Using brakes to stop tire rotation

[digits]

An interesting question. From a physics standpoint, if the momentum of just the tire's mass
rotating on the rim is sufficient to rotate the tire on the wheel and shear the valve (when
brakes are applied to the wheel) ...

What about during heavy braking? The entire weight of the aircraft would try to rotate the
tire on the rim, and shear the valve, correct?

Which exerts more force? The mass of the spinning tire, or the entire aircraft? From a kinetic
energy standpoint, how fast would the tire have to spin, for the force to be equal? Obviously
a tire rotating at 1 mph would be no problem. At 1000 mph?

We need Photofly. He eats this shit with a spoon, and has four degrees. I only have one,
and I am also much lazier :^)

Comparing energy on a 737, as that was the easiest plane to find tire weights from. Looking at main wheel tires:
* Tire weight: 94 kg (source: https://www.bridgestone.com/products/sp ... 21_01.html)
* Tire diameter: 44 inch => 1.1 m, tire radius r of 0.55 m
* Speed during landing: 155 knots, or 290 km/h (80 meter per second)

At this speed, a tire will rotate 23.3 times per second, which is a radial velocity (omega) of 144 radians/s

If we simplify the tire as a circle with all the mass of the circle on the outside, the rotational kinetic energy becomes:
KEr = 1/2 * M * r^2 * omega^2 = 294 kJ


* Amount of main wheels on a 737: 4
* Let's assume a mass of 50 ton

Kinetic energy of a plane in that configuration: 160 MJ. Per wheel, that gives 40 MJ.

Even if more than half of the braking happens aerodynamically or with thrust reverse, there is still much more energy being dissipated during the landing roll than during a brake application in the air.

For a movement of the tire around the rim, energy is not the only element to the story. It depends more on the impulse/time it takes to dissipate that energy.

If, when airborne, you apply the brakes for 2 seconds, you dissipate 147 kJ/s
If the landing roll takes 40 seconds, and you burn off half the energy with the tires, you get 500 kJ/s

The extra pressure of the weight of the plane on the tire might make it harder for the tire to move, but the difference is still so big that I find it highly unlikely you would ever be able to creep the tire on the rim by gently applying brakes after take-off.

Or, to look at it in a different way: upon touchdown the wheels violently accelerate from zero to over 200kph in a very short time. If they can survive that, a slight tap on the brakes after take off to do the opposite in a longer period of time, should not hurt the wheels at all.

[Colonel]

Hm. How many degrees did you say you have, again?

j/k

Nice work!

PS. I will admit I am guilty ... if there is a vibration after takeoff, I will touch the brakes
to see that makes it go away. Vibrations in aircraft bug the shit out of me, because I don't
know if they're going to get worse.

Does anyone know why nobody bothers to balance aircraft tires? We balance them on
cars and motorcycles. It's really not very hard to do, and doesn't require expensive
equipment.

[Chuck Ellsworth]

Amazing.

The only math I know is that metric for 69 is 181.

[Slick Goodlin]

Does anyone know why nobody bothers to balance aircraft tires?

The rice rockets I flew had balance weights on all wheels. They were the glue-on type and our shop would cover them with a strip of silver speed tape to keep the elements off. A couple TC guys hated that tape but we kept using it anyways, seemed to let the weights stay on longer before they inevitably disappeared.

[AvCanada Joe]

That's grade seven math where digits is from.

[Slick Goodlin]

That's grade seven math where digits is from.

As long as it’s not home schooled math...

[digits]

A bachelor in youtube and a master in Google is all you need in life.

[Colonel]

The rice rockets I flew had balance weights on all wheels

Interesting. And probably the most normal thing about those aircraft, which
I always suspected were designed by a team of engineers that had never seen
an aircraft before.

The first time I refuelled one, I couldn't believe it.

[digits]

Does anyone know why nobody bothers to balance aircraft tires? We balance them on
cars and motorcycles. It's really not very hard to do, and doesn't require expensive
equipment.

This is speculation, but I can see a few reasons:

1) aircraft wheels are seldom/never (?) operated at a constant high speed. They are either operated at a slow speed, or accelerating quickly or decelerating rapidly. Therefore, even if there is a limited inbalance, there is simply not enough time to do damage the wheel assembly.
2) I've driven my car a couple of times with snow stuck on the wheels. I usually notice it at constant speed, not while accelerating or braking. I think the change in speed breaks up the vibration.
3) during normal operation aircraft receive way more sideway loads, such that even a big imbalance would fail in comparison.

[Colonel]

the change in speed breaks up the vibration

True - there are almost certainly resonances at certain speeds.