Principles of flight?

[fivebob]

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?

Explain why it can, or cannot, take off?

[Alex B]

No, it will just spin the wheels

[V8MOFO]

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?

Explain why it can, or cannot, take off?

No it will not 'take off'

The factor that creates lift and makes the plane fly is the difference in speed between the airplane and the air particles. Different speeds are needed for different planes. ( Note: different opposing wind speeds )

Many people think that a plane takes off by air particles pushing up on the wings but it is actually due to 'how' the air passes by the wing.

You may or may not know that a plane's wing is flat on the bottom and curved on the top. This means that air particles that are forced to go above the planes wing has a longer distance to travel past the wing. The faster the air particles move the less pressure there is. Thus, the wing moves into the low air pressure ( which is up ) bring the plane with it.

If there is no partical movement ( like a vacuum, where there is none ) a plane will not lift.

[RomanV]

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?

Explain why it can, or cannot, take off?

Have you been hitting the LSD again Fivebob.
I cant believe that I'm the only person that has voted 'yes' so far.

Yes of course it can take off, the wheel speed to the ground is irrelevant.

The turbines that propel the plane foward dont give a shit what the wheels are doing, they're going to propel the plane fowards regardless.

eg. If the plane was taking off from a sheet of ice, and the wheels were locked up the whole time, it could still take off. I dont see why a travelator type scenario would be any different, it doesnt matter if the wheels are spinning at 300mph or 0mph, that plane is getting airborne.

Its the pressure differential between in front of, and behind the turbines that provide the foward motion, if the motors were going at full bore, that plane is going to move fowards regardless of what ever else is going on.
And it is still going to acheive the minimum speed (relative to the earth, not the conveyor) required for take off.

[UZZ30]

Many people think that a plane takes off by air particles pushing up on the wings but it is actually due to 'how' the air passes by the wing.

You may or may not know that a plane's wing is flat on the bottom and curved on the top. This means that air particles that are forced to go above the planes wing has a longer distance to travel past the wing. The faster the air particles move the less pressure there is. Thus, the wing moves into the low air pressure ( which is up ) bring the plane with it.

If there is no partical movement ( like a vacuum, where there is none ) a plane will not lift.

Well said Mofo, some people i know dont even know this, its pretty basic stuff! to me any way

[ollieboy]

We had this question in a physics exam and the whole class apart from me and another guy got it wrong, the people who got it wrong still didn't grasp the concept after over an hour of explaining.

[RomanV]

Lets say that the speed of the plane, relative to the earth, is 100kph.

This means that the speed of the travelator relative to the plane will be 100kph also. (in the opposite direction of course)

The plane will be traveling at 100kph relative to the earth, and 200kph relative to the travelator.

The whole travelator thing is just a mind $&#$%, it doesnt make any difference at all.

Except you might have to replace wheel bearings more often.

[Bridget]

wtf are you on about roman, lol

[V8MOFO]

Lets say that the speed of the plane, relative to the earth, is 100kph.

This means that the speed of the travelator relative to the plane will be 100kph also. (in the opposite direction of course)

The plane will be traveling at 100kph relative to the earth, and 200kph relative to the travelator.

You have me thinking now Roman. It will be interesting to see where this goes. It is true that relative to the travelator the plane will be moving at a set speed. 300kmph or 400kmph, whatever. But once ( and IF ) it leaves the ground, it's relative speed to the ground has nothing to do with the plane flying. Off the ground it will be motionless in air but it's wheels will be spinning.
That is why a normal boeing 747 has to be traveling ( by traveling I mean into the opposing drag force / the 'wind' ) at roughly 300kmph.

I'm guessing fivebob has some sort of slimy explanation behind this...

[V8MOFO]

The plane will be traveling at 100kph relative to the earth, and 200kph relative to the travelator.

I think you may be confused here. It is stated that the travelator is moving at the same pace as the plane. If the wheels are traveling at 100kmph left, and the travelator is traveling 100kmph right. They cancel each other out. Meaning the plane relative to the earth is doing 0kmph.

This about it this way, if you jacked the plane up so the wheels are off the ground, started the engines and got the wheels going to 100kmph ( for arguments sake ). And just kept turning them faster and faster, until top speed. Would the plane take off? No, because the engines alone arn't powerful enough to levitate the plane from a stop start ( relative to the drag force ).

[no_8wire]

No as the plane is not moving/ air is not flowing around wings fast enough to create lift...

[Cahuna]

This about it this way, if you jacked the plane up so the wheels are off the ground, started the engines and got the wheels going to 100kmph ( for arguments sake ). And just kept turning them faster and faster, until top speed. Would the plane take off? No, because the engines alone arn't powerful enough to levitate the plane from a stop start ( relative to the drag force ).

But the engine in a plane isn't driving the wheels, it is driving a propellor or is a jet engine, so in your example of having the plane jacked up on blocks the plane would be pushed forward off the blocks by the engines but wouldn't have any way of turning the wheels, despite the wheels hanging in free air.

Think of it this way:
* The engines push the aircraft along at 200km/h relative to the earth around it.
* The conveyor belt does its thing, spinning at 200km/h in the opposite direction to the aircraft.
* If the wheels were doing the driving on the aircraft then nothing would happen as the two would cancel each other out, but...
* Because the plane is being propelled by thrust (not its wheels) it moves relative to the ground, regardless of what is below it (as RomanV said). So even though the plane is doing 200km/h relative to the ground around it the wheels are doing 400km/h because of the conveyor belt. If the plane did 300km/h relative to the ground the wheel speed would be 600km/h.

It is very similar to being on the mother of all oil slicks or slippery ice. If you are in a car and put the throttle down the wheels will just spin and you will not go anywhere because the tyres cannot get a grip on the ground to apply an accelerative force in order to move forward. If you are in a plane then it will happily move off because it is applying its accelerative force to the air, not the ground, so the lack of grip on the ground makes no difference to it.

If you wanted a situation like this where you could not get the aircraft to take off then substitute "tailwind" for "conveyor". The plane would get up to some horrendously high speed on the ground but because the airspeed over the wings would be zero (plane speed - tail wind it is travelling with) there would be no lift. That is why planes take off into a headwind, because the groundspeed necessary to take off is less than with a tailwind.

[snwtoy]

Crikey, seems pretty elementary to me.
As long as the wheels aren't locked in place, the thrust provided by the engines doesn't care what they're doing. The wheels are simply an anti-friction device, same as floats on water or skis on ice.

In practice, because the wheels aren't driven, I'm not entirely sure what will happen.
I guess once the plane starts moving the conveyor senses the movement and tries to compensate.
But then how do you measure the speed of the wheels by themselves? Is it the speed relative to the surface of the conveyor? If this is the case then the wheels and the conveyor would accelerate at an insane speed, completely disproportonate to the speed of the plane.
In fact, given the limits of the tyres on the wheels, some time bofore the plane reaches lift-off speed they would pobably be spinning so fast they tyres would disintergrate, not to mention the conveyor self destructing, thus in turn causing the plane to crash and burn before it leaves the ground.

Hrmm.. can I reverse my vote?

[malc]

From what I make of the wording in the question,
the large conveyor belt is not driven by the wheels on the aircraft, in the same way that the wheels on a car drive the rollers on a rolling road.
The conveyor belt simply is matching the speed at which the aircraft's wheels are turning.
It does not really matter how the contraption works, all we need to know is that it does, and in theory if the wheels were to lock instantly, then the belt would also stop immediately.
So, for a plane to take off it must have an airspeed, not to be confused with a ground speed. The speed at which the air moves past the aircraft or which the aircraft moves through the air is the airspeed, the speed at which the aircraft moves over the ground is called the ground speed.
For a plane to take off it must move down a runway and get to a certain speed, at the same time, the wheels of the plane are spinning around letting the plane move along. If the plane is on a large belt which is spinning in the opposite direction of the wheels which is matching the speed of the wheels at all times, then the plane will not be moving in a forward or backward direction, it will stay in the same place. It will have a ground speed relative to the rolling belt, but it will not have an airspeed, which is required for an aerofoil to work.
A similar example would be if you are running on a treadmill in a room with no wind, you are staying in the same place relative to the air around you, and thus you cannot feel any airflow around your body (that is to say if the treadmill would match the speed that you are running).

[Bridget]

Razz is exactly right...

[RomanV]

Hmmm, should I spoil their fun?


This article brings up an interesting point.

Imagine, if instead of a plane, it was a hovercraft sitting on the conveyor belt.

As the hovercraft moved fowards at 20mph, the conveyor moved backwards at 20mph.
Since the hovercraft floats above the ground, it is irrelevant how fast the conveyor is going underneath it....
The propellers push against the AIR, not the ground....
The same thing happens with the aircraft.

So relative to the ground, the hovercraft is travelling at 20mph.
But relative to the conveyor, the hovercraft is travelling at 40mph.

In other words, if someone jumped off the hovercraft and hit the ground, he would hit it at 20mph.
If he jumped off and hit the conveyor, he would effectively hit it at 40mph.

The friction generated by the wheels of the plane is so marginal, there's no way that it is going to counteract the thousands of pounds of thrust generated by the engines of the plane.

I had a quick search of the net about this problem, and one thread waffled on for 150 odd pages.
But concluded in my favour towards the end.

[vvega]

aside from the wheels poping from exessive rotational speed it would take off

Those of you who think the treadmill would effect the takeoff run somehow have it in your mind (even if analytically you can state otherwise) that the wheels somehow propel the plane up to takeoff speed. This is simply not true.

Planes take off on floats and skis all the time with no wheels spinning at all. The analogies that various posters tried to make with those things are completely valid. There is no solid connection between the surface of the beltway and the CENTER OF GRAVITY of the airplane. The only physical connection is at the bottom of the tires. Resistance there does not hold back the axles, does not hold back the landing gear legs or trunnions, does not hold back the center of gravity of the airplane - it just makes the wheels spin. If we've solved the tire speed limit then those wheels can spin as fast as they want.

While the tires are spinning at whatever speed they want (around their own axles) the plane accelerates happily down mister runway and takes off as usual and we all get to Fresno on time.

see the problem is physics
most importantly newtons 3rd law
have a thing about it

[ollieboy]

According to the New Zealand institute of Physics Test Paper. The Plane would not take off because of the fact there is no air movement over the wings.

It is true that planes take off on skis etc but they are still moving when they take off, they are being thrust by props or jets which push the plane along and cause air movement under the wings which creates lift.

If the plane on the conveyor belt was moving at huge speeds it still remains in the same position in terms of displacement relative to the earth beneath the conveyor belt. This means there is no air movement across the wings to create lift.

[snwtoy]

According to the New Zealand institute of Physics Test Paper. The Plane would not take off because of the fact there is no air movement over the wings.

It is true that planes take off on skis etc but they are still moving when they take off, they are being thrust by props or jets which push the plane along and cause air movement under the wings which creates lift.

If the plane on the conveyor belt was moving at huge speeds it still remains in the same position in terms of displacement relative to the earth beneath the conveyor belt. This means there is no air movement across the wings to create lift.

I want a link to this test you harp on about.
The plane engines create thrust agains the surrounding air, generating air speed and thus lift as the pressure difference above and below the wing increases.
To say that the plane never takes off is just wrong. It doesn't matter if the ground is a conveyor belt, an oil slick or a runway - so long as there is a friction reducer involved the plane will take off.

[vvega]

According to the New Zealand institute of Physics Test Paper. The Plane would not take off because of the fact there is no air movement over the wings.

It is true that planes take off on skis etc but they are still moving when they take off, they are being thrust by props or jets which push the plane along and cause air movement under the wings which creates lift.

If the plane on the conveyor belt was moving at huge speeds it still remains in the same position in terms of displacement relative to the earth beneath the conveyor belt. This means there is no air movement across the wings to create lift.

so explane how your convayer belt disobays one of the most fundimental laws of physics and stops the thrust from the engines form pushing on the air particals behind the plane providing a forward thrust ???

the only thing it stops is the wheels form spinning at the attually speed the plane is traveling
and as the wheels DO NOT propell the plane it has no affect on the planes movement

i ask you this
if the planes wheels are so important to taking off
how dose a harrier do it

how do other VTAL aircraft do it

v