Wing wheel from Craggy Aero

On Sunday, May 3, 2020 at 6:17:40 AM UTC-7, wrote:

Scrubbing is what happens when you turn a tire on a radius.

If the wing wheel is biased forward, so that it is inline with the main gear axle, there will be no scrubbing as the geometry is correct for any radius turn.

Ignoring who said and did what, I just learned a useful geometry lesson.

Assuming 'scrubbing' is when the rubber on a tire rubs instead of rolls over the pavement.

Another symptom is 'hopping' where the wheel tracks wrong and rolls out of position flexing the mount. Eventually, there is enough force from the flex to make the wheel hop back into position. Definitely some 'scrubbing' involved as well. Definitely a worrying situation because the flex is putting weird loads on the flying parts.

If the wing wheel is vertical and in line with the line of travel, why would there be scrubbing when you are going straight?

If in addition, the axle of the wing wheel points to a line directly above the main wheel, why would there be scrubbing when the glider makes a turn?

Maybe in the limit, neither one of these can be aligned perfectly, but if you put the wing wheel in the right place it ought to work pretty good.

That explains why my old wing wheel still hopped on curves even after I adjusted it so it points straight ahead. Also why the new MM one doesn't.

Guess a swivel might be another out of the hole, but not the only way.

To be correct geometry for a specific radius turn, the wing axle must point towards center of the turn radius. The axle lines of the main and wing wheel intersect at this center. If the main wheel is the center, then the wing axle must point towards it. A swiveling wheel will align itself to this geometry. For a non swiveling wheel to be correct geometry for *any* radius, the axles have to be in line (more specifically their projection to the ground plane has to be on a line), guaranteeing that both axle lines intersect at the circle center regardless of radius. The wing wheel will then not scrub, even if it is the center of the turn. It will turn about its vertical axis, just as the main gear (or indeed the swiveling wheel) must, in other circumstances. That is not "scrubbing" as the term is usually applied.

And now, back to the popcorn....

On Sunday, May 3, 2020 at 4:48:52 PM UTC+1, jfitch wrote:
On Sunday, May 3, 2020 at 6:17:40 AM UTC-7, wrote:

Scrubbing is what happens when you turn a tire on a radius.

If the wing wheel is biased forward, so that it is inline with the main gear axle, there will be no scrubbing as the geometry is correct for any radius turn.

Ignoring who said and did what, I just learned a useful geometry lesson..

Assuming 'scrubbing' is when the rubber on a tire rubs instead of rolls over the pavement.

Another symptom is 'hopping' where the wheel tracks wrong and rolls out of position flexing the mount. Eventually, there is enough force from the flex to make the wheel hop back into position. Definitely some 'scrubbing' involved as well. Definitely a worrying situation because the flex is putting weird loads on the flying parts.

If the wing wheel is vertical and in line with the line of travel, why would there be scrubbing when you are going straight?

If in addition, the axle of the wing wheel points to a line directly above the main wheel, why would there be scrubbing when the glider makes a turn?

Maybe in the limit, neither one of these can be aligned perfectly, but if you put the wing wheel in the right place it ought to work pretty good.

That explains why my old wing wheel still hopped on curves even after I adjusted it so it points straight ahead. Also why the new MM one doesn't.

Guess a swivel might be another out of the hole, but not the only way.

To be correct geometry for a specific radius turn, the wing axle must point towards center of the turn radius. The axle lines of the main and wing wheel intersect at this center. If the main wheel is the center, then the wing axle must point towards it. A swiveling wheel will align itself to this geometry. For a non swiveling wheel to be correct geometry for *any* radius, the axles have to be in line (more specifically their projection to the ground plane has to be on a line), guaranteeing that both axle lines intersect at the circle center regardless of radius. The wing wheel will then not scrub, even if it is the center of the turn. It will turn about its vertical axis, just as the main gear (or indeed the swiveling wheel) must, in other circumstances. That is not "scrubbing" as the term is usually applied.

And now, back to the popcorn....

That is true assuming that the leg of the dolly is vertical and a non-vertical leg would make a negligible difference on a single tight turn. However with a noticeably non-vertical leg and a fixed wheel (e.g. and in my case, on the dihedraled outer part of a D2C or V3 wing) then on a straight tow, no matter how accurately the cuff is alinged, the wheel will tend to turn itself (like how you lean to turn a bike). It will then either relieve itself by bouncing, or by skidding back to position, or it will twist the dolly until you stop and go and sort it - depending on the surface and the weight on the dolly from ballast.

On Sunday, May 3, 2020 at 9:54:13 AM UTC-7, wrote:
On Sunday, May 3, 2020 at 4:48:52 PM UTC+1, jfitch wrote:
On Sunday, May 3, 2020 at 6:17:40 AM UTC-7, wrote:

Scrubbing is what happens when you turn a tire on a radius.

If the wing wheel is biased forward, so that it is inline with the main gear axle, there will be no scrubbing as the geometry is correct for any radius turn.

Ignoring who said and did what, I just learned a useful geometry lesson.

Assuming 'scrubbing' is when the rubber on a tire rubs instead of rolls over the pavement.

Another symptom is 'hopping' where the wheel tracks wrong and rolls out of position flexing the mount. Eventually, there is enough force from the flex to make the wheel hop back into position. Definitely some 'scrubbing' involved as well. Definitely a worrying situation because the flex is putting weird loads on the flying parts.

If the wing wheel is vertical and in line with the line of travel, why would there be scrubbing when you are going straight?

If in addition, the axle of the wing wheel points to a line directly above the main wheel, why would there be scrubbing when the glider makes a turn?

Maybe in the limit, neither one of these can be aligned perfectly, but if you put the wing wheel in the right place it ought to work pretty good..

That explains why my old wing wheel still hopped on curves even after I adjusted it so it points straight ahead. Also why the new MM one doesn't.

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

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

Tom has sent me a private email accusing me of making a libelous statement against him.

In the interest of keeping the peace, I apologize for any statement I have made.

I'm bored with cabin fever, so why not talk about how rubber meets...


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...

Here's the logic I'm hearing here.
The glider has momentum which must be countered to make it change direction.
The wing wheel provides this because there is no other place for it to go.

Given that a glider with a castering wing wheel can still turn, perhaps the main wheel can provide most of the turning force?

On Mon, 04 May 2020 05:23:58 -0700, stu857xx wrote:

I'm bored with cabin fever, so why not talk about how rubber meets...


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...

Here's the logic I'm hearing here.
The glider has momentum which must be countered to make it change
direction.
The wing wheel provides this because there is no other place for it to
go.

Given that a glider with a castering wing wheel can still turn, perhaps
the main wheel can provide most of the turning force?

Yes, came here to say that. The two places that carry the main loads
while a glider is being towed are the main wheel and the tow ball.

Side loads when turning or towing cross-wind will be largely carried by
the main wheel while the tow ball adsorbs rotary momentum (of wings and
rear fuselage) at the start and end of a turn or when swivelling the
glider in place.

If the wing wheel is fairly close to the spanwise line through the main
wheel's contact point with the ground and its axle is parallel to the
spanwise line, then there should be little or no side force on the wing
wheel: if your wing wheel cuff can't guarantee that the main and whng
wheel axles are parallel then, or course, all bets are off.

OK, heres an analysis of a light glider (my 201 Libelle). My wing wheel's
axle is roughly under the LE when fitted, so is probably close enough
fore/aft to the main wheel's contact point for scuffing not to be an
issue. The cuff is 280mm (11") wide and a snug fit everywhere including
at LE and TE, so the wing wheel's axle will be fairly accurately parallel
to the main wheel axle. I've never seen any tendency for the wing wheel
to scuff when towing the glider on either grass or hard surfaces.

My wing-wheel assembly weighs 5.4 kg and is on the lighter wing: in still
air with the glider stationary its always the other tip that slowly drops
if I let go of the wing and during airframe weighing the other wing was
around 4kg heavier. Its quite obvious that there's very little load on
the wing wheel when towing because, on our grass airfield, running over a
small clump or a heap of cuttings will make it leave the ground an inch
or two and only briefly.


--
Martin | martin at
Gregorie | gregorie dot org

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.

On Sunday, May 3, 2020 at 9:02:02 PM UTC-7, wrote:
Tom has sent me a private email accusing me of making a libelous statement against him.

In the interest of keeping the peace, I apologize for any statement I have made.

Thank you.