Plane on a Conveyor Belt - Page 3

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!

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

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:

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.

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.
 

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.

Quote:

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.
 

Quote:

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.

Quote:

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.

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

Quote:

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.

Quote:

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.