A harmless little riddle

This is where I diverge with "scientists" who live in academic ivory towers without regard for plain common sense. You're talking about two different planes of motion...the "runway", or in this case, the treadmill, and the air passing under, or in this case, sitting motionless around the wings. While one plane may provide the theoretical force required to generate enough momentum to allow liftoff, it's the air speed, not the ground speed, that is what generates enough uplift to propel the object vertically at a rate greater than the downward force of gravity. But, since no air is moving--only the treadmill--no vertical force is generated...only horizontal. Think of this in more than one plane. By reducing this to a single variable (horizontal velocity) the problem becomes unrealistic and cannot truly echo an actual case.

This is ground control to major tom:

Do the thrust of the engines have a vertical component?

If they do not, then you will roll on that treadmill forever and never take off.

will wrote:

This is where I diverge with "scientists" who live in academic ivory towers without regard for plain common sense. You're talking about two different planes of motion...the "runway", or in this case, the treadmill, and the air passing under, or in this case, sitting motionless around the wings. While one plane may provide the theoretical force required to generate enough momentum to allow liftoff, it's the air speed, not the ground speed, that is what generates enough uplift to propel the object vertically at a rate greater than the downward force of gravity. But, since no air is moving--only the treadmill--no vertical force is generated...only horizontal. Think of this in more than one plane. By reducing this to a single variable (horizontal velocity) the problem becomes unrealistic and cannot truly echo an actual case.

GODDAMMIT!! ANSWER THE QUESTION! WHAT FORCE IS KEEPING THE PLANE FROM MOVING??

Here's an analogy. You know those conveyor belt-walkway things in airports? What if you put a jet pack on your back and got on a skateboard on the conveyor? No matter what speed the conveyor is going, you could certainly counter act the ROLLING RESISTANCE AND FRICTION with a jetpack, right? It's the same think here!!

You are the one confusing the reference frames!! Air will move over the wings of the airplane because the airplane can move forward relative to the ground no matter how fast the freakin' treadmill is spinning!! The spinning treadmill will only make the wheels go faster and will impart no significant force on the airplane!!!

No please stop making a fool of yourself! Don't you know this message board is archived for posterity?

Someone please explain what FORCE is keeping the plane STATIONARY relative to the GROUND.

Do you know why you can't fulfill my request??

BECAUSE THERE IS NO FORCE THAT WILL KEEP THE PLANE FROM TAKING OFF! THE PLANE WILL TAKE OFF!

You guys are driving me to ALL CAPS! I'm going MAD over here! Christ.

I wish Bill Nye the Science Guy were here to explain this to you simpletons.

call me stupid... but... gravity?

At this point I just have to laugh and give up. Some people will just never get it I guess.

MarathonMind wrote:

This is ground control to major tom:

Do the thrust of the engines have a vertical component?

If they do not, then you will roll on that treadmill forever and never take off.

Does engine thrust need a vertical component? NO!!! The lift generated by the motion of air over the wings of the plane - the plane which will have both airspeed and groundspeed (approximately twice the airspeed) - will overcome the gravitational force just like with every other plane in the known world.

Answer this: Does the treadmill impart a horizontal force on the plane? (Hint: NO!!! - or at best a very small read:trivial one) If not, how does the treadmill stop the plane from moving? (Hint: IT DOESN'T!!!)

Like a brick wall I swear.

pelican wrote:

call me stupid... but... gravity?

You are stupid.

How does GRAVITY keep the PLANE from MOVING FORWARD!! GRAVITY AND THRUST FROM THE ENGINES ACT PERPENDICULAR TO EACH OTHER!!

TRY AGAIN STUPID!!

You don't think people ask deliberately stupid questions to watch the "enlightened" get enraged and show how much they need to convice others they are right? Just maybe? ; ) Why do you think posterity goes back to read threads like this...not to learn truths that are commonly known, but to see the comical interaction that ensues between fools and wise men. Shakespeare had nothing on letsrun.

will wrote:

You don't think people ask deliberately stupid questions to watch the "enlightened" get enraged and show how much they need to convice others they are right? Just maybe? ; ) Why do you think posterity goes back to read threads like this...not to learn truths that are commonly known, but to see the comical interaction that ensues between fools and wise men. Shakespeare had nothing on letsrun.

Oh of course. You knew the right answer all along. You're so sly...

Proviso: if the flaps are not angled, no the plane will never get off the ground, no matter how fast it goes. Assuming the plane has flaps facing up, sure, why not. Does that make you happy? Cornoary averted yet?

ok-my two cents. I have discuessed this with teamates long ago. I have since changed my initial opinion. I originally belived it would never take off due to the treadmill matching the wheels of the plane.

Here is the problem, that can never be the case.

Acceleration due to the force created by the engines will propell the plane forward and cause the wheels to spin faster than the treadmill. even if the treadmill instantly tries to catch up, the plane can take off, because the force will be exerted regardless of the free spinning wheels.this causes acceleration. thus the problem with wording. the actual scenario could never take place in the real world.

Now i do agree in actuallity, if this is the world we are living in, that some real issues with heat and friction would occur here, and the axles of the plane would probably be destroyed before actual take off.

david bowie wrote:

Here's an analogy. You know those conveyor belt-walkway things in airports? What if you put a jet pack on your back and got on a skateboard on the conveyor? No matter what speed the conveyor is going, you could certainly counter act the ROLLING RESISTANCE AND FRICTION with a jetpack, right? It's the same think here!!

No, given a fast enough conveyor belt, you will not go forward. 'You could certainly counter' is just incorrect. Look at the free body diagram and actually do the math.

Here's an analogy:

How are these three cases different?

1) Airplane with landing gear up resting on the fuselage

2) Airplane with landing gear down and freely rolling

3) Airplane sitting on ice treadmill with low friction skids

Will all three of these planes take off given the other constraints of the original riddle?

so could a plane on ice take off?

As long as you're admitting that according to the restraints of the question, the plane will take off, I'm happy.

Yes.

No, that's not what I'm admitting at all. If a fast enough treadmill can keep a plane resting on its fuselage from taking off, then logically a faster treadmill can keep a plane from taking off on its landing gear. So your "certainly counter" argument falls apart in the context of the original question too.

2) and 3) would definetly take off, happens everyday.

1) Yes, but you would have to make an awefully strong engine and an awefully strong fuselage. If this were a commercial plane the heat and friction involved in this case would be huge.

I see where you are going with this, but you can only run the treadmill at the same speed as the plane's wheels roll foward. Therefore you can't run the treadmill at 6 bazillion mph and make the friction forces huge, because the plane would take off when the wheels were only rolling at around 200mph (a which point the planes airspeed would be 100mph enough for most planes to take off).

I probably missed some of the 1000 posts or so, so I appologize if I restate what someone else may have pointed out.

Short answer: the plane will not take off.

Long answer: the engines on the plane are are not what really get the plane airborne - they are simply used to move the plane forward (down the runway for example) and force air over the top and bottom of the wings. Since the air must travel at the same speed across the two sides of the wings and the path across the top of the wing is longer than the path across the bottom of the wing, the pressure on the bottom of the wing is greater than the pressure on the top of the wing. This is called lift. Lift is an upward force that acting on the plane and is what allows planes to fly. You need fluid motion over the wings to generate lift and have flight - this means that air (our fluid) must be traveling over the wings. In the proposed experiment, there is no air traveling over the wings.

As an aside, I like the comment that stated if this did work, why wouldn't the navy use treadmills on aircraft carriers instead of the slings that they currently use. Maybe they just don't know ...

late comer wrote:

I probably missed some of the 1000 posts or so

Haha wow yeah you did