Airplane physics question

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I read that the mythbusters test was incorrect as the speed of the conveyor belt was not the same as the plane, resulting in the plane moving faster and having sufficent airflow through the wings in order for it to take off.

Surely it would be like running with a kite on a treadmill. Your not actually moving so there is no wind force to lift the kite.
 
Read my post above with the picture of the airplane on top of the car. There is no way that the treadmill can ever cancel the forward motion of the plane.
 
Duke
Read my post above with the picture of the airplane on top of the car. There is no way that the treadmill can ever cancel the forward motion of the plane.
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I just re-read your post and things are starting to become a bit clearer. If the plane had a speedometer it would still be moving at say 40mph relative to the belt. So, the plane is moving creating enough airflow to give lift.

One other question. If they installed a large conveyor belt on a battleship, could the plane use the conveyor belt to gather enough airspeed to lift off and then continue to fly off the boat?
 
Mark T
I just re-read your post and things are starting to become a bit clearer. If the plane had a speedometer it would still be moving at say 40mph relative to the belt. So, the plane is moving creating enough airflow to give lift.

One other question. If they installed a large conveyor belt on a battleship, could the plane use the conveyor belt to gather enough airspeed to lift off and then continue to fly off the boat?
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The belt has no effect on the plane's speed or takeoff distance requirements.
 
A fair chunk of those who believe the plane will never take off will still not change their mind after this Mythbusters episode, for one simple reason: there are two versions of the actual question at hand, and they'll just switch to the other version.

In one, which is the one the Mythbusters addressed, the belt matches the speed of the plane. The argument that they actually didn't match speeds exactly is a minor flaw and has no effect on the general outcome of the experiment.

In the other version however, in which the belt matches the speed of the planes wheels, the conveyor will reach infinite speed after a short while by definition. And whichever effort you put into a real life experiment of this, there is no way to accurately match it. On that basis, there is no way to prove the plane takes off if you think that this is the correct way to ask this question.
 
But if the conveyor has no effect on the takeoff distance required, then it's clear that the speed of the conveyor is independent of the takeoff problem. So it absolutely doesn't matter what speed the conveyor goes.

It's pretty open and shut.

Plus, theory supports that conclusion.
 
Famine
The belt has no effect on the plane's speed or takeoff distance requirements.
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I see what your saying. I suppose that if you hung a plane from a piece of string it would still move through the air, so it's the same as the plane moving on the conveyor belt. Is that correct?
 
Mark T
I see what your saying. I suppose that if you hung a plane from a piece of string it would still move through the air, so it's the same as the plane moving on the conveyor belt. Is that correct?
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Here's another way to imagine it:

You're standing on a skateboard on a treadmill that is designed to move opposite the rolling wheels of the skateboard at an identical speed. You're holding onto a ski tow rope.

The ski tow rope is tied to the trailer hitch of your buddy's jeep (starting to sound like a YouTube moment). The jeep is not on the treadmill.

When your buddy steps on the gas, nothing that treadmill can do will stop the jeep from towing you off the end of it. The treadmill may fly apart trying to keep up, and the skateboard wheels may burst into flames from the speed of the infinite feedback loop driving the treadmill. But you're going to follow the jeep down the road, pulled by your arms.
 
Duke
Here's another way to imagine it:

You're standing on a skateboard on a treadmill that is designed to move opposite the rolling wheels of the skateboard at an identical speed. You're holding onto a ski tow rope.

The ski tow rope is tied to the trailer hitch of your buddy's jeep (starting to sound like a YouTube moment). The jeep is not on the treadmill.

When your buddy steps on the gas, nothing that treadmill can do will stop the jeep from towing you off the end of it. The treadmill may fly apart trying to keep up, and the skateboard wheels may burst into flames from the speed of the infinite feedback loop driving the treadmill. But you're going to follow the jeep down the road, pulled by your arms.
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Thanks for that, it's all coming together. So the way the power is implemented determines how it moves. For example, if I hold a remote control car by its roof and spin the wheels, nothing will happen. If I hold a model plane and power it up I should feel some force as the planes is moving air.

If you had a remote control car, strapped some wings to it, would it be able to lift off even though the actual drive system is different?
 
Mark T
Thanks for that, it's all coming together. So the way the power is implemented determines how it moves. For example, if I hold a remote control car by its roof and spin the wheels, nothing will happen. If I hold a model plane and power it up I should feel some force as the planes is moving air.

If you had a remote control car, strapped some wings to it, would it be able to lift off even though the actual drive system is different?
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The wheels on the bottom moving freely prevent the conveyor from exerting any substantial force on you. The faster the conveyor goes, the faster the wheels turn, but you sit still. The only force that acts on you is the frictional force of the wheels rotating (which is negligible in this case).

If there were a drivetrain it would be different, but because the wheels are allowed to rotate freely, no force gets translated.

Edit: ^^ The above is referring to the airplane problem. Not the RC car.
 
Mark T
If you had a remote control car, strapped some wings to it, would it be able to lift off even though the actual drive system is different?
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No, it would sit in one place, because no matter how fast the wheels went, the treadmill could counteract it. But with a plane, the power is generated by pushing against the air, which is not affected by the treadmill.
 
Mark T
If you had a remote control car, strapped some wings to it, would it be able to lift off even though the actual drive system is different?
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Yes, as long as it's not on a conveyor belt rotating in the opposite direction...

This is because your winged car relies on friction between the driven wheels and the road surface for forward motion, so if the car is going forwards at 100mph, and the conveyor belt is going backwards at 100mph, the car's airspeed, assuming no wind, would be zero. Hence zero lift from wings, hence zero takeoff. I stress, this applies for your wheel driven car ONLY. Aircraft always, without fail, avoid this unlikely problem by pushing against air instead of against ground, which brings me neatly to the second flaw with your wheel driven winged car: Assuming it's not on the conveyor belt, as soon as it takes off it will lose thrust because it's driving wheels are no longer touching the ground. Without thrust, it will slow down, lose airspeed (and lift) and come back to earth again. So it will never really fly.
 
Alfaholic
Yes, as long as it's not on a conveyor belt rotating in the opposite direction...

This is because your winged car relies on friction between the driven wheels and the road surface for forward motion, so if the car is going forwards at 100mph, and the conveyor belt is going backwards at 100mph, the car's airspeed, assuming no wind, would be zero. Hence zero lift from wings, hence zero takeoff. I stress, this applies for your wheel driven car ONLY. Aircraft always, without fail, avoid this unlikely problem by pushing against air instead of against ground, which brings me neatly to the second flaw with your wheel driven winged car: Assuming it's not on the conveyor belt, as soon as it takes off it will lose thrust because it's driving wheels are no longer touching the ground. Without thrust, it will slow down, lose airspeed (and lift) and come back to earth again. So it will never really fly.
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Understood. I feel much better now that's cleared up.
 
Mark T
So the way the power is implemented determines how it moves.
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Thats pretty much what they said on Mythbusters. Since the plane is being pushed by the propeller pushing against air and not the wheels pushing against the ground, the ground is therefore independent and doesn't matter.
 
Loon
Since the plane is being pushed by the propeller pushing against air and not the wheels pushing against the ground, the ground is therefore independent and doesn't matter.
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Exactly. I don't know why people don't realize this.
 
Bernouilli's principle! I learned this not long ago in physics. Thats all we need to know here. I can't believe this questions still up, where's the aerospace engineer of the forum? I forget his username...
 
That would be Danoff, who posted about six posts above yours.
 
Keichi Tsuchiya
Bernouilli's principle! I learned this not long ago in physics. Thats all we need to know here.
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More important than how the aircraft generates lift is how it generates thrust. The fact that it doesn't push with it's wheels is what's critical here.
 
If, indeed, the Bernoulli Principle is true in that application. Which it might not be.

[/Stir]
 
To put this into perspective, a plane rolling on a conveyor belt moving at 25mph would be just like a plane rolling at 25mph (ground speed) on a runway with a 25mph tailwind.

As long as they can achieve sufficient air speed to take off, planes take off with tailwinds all the time.
 
Wolfe
To put this into perspective, a plane rolling on a conveyor belt moving at 25mph would be just like a plane rolling at 25mph (ground speed) on a runway with a 25mph tailwind.

As long as they can achieve sufficient air speed to take off, planes take off with tailwinds all the time.
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*places a phonecall to Air Traffic Controller brother, to tell him "YUOR DOING IT RONG"*
 
Famine
*places a phonecall to Air Traffic Controller brother, to tell him "YUOR DOING IT RONG"*
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Of course an ATC at a controlled airport would point a plane to the runway entrance that would give them a headwind. That doesn't mean that planes can't or don't take off with a tailwind.
 
TVRKing
How would you get a plane on a conveyor in the first place?
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They a huge sheet of....some kind of material with one end attached to a truck. The truck and plane simultaneously took off in opposite directions.
 
Loon
They a huge sheet of....some kind of material with one end attached to a truck. The truck and plane simultaneously took off in opposite directions.
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...Pilot is practical joker and hits wheel brakes as hard as he can just before VR. Truck driver suffers the mother of all faceplants.
 
there is really only 2 things that come into play

force from the engines (F)
resistance in the wheel bearings (R)

as long as F>R then the conveyor belt is going to make bugger all difference to the plane.

and iam gonna take a guess here that the hundreds of thousands of bhp a 747's engines throw out will overcome the wheel bearing resistance fairly happily.

the most concerning part of this is famines post about how this affects helicopters a couple of years ago, very frightening.
 
Three things strike me about this thread:

1. Sage's rather odd resurrection of it;
2. The fact that this question is still being asked. Seriously, it makes me want to smash my brains out with a hammer every time I see it. The answer is so simple, and yet so many people have such difficulty comprehending it; and
3. That in 2005, Famine was one such person.

Can we please consign this question to the ranks of those which should never be asked? Seriously, it's up there with "Hello young lady, have you been putting on weight recently?"; "Hi Nelson, so that apartheid thing was all a bit of a fuss about nothing then, wasn't it?"; and "Seven days to build the Earth? Really? Are you sure?"
 
A different question then :p

What if you were flying a plane, and the speedometer shows your doing 100 knots, however, you have headwind, and the speed of the headwind is estimated at a 120 knots. Is the plane flying backwards?

Probably a "obvious answer" question, but I heard some mates of me discussing it the other day...
 
Bram Turismo
A different question then :p

What if you were flying a plane, and the speedometer shows your doing 100 knots, however, you have headwind, and the speed of the headwind is estimated at a 120 knots. Is the plane flying backwards?
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Well, if you're doing 100, and the wind does 120 in the opposite direction, you're doing -20, which means that the plane actually moves backwards in respect to the ground, even if it moves forward in respect to the surrounding air.
 
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