Plane on a treadmill

Fratton Fred

The engines don't make it fly though, the wings do. I get your point, but the plane has to trundle forward on the wheels before there is enough air passing over the wings to create lift.

What I think you are talking about, is the plane effectively outrunning the treadmill which it needs to do if it is dragged off. Don't forget, the plane still weighs the same until the wings start to lift it off the ground, which they will not do until the reach a certain velocity. Until this happens, the plane may as well not have wings at all.

Biro

Fratton Fred wrote: »

The engines don't make it fly though, the wings do. I get your point, but the plane has to trundle forward on the wheels before there is enough air passing over the wings to create lift.

What I think you are talking about, is the plane effectively outrunning the treadmill which it needs to do if it is dragged off. Don't forget, the plane still weighs the same until the wings start to lift it off the ground, which they will not do until the reach a certain velocity. Until this happens, the plane may as well not have wings at all.

I know the engines don't make it fly. What I'm saying is the engines will bring it forward at the same rate as they would regardless of the treadmill. The plane doesn't have to outrun the treadmill at all. The wheels do, but they're free spinning anyway, and therefore not part of the equation.

A320-200

I know the engines don't make it fly. What I'm saying is the engines will bring it forward at the same rate as they would regardless of the treadmill. The plane doesn't have to outrun the treadmill at all. The wheels do, but they're free spinning anyway, and therefore not part of the equation.

Exactly. For example, B777 roll's down the treadmill, Vr of 140KIAS, at Vr the wheels will be at a rotational speed of 280Kts.

Fratton Fred

Biro wrote: »

I know the engines don't make it fly. What I'm saying is the engines will bring it forward at the same rate as they would regardless of the treadmill. The plane doesn't have to outrun the treadmill at all. The wheels do, but they're free spinning anyway, and therefore not part of the equation.

This is a really good question, I see why the OP was so confused now.

I get what you mean and I can see that happening.

Does it not depend on how how fast you win up the engines? if you start off slowly and slowly increase the power, the plane will think it is moving forward and eventually not go anywhere, but if you increase the power quickly, I can see the power dragging it forward.

It was a long time ago i did my physics exams, but is there not something about initial momentum, initial velocity etc? if it can achieve that then it will go forward I guess. Newton would know the answer.

Jesus, my head hurts.:D

civdef

Won't the plane just roll off the treadmill and take off normally? Even if the treadmill was infinitely long, the plane would move fiorward along the treadmill at close to normal takeoff speed- allowing for a bit of extra friction.

The treadmill won't keep the plane fixed in place wrt the air around it since the aircraft wheels freewheel.

Fratton Fred

I got ye,

What you are saying is that rather than trundling down the runway on the wheels, the plane is dragging them along the ground behind it (Effectively anyway).

I'm still pretty sure that if you wound it up slowly, it would stay where it was, the plane needs to generate enough power to overcome the forces holding it down, a bit like the initial effort required to push a box along the floor, so the plane would use a load of fuel to take off, but it would eventually.

mikedragon32

The quick answer is yes it will.

The threadmill is "runway length" so there is no issue about the plane falling off. As some have stated, the jets/props provide the forward motion, not the wheels. The wheels are just there to keep the belly of the plane off the ground. So the plane will be moving through the air.

So, if the plane needs to be doing 200kph to take off, it's speed through the air will be 200kph. The speed of the threadmill will be 200kph, so the speed of the wheels will be 400kph. The plane is still moving through the air and it's the that speed that counts!

Tazzle

Hi, I just wandered into this forum by spotting it on the front page. Here's my two cents. Don't think of it in speed. Think of it as force. The force pulling the plane forward is the thrust from its propellors. Some forces resisting it are its drag from the air and its drag from the ground. The drag from the ground is reduced using wheels, you move the ground backward all you do is increase the drag on the wheels. I think it's a fair bet that considering a plane can take off from water that a Boeing on a ginormous treadmill is going to take off, unless the speed is so great the wheels ignite and make the plane splode.

Biro

Fratton Fred wrote: »

I got ye,

What you are saying is that rather than trundling down the runway on the wheels, the plane is dragging them along the ground behind it (Effectively anyway).

I'm still pretty sure that if you wound it up slowly, it would stay where it was, the plane needs to generate enough power to overcome the forces holding it down, a bit like the initial effort required to push a box along the floor, so the plane would use a load of fuel to take off, but it would eventually.

Yep, now you're on board!
I see what you're saying about the initial movement, but what would likely happen is that at lets say 5% thrust under normal circumstances the plane would start rolling forward to lets say 10mph, if it were moving backwards on a treadmill before you applied 5% thrust it would probably slow down from minus 10mph to zero then accelerate to 10mph, but after that point accleration from then on would be the same, minus the extra friction from the wheels, which probably would be negligable enough.

Biro

tracker-man wrote: »

Mythbusters did this.... but they did it WRONG. They used a microlight but it was clearly not stationary on their "treadmill", it was moving, air was flowing over the wings, lift was produced and he took off. On a single engine aircraft or an A380, the airflow over the wings produced by the engine alone is not enough to get the plane airborne. Simple as. The engines PULL the plane through the air, generating lift as the air and wind move around the wing at speed.

Hope this answers your question.

They actually did it right. No one said the plane had to be stationary, and we know that the engines don't even come close to providing any lift.

Fratton Fred

My old Mech Science lecturer would have been hard reading this thread. I can just see him doing diagrams etc on the board and scribbling all sorts of equations.

Thinking of the wheels keeping the belly of the plane off the ground rather than the plane moving on its wheels makes it a lot clearer.

cormy

I can't believe this debate is still going on. The plane will take off regardless of the treadmill speed. The reason this gets confusing is because the wiring in our brains seems to make us attribute too much significance to the speed of the wheels. The engines are pushing against the surrounding air and so the craft *will* move forward/accelerate ... simple as that ... (forget about the wheels - they are irrelvant). Think about it another way: imagine landing a plane on this notional treadmill, or performing a touch-and-go on it. Would the plane stop dead as soon as it touched the treadmill? Nope. The wheels would spin at the speed of the craft but this is completely irrelevant, the engine(s) are providing thrust against the surrounding air and this is keeping the craft moving forward - just as would be the case when initial takeoff began.

MrCreosote

cormy wrote: »

I can't believe this debate is still going on. The plane will take off regardless of the treadmill speed. The reason this gets confusing is because the wiring in our brains seems to make us attribute too much significance to the speed of the wheels. The engines are pushing against the surrounding air and so the craft *will* move forward/accelerate ... simple as that ... (forget about the wheels - they are irrelvant). Think about it another way: imagine landing a plane on this notional treadmill, or performing a touch-and-go on it. Would the plane stop dead as soon as it touched the treadmill? Nope. The wheels would spin at the speed of the craft but this is completely irrelevant, the engine(s) are providing thrust against the surrounding air and this is keeping the craft moving forward - just as would be the case when initial takeoff began.

Maybe this is a good way of thinking about it- thinking about what would happen in the opposite situation i.e. a plane landing? Thanks for all the replies! It's been getting on my nerves for a few days now...

asdasd

Good thread. Well done biro.

youtheman

I thought this was a simple problem, but the more I thought about it, the more I realised it was a little more difficult.

I've applied my superior engineering brain to come up with the following logic (and I welcome it being shot out of the air):

Assume the plane has a take off speed of 10 mph.

Step 1
Put the model plane on the threadmill. Leave the engine off. Turn on the threadmill. The plane will move backwards. No argument there.

Step 2.
Put the model plane on the threadmill. Leave the engine off. Turn on the threadmill to 8 mph. Keep a push on the back of the plan with a force 'F' to keep it stationary (i.e. to balance the friction in the undercarriage).

Step 3.
Put the model plane on the threadmill. Turn the engine on. Turn off the threadmill. If the engine applies a force of 'F' then the plane will reach 8 mph (as confirmed by Step 2). This assumes no friction in the air. In reality you will have to apply a force of 'F' plus a % to reach the 8 mph (the extra allowing for the air friction). You will have to apply a force of 'F' plus %% to reach the take off speed of 10 mph (to overcome the total air friction and the undercarriage friction). The action/reaction is between the aircraft engine and the surrounding air.

Step 4
Now to the original query. Turn on the threadmill to 10 mph. As long as the engine can develop a thrust of 'F' plus %% then the aircraft will reach 10 mph and take off ('F' will get it to 8 mph allowing for the undercarriage friction, the extra % will allow for the air friction and the extra %% will allow for 8 mph to 10 mph).

So its not about the threadmill speed, the power is not being applied throuh the threadmill. It's about having enough power to get the plane to the take off speed (allowing for the various frictions involved).

asdasd

If the engine applies a force of 'F' then the plane will reach 8 mph (as confirmed by Step 2)

Thats not quite right. The only force you are acting against when the treadmill is going backwards is the friction force on the wheels, going forward at the same speed you need to accelerate the large object to eight miles an hour which needs the equivalent thrust backwards.

If in fact you applied that force instead of just holding the plane when the treadmill was going backwards at 8 miles ah hour, the plane would go forward at 8 miles an hour and the wheels spin as if they were going forward at 16 miles an hour.

In the grand scheme of things the wheel friction in negligible.

FOGOFUNK

I cant believe this thread lasted so long.

Basically take off speed has no relation to the undercarriage or surface the plane is sitting on.

Think of seaplanes, if a current is flowing the opposite direction, the take off speed doesnt change.

Takeoff speed is only relative to airflow over the wings.

asdasd

I can't believe you didn't understand any of the thread.

FlutterinBantam

FOGOFUNK wrote: »

I cant believe this thread lasted so long.

Basically take off speed has no relation to the undercarriage or surface the plane is sitting on.

Takeoff speed is only relative to airflow over the wings.

Weeeeeey.:D:D:D:D:D


One shiny new Euron to the man with the Phil Lynott haircut.

Manic Moran

Also depends on the airplane.

Harriers, Forgers and Freestyles will be able to take off with little trouble regardless of what the treadmill is doing, for example.

There are also fixed-wing aircraft designed to have the airflow from the engine pass directly over the wing, thereby creating lift even without forward movement. Usually you need to get a little forward motion, but if the engine is strong enough and the aircraft light enough, you will get lift without forward velocity of the fuselage compared to the ground.

NTM

FlutterinBantam

Good point MM

Delta Kilo

Right, I get it, and yes, it will fly.

I think I have a good way of describing it too.

Think of this. You are on a skateboard which is on a treadmill. There is a rope tied to the handle of the treadmill and you are holding onto the other end of the rope. You turn the treadmill onto max speed, standing on the skateboard and holding yourself on the treadmill with the rope. You then pull the rope and you will still move forward, because the wheels are free to move.

You pulling the rope is the same as the propeller pulling the plane through the air. Therefore, the plane will be able to move forward and generate the lift.

Biro

Manic Moran wrote: »

Also depends on the airplane.

Harriers, Forgers and Freestyles will be able to take off with little trouble regardless of what the treadmill is doing, for example.

There are also fixed-wing aircraft designed to have the airflow from the engine pass directly over the wing, thereby creating lift even without forward movement. Usually you need to get a little forward motion, but if the engine is strong enough and the aircraft light enough, you will get lift without forward velocity of the fuselage compared to the ground.

NTM

You misunderstood the whole thread...

Manic Moran

No, I didn't. I just thought outside of the box.

And I didn't even mention RATO. Or if the treadmill was mounted on an aircraft carrier deck doing 35kts into a 15kt headwind....

NTM

Capt'n Midnight

http://xkcd.com/473/ hover the mouse over the cartoon

We actually divorced once over the airplane/treadmill argument. (Preemptive response to the inevitable threads arguing about it: you're all wrong on the internet.)"

Celtic Mech

Here's my 2 cents...now back to Basic Aerodynamics that was studied a few years back!
i think firstly we have to question how any aircraft produces Lift. The shape of the airfoil is designed in such a way that the air passing over the wing causes a drop in pressure. The pressure below the wing is Higher than that of the pressure Above the Wing...it is this pressure difference that is the fundamental in causing Lift. It causes an upward force from the High Pressure below to the Low Pressure above.
What is essential in causing this Upward force is the relative Airflow to the Airfoil...i.e the Speed of the Airflow.
Now take our treadmill query. (In this example we will ignore friction). Lets say we have an Airbus A320 with a Take off speed of 140 Kts. Treadmill Spins rearwards to a speed of 140Kts. In return to keep the aircraft Stationary on the treadmill, we must have the engines produce Thrust, thus allowing us to combat this rearward 140Kts. Ok, engines up to take off power, aircraft is now stationary. rearward force of treadmill is equaled to be the Thrust of engine in Fwd Direction. We MUST now look at the Lift needed. As we go back to basic Aerodynamics, for an aircraft to Take Off (i.e. Climb) Thrust Must overcome Drag AND Lift MUST Overcome Weight! As i have shown above, we know how to produce Lift..its the airfoil shape and the Airflow in relation to this...i.e. the relative wind. basically, with the aircraft on the treadmill, we have a Stationary Aircraft..i.e The speed of the air over the upper surface is ZERO. The pressure above and below the Wing is the same. no lift is created. The Aircraft Cannot Take off!!!
Looking back thru the posts, a lot of people are getting hung up on the speed of the wheels. they have nothing to do with it. Wheels are being spun up to 140kts but the actual airframe is going nowhere as its stationary. I think some are confusing the situation with Momentum. if, all of a sudden we stopped the treadmill, the aircraft would immediatly be propelled forward and may even take off but it will NOT take off from a stationary position (Before i get lept on...Im taking an example of a passenger jet...i know there are military jets that can take off from stationary etc).
What we must remember is that when an A320 or B737 is tearing down the runway at V1...the wheels are spinning at a speed of 140Kts BUT the Airflow against the Wings is ALSO going at 140Kts due to the Fwd motion of the Aircraft. The Fwd momentum of the aircraft...Trust has overcome Drag i.e the Engines accelerating us to our rotation Speed, but more importantly the 140kts of the airflow is causing the Low Pressure area Above the Wing Causing Lift. it will be the Positive Forces of Lift and Thrust that will overcome the negative Forces of Drag and weight allowing us to Take off.

Thomas_S_Hunterson

Celtic Mech wrote: »

Now take our treadmill query. (In this example we will ignore friction). Lets say we have an Airbus A320 with a Take off speed of 140 Kts. Treadmill Spins rearwards to a speed of 140Kts. In return to keep the aircraft Stationary on the treadmill, we must have the engines produce Thrust, thus allowing us to combat this rearward 140Kts. Ok, engines up to take off power, aircraft is now stationary. rearward force of treadmill is equaled to be the Thrust of engine in Fwd Direction. We MUST now look at the Lift needed. As we go back to basic Aerodynamics, for an aircraft to Take Off (i.e. Climb) Thrust Must overcome Drag AND Lift MUST Overcome Weight! As i have shown above, we know how to produce Lift..its the airfoil shape and the Airflow in relation to this...i.e. the relative wind. basically, with the aircraft on the treadmill, we have a Stationary Aircraft..i.e The speed of the air over the upper surface is ZERO. The pressure above and below the Wing is the same. no lift is created. The Aircraft Cannot Take off!!!
Looking back thru the posts, a lot of people are getting hung up on the speed of the wheels. they have nothing to do with it. Wheels are being spun up to 140kts but the actual airframe is going nowhere as its stationary. I think some are confusing the situation with Momentum. if, all of a sudden we stopped the treadmill, the aircraft would immediatly be propelled forward and may even take off but it will NOT take off from a stationary position (Before i get lept on...Im taking an example of a passenger jet...i know there are military jets that can take off from stationary etc).

No-one's saying it will take off stationary. What people are saying is that the motion of the treadmill will not significantly affect the ability of the plane to gain speed. It will move at the same speed relative to a stationary point and exactly twice that speed relative to the treadmill.

The wheels serve only to remove friction between the plane and the runway. If I hold a toy car on a treadmill, it will not move. If I push it against the motion of the treadmill, it will move forward despite the motion of the treadmill, I do not have to overcome the speed of the treadmill.

Manic Moran

I think some are confusing the situation with Momentum. if, all of a sudden we stopped the treadmill, the aircraft would immediatly be propelled forward and may even take off but it will NOT take off from a stationary position

The formula for calculating momentum is mass multiplied by velocity. Given that the relative velocity of a stationary object is '0,' there is no momentum at all, and thus no reason whatsoever for the aircraft to go forward at any notable speed if the treadmill is stopped.

Now take our treadmill query. (In this example we will ignore friction). <snip> Treadmill Spins rearwards to a speed of 140Kts. In return to keep the aircraft Stationary on the treadmill, we must have the engines produce Thrust, thus allowing us to combat this rearward 140Kts.

Wrong! Friction is the only conceivable reason one would need to produce thrust (Assuming a still day, no notable coriolis forces, and a level treadmill etc). The only relevance to the treadmill as regards an airplane's engines is that the engines need to power up only to the extent required to equal the friction caused by the wheel bearings which, on a modern aircraft, is probably somewhere just on the notch above 'idle'. In effect, you're looking at something akin to the old trick of whisking away a tablecloth from under the plates: Just because the 'surface' is doing something does not necessarily overcome inertia.

NTM

ian_m

https://www.youtube.com/watch?v=YORCk1BN7QY

*Kol*

If the aircraft balances it's engine power to equal the friction between the treadmill and the wheels (the force that is trying to push the plane backwards essentially and off the tread mill) it will be stationary and never take off no matter what speed the treadmill is at. This would not be a lot of thrust, probably the same breakaway thrust to get the aircraft moving from a standstill when going to taxi.

Once the plane overcomes that friction by increasing engine thrust it can move along the treadmill and accelerate to takeoff speed (assuming its a runway length treadmill). I think a lot of people are visualising a small tread mill rather than a runway length one. It doesnt matter if the treadmill is going at a thousand miles an hour as long as the plane can overcome the friction between it and the treadmill and move forward to a sufficient speed to get lift.