toe-in or toe-out with gear deflection under load???

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lots of smart people on here....please give some feedback? Thank you.
I have a design underway for a very light tractor, tail-dragger biplane that is currently configured with a J3 style shock absorbing gear. The issue is that the gear is more like a tri-pacer in that the main struts are virtually normal to the lower longeron in the side view and the secondary struts are angled back from the outboard lower corners of the firewall to the axles.
On the pipers, both upper pivots are in a line parallel to the centerline of the aircraft in both side and top view. In the current configuration I am playing with, the virtual hinge axis is slanted about 3 degrees in the side view and a similar amount in the top view. I am concerned that under loading/deflection the wheels will toe in. Is toe in under load beneficial or disastrous?
If it is disastrous I can penalize aesthetics and make the firewall lower and wider and also bring the main upper hinge point inboard under the longeron rather than in the load line to the center of the longeron.
all designs are a boat full of compromises but aesthetics matter some to me as does the added weight resulting from the increase in size of the forward fuse...
Thanks!
 

Dana

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It's best if the wheels stay straight. Consider: the aircraft touches down banked slightly left, causing the left gear to deflect. If it toes in, it wants to drive the wheels to the right, but the aircraft CG is above the wheels, so it banks left more, which deflects the gear more, resulting in more toe in, and...
 

Daleandee

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It's best if the wheels stay straight.
Thatone

IIRC, plans for the Sonex give a slight toe in (less than 1º) when unloaded so that when loaded (gear flex) the wheels are straight.
 
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Thanks to all. Ever since posting I have been running "what if" structural configs to make sure that the toe stays straight regardless of gear travel. Less attractive aesthetically but no liklihood of hunting for my shorts after a bad landing!
Thanks again.
 

wsimpso1

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Let's not get all wrapped around the axle :D. Issues in taildraggers of toe-in vs toe-out vs tracking true is a controversial issue. Pazmany indicates in his fine book on landing gear that at operating weight and configuration, even taildraggers should probably be set with zero toe.

Getting into the above discussion is all well and good, but seems to be off topic just a bit. Based upon the OP's mention of the positioning of the gear pivots on the fuselage, that he is concerned with the amount of change in alignment as the gear deflects... which brings up a couple points. In many types of conventional landing gear, and certainly in Cub style gear, there is always a substantial change in camber as the gear moves through its travel. To define terms, positive camber is where the top the tire is canted outboard, and toe-in is where the front of the tires are closer together than the rear tires.

Good control while near and above stall speed with minimum tendency to ground loop seems to be the goal. The ground loop is possible in traildraggers both with the tail up as well as with the tailwheel loaded. The mains have to be loaded for round loop to occur. We want it to stay docile and under control from wheel landings at max weight on down to a walk at min weight and all the way from tail wheel firmly on the ground up to level flight attitude during takeoff or wheel landings.

What could mess this up? Well, changing back and forth between toe-out and toe-in as the gear leg swings would certainly do it. No matter where you land on the toe issue, staying on one side of the zero toe line seems important to me. So if your suspension geometry changes toe as it swings and as the tail goes up and down, you might make sure it stays on one side of the line over the range of operation.

Next up is that camber tries to steer the wheels too. Positive camber produces camber thrust to the outside, while negative camber thrusts to the inside. Load one wheel more than the other, and that wheel dominates. If camber crosses back and forth across zero, that could make things squirrely too.

Last effect is camber and toe changing from tail high to tail low. Zero camber changes toe not one bit, but positive camber/zero toe/tail high becomes slightly toed-in as the tail comes down, opposite with negative camber.

It seems to me that zero camber/zero toe would be ideal, but difficult to maintain through the range of suspension travel and range of deck angle.

Me? I am building a tricycle gear on my bird (folks are thinking "wimp", "loser", etc, but with 8.7 hours of taildragger and 2200 of nosewheel, which would you build?), but if I were building a taildragger, I would aim for slight toe-out that never quite goes across the line of zero-zero to avoid it swapping behaviours on me. YMMV.

Billski
 
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Pops

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In designing the LG on the SSSC and the JMR. I preload the LG die springs for the EW of the aircraft. I have a large rod end on one end of the shock strut. ( just like on the LG of the Bearhawk) I adjust the rod end for the axle to have Zero camber and Zero toe with my average flying weight. Makes for a easier handling traildragger.
Picture of the Bearhawk shock strut.
Picture of the SSSC LG with a rod end on the upper end of the shock strut.
Picture of the BH axle jig. Since we built 4 BHs' I make this axle jig. Clamp the LG in the jig and place on the inverted fuselage and bolt the LG brackets to the LG and place the brackets so all the measurements are correct and weld the brackets.
 

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Turd Ferguson

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This sounds like much ado about nothing. The wheels toe in/out with camber changes. Toe value changes on a tailwheel with the fuse level vs three point attitude. Tires scrub across the surface. Adjust the main wheels for 0 - 1/4 degree toe-in the takeoff/landing attitude and let 'er rip.
 
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so...sifting through all of this, if I aim for a net zero scrub/camber at 1G at gross, then the effects of a less than graceful arrival should be relatively minimal/benign? the 1/4 degree toe-in (helps aesthetically) level is attractive for takeoff roll and wheel landings but the 3 point setting will then be slightly toe-out, correct? also, thumping down slightly wing low would cause that wheel to go toe-out?
again, thank you all for the very helpful inputs! Trade studies suck!
 

rv7charlie

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This sounds like much ado about nothing. The wheels toe in/out with camber changes. Toe value changes on a tailwheel with the fuse level vs three point attitude. Tires scrub across the surface. Adjust the main wheels for 0 - 1/4 degree toe-in the takeoff/landing attitude and let 'er rip.
And you may well get your wish....
 

Dan Thomas

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I have a bunch of time in a Champ that had its left wheel toed in about three degrees. Very unpleasant on landing. I took to touching down with the nose a degree or two to the left, to even out the toe-in between the wheels. It was better but not satisfactory. Still too much toe-in. We fixed it properly after that. Got the wheels to zero.

A customer brought his 170 to me with complaints of poor ground handling in landing and takeoff. He'd just bought the airplane. I set it up and took measurements and found the mains WAY outside Cessna's specs, which call for zero to slightly toed-out. Its camber was also way off. I spent several hours measuring the Cessna tapered shims and doing trig calculations and got the gear within spec, and the customer said it was a different airplane, a joy to fly, after that.

Wheel alignment matters. You don't just build it and hope for the best. Sooner or later you will have a difficult landing, such as in a gusty crosswind, and adding in the factor of misaligned gear just might break the camel's back.

Besides that, tire wear is greater with misaligned gear. They wear fast enough if everything is right.
 

Pops

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When I was a student pilot I ground looped a Smith Mini-plane. Just as the tailwheel was coming down I hit the rough place in the runway where everyone had been touching down for many a moon. The airplane made a small bounce and hit a little ways and off it went. Got the turn stopped with brakes but I was pointing 90 degrees to the runway and ran out in a hay field about waist high. It stopped very quickly and had fresh cut hay over the wings and everything and the prop had chopped a hole out the hay field. I waited for a little for friends at the end of the runway to come and push it out but no one came so I turned it around on one brake and chopped more hay getting back on the runway. No damage except for my ego, but had a lot of fresh cut hay and grass stains to clean off. My instructor owned a Pitts and said that the Mini-Plane was the worse handling airplane that he had flown. My instructor finally ground looped it when he was flying it to the new owner and it got destroyed. The LG had been bent and it had lots of toe-out and the owner never fixed it.
Make it right, or be like me.
 

jedi

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I know of a Tripacer converted to a tail wheel that was sold 3 or 4 times in quick succession because of bad geometry on the conversion. Every new owner ground looped it and resold it until it was finally fixed.
 

Dan Thomas

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I know of a Tripacer converted to a tail wheel that was sold 3 or 4 times in quick succession because of bad geometry on the conversion. Every new owner ground looped it and resold it until it was finally fixed.
Just another example of the lousiness of the maintenance. I have had the pleasure and pain of fixing so much stuff that was just plain wrong. Pain, because it takes time and costs money. Pleasure, because the airplane then performs as it was intended to, and the students or owners are very pleased.
I used to get feedback from former students at the flight school. They went on to get jobs as commercial pilots or instructors, flying airplanes that got shoddy maintenance, and they'd say that they were scared half the time and the airplanes flew sloppily or like dogs. Rough running, poor power, controls stiff or slack, everything out of rig. Stuff in the panel that didn't work or was almost useless. Bad brakes, horrible shimmy. An endless list.
 

wsimpso1

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so...sifting through all of this, if I aim for a net zero scrub/camber at 1G at gross, then the effects of a less than graceful arrival should be relatively minimal/benign? the 1/4 degree toe-in (helps aesthetically) level is attractive for takeoff roll and wheel landings but the 3 point setting will then be slightly toe-out, correct? also, thumping down slightly wing low would cause that wheel to go toe-out?
again, thank you all for the very helpful inputs! Trade studies suck!
First, I can not even imagine how toe-in helps aesthetically. Are you confusing camber with toe-in? Positive camber is top of wheels out while toe-in is present when the front edges are closer together than the aft edges.

Put some math on it and see how big things have to be to cross the boundaries. If you have 2 degrees of camber (top outboard) with the tail high and then drop the deck angle 12 degrees to get the tailwheel on the ground, the toe-in changes from 0 degrees to arcsine(sine(2 degree)*sine(12 degrees)) = arcsine(0.0349*0.2079) = arcsine(0.0073) which is sine of 0.415 degrees. If you start with ZERO toe-in and drop the tailwheel, you now have less than half a degree of toe-in.

If instead you had 1/2 degree of toe-out with the tail high, you would still be toe-out a tiny bit. And that positive camber makes the tires behave as if they were slightly toed-out too.

And if you start with 2 degrees negative camber and zero toe with the tail high and drop the tailwheel, you will have about half a degree of toe-out, but now the camber thrust is working against you.

The rest of the world knows something about toe. The tire drags and makes some force aft. Push an airplane and you know what the total tire drag is. In a taildragger, the little wheel has little drag, and about half of of the tire drag is on each main wheel. That force aft is very slightly deforming the tire, the landing gear parts, and moving through any lash or free play at the hinge. I bet that half degree of toe-in that is being advocated by some is converted to near zero toe or even some toe-out when you are rolling along a little below landing speed. The zero toe built in and advocated by other knowledgible folks puts you in toe-out when you drop the tail. Sounds like somewhere in that area is good territory.

The big issue is that you seem to have some geometry issues that might change toe as it swings. Here again, we get some guidance from the real worlds. Piper type bungee gear is typically set up with preload and is on the top stop in flight. If you do this per typical practice, the gear is still on the top stop sitting on the ramp empty. Add fuel and people and it moves a bit. Pops does this with his planes and they behave nicely. He sets zero zero for alignment with weight in the bird. Under load and rolling, I bet it is slightly toed out. Hardly moves between touching on a wheel landing and carrying the whole weight with the tailwheel down.

Your alignment will not change much even if the geometry is wonky during travel. Where it will change is when you use the springs to arrest some vertical velocity. Then the gear deflects past steady state position and rebounds back towards steady state. Quickly. Not much time spent with large suspension travel. The tires are scrubbing back and forth while it does that. Work the trig for angle changes see how much difference in steering it makes during travel between zero load and static load, then how much it does for that 0.1 sec it is near the end of travel. Maybe compare that the tire track angle as the tires scrub during the landing stroke. I suspect that the toe angle changes during the landing stroke as still small compared to the change in direction the tires make absorbing the stroke and then rebounding to static position.

In the end, I still suspect you will be near the classic zero to modest toe-out to the tiniest amount of toe-in advocated by various folks. Have fun, but please, go build. With a big bar and a torch or two, you can change it later. One my bird, with Grove axles, I can buy or make angled shims to adjust things.

Billski
 

speedracer

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When I set up the main gear on a Long EZ I first snap a chalk line on the floor under the exact center of the plane. Then Put an 8' straight edge on each wheel and toe it in 1/8" (in 8'). they always track straight and true. I had one Long EZ that liked to wander a bit. I guessed on toe out and I was right. I fixed it.
 
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