Wing wheel from Craggy Aero

On Monday, May 4, 2020 at 8:25:05 AM UTC-7, jfitch wrote:
On Sunday, May 3, 2020 at 8:45:27 PM UTC-7, 2G wrote:
On Sunday, May 3, 2020 at 3:05:50 PM UTC-7, RR wrote:
Slightly related, bicycle and motorcycle dynamics (or any two wheeled vehicle) are quite a complex subject and only recently fully described mathematically (in the last ten years or so). The solutions are too complex to solve so numeric methods are used.

Not only complex but very few people who ride bikes or motorcycles even know how they initiate a turn. Most assume that if you want to turn left, you pull back on the left handlebar, but in fact to turn left (while moving at a reasonable speed) you push on the left handlebar. The dynamics cause the bike to lean left, and the geometry causes the wheel to turn left to compensate. It is one of the reasons it is hard to learn how to ride a bike, the handling changes with speed.

Try it, push lightly with your fingertips, the handlebar will push back after the turn is initiated. Hang glider turning dynamics are related.

Rick

All turns, whether on the ground or in the air, have one immutable thing in common: there must be a lateral force to cause an object with momentum in a straight line trajectory to change that trajectory. A wheel can have its axle turned and the tire generates this force by friction with the ground, as occurs while executing a turn in a car. If some other force is applied to the car to turn it, such as a wind gust, the tire will resist this turning force until the applied force exceeds the grip of the tire. The car will then skid. The same thing is going on with a glider's wing wheel as it is being forced to turn around a curve by the towing vehicle. This lateral force will be the same regardless of the design of the wing wheel, it will just be less apparent with some than with others. This force MUST be transmitted to the wing - there is no other place for it to go.

Tom

Unless you are doing some high speed slalom with your tow bar, the sideways forces on the glider are negligible. This is not what creates the scrubbing and hopping that is being discussed. If you ARE doing high speed slalom with your glider, then the forces contributed by each component depend on the tire traction and slip angles. Since the glider is nearly balanced on the main, it will carry perhaps 95% of the vertical load, the wing wheel carries only slightly more than it's own weight. The main also has a much larger contact patch than the wing wheel, and much stiffer mountings. So it (and the tow bar) would carry nearly all of the lateral force. Which, for normal tow out ops is nearly zero.

I load the wing wheel with a gallon of water to prevent gusts from dropping the opposite wing. I think it is near impossible to get perfect alignment of the axle of the wing wheel with the axle of the main, but this is negligible. The other issue is the wing dihedral tilting the wing wheel off of vertical. I have observed the Cobra wheel tilting badly in a turn. If there were no lateral forces on the wheel it would maintain its vertical position regardless of poor the clamping was to the wing. Perhaps once the wing wheel starts to tilt the forces on the wheel increase, aggravating the situation. But scrubbing is taking place, which I showed by taking the wing wheel to the limit where it is at the center of the turn. At this position the wheel isn't rotating at all, but is being scrubbed as it is rotated. This is reduced, but not eliminated, as the wheel is moved away from the center of rotation.

Tom

But scrubbing is taking place, which I showed by taking the wing wheel to the limit where it is at the center of the turn. At this position the wheel isn't rotating at all, but is being scrubbed as it is rotated. This is reduced, but not eliminated, as the wheel is moved away from the center of rotation.

Tom[/quote]

Sooo much talk about scrubbing. Who really cares? Who has ever had a wing wheel tyre (tire to you guys) wear out? Sooo many people offended by you Tom.
It's actually International Hand Scrubbing Day today. It might be more constructive to discuss that in the context of todays issues.

I've worn out two or three tyres (sic). But I have to tow a kilometer each way on asphalt. (and the tire just shows wear down the middle. Motorcycle racers call that sort of wear "the chicken strip.")

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 just replaced a wing wheel tire on Saturday. When I went to pump up the tire I noticed exactly what Mark said - a "chicken strip" of wear with some wear down to the cords.

Imagine my surprise when I realized the wheel bearings were very"crunchy". (I never noticed any resistance while in use.)

After some TLC, she is now spinning like a top.

It might be worth giving your wheel a spin and listening.

Lou

On Monday, May 4, 2020 at 11:06:34 PM UTC-7, 2G wrote:
On Monday, May 4, 2020 at 8:25:05 AM UTC-7, jfitch wrote:
On Sunday, May 3, 2020 at 8:45:27 PM UTC-7, 2G wrote:
On Sunday, May 3, 2020 at 3:05:50 PM UTC-7, RR wrote:
Slightly related, bicycle and motorcycle dynamics (or any two wheeled vehicle) are quite a complex subject and only recently fully described mathematically (in the last ten years or so). The solutions are too complex to solve so numeric methods are used.

Not only complex but very few people who ride bikes or motorcycles even know how they initiate a turn. Most assume that if you want to turn left, you pull back on the left handlebar, but in fact to turn left (while moving at a reasonable speed) you push on the left handlebar. The dynamics cause the bike to lean left, and the geometry causes the wheel to turn left to compensate. It is one of the reasons it is hard to learn how to ride a bike, the handling changes with speed.

Try it, push lightly with your fingertips, the handlebar will push back after the turn is initiated. Hang glider turning dynamics are related..

Rick

All turns, whether on the ground or in the air, have one immutable thing in common: there must be a lateral force to cause an object with momentum in a straight line trajectory to change that trajectory. A wheel can have its axle turned and the tire generates this force by friction with the ground, as occurs while executing a turn in a car. If some other force is applied to the car to turn it, such as a wind gust, the tire will resist this turning force until the applied force exceeds the grip of the tire. The car will then skid. The same thing is going on with a glider's wing wheel as it is being forced to turn around a curve by the towing vehicle. This lateral force will be the same regardless of the design of the wing wheel, it will just be less apparent with some than with others. This force MUST be transmitted to the wing - there is no other place for it to go.

Tom

Unless you are doing some high speed slalom with your tow bar, the sideways forces on the glider are negligible. This is not what creates the scrubbing and hopping that is being discussed. If you ARE doing high speed slalom with your glider, then the forces contributed by each component depend on the tire traction and slip angles. Since the glider is nearly balanced on the main, it will carry perhaps 95% of the vertical load, the wing wheel carries only slightly more than it's own weight. The main also has a much larger contact patch than the wing wheel, and much stiffer mountings. So it (and the tow bar) would carry nearly all of the lateral force. Which, for normal tow out ops is nearly zero.

I load the wing wheel with a gallon of water to prevent gusts from dropping the opposite wing. I think it is near impossible to get perfect alignment of the axle of the wing wheel with the axle of the main, but this is negligible. The other issue is the wing dihedral tilting the wing wheel off of vertical. I have observed the Cobra wheel tilting badly in a turn. If there were no lateral forces on the wheel it would maintain its vertical position regardless of poor the clamping was to the wing. Perhaps once the wing wheel starts to tilt the forces on the wheel increase, aggravating the situation. But scrubbing is taking place, which I showed by taking the wing wheel to the limit where it is at the center of the turn. At this position the wheel isn't rotating at all, but is being scrubbed as it is rotated. This is reduced, but not eliminated, as the wheel is moved away from the center of rotation.

Tom

There may be lateral forces on the wing wheel, they not due to centripetal forces in a slow speed turn. It is due to misalignment of the wing wheel axle with the instantaneous direction of travel. All you need to do is draw the axle extensions of all the wheels involved on a piece of paper - main gear, wing wheel, tow vehicle wheels. If these do not meet at a single point, there is scrubbing on the axle that doesn't (traveling in a straight line, that single point is at infinity). It is geometrically impossible to achieve this for any turn radius (less than infinite) unless: the wing wheel axle is allowed to caster, or the wing wheel axle lies on the same line as the main gear. On some gliders the latter is difficult as the main gear is well ahead of the wing and a castering wheel is your only option. On most though, all that is necessary it to tilt the wheel fork forward a bit to get the wheel approximately under the leading edge which is typically the location of the main gear. The further aft of the main it is, the greater the problem. If you cannot keep the axle pointed in the direction you want, then bets are off. The Cobra wheel is not the best in this aspect.

This isn't really rocket science - vehicle builders have been doing this for millennia - it is true for oxcarts as well as race cars. Ackermann described it in the early 1800's for horse drawn carriages, and he stole it from others.

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

On Tuesday, May 5, 2020 at 5:43:09 AM UTC-7, Brett wrote:
But scrubbing is taking place, which I showed by taking the wing wheel
to the limit where it is at the center of the turn. At this position the
wheel isn't rotating at all, but is being scrubbed as it is rotated.
This is reduced, but not eliminated, as the wheel is moved away from the
center of rotation.

Tom

Sooo much talk about scrubbing. Who really cares? Who has ever had a wing
wheel tyre (tire to you guys) wear out? Sooo many people offended by you
Tom.
It's actually International Hand Scrubbing Day today. It might be more
constructive to discuss that in the context of todays issues.




--
Brett

Obviously not you. What do you tow on, grass? I have worn out two tires in a few years. Sorry if you're offended - suggest you stop reading this thread as a remedy. Who wudda thought that wing wheels would be so controversial? Welcome to RAS.

Tom

A few years back, someone showed me his unique design for a w/w. He mounted a basket-ball sized, round utility ball (with a bladder) that freely rotated against an upside-down "cup" which housed a dozen or so ping-pong ball sized "bearings". The ball allowed easy movement in any direction. (Picture one of those roller-type deodorant applicators in motion). He made a brake for it from a metal band that was actuated by a thumbscrew. Clever

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.