Plane and a Treadmill - DPChallenge Forums

12/06/2006 11:52:59 AM · #26

If the plane had no wheels, it would scrape along the ground, so you would need more thrust to get it to move and take off (and probably destroy the fuselage in the process).

So we give the plane wheels, so now it can move down the runway with less thrust. The movement down the runway (facilitated by the wheels) allows air to pass over and under the wings and provides lift.

Now, we place a treadmill under the wheels that magically matches the forward speed the plane would otherwise have. This makes the plane no longer move forward relative to the runway or the air (not accounting for any wind :) ). We apply more thrust as we did without any wheels to try to break free of the treadmill, but magically the treadmill also speeds up, keeping the plane stationary.

I see no way (other than wind) for air to move over the wings and provide lift.

12/06/2006 11:55:45 AM · #27

Originally posted by lesgainous:

.

Aerodynamically speaking, the plane would not be able to obtain enough lift, assuming there is nothing else (no other variables) that you left out of the scenario. There would have to be something that produces a lower air pressure above the wing (as compared to higher pressure under the wing) in order to generate lift.

The mistake is assuming the treadmill can stop the plane from moving. It cant. Planes do not take off because of their wheelspeed moving the plane forward.

The 'other variable' is the big honking jet engine pushing the plane forward by pushing air. The treadmill can't stop that. It can destroy the tires turning them incredibly fast, but planes aren't accelerated by their wheels.

Consider water planes, or a plane on skids, instead of wheels.

Another way to think about it, you are on a treadmill, on rollerskates, the treadmill turns fast enough to keep you stationary. Now someone throws you a rope tied to the wall. Can you pull yourself forward - no matter how fast the treadmill goes ? Yes you can - because that's an external force, nothing to do with the treadmill/wheel boundary - just like a jet engine/ propeller against air.

_eug

12/06/2006 11:58:18 AM · #28

Originally posted by eqsite:

Originally posted by _eug:
And y'll think all that engine thrust is going no where?

The thrust is causing the plane to move relative to the surface of the treadmill, but because the treadmill itself moves at the same speed of the plane, the plane remains stationary in relationship to the surrounding air (the engines do not move the air over the wings). No air motion over the wings means no lift.

But the thrust is being APPLIED to the air, NOT to the ground.

12/06/2006 12:01:35 PM · #29

OK, think of this: The airplane is turned off. No thrust. What happens if the treadmill starts moving? Will the wheels on the plane just turn and the plane will stay in place, or will the plane move along with the treadmill.

Given the friction of the wheels along the treadmill and the inertia of plane itself, I speculate that the plane will move with the treadmill.

Now, in order to keep the plane from sliding off the treadmill, I turn the engines on and apply thrust. I should be able to come into equilibrium with the treadmill and remain stationary.

Now the question is, once I've achieved that equilibrium and I apply more thrust, will the plane move forward (having broken the friction and inertia) or not?

If all of the force of the thrust is translated through the wheels into the treadmill, then the plane will not move forward. But if only a small portion of the force goes to the treadmill, then the plane will move foreward regardless of the treadmill and could obtain lift.

So I guess it might be plausible after all.

LoudDog

Canon EF 17-40mm f/4.0L USM

12/06/2006 12:01:50 PM · #30

think of pulling a table cloth out from under mom's fine china really fast and the china not move. Same principal. The China's air speed remains constant and by pulling fast you eliminate the friction (which the wheels on the landing gear would do when the treadmill moves).

The thrust moves the plane relative to the air. Ground has no bearing on the air speed other then creating resistance due to friction. True, there will be some resistance from the wheels needing to turn roughly twice the required speed (thanks to the treadmill) but that will be small compared to the power required to take off and the plane's engine(s) should be plenty strong to counter that.

Canon EF 24-105mm f/4.0L IS

12/06/2006 12:02:05 PM · #31

or another way to realise why it takes off. Put a small toy truck on a treadmill, with a rope tied to it, keeping it stationary wrt to the treadmill. Turn the treadmill up really fast so the trucks wheels spin really fast and it stays in the same place.

Now pull the rope (turn on the engines, start the propeller) all the same, all an external force acting on the truck, nothing to do with the wheels or treadmill. If you like, have the treadmill accelerate as fast as it possibly can at the same time.

Can you pull the truck of the treadmill - of course you can. Because the treadmill/wheels interface isn't where the force is applied.

The plane takes off.

DrAchoo

Panasonic Lumix DMC-LX5

12/06/2006 12:03:40 PM · #32

Think about this experiment:

You have a matchbox or Hot Wheels car on a long track. The important thing to remember about Matchbox cards is the wheels are not attached to any sort of drive mechanism. They just spin. You tie a string to the front of the car and stand ten feet in front of it.

If someone behind the car started pulling the track out from under the car, could they ever pull it fast enough to prevent you from pulling the car to you by pulling in the string? The answer is no.

In the plane experiment the air is the string. The wheels are exactly like a matchbox and not attached to any drive mechanism. No matter how fast that treadmill goes, the string is still holding the plane in place and pulling it forward. Eventually it pulls it forward fast enough to cause it to fly and then the treadmill instantly removes itself from the equation.

EDIT: Ha. Apparently Gordon and I hit the same analogy at the same time.

Message edited by author 2006-12-06 12:05:40.

LoudDog

Canon EF 17-40mm f/4.0L USM

12/06/2006 12:05:40 PM · #33

Originally posted by eqsite:

If all of the force of the thrust is translated through the wheels into the treadmill, then the plane will not move forward. But if only a small portion of the force goes to the treadmill, then the plane will move foreward regardless of the treadmill and could obtain lift.

the wheels only provide minimum resistance to the thrust and they are designed to minimize resistance. If the brakes on the wheels were applied it would be rough for the plane to take off in this situation. However, an over powered plane like an F18 or F15 could probably still take off even with the brakes on though.

_eug

12/06/2006 12:07:47 PM · #34

Originally posted by DrAchoo:

EDIT: Ha. Apparently Gordon and I hit the same analogy at the same time.

I was thnking of a similar analogy earlier, but scrapped it. I didn't want to get it wrong. lol

mamba

Canon EF-S 18-55mm f/3.5-5.6 II

12/06/2006 12:13:10 PM · #35

depends on the plane. am i allowed a harrier jump jet? if i am then yes it does as it can take off vertically. :P

Marjo

Canon EOS-7D

Canon EF 70-200mm f/2.8L IS USM

12/06/2006 12:15:13 PM · #36

Now I'm wondering exactly how an aircraft is catapulted off a carrier.

12/06/2006 12:15:26 PM · #37

Originally posted by mamba:

depends on the plane. am i allowed a harrier jump jet? if i am then yes it does as it can take off vertically. :P

They all would. A harrier is a good example of why though - the thrust is applied in a different vector, so the treadmill is obviously unconnected. Same is true for any other plane, it just appears that the treadmill might have an affect (which it would say to a car or person, where the force is applied through the legs/wheels.)

12/06/2006 12:16:17 PM · #38

Originally posted by Marjo:

Now I'm wondering exactly how an aircraft is catapulted off a carrier.

Big rubber band applying lots of force. (like the rope/ person pulling in the toy truck example)

Wheels just there to reduce friction again.

Planes don't drive down a runway to take off.

Canon EF 28-135mm f/3.5-5.6 IS USM

12/06/2006 12:31:18 PM · #39

Originally posted by LoudDog:

I am in total agreement with this. Having thought it through, I now believe that the plane will still move forward and therefore could get lift.

Of course, I just tried to walk out the "in" door at the grocery store, so obviously my brain is not functioning at full capacity today :)

xstream

Canon EOS-40D

12/06/2006 12:33:49 PM · #40

Originally posted by Marjo:

Now I'm wondering exactly how an aircraft is catapulted off a carrier.

Wonder no more:
HowStuffWorks

It's basically a giant cylinder filled very rapidly with steam from the reactors on the carrier.

_eug

12/06/2006 12:34:16 PM · #41

Originally posted by eqsite:

Of course, I just tried to walk out the "in" door at the grocery store, so obviously my brain is not functioning at full capacity today :)

Ouch. Talk about bruising ones ego. Was anyone around to witness it?

Canon EF 100mm f/2.8 USM Macro

12/06/2006 12:43:06 PM · #42

Originally posted by xstream:

Originally posted by Marjo:
Now I'm wondering exactly how an aircraft is catapulted off a carrier.

Wonder no more:
HowStuffWorks

It's basically a giant cylinder filled very rapidly with steam from the reactors on the carrier.

There's a fine example of panning on that page too!

Spork99

OM System OM-1

12/06/2006 12:44:11 PM · #43

The plane would take off.

The thrust from the engines pushes against the air, not the ground.

The wheels would turn twice as fast on the treadmill, but the motion of the plane relative to the ground and air would not change.

Canon EF 100mm f/2.8 USM Macro

12/06/2006 12:44:14 PM · #44

Originally posted by _eug:

Ouch. Talk about bruising ones ego. Was anyone around to witness it?

Just the Salvation Army guy. It probably happened because I didn't have any change to put into his kettle. Or because I was preoccupied with some stupid plane and treadmill!

Spork99

OM System OM-1

12/06/2006 12:46:16 PM · #45

Originally posted by xstream:

I believe that the newest systems use an electromagnetic catapult similar in concept to a rail gun. I don't know if they are in use yet, but they are supposed to be faster and more efficient.

Apple iPhone 5

Nikon AF-S DX Zoom-Nikkor 18-55mm f/3.5-5.6 ED

12/06/2006 12:48:50 PM · #46

Okay, right now I go with the idea that the plane will take off, because of the thrust of the engines (the 'rope attached to the front' theory)

But... the bit I can't visualise is what happens to the treadmill? - As soon as the planes wheels start turning, the treadmill matches their speed.

For example, could the wheels quickly accelerate to a point where the friction *does* become significant enough to prevent the thrust from moving it forward?

Marjo

Canon EOS-7D

Canon EF 70-200mm f/2.8L IS USM

12/06/2006 12:54:41 PM · #47

Originally posted by Spazmo99:

Originally posted by xstream:

Originally posted by Marjo:
Now I'm wondering exactly how an aircraft is catapulted off a carrier.

Wonder no more:
HowStuffWorks

It's basically a giant cylinder filled very rapidly with steam from the reactors on the carrier.

I believe that the newest systems use an electromagnetic catapult similar in concept to a rail gun. I don't know if they are in use yet, but they are supposed to be faster and more efficient.

thanks for the link. :)

electromagnetic would have to be safer...

Jason_Cross

12/06/2006 01:01:40 PM · #48

Originally posted by Southern Gentleman:

No! because what make a jet/plane airborne is the wind going over the the wing making an updraft under the wing giving it lift. That's why most take offs are into the wind and landings with the wind.

I agree, you need lift, and the experiment does not say anything about what could be done to achieve lift. If you aim the motors down you could have lift, but that is not stated.

LoudDog