwards towards a toe-out configuration ..."

 

A Pitts can be decidedly ornery with too much of either, especially with the bungee gear - you can see the effect just pushing it backwards and forwards on the ground..

 

The 8KCAB Service Manual specified nil toe-in/out and rectified by shims (which can also be used to fix the camber). You can buy shims at https://www.aircraftspruce.com/catalog/lgpages/tapershim.php with this advice: "For wheel alignment. Ideal setting is zero toe-in and zero camber at normal operating weight. shims may be rotated to any one of four positions to obtain desired result."

 

 

[facthunter]

I think a leaf spring set up is very different from an oleo with scissor s links  on the mains.  When things are going astray there's a lot of potential weight transfer so unless  a wheel is loaded (carrying weight at the time) it's not doing much, and the other wheel would do more. The leaf spring goes wider track when compressed as do many pivoting bungee types. Toe in would inhibit this to some extent but the effect would be very assymetric in the tug of war with markedly different forces acting through each leg if say you have bounced in a x wind situation. and the legs are loaded favouring  one side. (all the more reason to keep the into wind wing DOWN always. Vev  I'd go for no toe In or out , if I had to decide NOW and at least as a start point, but I may have used a bit of toe out on the Citabria for the reason  that I tried to explain above. Investigate further ol mate as I must have had a reference for that.   EAA USA  would have something on it. Nev

 

 

[Vev]

G'day Nev/David,

 

Thanks for the input as always.

 

I'm dealing with 2 ac, one a Jab I flew for a mate and was concerned with the poor ground handling...  compared to the 600 + hours I have in Jabs this is the worst I have experienced and wanted to help him sort it out. I'm thinking a starting point for this one might be to set it up with a small amount of camber and no toe-in and suck it and see? Let me know if this doesn't sound like a good starting point?

 

The other ac is an antique tail dragger with oleos which has an independent system, that is, no connection links via scissors to each side. There seems to be a significant amount of toe-in (3 deg) and is showing signs of scrubbing from taxiing, I use the grass for landings which I guess is masking any problems. This one is causing more concern, yet the guys in the US have suggested anywhere between 2-4 deg toe-in which seems like a lot ... they are saying I am within spec, albeit it doesn't feel right to me. Any thoughts?

 

Cheers

 

Vev

 

 

[facthunter]

 How's it steering after touching down.  and rolling.? You are talking about the Curtis, I presume. It's tyres are treadles aren't they?  Nev

 

 

[Vev]

Nev,

 

Yes it's the Curtiss .. it's steering fine on touch down and roll out too. Yes the tyres are slick.

 

 

[billwoodmason]

Re the Jabiru - it is covered in the construction manual (available on the Jabiru web site), they recommend a slight amout of toe in as this will tend to straighten at max gross weight. A slight amount of toe in is better than toe out as has been already been pointed out. This is achieved by rotating a bevelled washer on the axle attach point. Cheers.

 

 

[Geoff_H]

I believe that the toe in/out depends upon whether the craft is a tricycle or tail wheel.  In a tail wheel the C of g is Infront of the wheels, this is critically stable and can lead to instability very easily.  I understand that on a tail dragger toe out is better as it acts quicker and pulls the craft back to a more straight line during landing.  Landing is where stability is critical, taxying can be less important and whilst annoying I think that most manufacturers look to landing characteristics.  Tricycle craft have the C of g in front of the wheels, somewhat more stable arrangement for landing.  It is my understanding that manufacturers will arrange toe in/out on tricycle craft so that from touchdown to relatively slow motion that the wheels are close to zero toe in/out.  The toe in/out at this point of relatively low loading depends on the way the undercarriage moves with camber and caster and forward alignment during the process of undercarriage loading.  I think that as you can only adjust the undercarriage alignment during stationary but light to medium loading is critical then it is trial and error, unless you have a finite element program and lots of time to calculate a variety of options, and of course springs vary so much that you would have to test the spring rate.

 

I have no idea what your friends problem is caused by, I suspect spring geometry or an undercarriage wear problem or change from original, but if one craft behaves differently to another of the same make and model I would carefully examine the whole system including location to the frame of the craft.

 

 

[WayneL]

I understand that on a tail dragger toe out is better as it acts quicker and pulls the craft back to a more straight line during landing.

Surely you meant to say Toe-IN!

 

 

[Old Koreelah]

My little Jodel taildragger had slight toe-in and was all over the shop until I reversed the u.c. legs. 

 

Now, with slight toe-out, there are no more landing dramas.

 

 

[Geoff_H]

No toe out will help taidraggers.  Race cars use it as it responds quicker.  Also consider that the C of g is in front of the wheels. A drag to the off side will pull the craft the other way correcting the off course deviation.  Toe out on tricycle craft will make the off track worse.  It is all in where the C of G sits.  All forces produce moments about the C of G.

 

 

[facthunter]

C of G is BEHIND the wheels on a Taildragger. That's why it's directionally unstable. The further forward the mainwheels, the moreso.. A sideload due to a turn initiating (acting through the C of G )increases the turn couple. I'm inclined to agree that (sometimes) toe out is helpful at the point of touchdown where the U'C might be considered to "pivot" at the fuselage and the track increases.. and weight transfer happens, unevenly. Nev

 

 

[Kenlsa]

Vev, I have set my SP 500 Jab with no toe in or out and zero camber at 310 kg, (no pax and half fuel) so at normal flying (solo and about half fuel, tools etc @ 400kg) I have perfectly worn tyres. Note that this is with the old straight leg U/C. Ken

 

 

[kaz3g]

I believe that the toe in/out depends upon whether the craft is a tricycle or tail wheel.  In a tail wheel the C of g is Infront of the wheels, this is critically stable and can lead to instability very easily.  

I think you meant CoG is behind the wheels...kaz

 

 

[poteroo]

My experience with taildragger Cessnas, (152TT,170,180,185), has been that about 2 deg toe-in is needed because when the weight comes onto the main wheels, there is a force applied to the tyres which effectively 'pushes' them aft. This then straightens the toe-in and the directional control is good. But, the wheels need to be set so that it doesn't allow movement back to a toe-out setting, or directional control becomes difficult. This makes for some interesting excursions when landing on one wheel in crosswinds. If the aircraft is pushed backwards on the ground, you'll see that the wheels will toe-in if setup correctly.

 

The C152 Texas Taildragger was one real s-o-b for directional control. Eventually, we had a look inside the gear boxes and found it had been 'adjusted' with wooden blocks - which didn't hold the toe-in for long. On the ground, it was obviously toed-out, and even when pushed backwards, it never reached even neutral toe-in. Wheel landings were a real adventure in that aircraft! It was a shocker for 3 pointers too - the rudder was sweptback, and of limited effectiveness, and because the gear legs didn't include a vertical section,(as in the original Bolen conversion), the tailwheel usually hit before the aircraft fully stalled onto 3 wheels.

 

My trusty C170 suffered from sagging gear legs, (bowed inwards), so I had the legs re-tempered, giving them the classic C180 outwards bowing result.  The I fitted 8.00 tyres. It was just slightly toed in, and was the sweetest handling tailwheel Cessna I've ever flown. The height of the door sill was thus lifted to exactly the same as a C180, thus countering the bad effect of fitting a larger Scott tailwheel in place of the ancient small, solid one. This allowed the C170 to do 3-point landings without hitting the t/w 1st.

 

happy days, and all the best for New Year.

 

 

 

[djpacro]

Cleaning out my office and found this article on the subject 

 

 

 

 

 

[Vev]

Thanks Dave ... very useful.

 

Cheers

 

Vev