Riddle of the day !!!

thirstywork2

An airplane is sitting on an enormous treadmill. As the plane starts its engines, the treadmill runs in the opposite direction at the same speed the plane is moving. Can the plane take off?

Thud

no

RayCun

http://www.boards.ie/vbulletin/showthread.php?t=2056067540

:pac:

ThisRegard

https://www.youtube.com/watch?v=0KsdMuhYJPw

Off topic, but Mythbusters just isn't the same with the American voice over.

sconhome

Yes!

And I'm putting my explanation in a spoiler.

Its to do with airspeed over the wings and not ground speed

ThisRegard

Exactly Amphking.

sconhome

See the synchronicity in the thinking there.

dna_leri

amphkingwest wrote: »

Yes!

And I'm putting my explanation in a spoiler.

Its to do with airspeed over the wings and not ground speed

Surely there is no airspeed over the wings as the plane is static relative to the air?
In fact if

the treadmill runs in the opposite direction at the same speed the plane is moving

the treadmill is not actually moving as the plane is not actually moving and it is just sitting there with engines idling and therefore will not take off.

sconhome

dna_leri wrote: »

Surely there is no airspeed over the wings as the plane is static relative to the air?
In fact if the treadmill is not actually moving as the plane is not actually moving and it is just sitting there with engines idling and therefore will not take off.

Airplanes do not have driven wheels so they will spin happily on the treadmill. The motion comes from the engines pushing air backwards resulting in the plane being moved forwards (equal and opposite reaction).

Take off comes from the air flowing over the wings.

BobMac104

therefore should runners be fitted with airspeed meters and when this distance reads whatever required distance the race is, then thats the time. This would eliminate the issue of wind assistance.

also if a runner runs too fast on a treamill will they take off?

BrokenMan

BobMac104 wrote: »

also if a runner runs too fast on a treamill will they take off?

No but if they run too slow they will

BobMac104

:pac:

sconhome

RayCun wrote: »

http://www.boards.ie/vbulletin/showthread.php?t=2056067540

:pac:

My head hurts!!

shels4ever

thirstywork2 wrote: »

An airplane is sitting on an enormous treadmill. As the plane starts its engines, the treadmill runs in the opposite direction at the same speed the plane is moving. Can the plane take off?

Answer is 100% yes as the plane is a harrier jump jet.

Ecoenergy

The engines only provide forward motion, its the air passing over the wings which gives lift, therefore there is no take off in this case. The wing is designed so that air passes over the top of the wing faster than under the wing and this creates the lift. The engines and hence forward motion will provide this air movement on a runway but not on a threadmill.

PaulieC

the real riddle is why you call it a treadmill. Surely everyone knows it's called a threadmill ?

hardCopy

cambo_gueno wrote: »

the real riddle is why you call it a treadmill. Surely everyone knows it's called a threadmill ?

Threadmills suck, it's better to run on the footpad

Pherekydes

Ecoenergy wrote: »

The engines only provide forward motion, its the air passing over the wings which gives lift, therefore there is no take off in this case. The wing is designed so that air passes over the top of the wing faster than under the wing and this creates the lift. The engines and hence forward motion will provide this air movement on a runway but not on a threadmill.

The engines thrust air backwards, so they must go forwards. They are attached to the wings, so the wings go forwards. Simple, really.

lintdrummer

If this experiment were to be carried out correctly then the plane wouldn't take off.

Think about if you were to try it with an R/C plane on a treadmill. As the treadmill runs, the plane would be dragged backwards. As the power is applied to the engine, you would reach a point where the force pulling the plane forward would match the speed of the threadmill and the plane would stay stationary. You may even get it up to full throttle but be going nowhere.
As a result, there would be very little airflow over the wings and it just wouldn't be physically possible to take off.

In the mythbusters clip the plane is able to outpower the truck by some way and so it moves forward. As it's such a light aircraft it would require very little speed to get in the air. He would have been pointing into the wind too which would help.

That's my 2 cents. Can of worms officially opened :pac:

plodder

The force of the engines acts on the air. Therefore, they are pushed forward relative to the air, not the ground (the treadmill). The plane will just accelerate forward on the treadmill no matter how fast it tries to spin the wheels. At least that's how I see it now, having changed my mind twice already :pac:

dna_leri

Pherekydes wrote: »

The engines thrust air backwards, so they must go forwards. They are attached to the wings, so the wings go forwards. Simple, really.

Ok, so the plane then moves forward relative to the ground (as the wheels are not driven and are just spinning freely) and falls off the treadmill and crashes through the wall of the gym, but does not take off from the treadmill.

dev123

amphkingwest wrote: »

Airplanes do not have driven wheels so they will spin happily on the treadmill. The motion comes from the engines pushing air backwards resulting in the plane being moved forwards (equal and opposite reaction).

Take off comes from the air flowing over the wings.

While airplanes do not have driven wheels they do rely on the friction generated between the wheels and the static ground to gather speed for take off.

A plane has zero speed before take off and must get up to in and around 170mph to take off so that the airflow over the wings can provide lift.

This increase in speed happens by the interaction of the spinning wheels with the ground.

If you put the plane on a rolling road the forces generated by the engine would be converted into frictional forces between the wheels and the rolling road. The rolling road will happily swallow up all your forward thrust leaving the plane in the same position.

plodder

dev123 wrote: »

While airplanes do not have driven wheels they do rely on the friction generated between the wheels and the static ground to gather speed for take off.

But a plane can take off easily (easier maybe) on a frictionless surface (eg ice)

dna_leri

Related question:
If you stand on a skateboard on a treadmill, what will happen?
Now if you put a jetpack on your back (while still on the skateboard on the treadmill), what will happen?

dev123

plodder wrote: »

But a plane can take off easily (easier maybe) on a frictionless surface (eg ice)

The rubber wheels on ice will not be frictionless.

I suppose the key things is that the ice does not move relative to the plane. The plane can get from point A, the apron, to point B, the end of the runway all the time gathering speed relative to the ground.

A guy paddling a canoe upstream at the same speed the water is flowing against him.

How long will you wait for him a mile upstream?

Pherekydes

dev123 wrote: »

While airplanes do not have driven wheels they do rely on the friction generated between the wheels and the static ground to gather speed for take off.

No they don't. The friction you mention acts as a type of drag and must be overcome by the a/c while taking off. This friction is quite small compared to air drag or the weight of the a/c.

A plane has zero speed before take off and must get up to in and around 170mph to take off so that the airflow over the wings can provide lift.

This increase in speed happens by the interaction of the spinning wheels with the ground.

Nope. Completely wrong. An a/c must reach a certain airspeed, i.e. its speed relative to the air, to overcome its weight and the drag mentioned above.

How would a/c take off from ice, water or very short runways, like a/c carrier decks, if the increase in speed was happening by interaction of spinning wheels?

If you put the plane on a rolling road the forces generated by the engine would be converted into frictional forces between the wheels and the rolling road. The rolling road will happily swallow up all your forward thrust leaving the plane in the same position.

Again, the thrust is far, far greater than any frictional forces generated by the wheels and any surface they rest on.

Pherekydes

dev123 wrote: »

The rubber wheels on ice will not be frictionless.

This may or may not have occurred to you, but a/c take off from ice without wheels in certain circumstances. They use skis instead.

A car exerts its motive force against the road, a canoeist exerts his motive force against the water, but an a/c exerts its motive force against the air.

louthandproud

if the threadmill is a few hundred yards long then the plane will move forward and take off, doesn't really matter what the surface of the threadmill is doing, as long as nothing is blocking the plane from moving forward, like the giant wall of the giant gym.

dev123

Pherekydes wrote: »

No they don't. The friction you mention acts as a type of drag and must be overcome by the a/c while taking off. This friction is quite small compared to air drag or the weight of the a/c.



Nope. Completely wrong. An a/c must reach a certain airspeed, i.e. its speed relative to the air, to overcome its weight and the drag mentioned above.

How would a/c take off from ice, water or very short runways, like a/c carrier decks if the increase in speed happening by interaction of spinning wheels?



Again, the thrust is far, far greater than any frictional forces generated by the wheels and any surface they rest on.

The wheels facilitate the movement of the plane relative to the ground. Would you agree that it is this relative movement that allows the plane to gather enough speed for the wings to generate lift?

Replace the wheels with skis for water take off. Plane accelerates against the flow of water. The water responds to the planes increase in thrust by increasing the flow of water. What does your force vector diagram look like? One big arrow pointing left, from the engines, and one pointing right, the drag on the skis from the flow of water . What is the magnitude and direction of the total force acting on the aircraft?

How does relaitve movement with the air occur if the plane is stationary?

dev123

Pherekydes wrote: »

This may or may not have occurred to you, but a/c take off from ice without wheels in certain circumstances. They use skis instead.

A car exerts its motive force against the road, a canoeist exerts his motive force against the water, but an a/c exerts its motive force against the air.

Why does it have wheels?

meijin

So how a rockets can fly in space (assuming vacuum) with no contact with any ground, or treadmill? :P

sconhome

dev123 wrote: »

While airplanes do not have driven wheels they do rely on the friction generated between the wheels and the static ground to gather speed for take off.

A plane has zero speed before take off and must get up to in and around 170mph to take off so that the airflow over the wings can provide lift.

This increase in speed happens by the interaction of the spinning wheels with the ground.

If you put the plane on a rolling road the forces generated by the engine would be converted into frictional forces between the wheels and the rolling road. The rolling road will happily swallow up all your forward thrust leaving the plane in the same position.

I can sort of see what you mean, because of the logic.

The problem I see is that airplane speed in measured in knots which is an indication of air speed relative to ground speed. It is relative because at various altitudes the air provides less resistance to the forward motion of the plane.

The plane relies on the wheels to support the body off the ground. The movement of the wheels is a response to the thrust of the engines pushing the plane forwards. The friction, between wheel and ground, in my opinion, is just an occurrence that results in more force being required to get the plane off the ground, hence why less throttle is needed to keep it in the air.

I am curious as to how the forced movement of air by the engines can be
converted to ground friction as the wheels serve no purpose other that to act as a neutral (bar friction) interface between the two.

menoscemo

thirstywork2 wrote: »

An airplane is sitting on an enormous treadmill. As the plane starts its engines, the treadmill runs in the opposite direction at the same speed the plane is moving. Can the plane take off?

I bet you regret asking this now?

pgmcpq

amphkingwest wrote: »

Airplanes do not have driven wheels so they will spin happily on the treadmill. The motion comes from the engines pushing air backwards resulting in the plane being moved forwards (equal and opposite reaction).

Take off comes from the air flowing over the wings.

So the answer is : it depends ? .... on whether the forward thrust caused by the engine causes sufficient airflow over the wings without the forward motion lost by the treadmill running in the opposite direction. Have I go this right ?

If so my guess would be "no" .... but would need to see the math to be sure.

Pherekydes

dev123 wrote: »

The wheels facilitate the movement of the plane relative to the ground. Would you agree that it is this relative movement that allows the plane to gather enough speed for the wings to generate lift?

Nope. You're not paying attention. An a/c takes off due to its speed relative to the air.

If the a/c groundspeed was important then it wouldn't matter whether a/c took off into the wind or with it, but they always take off into the wind, which increases airspeed.

GT_TDI_150

thirstywork2 wrote: »

An airplane is sitting on an enormous treadmill. As the plane starts its engines, the treadmill runs in the opposite direction at the same speed the plane is moving. Can the plane take off?

wasnt this exactly what they did on mythbusters?

Pherekydes

dev123 wrote: »

How does relaitve movement with the air occur if the plane is stationary?

It's windy out?

T runner

meijin wrote: »

So how a rockets can fly in space (assuming vacuum) with no contact with any ground, or treadmill? :P

Newtons third Law means the rocket goes with force equal and opposite in direction to rocket (boosters?)

No wind to slow it down

dev123

Pherekydes wrote: »

dev123 wrote: »

The wheels facilitate the movement of the plane relative to the ground. Would you agree that it is this relative movement that allows the plane to gather enough speed for the wings to generate lift?

Nope. You're not paying attention An a/c takes off due to its speed relative to the air.

If the a/c groundspeed was important then it wouldn't matter whether a/c took off into the wind or with it, but they alwys take off into the wind, which increases airspeed.

How does it generate enough speed relative to the air for enough lift to be generated? It has to move relative to the ground, ice, lake etc. Ground speed comes before air speed. It must have been rag week when they covered that ;-)

Pherekydes

dev123 wrote: »

How does it generate enough speed relative to the air for enough lift to be generated? It has to move relative to the ground, ice, lake etc. Ground speed comes before air speed. It must have been rag week when they covered that ;-)

I didn't cover that in college; I covered it in the a/c industry.

All a/c speed is airspeed. Groundspeed is mostly irrelevant.

And it doesn't have to move relative to the ground. Helicopters take off because their rotors are moving relative to the air and generating lift.

bart simpson

dev123 wrote: »

How does it generate enough speed relative to the air for enough lift to be generated? It has to move relative to the ground, ice, lake etc. Ground speed comes before air speed. It must have been rag week when they covered that ;-)

no ground speed doesnt come before airspeed, a/c work on thrust, so from the very start the a/c is pushing itself against the air around it, this is what trust is, a car works on the torque, the torque of the wheels against the road.
in op scenario, the plane would take off as normal using its thrust, the planes wheels would spin twice as fast before take off however.

now what would happen if a plane tried to take off with a 170mph tailwind? would the plane be able to get thrust to take off???

dev123

Pherekydes wrote: »

All a/c speed is airspeed. Groundspeed is mostly irrelevant.

Airspeed, ground speed, they are both speed. So to save confusion lets call it velocity.

What about this scenario:
A married couple fly to an island for their honeymoon. The seaplane lands and and pulls up to the dock. They get out and just as they are about to take out their bags the current picks up. The captain increases the thrust on the engines just a little to keep the plane in the same location.

The bags are out and the husband leans in to tip the captain. All of a sudden the current increases massively and the captain has to gun the engines just to hold the plane in place. The husband ties the plane to the dock and the captain kills the engines.

The current doesn't die down and the captain gets out. The husband tips him ans wishes him all the best.

The captain turns around and says he can't take off with that current. The husband doesn't understand. The captain draws a big arrow in the sand one pointing left, and one pointing right. He explains that all the thrust from the engines, the arrow point left, is being used up to keep the plane from going backwards due to the current (the arrow pointing right).

So in this scenario how useful are the wings, how much lift will the plane be generating and what is his airspeedvelocity when the engines are at full tilt fighting the current?

The answer to three questions above is the same: zero

dev123

bart simpson wrote: »

no ground speed doesnt come before airspeed, a/c work on thrust, so from the very start the a/c is pushing itself against the air around it, this is what trust is, a car works on the torque, the torque of the wheels against the road.
in op scenario, the plane would take off as normal using its thrust, the planes wheels would spin twice as fast before take off however.

now what would happen if a plane tried to take off with a 170mph tailwind? would the plane be able to get thrust to take off???

Why twice as fast?

They plane would need to reach 340mph to achieve a wind speed magnitude of 170mph over the wings (340 -170). Would this be right?

Pherekydes

dev123 wrote: »

Airspeed, ground speed, they are both speed. So to save confusion lets call it velocity.

There is no confusion.

Airspeed is speed relative to the air; groundspeed is speed relative to the ground. They are very different.

I'm afraid the rest of your post was nonsense.

dev123

Pherekydes wrote: »

I'm afraid the rest of your post was nonsense.

I will have to conceed to the superior debating skills on display and call it a day.

donothoponpop

Pherekydes wrote: »

There is no confusion.

Airspeed is speed relative to the air; groundspeed is speed relative to the ground. They are very different.

I'm afraid the rest of your post was nonsense.

What forces are acting upon the wings? There's a normal reaction force owing to the air, is that it? (I don't know anything about forces in fluids).

I presume if the riddle was phrased as a rocket on wheels on a treadmill, the rocket would stay still relative to the ground?

Pherekydes

donothoponpop wrote: »

What forces are acting upon the wings? There's a normal reaction force owing to the air, is that it? (I don't know anything about forces in fluids).

I presume if the riddle was phrased as a rocket on wheels on a treadmill, the rocket would stay still relative to the ground?

Lift, weight, thrust and drag.

A rocket will, of course, move forward, due to thrust, but will not generate any lift (or very little).

donothoponpop

Cheers, I see. The plane (or my rocket) exerts forces against the fluid (air), and reacts by lifting (or moving forward). The treadmill is a red herring designed to make the problem look like a closed system- like to a car accelerating on a treadmill, while staying still relative to the ground.

Pherekydes

donothoponpop wrote: »

Cheers, I see. The plane (or my rocket) exerts forces against the fluid (air), and reacts by lifting (or moving forward). The treadmill is a red herring designed to make the problem look like a closed system- like to a car accelerating on a treadmill, while staying still relative to the ground.

Spot on.

Next riddle! :pac:

dna_leri

For more analysis see:
http://www.airplaneonatreadmill.com/

donothoponpop

Pherekydes wrote: »

Next riddle! :pac:

Coe and Ovett in their heyday, are to race 1500m against the clock in separate stadiums on either side of a city. The gun is simultaneous for both races, however neither will receive any info on how the other is running, during the race.

If they both finish under 4 minutes, they both get €10,000. However, if one finishes in over 4 minutes, he gets €1,000,000, and the other gets nothing. If both finish in over 4 minutes, they both get nothing.

How do the races progress? (Edit: they can discuss tactics prior to the races.)