Interesting Article

[iflyforpie]

Isn't that neat! it boils down to an incredibly simple sine
of the angle of bank, regardless of speed.

I didn't realize that you were relying on the tire to support the entire sideways force of the aircraft. I thought there was some cross control (or cross wind) in there somewhere which is why I thought the loading was so light.

[Colonel Sanders]

My mistake was thinking that the lift of the wings
would help reduce the weight on the wheel and thus
reduce the sideload, when in fact it doesn't help at all.

[Blakey]

My mistake was thinking that the lift of the wings
would help reduce the weight on the wheel and thus
reduce the sideload, when in fact it doesn't help at all.

I'm a little unclear on why it would not and I often do this in the Champ without compressing the oleo so there can't be too much weight on the tire at that point. Also, while doing this, I can easily "steer" the aircraft from one side of the runway to the other. This leads me to believe that there is no need to "scuff" a tire while doing this and no need for any excessive sideloading. In short, I can sideload the tire outboard or I can sideload the tire inboard, why is there not a point where the tire is running "true" (albeit tilted slightly.) with no significant sideload?

I love to use this method (On long runways.) to teach crosswind landings and I've never had a problem with "rim-creep", scuffing or overload. I would be very interested to see a close-up picture of a tire during this manoeuvre.

[Colonel Sanders]

It doesn't hurt anything, otherwise I would have
broken every light airplane I've ever flown

[photofly]

I can sideload the tire outboard or I can sideload the tire inboard, why is there not a point where the tire is running "true" (albeit tilted slightly.) with no significant sideload?

Leaving out words like "slightly" and "significant" (which imply value judgements) your question is "why is there not a point when the tire is running true?"

bank.jpg (234.26 KiB) Viewed 1269 times

Draw a vector diagram of the forces on the aircraft. There are four that matter:

A. Lift (pitched to the vertical at the bank angle)
B. Force of the runway on the tire perpendicular to the wheel axle ("normal" load on the tire)
C. Force of the runway on the tire parallel to the wheel axle ("side-load")
D. gravitational weight on the aircraft acting through the centre of mass

Since the wheel axle is parallel with the wing span, A and B are parallel, and both have a sideways (ground-horizontal) component towards the lower wing.

D acts vertically downwards. It has no ground-horizontal component.

If the aircraft is moving in a straight line then it's not accelerating sideways. There must be a force on it opposing the sideways component of (A+B). The only place this force can come from is C. (I thought about the rudder, but it's small and doesn't cause much sidways force. It's also deflected in the wrong direction to help.)

If there is a crosswind then there is a fifth force E, the force of the crosswind on the aircraft. This horizontal force will act opposite to the horizontal component of A+B. You can roll down the runway on one wheel without side-loading it when you have the correct bank angle for the crosswind. This is a good crosswind landing.

If there is no crosswind, the only angle of bank that gives no side-load is zero.

If you steer in curves down the runway then you have to take into account the force to accelerate the aircraft in a curved path. If you steer towards the low wing then the sideload will decrease or go negative (to the other side); if you steer towards the high wing it will increase.

I suspect the reason the manoeuvre doesn't cause any damage is because rolling tires are much better at resisting sideloads than you might otherwise think.

Imagine that you are on the roll in a strong crosswind. Just before you rotate you have a significant side-load, the same as you would have rolling down the runway, in a bank, with no crosswind. Albeit in that case, the side-load is shared between two (or three) tires.

[photofly]

I thought there was some cross control (or cross wind) in there somewhere which is why I thought the loading was so light.

A good point.

If you fly the manoeuvre like a forward slip so the longitudinal axis of the aircraft is no longer aligned with the direction of travel then yes, you can reduce the side-force (and the force in general) on the tire to any arbirary amount or to zero. However in that case the tire will be being dragged (in a skid) on its outside edge and lose rubber.

That would explain the Colonel's scuff marks.

[Beefitarian]

Very nice, photofly. What is causing "B" to not be vertical?

I'm probably incorrectly thinking it should directly oppose "D".

[photofly]

Very nice, photofly. What is causing "B" to not be vertical?

B and C are two components of the single net force of the runway upon the tire. You can resolve this single force into any two mutually perpendicular directions of your choice. Since we're concerned with the side-load and normal load on the tire the two obvious directions to choose are parallel to, and perpendicular to, the wheel axle.

You could instead resolve the tire force in the earth-horizontal and earth-vertical directions but because the tire axis is tilted to the earth-horizontal the side-load on it would be a combination of the two, and the result would not be as obvious.

I'm probably incorrectly thinking it should directly oppose "D".

No. The tire can be pushed upwards (to oppose D), but also sideways, resisting a force across the runway. Otherwise you'd never be able to taxi in a straight line - the slightest wind would push you sideways unless the tires could resist.

[Beefitarian]

I'm pretty sure I'll type this wrong but.. The side force "C" is causing a torque or twisting of the vertical force so that "B" is a force slightly away from "C"?

[Colonel Sanders]

That would explain the Colonel's scuff marks.

I suspect that the scuff marks are more a function of the
angle of bank rather than alignment, but I might be wrong.

To get the longitudinal axis of the aircraft significantly
misaligned with the runway, I would have to present an
awful lot of the fuselage side as drag, which would have
to be opposed by a simply enormous rudder - one probably
larger than the one that I have! Prop slipstream would help
increase rudder effectiveness, but even that might not do it.

PS B and C are simply vectors chosen at convenient angles
with respect to the aircraft frame of reference. The vector
summation of the two is the total force acting on the wheel.

Tip to tail makes vectors make sense.

I suspect the reason the manoeuvre doesn't cause any damage is because rolling tires are much better at resisting sideloads than you might otherwise think

Yup.

[photofly]

We haven't considered torques. Torques would act to roll or yaw the aircraft rather than accelerate it (by sliding) sideways. Obviously the torques are in balance since the aircraft is in equilibrium. Because the pilot is constantly adjusting the ailerons and rudder (major contributors to roll and yaw torques) I don't think considering torques is important unless, for instance, you want to examine how much extra aileron or rudder you need to maintain a given angle of bank with one wheel on the ground.

So to answer your question, B and C are just two components of a single force, the runway-pressing-on-the-tire force. They don't do anything to each other, but if you add them together (as vectors) the resultant is what the tire "feels" from the runway.

[Beefitarian]

Torque is for sure the wrong word but I don't know how to describe what I meant.

All three vector forces "A" "B" and "C" are going to add up to equal the vertical force of gravity which is "D" or the mass. I suspect they would all be slightly dynamic and affected by any change in wind speeds and directions as well as control surface deflection as the plane continued in motion.

I'm probably incorrectly thinking it should directly oppose "D".

No. The tire can be pushed upwards (to oppose D), but also sideways, resisting a force across the runway. Otherwise you'd never be able to taxi in a straight line - the slightest wind would push you sideways unless the tires could resist.

I understood right away how "C" opposes "A" to prevent "A" from dragging the plane sideways.

Initially I was thinking of "B" as the direct reaction to the mass but now I am considering how part of the force "C" which is directed to the side opposing "A", will be re-directed by the tire and axle to cause "B" in a direction similar to "A". It will be affected mostly by the slight changes in "A" and "C" as the plane and tire moves in relation to the runway surface.

[Beefitarian]

PS B and C are simply vectors chosen at convenient angles
with respect to the aircraft frame of reference. The vector
summation of the two is the total force acting on the wheel.

Tip to tail makes vectors make sense.

Yes, initially I was making "B" into the sum of everything that needs to directionally oppose "D" instead of a sneaky component.

[photofly]

Yup.

A tire with an outside radius of 20cm travelling at 100km/hr (about 55kts) is pulling about 350 times earth gravity at the tread. (Is my math correct?) maybe it's just that much harder to generate a bead-breaking deflection against that kind of centrifugal force while rolling.

[Colonel Sanders]

I think the 30 psi in the tube/tire would also
probably deal with that sideforce, even when
it wasn't moving.

For grins figure out how fast a 4 inch tailwheel
is spinning at 80 mph touchdown ...

[photofly]

There is certainly a lot of energy in a spinning tire (enough to rip a Concorde wing to shreds, for example) and where there's large energies large forces can often be found.

I once figured out how much energy was stored in the drum of my spin dryer as it rotated at top speed, full of damp clothes, and how much damage it would do to my house if the bearings suddenly decided to let go. Let's just say that closing the door to the laundry room wouldn't have made the slightest difference.

[pdw]

For grins figure out how fast a 4 inch tailwheel
is spinning at 80 mph touchdown ...

6500 rpm

[photofly]

Assuming you're talking about a 4 inch radius, 3360 rpm.

[akoch]

Funny. And one of the original points was about using GPS vs the paper map by inexperienced pilots. While we are taking hours or days to figure a grade 9 physics problem. I'd say for all of us needing a piece of paper and a few quiet minutes to figure this out (myself included) we really should use GPS and other aids and apply of what is left of our brain power to enjoy the scenery outside Those who completed this test problem mentally in a few seconds should go practicing 30-degree bank one-wheel landings on a low-wing airplane!

[Trematode]

Re: One-wheel landings

Ok, I've been a bit confused about this from the beginning. With all the never-ending lip service being given to this exercise, maybe someone can clear this up for me:

I can do these all day in a crosswind -- but can somebody explain to me how you aren't forward slipping if you are doing this in zero wind and rolling 1000 feet down the runway in a straight line? Would you not need opposite rudder to keep going straight? If so, I don't see how you wouldn't be dragging the wheel sideways down the runway, even if only slightly.

Looking at some of hoover's examples, it seems he allows the aircraft to turn torwards the low wheel, and then alternates to the opposite wheel, in order to turn back in the opposite direction and stay on the landing surface. This makes sense to me. But if you guys are going straight down a runway for 1000 feet on the same wheel in no wind -- you gotta be dragging that tire, right?!

Maybe that's where the scuff marks are coming from.

I personally never found any value in teaching this without the crosswind, but that may just be ignorance brought about from inexperience.

[akoch]

My wheel was certainly dragging and skidding, wheel pant touching the surface as well.
I don't think there is any value in the maneuver itself, but it gives you something to do and to work on. It does required good stick and rudder work, and is challenging. And it translates into better control during normal landings. That's the value I understood.

[Trematode]

Could you not just boil it down to a simple rule? How about "always keep your longitudinal axis, and your wheels, aligned with the runway?"

How is this one-wheel landing exercise, without the context of a crosswind, emphasizing good control habbits? To me the thought of it sounds unnatural without the crosswind.

If you want the student to be comfortable handling the airplane in a crosswind, spend more time practicing crosswind landings. If it's confidence and comfort with crossed controls you're looking to instill, spend more time on slips in general.

If the end goal is to teach the ab initio student to keep the nose straight in any landing -- having them purposefully misalign it to keep one wheel in the air for shits and giggles sounds to me like it might be defeating the purpose.

[photofly]

but can somebody explain to me how you aren't forward slipping if you are doing this in zero wind and rolling 1000 feet down the runway in a straight line?

Forward slip: bank the aircraft and turn the fuselage to one side so the increased aerodynamic force on one side vs. the other offsets the sideways part of the lift vector.

This exercise: bank the aircraft and keep the fuselage aligned with the direction of travel; offset the sideways part of the lift vector with a sideways force on the grounded wheel.

It was presented to me as an exercise in aircraft control requiring coordinated use of rudder elevator and ailerons along with a feel for what the wheel(s) is/are doing on the ground at the same time.

[akoch]

I would see it as a bit more advanced exercise than just the initial student practice.

My POH does have a sentence saying land and roll on good tire if one of the tires is deflated. I had an issue with one of my tires a couple of times, so I tried this before it even came up on this thread. And for practical purposes there was no difference if I landed on the good tire or both. Felt the same and no appreciable difference in handling other than the tendency to turn a little bit. More noticeable when taxiing than during take off/landing run.

[Trematode]

Forward slip: bank the aircraft and turn the fuselage to one side so the increased aerodynamic force on one side vs. the other offsets the sideways part of the lift vector.

This exercise: bank the aircraft and keep the fuselage aligned with the direction of travel; offset the sideways part of the lift vector with a sideways force on the grounded wheel.

I am just trying to picture how you are setting it down on one wheel, with no wind, while maintaining your centerline. By definition, if the right wing is low, you will be turning to the right unless you oppose it with the rudder, yes? How would this be any different than adding TOO MUCH crosswind correction? Wouldn't you just be misaligning your longitudinal axis in the other direction?

Do you touch down on both wheels first and then raise one? If that's the case does the aircraft remain relatively aligned with the runway? If not, how do you correct it?