On Wednesday, May 6, 2020 at 9:02:12 PM UTC-7, 2G wrote:
On Wednesday, May 6, 2020 at 9:37:53 AM UTC-7, jfitch wrote:
On Tuesday, May 5, 2020 at 9:42:44 AM UTC-7, Andy Blackburn wrote:
On Tuesday, May 5, 2020 at 6:31:53 AM UTC-7, wrote:
Sooo much talk about scrubbing. Who really cares?

It's not the tire wear we care about. It's the nasty torque load imparted to the wing trailing edge and control surface that occurs when the wing wheel skips from side to side if it is not aligned correctly with the direction of travel.

I think alignment parallel to the main wheel (and path of travel) is the key thing. I've run through the geometry and I don't think turning generates appreciable (or any) lateral loads on the wheel at the tip if it is aligned properly when you put it on. Certainly it will travel a shorter distance on the inside of a turn and longer distance on the outside of the turn, but that just means it'll spin on its axel a bit less or more (it's why cars need a differential). It's free to do this. Try drawing two concentric arcs and a radius through both to represent the angular offset in a turn. Both arcs are perpendicular to the radius. Sure, if the tip wheel is mounted so it trails the wingtip by many feet (imagine tens of feet and the main wheel going in a circle so small that the glider is essentially pivoting on the center of the main wheel and you get the picture - the tip wheel would look like an additional tail boom and would scrub), but that's not the situation we are talking about here. Towing your glider with your car generates pretty gentle turns and all tip wheels I know of don't mount very far off the perpendicular line from the main wheel. All the scrubbing I've seen is due to wheel misalignment when you put it on.

That's not to say a castering tip wheel isn't useful. It is. It allows the user to be more casual in aligning the wheel when mounting it. I have a Cobra wheel and the fancier IMI wheel. I like the design of the IMI wheel a bit better - but it is also a more expensive design.

Andy Blackburn
9B

Andy, not sure what you drew and this antiquated site will not allow pictures. If the wing wheel is behind the main gear, the arcs they are trying to describe are not concentric. The worst case is a turn towards the wing wheel. Let us suppose an 18m glider turning a 40' radius (center of turn is about 10' beyond the tip). Further assume the main is at the leading edge, and the wing wheel 1' aft of that, centered under the wing, and 25' out from the main. The wing wheel will be 3.8 deg off of the correct Ackermann geometry, and that is a significant difference: in just 10 deg of turning it will try to move 8" towards the fuselage, or experience 8" of sideways scrubbing. You would never mount your wing wheel 3 deg off, this is easily seen by eye. Turning away from the wing wheel things are not so bad, less than a degree misalignment.

This is also easy to observe: if the wing wheel is mounted accurately and securely, it will still scrub noticeably in turns, if aft of the main gear axle line. Even spinning the glider stationary on its main gear will cause scrubbing with these assumptions: the wing wheel is over 2 degrees off of the correct angle and each 10 degrees of turn will require 6" of scrubbing.

The only solutions a 1) live with it, 2) move the wheel in line with the main, or 3) caster the wing wheel. Most opt for 1 as it is typically a manageable problem. But 2 is pretty easy to do, or at least get closer to, if your wing wheel is adjustable. Moving the axle just 6" forward in the example reduces the scrubbing by 50%. C is also a solution, but requires a more complex, heavy, and costly wheel.

I went with 3 and it was cheaper and easier than 1 or 2.

I have long noted that when towing trailers the trailer tires track inside of the vehicle tire tracks. I have explained this to my wife, but last year she made a turn in a parking lot that caused the glider trailer tire to bounce over a curb that the towing vehicle easily made it around. This blew the tire. Needless to say, I wasn't happy.

This same geometry affects towing of gliders, but we don't notice it as much because we have big ramps. This paper puts theory to this principal:
https://epa.oszk.hu/02400/02461/0002...02_025-037.pdf
Well, if the trailer pulling angle is different than the towing vehicle turning angle, the center of rotation of the trailer has to be different than that of the vehicle. This must result in scrubbing of the trailer tires and, by extension, the wing wheel. The experiment in this paper shows a substantial difference between the turning angle and the trailer towing angle (4°, fig. 10) that is constant once the turn is established.

Glider trailer tires will not scrub in a normal turn of any radius. If you draw a line through the axles of the trailer and vehicle wheels, they will intersect at a point - the center of the turning circle. The radii of each of the 6 wheels will be distinctly different, but the center of turn the same and no scrubbing. This will not be true of a two axle glider trailer, those axles are parallel and lines drawn through them cannot intersect, hence they will scrub. The scrubbing is why a dual axle trailer is difficult to turn. Similarly a fixed wing wheel that is not in line with the main gear will scrub, for the same reason. Get it in line with the main and it will not.

I was able to alter my Cobra wheel to get it nearly in line for about $20.