Who's right?

tim_enchanter · 20 Jan 2007 at 03:44

hehe sorry can't help myself :eek:

Also it matters not if you went down a vocational or educational route as long as you learn along the way and apply yourself to whatever you do.

VIRII · 20 Jan 2007 at 03:46

blighter said:
i was at grammar till i was 16 and had the choice to stay on for a-levels, but i just see them as another 2 years of stuff you'll never necessarily use in your life, so i went to a more future-career-specified-route and i must admit, i don't regret not doing a-levels one bit (seeing that my course - a btec national - counts as about the same UCAS points etc as 3 A-Levels

lol @ tim for carrying on with the OT

Hehe I used to think I'll never use calculus in my job. WRONG lol.

tim_enchanter · 20 Jan 2007 at 03:48

VIRII said:
Hehe I used to think I'll never use calculus in my job. WRONG lol.

I leant it but I'm glad I've not had to use it

tim_enchanter · 20 Jan 2007 at 03:52

Although a large part of my job is helping people to diagnose problems and I've not done very well at that on here tonight

JRS · 20 Jan 2007 at 04:07

VIRII said:
Then the plane would not move forward at all would it.....

Of course it will. What's stopping it?

The wheels can't, they're freewheeling. And since the treadmill's only point of contact with the plane is those wheels....

VIRII said:
However if we accept that the plane is indeed moving forward then the wheels are rotating, the treadmill is countering the rotation exactly according to the question so there will be skidding.
There is either skidding or the plane doesn't move.
The plane does move...

Think about the following scenario:

You have a toy car, and a treadmill. You set the treadmill running backwards, and push the car along it - against the treadmills's movement but with the car moving forward relative to the rest of the world. Do the wheels skid?

Nope. They just rotate like crazy. And it's exactly the same for the hypothetical plane in the original post. If the rotational speed of the wheels was limited, then you probably would see them skid at some point. But for the purposes of this question their rotation is completely unchecked.

fornowagain · 20 Jan 2007 at 04:30

VIRII said:
What is needed is a good explanation as to why the wheels and treadmill have no or little effect on moving the plane forward or stopping it moving forward.
A car on a rolling road has direct drive to the wheels and that drive is a rotational force not a forward thrust hence the treadmill or rollers are turned.
Strap a batman style rocket on the car and it's going to overcome the rolling road.

I can't adequately verbalise why the wheels (which are not direct driven) on the plane have little effect on a treadmill but the forces involved are very different in size and direction. I guess there comes a point where the downwards force (mavity) keeping the wheels in contact with the treadmill is simply insignificant compared to the forward thrust so the wheels will simply skid forward / lose grip on the treadmill even if still turning.

JRS said:
They won't skid or lose grip - the wheels are free to rotate as fast as they like. The friction force (co-efficient of friction between wheels and treadmill multiplied by the reaction force, i.e. weight of the aircraft) would only come into play if the wheels were being driven or braked, and they aren't.

Isolate the wheel and strut. The strut transfers drag to the fuselage.

The wheel gains angular velocity (acceleration) and energy in reaction to the conveyor. Ignoring gyroscopic forces. What forces are being transfered to the strut? Because of the weight of the plane down on the strut, the tyre is pushed against the conveyor. The tyre deforms as it rotates, also the tyre surface moves against the conveyor surface and causes friction. These create an angular force forward and down. The conveyor is solid and pushes back in the opposite direction. This reaction can be resolved at the wheel centre into a vertical "weight" and a horizontal "drag" on the strut. The drag element is very small compared to the weight element. Both decrease as the plane becomes lighter from wing lift. As the wing lift equals mavity all thats left is tyre friction.

This is the small bit. At the outer radius of the tyre the angular speed of the tyre is the same as the conveyor linear speed. Small amount of energy are still transferred by the conveyor to overcome bearing friction and air drag on the tyre. The transfer of energy is not perfect and causes small changes in angular velocity, wheel slip. Seen as friction at the contact point. The friction force has a horizontal reaction component or drag parallel to the surface of contact. Acting at the wheel center and thus transfered to the strut/fuselage. This is however a very very small force.

Mickey_D · 20 Jan 2007 at 04:31

Ok, to end all arguement in this silly thread, if the wheels prevent/make a plane go into the air, then how the hell do ski planes work in the arctic? How the hell do float planes work, then?

In the original question, it is stated that the treadmill would counter any forward velocity of the plane by speeding up. There's one problem - it wouldn't be able to.

If you rolled a plane from the tarmac onto a huge treadmill already doing 3 times the plane's take-off speed, it would only take a small portion of the plane's engine power to counter the extra friction to hold it still. Meaning the plane would maintain its ground position with barely ANY of its power no matter the treadmill's speed. It would then only need to increase its throttle ever so slightly to move forward. Any and all increase in the treadmill's speed (until the surface of the treadmill was doing supersonic speeds) could be counteracted by a small increase in throttle. The plane would therefor only need to wait until the treadmill blew itself to pieces by trying to go so fast, then it could just take off as normal. Or it could just apply full power and go for takeoff anyways.

Ground speed and airspeed are two totally different things and are as irrelevant to each other as windspeed and ground speed are to a car. Think about the opposite. Put a car in a wind tunnel. Put a lever against the front bumper that pushes on the wind tunnel's throttle. What do you think would happen when you got in the car and put it in gear? Exactly. The car would snap that throttle lever off as the wind tunnel went to hypersonic speeds trying to push the car backwards. Why? Because the car is driven from the ground and the air speed really means nothing to it. Exactly the same thing with the plane. The ground speed is completely irrelevant to the plane as the ground IS NOT what makes the plane move. The air is.

In the hypothetical case of the plane on the treadmill about the only way the treadmill could prevent the airplane from taking off is if the treadmill got going SO FAST that the bearings in the wheels siezed up and acted like the plane's brakes. Even then there are a majority of planes out there will overpower thier brakes anyways.

Tell you what, I've got a friend who has an R/C plane. I've got a treadmill. I'll get together with him and film the plane on the treadmill as it just nonchalantly rolls of the front of the treadmill at full speed, ok?

THE PLANE WILL TAKE OFF!!

ken1307 · 20 Jan 2007 at 04:36

I havent read the last few posts but here is what I think

The plane will take off assume that:

- The wheels are FREELY rotating (1)
- There is friction between the treatmill and the wheels (2) <== this is just to make the wheel spins otherwise the plane + wheels would just "sliding" ontop of the treatmill
- The treatmill is long enough for the plane to accelerate to its take off speed (3)

1st case [No thrust from the engines] It doesnt matter which direction the treatmill moves (backward or foward), the plane will be stationary because of (1). Although the wheel is rotating, it does not extert any forces onto the plane (1.1) and from Newton's First Law of Motion:
I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

==> The plane is stationary and the wheels will just rotates freely at the SAME speed of the treatmill.

2nd case [Thrust from the engines]: From the first case you can see that the movement of the treatmill and the wheels has NOTHING to do with the plane, as soon as the engine pushing it (which is considered as external force and it only applies to the plane), it will start moving and take off with assumption (3) as usual. Compare with the real scenario, I believe the plane would accelerate faster as there is basically no friction. As far as the wheel goes, it still spins at the same speed as the treatmill like it was the 1st case

its late now so please ignore my spelling

***** OMG, another 2 posts while I was typing , you guys are fast

tim_enchanter · 20 Jan 2007 at 04:57

ken1307 said:
I havent read the last few posts but here is what I think

The plane will take off assume that:

- The wheels are FREELY rotating (1)
- There is friction between the treatmill and the wheels (2) <== this is just to make the wheel spins otherwise the plane + wheels would just "sliding" ontop of the treatmill

1st case [No thrust from the engines] it doesnt matter which direction the treatmill moves (backward or foward), the plane will be stationary because of (1), the wheel does not extert any forces onto the plane. It will just rotates freely at the SAME speed of the treatmill

2nd case [Thrust from the engines]: From the first case you can see that the movement of the treatmill has NOTHING to do with the plane, as soon as the engine pushing it (which is considered as external force because it only applies to the plane), it will start moving and take off as usual. Compare with the real scenario, I believe the plane would accelerate faster as there is basically no friction. As far as the wheel goes, it still spins at the same speed as in the 1st case

its late now so please ignore my spelling

***** OMG, another 2 posts while I was typing , you guys are fast

Almost correct but in your first point the plane would move along with the treadmill forward or backward depending on the direction of the treadmill.
A small amount of thrust would be required to counter the friction of the wheels/bearings and the weight of the plane holding it down on the treadmill.

ken1307 · 20 Jan 2007 at 05:01

tim_enchanter said:
Almost correct but in your first point the plane would move along with the treadmill forward or backward depending on the direction of the treadmill.
A small amount of thrust would be required to counter the friction of the wheels/bearings and the weight of the plane holding it down on the treadmill.

I have updated my post abit

but with regard to your point, the plane WILL NOT move AT ALL because the wheel DOES NOT extert any forces onto the plane (its a FREELY ROTATING WHEEL which I think is the crucial point in this discussion) and please remember, there is nothing can just move by itself

tim_enchanter · 20 Jan 2007 at 05:08

Are you saying it doesn't move on the treadmill or it doesn't move in relation to solid ground?
I'm talking as an observer stood away from the treadmill.
If I stand and watch the plane it will move away fom my fixed position but will remain stationary on the moving belt.
Until some thrust is applied.

Exactly the same as if you put a trolley on a conveyor, stick it on there and it will move away with the conveyor, it's wheels will not turn.
However if you stand off the end of the belt and hold the trolley the wheels will rotate and the trolley will stay still.
This is the same as applying some engine thrust.
I stand by my point

Mickey_D · 20 Jan 2007 at 05:13

I think some of you are putting WAY too much faith in the friction applied by the wheels on a plane. Have none of you seen those strong man competitions where a guy pulls a 747? Have none of you seen a rather whimpy looking guy move a Cessna around by himself? It takes little to no effort to move a plane.

The average person can supply about 500W of power for a brief period of time. That's about 2/3-3/4 of a hp. Even the old biplanes had over 50hp engines.

No amount of action from the treadmill is going to prevent a plane from rolling forward. And as soon as the plane rolls forward, the treadmill's already lost. In which case the whole question becomes irrelevant at that point because it is no longer holding the plane still.

tim_enchanter · 20 Jan 2007 at 05:20

I know but I've got nothing better to do other than go to sleep and I can't be bothered to do that

ken1307 · 20 Jan 2007 at 05:23

tim_enchanter said:
Are you saying it doesn't move on the treadmill or it doesn't move in relation to solid ground?
I'm talking as an observer stood away from the treadmill.
If I stand and watch the plane it will move away fom my fixed position but will remain stationary on the moving belt.
Until some thrust is applied.

no, it is stationary relative to you as well. It does not move at all

please read my updated post again

tim_enchanter said:
Exactly the same as if you put a trolley on a conveyor, stick it on there and it will move away with the conveyor, it's wheels will not turn.

We are talking about two different things here. The wheels of your trolley is not FREELY WHEEL

In your case, when the conveyor starts moving, it applies a force onto the wheel (as in it pulls the wheel toword its moving direction). Since your wheels is CONNECTED (as opposied to FREELYWHEEL) to the trolley, it also pulls the trolley with it. With regarding to the wheel not turning, it is because the friction between it and the trolley is greater than the friction between it (the wheel) and the conveyor. If you have a really really smooth bearing and the conveyor accelerates fast enough, it will turns

tim_enchanter said:
However if you stand off the end of the belt and hold the trolley the wheels will rotate and the trolley will stay still.

and when you do that, you basically create an counter-force which technically makes your trolley's wheel an FREELY ROTATING one and it also prove my point

tim_enchanter · 20 Jan 2007 at 05:35

ken1307 said:
In your case, when the conveyor starts moving, it applies a force onto the wheel (as in it pulls the wheel toword its moving direction). Since your wheels is CONNECTED (as opposied to FREELYWHEEL) to the trolley, it also pulls the trolley with it. With regarding to the wheel not turning, it is because the friction between it and the trolley is greater than the friction between it (the wheel) and the conveyor. If you have a really really smooth bearing and the conveyor accelerates fast enough, it will turns

Thats all very well but unfortunately we don't have the luxury of frictionless bearings and therefore can't escape the fact that no matter what speed you start the belt at, the friction and mass of the plane will always end up causing it to start moving along the belt unless another force is applied to hold it still.
As someone like blighter said pages ago this may only need to be a guy holding onto it.

ken1307 · 20 Jan 2007 at 05:38

tim_enchanter said:
Thats all very well but unfortunately we don't have the luxury of frictionless bearings and therefore can't escape the fact that no matter what speed you start the belt at, the friction and mass of the plane will always end up causing it to start moving along the belt unless another force is applied to hold it still.
As someone like blighter said pages ago this may only need to be a guy holding onto it.

we all talk about a hypothetical (I hope this is the right word

) situation here

As long as the wheel could FREELY ROTATE, the treatmill or wheels 's movements is irrelevant

The treatmill would need to be long enough so the plane can gain enough speed for take off though

im off to bed now btw, c u all tmr

Vonhelmet · 20 Jan 2007 at 07:17

Oh man, how did you get 6 more pages in the 6 hours between 1 and 7 on a Saturday morning?

Da_Big_Jeff · 20 Jan 2007 at 08:02

If the assumption that the runway belt matches to the speed of the wheels relative to the ground then the plane will take off as discribed in the trolly model.
But if the runway matches speed realtive to the surface of the runway belt then it will increase its speed indefinatly and approach the speed of light where apon it will increase its mass and therefore, gravitational attraction with the plane meaning it will increase in wieght and not be able to take off. Either that or physics will go a bit mental. But then I have been up all night and could be a bit off.

gurusan · 20 Jan 2007 at 09:36

wow....this is still going.

gurusan · 20 Jan 2007 at 09:56

Mickey_D said:
Tell you what, I've got a friend who has an R/C plane. I've got a treadmill. I'll get together with him and film the plane on the treadmill as it just nonchalantly rolls of the front of the treadmill at full speed, ok?

Please do, if people can't understand using a brain cell or two then seeing it should help them.

Although I'm sure some will come up with all sorts of excuses about how the experiment was flawed.