Airplane: Does it take off?

[craigwatanabe]

Re: Airplane: Does it take off?

Mythbuster's episode already on Youtube well the sneak preview anyway

[craigwatanabe]

Re: Airplane: Does it take off?

so it takes off but the plane is moving forward as opposed to being stationary. If the plane is moving forward of course wind will be generated over and under the wings to create lift thus liftoff.

The whole point of the conveyor belt is to keep the plane stationary while the wheels spin below. The plane never stayed stationary, it moved forward.

I'm sorry but that episode never really convinced me.

[zff]

Re: Airplane: Does it take off?

The plane never stayed stationary, it moved forward.

That's what I've been saying!

The whole point of the conveyor belt is to keep the plane stationary while the wheels spin below.

How could you possibly get that interpretation from this question?

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The treadmill wasn't there to keep the airplane in place, the treadmill was there to see if you understand how airplanes generate forward thrust.



Let me just do a simple substitution of words and concepts.

A car is sitting at one end of a long wind tunnel. The car moves in one direction while the wind blows in the opposite direction. This wind tunnel has a control system that tracks the speed of the car and tunes the speed of the wind to be exactly the same (but in the opposite direction).

Would you interpret this to mean the wind prevents the car from moving forward? If not, please point out how the grammar in my question differs enough from the grammar in the original question to give you the opposite impression.

BTW, you *do* understand how difficult it would be for an airplane to taxi in that wind tunnel, right? It's be just as difficult as a car on the treadmill. I really thought you were kidding around and understood the trick nature of the question. Maybe I will take a few of those 93¢ bricks.

[helen]

Re: Airplane: Does it take off?

I missed the part where they used a model plane to demostrate the process which I understand it did lift off.

I did see the part about them using the ultralight plane which they did a good job of recording that the plane needed a runway length of 85 feet with a takeoff speed of 25 mph. However I do have an issue what they used for the conveyer belt, what drove it and not revealing the length it took, the speed it needed to take off or the amount of time it took from zero to take off for both flights.

[craigwatanabe]

Re: Airplane: Does it take off?

so the "trick" part of the question is that the plane is "moving" in an opposite direction of the conveyor belt.

It never said the plane was stationary. BUT if the conveyor belt is counteracting any forward motion with equal counter action the plane should stay stationary, like walking on a treadmill. You only move forward if your rate of speed exceeds the rate of speed of the treadmill.

BUT the conveyor belt is tuned to the same speed of the aircraft.

Yeah this is a trick question, obviously because if you were to consider keeping the plane stationary while the thrust counters the treadmill it won't fly, but if the thrust exceeds the treadmill the plane moves forward thus generating lift.

One thing Mythbusters failed to take into account is drag on the conveyor belt/tarp. The tarp will create some sort of wind movement simply because of the friction of the tarp's surface against air within it's proximity. This friction causes the air molecules to drag and be pulled along with the tarp creating motion in that area.

Any motion below the wing vs no motion above the wing produces the same result as wind blowing over/under the lifting surface.

The bottom line is that it takes differing wind speeds to make an aircraft to rise or fall. The conveyor belt is creating wind like a fan (albeit not an effective one).

[GeckoGeek]

Re: Airplane: Does it take off?

BUT if the conveyor belt is counteracting any forward motion with equal counter action the plane should stay stationary, like walking on a treadmill.

Ah, but that's the trick. The treadmill CAN'T counteract the plane's motion because the wheels of an airplane freewheel just like a trailer. Walking on a treadmill is not a valid comparison because the walker is pushing against the treadmill to go forward. Same with a car.

Think about this: The plane's motion is going to be determined by F=ma. What forces are acting on the mass of the plane? Well, you've got the force of the prop sending the airplane in the direction of take off. What about the treadmill? If the wheels are frictionless, they just spin. The treadmill can't apply any counteracting FORCE to oppose the prop. All the treadmill does is make the wheels spin faster.

[zff]

Re: Airplane: Does it take off?

A trailer is a good analogy. The entire airplane is a trailer, and the propeller is towing it. It really doesn't matter if the treadmill is on or off.

[craigwatanabe]

Re: Airplane: Does it take off?

But that means the propeller is pulling wind over the wings creating lift. Nowhere in the question did it say that Thrust was key in keeping up with the conveyor.

For arguement's sake let's say the plane was a glider with no thrust or prop capabilities just wheels that spin in relation to the conveyor belt. This is a more realistic repesentation of the question.

[salmoned]

Re: Airplane: Does it take off?

Ummmmm.........couple points..... if the wheels on the plane were friction free, then the wheels wouldn't rotate at all.

Based on this statement, I don't believe you understand the principles of friction. There is no friction between the ground and the wheels unless there is slippage (skid-marks being an obvious result of friction). Obviously there is static friction, but since that doesn't result in any energy gain or loss it can be disregarded. The only energy loss in the wheels to ground component of the system is the rotational momentum and internal friction (bearings) of the wheels. Assuming these additional losses are relatively small with regard to the thrust of the aircraft, the length of the conveyor belt would need to be only a bit longer than the normal take-off length.

[craigwatanabe]

Re: Airplane: Does it take off?

Based on this statement, I don't believe you understand the principles of friction. There is no friction between the ground and the wheels unless there is slippage (skid-marks being an obvious result of friction). Obviously there is static friction, but since that doesn't result in any energy gain or loss it can be disregarded. The only energy loss in the wheels to ground component of the system is the rotational momentum and internal friction (bearings) of the wheels. Assuming these additional losses are relatively small with regard to the thrust of the aircraft, the length of the conveyor belt would need to be only a bit longer than the normal take-off length.

Ahhh but there is...it's called surface tension

[zff]

Re: Airplane: Does it take off?

For arguement's sake let's say the plane was a glider with no thrust or prop capabilities just wheels that spin in relation to the conveyor belt. This is a more realistic representation of the question.

I (and I think everyone else here) will agree with you that if an airplane is kept stationary, it will not take off no matter how fast its wheels are spinning. I agree that the unpowered glider will never get airborne. However, if you believe that an unpowered glider is a more accurate representation of the question, then you and I have completely opposite interpretations of the original question.

I already know the point you're trying to make because I initially made the same mistake when I first saw this question. I thought the question was designed to see if anyone thought that an airplane spinning its wheels in place could somehow get airborne. If anyone misunderstood how an airplane generates lift, they might make that mistake. If you read the question carefully, however, you'll see that's NOT what it's asking.

The question is testing to see if you understand the physics behind how an airplane generates forward motion while on the ground. If you can't see that, then maybe you don't fully understand the physics behind how an airplane generates forward motion while on the ground, and specifically, how it differs from a car.

Do you understand the fundamental difference between how an airplane taxis and a car drives? Do you understand why a treadmill is not able to hold an airplane stationary?

[craigwatanabe]

Re: Airplane: Does it take off?

I thought the question was designed to see if anyone thought that an airplane spinning its wheels in place could somehow get airborne.

The question is testing to see if you understand the physics behind how an airplane generates forward motion while on the ground. If you can't see that, then maybe you don't fully understand the physics behind how an airplane generates forward motion while on the ground, and specifically, how it differs from a car.

Do you understand the fundamental difference between how an airplane taxis and a car drives? Do you understand why a treadmill is not able to hold an airplane stationary?

Ahem...that question you are quoted as being never indicated thrust as a component of the equation. It never indicated the aircraft used thrust...at all.

So with that assumption taken out of the loop, the wheels will spin in relation to the counteraction of the conveyor belt moving in the opposite direction. Because no thrust is applied the plane either stays stationary or will begin to pull backwards along with the treadmill.

In the best case scenerio, the plane remains stationary, and yes I do understand the physics of aerodynamics, you need thrust to gain lift via differing air pressure over and under the airfoil. Without the component of it (never indicated thrust in the question) liftoff cannot be achieved.

The question of how an aircraft can generate forward motion while being on the ground is very simple, thrust generated in one direction will push that aircraft in the other direction. But thrust alone cannot generate the lift necessary because thrust must generate wind currents over the airfoil to generate that lift.

If thrust can overcome the weight of the mass it is pushing against forward motion occurs. Enough of this forward motion over this mass (being the airfoil) will generate lift. But it takes forward motion to achieve that lift. Spinning wheels alone will not do it, spinning wheels along with thrust will.

It's all about air speed not ground speed and that's why the pitot tube on an aircraft is so vital. Ground radar cannot tell how fast the aircraft is moving in relation to a head wind it only tells ground speed. Wind speed tells how much more thrust a plane will need to stay airborne.

There is no wind when a plane is stationary even when the ground is moving at a 100 MPH. If the plane is to achieve liftoff it must attain a forward speed of 100 MPH and then some to achieve forward motion to attain liftoff.

Remember where in the question did it say thrust was needed to move the plane forward? Thrust was only used to compensate for the opposite motion of the conveyor belt. EQUAL but opposing forces at work make motion null. No motion means no wind. No wind means no lift. Take the opposing force out of the loop and suddenly you have motion. And if it's in the right direction, you get lift.

[craigwatanabe]

Re: Airplane: Does it take off?

Here's a lively debate I grabbed from another forum. There is no consensus there either, so don't try and cheat.

The question is:

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

Does the plane ever take off, and why or why not?

So there's the original question by none other than Adrian who has since left the building. The plane can move so as to suggest forward thrust being applied. But how much thrust is being applied to the airplane?

It doesn't matter because the conveyor belt is constantly being adjusted to match the speed EXACTLY to the speed of the airplane.

In the case of Mythbusters, the conveyor belt never matched the speed of the plane as it accelerated but lagged resulting in the plane gaining forward momentum and resulting in lift.

In the case of the conveyor belt in relation to thrust, the conveyor belt's opposing force represents thrust in the opposite direction OF EQUAL SPEED of the thrust of the airplane. The two forces cancel each other and the result is a null factor of a big fat ZERO MPH in any direction negating any forward motion thus no lift.

[zff]

Re: Airplane: Does it take off?

Ahem...that question you are quoted as being never indicated thrust as a component of the equation. It never indicated the aircraft used thrust...at all.

I made an assumption that the pilot would attempt a conventional takeoff. We're all making assumptions here, aren't we? We assume there's no wind. We assume the tires and wheels can handle the extra takeoff speed. We assume we're not using a VTOL airplane. Why would we not assume the pilot will attempt a conventional full-throttle takeoff? With that assumption, thrust is a part of the equation. I think it would be far more unreasonable to assume the pilot is only going to crack open the throttle enough to prevent moving backwards with the treadmill.

So with that assumption taken out of the loop, the wheels will spin in relation to the counteraction of the conveyor belt moving in the opposite direction. Because no thrust is applied the plane either stays stationary or will begin to pull backwards along with the treadmill.

I agree.

But thrust alone cannot generate the lift necessary because thrust must generate wind currents over the airfoil to generate that lift.

I understand that, and hope I didn't imply that I thought the propellor would generate enough wind over the wings to generate lift. I agree it won't.

If thrust can overcome the weight of the mass it is pushing against forward motion occurs. Enough of this forward motion over this mass (being the airfoil) will generate lift. But it takes forward motion to achieve that lift. Spinning wheels alone will not do it, spinning wheels along with thrust will.

I agree, again, with the provision that I had forward thrust factored into my secenario.

It's all about air speed not ground speed and that's why the pitot tube on an aircraft is so vital. Ground radar cannot tell how fast the aircraft is moving in relation to a head wind it only tells ground speed. Wind speed tells how much more thrust a plane will need to stay airborne.

There is no wind when a plane is stationary even when the ground is moving at a 100 MPH. If the plane is to achieve liftoff it must attain a forward speed of 100 MPH and then some to achieve forward motion to attain liftoff.

Still no arugment from me on any of these points.

Remember where in the question did it say thrust was needed to move the plane forward? Thrust was only used to compensate for the opposite motion of the conveyor belt. EQUAL but opposing forces at work make motion null. No motion means no wind. No wind means no lift. Take the opposing force out of the loop and suddenly you have motion. And if it's in the right direction, you get lift.

I guess this is where our agreement ends. Because I assumed a normal take off, I assumed there was going to be way more than enough forward thrust to overpower any amount of drag the treadmill could create.

In the case of Mythbusters, the conveyor belt never matched the speed of the plane as it accelerated but lagged resulting in the plane gaining forward momentum and resulting in lift.

I'm a little confused here.

The pick up pulling the conveyor belt accelerated up to the airplane's take off speed, and I imagine the truck got to that speed before the airplane did. For the entire time the airplane was on the ground, the conveyor belt either matched or exceeded the airplane's speed. Does this not fulfill the conditions outlined in the original question? The conveyor belt is to match the airplane's speed, not its thrust.

I'm not completely clear on how a treadmill can match an airplane's thrust anyway. Let's say the treadmill surface was ice, and instead of wheels, the airplane had skates. How would this icy treadmill go fast enough to prevent the ice-skating airplane from moving forward during a normal full-throttle takeoff?

[craigwatanabe]

Re: Airplane: Does it take off?

I'm not completely clear on how a treadmill can match an airplane's thrust anyway. Let's say the treadmill surface was ice, and instead of wheels, the airplane had skates. How would this icy treadmill go fast enough to prevent the ice-skating airplane from moving forward during a normal full-throttle takeoff?

Magic and some anti-gravity generators.