lml@lancaironline.net Mailing List Archive

Gary,
I like using grease plates. Its simple, cheap and in my experience it has served me well.
As for side forces pushing the gear together or pulling the gear apart depending on toe and direction being rolled, believe it, they can be great enough to cause problems in some designs. However I don't think the test of seeing if the plane tries to roll back the other way will be very conclusive. You may try measuring the gear after rolling it one direction and then another. I can imagine that if you have severe toe in or out there will be a measurable difference but mostly I would expect it to show up in the distance between the mains, not their toe angle.
In the end I would suggest getting the mains set to your best measured zero or with slight toe in and go flying. If the tires wear unexpectedly post about it here on the LML and I'll be happy to try and figure out, and learn with you, what might be going on.

Matthew

On Mon, Jan 25, 2010 at 6:42 PM, Gary Casey <casey.gary@yahoo.com> wrote:

Good point, Matthew. But I think the question is whether or not the gear is able to "normalize" just rolling back and forth under weight or if the wheels need to be placed on greased plates to normalize. Certainly you can't just lower the plane down without moving it and expect the alignment to be anything close to correct. Do the tires develop enough side force to stress the gear legs just by rolling? I suppose it is possible, but I doubt it. One sign of that would be if you pushed (or pulled) the plane, let go, and it responded by rolling back the other way. If it is true, then I would measure the toe-in by pulling the plane forward and then block the nose wheel to keep it from rolling back and then measure the toe-in in the "stressed" condition. Cessna has built thousands of each version of tubular gear (swept forward and straight out) and I don't know of any special technique they use or recommend to measure alignment. They do sell shims that can be used to adjust both toe-in and camber, as I recall. Both my Cessnas exhibited positive camber angles in the air, but only slightly positive camber on the ground, which I ignored. The tires always wore on the outside and I attributed that to the scuff spinning the tire up on landing, not normal taxiing.

Another thought related to oscillation while braking. The "correct" geometry of the gear leg would be to have the tube pointed at the center of the tire contact patch. That eliminates the potential torsional load on the tube while braking. Any twist of the leg will significantly change the toe-in and I would think that would be bad. I admit I haven't looked to see if it is close the correct geometry, but I think on the ES it is close enough. The IV is a different story because of the shape of the tube - but it isn't different that any of the Cessna retractables, so what does that say? I don't know.

I don't know if there is a truly correct way to measure toe-in, but the way I do it is to place blocks the approximate height of the axle ahead and behind the wheel. Then I put a straight edge against the tire sidewall, resting on the blocks. I measure from the ends of the straight edge to the aircraft center line to get the toe-in measurement. I aim for zero, but should there be a slight toe-in so that brake drag will bring it back closer to zero? I don't think so, if only from the tire wear perspective.

Gary

Re: [LML] Re: L-IV Alignment
January 24, 2010 4:07:52 PM MST

From:
"Matthew Collier" <ivdiggs@gmail.com>

To:
"" <lml@lancaironline.net>

Gary,
I made the assumption that we were discussing the Lancair IV. The ES does not have the same issue since the angle of the leg is nearly straight down when viewed from the side. When viewing the IV from the side the legs go forward quite a ways from their attach point in the gearbox. This causes the toe and camber to change with load. The ES will almost only change in camber with changes in load. Toe being off by a degree or two (in opposite directions) will show excessive tire wear very quickly, while camber being off takes much longer and may never seem to be a problem.

Matthew Collier
Fibercraft Inc.