Plane on a Conveyor Belt

[iacas]

Originally Posted by Chipless

OK.  Looks like I misread the initial post, and now have my tail tucked between my legs.

Props to you for that. You wouldn't believe how many people dig in and insist that the plane isn't going to move forward.

[the chopper]

Hi, It's similar to the one about superman flying at the speed of light with a vanity mirror held out in front of him! can he see a reflection???

The reason i am saying NO the plane cannot take off is that i reckon air passing under the wings as the plane physically moves in a linear direction is ultimately what causes lift.  And all things being equal a treadmill does not cause wind in my face in the same way as physically running through air!  even although ground speed has been simulated.  Good teaser though!

[the chopper]

So it is a jet plane! would the same be true for a prop engine?

[jamo]

Hi, It's similar to the one about superman flying at the speed of light with a vanity mirror held out in front of him! can he see a reflection??? The reason i am saying NO the plane cannot take off is that i reckon air passing under the wings as the plane physically moves in a linear direction is ultimately what causes lift.  And all things being equal a treadmill does not cause wind in my face in the same way as physically running through air!  even although ground speed has been simulated.  Good teaser though!

[quote name="the chopper" url="/t/63320/plane-on-a-conveyor-belt/30_30#post_786877"]So it is a jet plane! would the same be true for a prop engine? [/quote] Read the above. It takes off. As would a plane with a propeller engine. It's not the same as running on a treadmill because airplanes don't attempt to take off using their wheels, just as you don't run with a jet pack on your shoulders and rollerblades on your feet.

[the chopper]

If prop thrust were enough to lift the plane up, then the wings would be no larger than the area behind the prop. Clearly, however, wings extend out much further than the props of plans. Consider also jets, which do not thrust any air at all on the wing, the thrust of a jet goes behind the wing.

Planes fly because there is more surface area on the top of the wing than on the bottom (the bottom is flat the top is curved). Because the air has to travel a greater distance over the top than on the bottom, the air gets spread out, as it were, and the air-pressure on the top of the wing is less than on the bottom. Because the air pressure is less, it generates upward lift.

It takes a whole lot of air moving at a rapid speed over a wing to generate lift. The props or jets aren't going to move the plane forward because the plane is on a belt. If the plane is not moving forward, then there is no motion of air over the wings (and remember, the prop blast won't cover the entire wing, so it is not enough). No air moving over the wings? No lift.

That dog won't hunt.

I found this post at the following site               http://www.kottke.org/06/02/plane-conveyor-belt  , It is certainly worth noting that the question demands a zero groundspeed no matter how much propulsive thrust is generated from an engine!! Airflow dynamics cause lift and flight not simply thrust from engines.

replies please!! i will put this to some US airforce people i know and get back to you, cheers! great question.

[Golfingdad]

If prop thrust were enough to lift the plane up, then the wings would be no larger than the area behind the prop. Clearly, however, wings extend out much further than the props of plans. Consider also jets, which do not thrust any air at all on the wing, the thrust of a jet goes behind the wing.  Planes fly because there is more surface area on the top of the wing than on the bottom (the bottom is flat the top is curved). Because the air has to travel a greater distance over the top than on the bottom, the air gets spread out, as it were, and the air-pressure on the top of the wing is less than on the bottom. Because the air pressure is less, it generates upward lift.  It takes a whole lot of air moving at a rapid speed over a wing to generate lift. The props or jets aren't going to move the plane forward because the plane is on a belt. If the plane is not moving forward, then there is no motion of air over the wings (and remember, the prop blast won't cover the entire wing, so it is not enough). No air moving over the wings? No lift.  That dog won't hunt.

I found this post at the following site               [URL=http://www.kottke.org/06/02/plane-conveyor-belt]http://www.kottke.org/06/02/plane-conveyor-belt[/URL]  , It is certainly worth noting that the question demands a zero groundspeed no matter how much propulsive thrust is generated from an engine!! Airflow dynamics cause lift and flight not simply thrust from engines.

replies please!! i will put this to some US airforce people i know and get back to you, cheers! great question.

You're thinking the same way I did when I first answered and the same way every non tst-er I have quizzed with this since. You are making the incorrect assumption that the conveyor belt is keeping the plane from actually moving. The conveyor belt will have ZERO effect whatsoever on the planes travel. You are correct that the pet propulsion doesn't provide the lift but it does provide ALL of the power to move the plane so the wind can do its job. Another way to think about it is to consider a water plane ... The water provides no friction against the floats yet those planes are still able to get up to speed and take off. Why? Because its the props (or jets) providing the power. Love this question though ... I'm relaying it to everybody. :)

[x129]

The plane will move forward and hence have airspeed over the wing. The fact that it is on a belt moving in the opposite is irrelevant.  Feel free to google any of the zillion a websites but the summation is that the wheels on the plane are not there for propulsive power. They are there to reduce friction. Think about how sea planes(or snow planes with skis) can still take off.  The wheels on this plane will spin like crazy bu the plane will still go forwards and generate airspeed over the wings.  If your a physics geek, you can draw up a free body diagram where the forces in the x direction are the  F = (thrust of the propeller- drag from the wheels). The drag on the wheels has nothing to do with thrust.

Quote:

If prop thrust were enough to lift the plane up, then the wings would be no larger than the area behind the prop. Clearly, however, wings extend out much further than the props of plans. Consider also jets, which do not thrust any air at all on the wing, the thrust of a jet goes behind the wing.

Planes fly because there is more surface area on the top of the wing than on the bottom (the bottom is flat the top is curved). Because the air has to travel a greater distance over the top than on the bottom, the air gets spread out, as it were, and the air-pressure on the top of the wing is less than on the bottom. Because the air pressure is less, it generates upward lift.

It takes a whole lot of air moving at a rapid speed over a wing to generate lift. The props or jets aren't going to move the plane forward because the plane is on a belt. If the plane is not moving forward, then there is no motion of air over the wings (and remember, the prop blast won't cover the entire wing, so it is not enough). No air moving over the wings? No lift.

That dog won't hunt.

I found this post at the following site               http://www.kottke.org/06/02/plane-conveyor-belt  , It is certainly worth noting that the question demands a zero groundspeed no matter how much propulsive thrust is generated from an engine!! Airflow dynamics cause lift and flight not simply thrust from engines.

replies please!! i will put this to some US airforce people i know and get back to you, cheers! great question.

[the chopper]

I do appreciate the wheels on the plane or any other combustion based engined vehicle do not propel the vehicle, the wheels provide a stable basis for the mass of the vehicle and minimize the drag of friction on the land surfaces. Where,  for now anyway, i am unconvinced is simply this. From what i have read about the laws of flight, Thrust alone will not cause stable flight!. And the fact the wheels would be whizzing round on the conveyor belt equal to the speed/ forward momentum created by the force of the thrust of the plane, makes the fact a plane is on a belt very relevant indeed. There would need to be an appropriate headwind in my opinion (100mph) for talking sake to get that plane to lift up in stable flight. The dynamics of air pressure on the wings is what causes flight, not thrust, It really depends on which aircraft too, i am thinking of a jumbo jet.

[x129]

You don't seem to understand that the plane will not be stationary. It will be going forward at some fast speed.  Again the force accellerating the plane is not applied to the ground.  Having the treadmill just makes the wheels spin a bit more.

Originally Posted by the chopper

I do appreciate the wheels on the plane or any other combustion based engined vehicle do not propel the vehicle, the wheels provide a stable basis for the mass of the vehicle and minimize the drag of friction on the land surfaces. Where,  for now anyway, i am unconvinced is simply this. From what i have read about the laws of flight, Thrust alone will not cause stable flight!. And the fact the wheels would be whizzing round on the conveyor belt equal to the speed/ forward momentum created by the force of the thrust of the plane, makes the fact a plane is on a belt very relevant indeed. There would need to be an appropriate headwind in my opinion (100mph) for talking sake to get that plane to lift up in stable flight. The dynamics of air pressure on the wings is what causes flight, not thrust, It really depends on which aircraft too, i am thinking of a jumbo jet.

[billchao]

Originally Posted by the chopper

I do appreciate the wheels on the plane or any other combustion based engined vehicle do not propel the vehicle, the wheels provide a stable basis for the mass of the vehicle and minimize the drag of friction on the land surfaces. Where,  for now anyway, i am unconvinced is simply this. From what i have read about the laws of flight, Thrust alone will not cause stable flight!. And the fact the wheels would be whizzing round on the conveyor belt equal to the speed/ forward momentum created by the force of the thrust of the plane, makes the fact a plane is on a belt very relevant indeed. There would need to be an appropriate headwind in my opinion (100mph) for talking sake to get that plane to lift up in stable flight. The dynamics of air pressure on the wings is what causes flight, not thrust, It really depends on which aircraft too, i am thinking of a jumbo jet.

Which aircraft is irrelevant. What type of engine it has is also irrelevant. What is important is that the engines will move air. The displacement of that air will create a low pressure area in front of the engine where the air used to be, which will cause more air to rush into it. Thus, air movement is created by the engine. Take a jet engine and mount it to some brackets to secure it and turn it on. It still sucks up the air in front of it and blows it out the back, yet and it's not moving laterally an inch. They do thrust tests on engines all the time by attaching them to 747s and putting on the brakes. The engine is still causing force to be applied to the airplane.

The very instant the thrust of the plane is strong enough to create lift, the conveyor belt is out of the picture and that plane is gone.

[Golfingdad]

I do appreciate the wheels on the plane or any other combustion based engined vehicle do not propel the vehicle, the wheels provide a stable basis for the mass of the vehicle and minimize the drag of friction on the land surfaces. Where,  for now anyway, i am unconvinced is simply this. From what i have read about the laws of flight, Thrust alone will not cause stable flight!. And the fact the wheels would be whizzing round on the conveyor belt equal to the speed/ forward momentum created by the force of the thrust of the plane, makes the fact a plane is on a belt very relevant indeed. There would need to be an appropriate headwind in my opinion (100mph) for talking sake to get that plane to lift up in stable flight. The dynamics of air pressure on the wings is what causes flight, not thrust, It really depends on which aircraft too, i am thinking of a jumbo jet.

Nope. Planes are different in that the power moves the air around them to propel them forward. Whereas the wheels on other engined vehicles (I.e. a car) ABSOLUTELY DO propel them. The power of the engine goes directly to the driveshaft and turns the wheels. You stick a car on a conveyor belt and that thing ain't hoping anywhere. Somehow we will make you understand ... The plane is still moving forward at the same speed regardless of if its on a conveyor belt. The plane is not stationary. Period. If the conveyor belt moved with the plane it wouldn't go twice as fast either. It would still be going the exact same speed and the wheels simply would not spin.

[Ernest Jones]

Originally Posted by the chopper

If prop thrust were enough to lift the plane up, then the wings would be no larger than the area behind the prop. Clearly, however, wings extend out much further than the props of plans. Consider also jets, which do not thrust any air at all on the wing, the thrust of a jet goes behind the wing.

Planes fly because there is more surface area on the top of the wing than on the bottom (the bottom is flat the top is curved). Because the air has to travel a greater distance over the top than on the bottom, the air gets spread out, as it were, and the air-pressure on the top of the wing is less than on the bottom. Because the air pressure is less, it generates upward lift.

It takes a whole lot of air moving at a rapid speed over a wing to generate lift. The props or jets aren't going to move the plane forward because the plane is on a belt. If the plane is not moving forward, then there is no motion of air over the wings (and remember, the prop blast won't cover the entire wing, so it is not enough). No air moving over the wings? No lift.

That dog won't hunt.

I found this post at the following site               http://www.kottke.org/06/02/plane-conveyor-belt  , It is certainly worth noting that the question demands a zero groundspeed no matter how much propulsive thrust is generated from an engine!! Airflow dynamics cause lift and flight not simply thrust from engines.

replies please!! i will put this to some US airforce people i know and get back to you, cheers! great question.

A stationary plane will not be able to produce lift regardless of how much thrust is created (due to the absence of airflow) but this plane is not stationary, the treadmill is a decoy.

[JetFan1983]

Here's the Mythbusters' episode on this... well, the last four minutes of it anyway.

[Missouri Swede]

Originally Posted by the chopper

And the fact the wheels would be whizzing round on the conveyor belt equal to the speed/ forward momentum created by the force of the thrust of the plane, makes the fact a plane is on a belt very relevant indeed.

Yes, for flight you need lift provided by airspeed, provided by thrust.  But I think the problem is that you are thinking that the conveyor belt going backwards provides some sort of force that keeps the aircraft from moving forward and accumulating airspeed and groundspeed, acting like a brake to keep the aircraft from moving forward. This isn’t so.

Let’s assume for a moment that it were true that the conveyor belt kept the plane from moving. (And for simplicity, let’s have wind zero, so that airspeed equals groundspeed.)

The moment the plane began to move forward and generate any positive groundspeed and the conveyor instantaneously responds with opposite velocity, then (if I understand your reasoning that this would keep the plane motionless) the groundspeed of the aircraft would stay at zero.  But if groundspeed has stayed at zero, the conveyor speed would also be zero.

So then you would have the engines up at max power (imagine an F15 in full afterburner; but it doesn’t matter whether it’s a single engine prop—same thing applies, just cooler with an F15 IMO), and there would be zero groundspeed. But with zero groundspeed, the conveyor is stationary. So the plane is sitting on afterburner generating 35000 lbs of thrust (or whatever), and the wheels and conveyor are not moving at all.

Where is all the force generated by the engines going? You’ve got a large mass of air being displaced providing forward thrust on the aircraft; it has to be acting on something. But in your reasoning, the airplane remains stationary and so the wheels don’t spin and the conveyor is motionless.

If an aircrew member came to me stating the above, I’d place him on DNIF (Duties Not Involving Flying). And I’d fly with someone else that day.

[iacas]

Originally Posted by billchao

Which aircraft is irrelevant. What type of engine it has is also irrelevant. What is important is that the engines will move air. The displacement of that air will create a low pressure area in front of the engine where the air used to be, which will cause more air to rush into it. Thus, air movement is created by the engine. Take a jet engine and mount it to some brackets to secure it and turn it on. It still sucks up the air in front of it and blows it out the back, yet and it's not moving laterally an inch. They do thrust tests on engines all the time by attaching them to 747s and putting on the brakes. The engine is still causing force to be applied to the airplane.

The very instant the thrust of the plane is strong enough to create lift, the conveyor belt is out of the picture and that plane is gone.

To be clear, the plane doesn't "pull air over the wings" so much as it uses the air to push or pull the wing (and the rest of the plane itself) forward through the air.

A prop on a plane works exactly (albeit a bit less efficiently given the different densities between liquid water and gaseous air) like the prop on a boat. It pushes or pulls against the air to MOVE THE PLANE FORWARD. Just like someone standing on a treadmill with skates could pull against a rope mounted to the wall to pull themself forward.

P.S. Also note that the pilot in MythBusters gets it wrong. He says (paraphrased) - "if I match my speed to the tarp AND THERE'S NO AIR OVER THE WINGS I should sit there like a brick." The capitalized part is not accurate. He succumbed to the same faulty logic as chopper and others.

[iacas]

Here's a sort of free-body diagram showing the resulting motion from the forces at play. Arrows not to scale because if they were you would not even see the friction of the wheels arrow it'd be so tiny.

[billchao]

Originally Posted by iacas

To be clear, the plane doesn't "pull air over the wings" so much as it uses the air to push or pull the wing (and the rest of the plane itself) forward through the air.

A prop on a plane works exactly (albeit a bit less efficiently given the different densities between liquid water and gaseous air) like the prop on a boat. It pushes or pulls against the air to MOVE THE PLANE FORWARD. Just like someone standing on a treadmill with skates could pull against a rope mounted to the wall to pull themself forward.

P.S. Also note that the pilot in MythBusters gets it wrong. He says (paraphrased) - "if I match my speed to the tarp AND THERE'S NO AIR OVER THE WINGS I should sit there like a brick." The capitalized part is not accurate. He succumbed to the same faulty logic as chopper and others.

I don't believe I said that? If I implied it, I apologize. What I was trying to illustrate is that a plane's engine will draw air even when stationary.

[iacas]

Originally Posted by billchao

I don't believe I said that? If I implied it, I apologize. What I was trying to illustrate is that a plane's engine will draw air even when stationary.

You did, and you kind of just did it again...? The purpose of the engines is to move the plane, not to move the air.