Randazzo

So, you have a plane on a conveyor belt...

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There is a plane parked on a conveyor belt. The plane fires up it's engine and attempts to take off. The belt runs in the opposite direction. Does the plane take off?

Go!

(Sorry mods.)

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So mythbusters has proven that a conveyor belt going less than half the forward speed of the plane will allow one to take off.

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Since all the action/reaction business is happening above the wheels, the belt should be pretty much irrelevant. The speed of the plane wings is what counts, not that of the wheels or ground. Do the same with a block of air surrounding the plane, however, and you will see different results. That last bit is exactly why pilots like to start into the wind and not along with it.

It surprises me that this is such a mystery to so many people.

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Since all the action/reaction business is happening above the wheels, the belt should be pretty much irrelevant. The speed of the plane wings is what counts, not that of the wheels or ground. Do the same with a block of air surrounding the plane, however, and you will see different results. That last bit is exactly why pilots like to start into the wind and not along with it.

It surprises me that this is such a mystery to so many people.

How do you propose to increase the velocity of air passing over the lift surfaces without forward movement?

Wheels count.

I do know the factually correct answers to this conundrum, but it's the discussions that always amuse.

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Assuming that the engines are able to push the plane at the same speed the conveyor belt is moving (both being a constant), the result is a function of the wheel bearing friction and plane weight.

If the wheel bearing friction is small and the plane is light, you get the results that the Mythbusters got. Increase the weight of the plane and/or the friction coefficient of the wheel bearings, and the belt will start to drag the plane with it, slowing the forward motion. At a certain point, the plane will not be able to accelerate, and therefore to lift off.

You can conduct a simple experiment dis/proving this, just start braking to increase wheel bearing friction and/or add something heavy for additional weight. If you have the belt and the plane, that is.

Did I amuse you enough with my post? :P

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How do you propose to increase the velocity of air passing over the lift surfaces without forward movement?

Wheels count.

I do know the factually correct answers to this conundrum, but it's the discussions that always amuse.

Get a plane, take away the wheels. It flies. Wheels are irrelevant to it's forwards motion. They can have skies, launch off a rocket/plane/ocean/carrier etc. If they wheels spin then the craft can move forwards. If you have friction on the wheels so as to move the aircraft back/slow it, then no it will not take off, but by that point your practically pushing it back, which is hard or impossible on a conveyor with wheels, you'd have to glue the wheels down to prevent the aircraft from taking off.

To make it easier to understand, change the question to "can a helicopter take off from a conveyor moving at any speed" the answer is also "yes".

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Assuming that the engines are able to push the plane at the same speed the conveyor belt is moving (both being a constant), the result is a function of the wheel bearing friction and plane weight.

If the wheel bearing friction is small and the plane is light, you get the results that the Mythbusters got. Increase the weight of the plane and/or the friction coefficient of the wheel bearings, and the belt will start to drag the plane with it, slowing the forward motion. At a certain point, the plane will not be able to accelerate, and therefore to lift off.

You can conduct a simple experiment dis/proving this, just start braking to increase wheel bearing friction and/or add something heavy for additional weight. If you have the belt and the plane, that is.

Did I amuse you enough with my post? :P

Smarty-pants. However, I'm confident that the internet will overlook your wisdom.

Get a plane, take away the wheels. It flies. Wheels are irrelevant to it's forwards motion. They can have skies, launch off a rocket/plane/ocean/carrier etc. If they wheels spin then the craft can move forwards. If you have friction on the wheels so as to move the aircraft back/slow it, then no it will not take off, but by that point your practically pushing it back, which is hard or impossible on a conveyor with wheels, you'd have to glue the wheels down to prevent the aircraft from taking off.

To make it easier to understand, change the question to "can a helicopter take off from a conveyor moving at any speed" the answer is also "yes".

You cannot compare the function of a rotary wing aircraft to a fixed wing aircraft. A rotary wing aircraft generates lift by spinning it's wings through the air, even from a stationary position. A fixed wing aircraft must move forward to generate lift. Until it's weight is borne on it's wings and not it's gear, the wheels do matter. I imagine a plane with skids might do worse, though you never know.

Your examples of rocket/plane/carrier all involve already moving forward, with the carrier also using a catapult system.

Furthermore, if wheels have no effect on the movement of an airplane...how do brakes work?

How is this a game? It's in forum games

Heh.. if it lives long enough, that will become clear.

Edited by Randazzo

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It's vertical weight is bared with it's gears. That's up and down. So movements in other directions do not apply. :)

Your imagination on skids is not very good. Try imagining with something more tasty. Like Jello, custard or icecream. Now make our conveyor out of Jello, or custard or icecream, and our wheels will be replaced by skids (made of ice cream cones of cause). As there is very little friction, our river of flowing dessert just does nothing to slow down our aircraft. It's like trying to take off from ice... it can be done. :)

So, unless the wheels melt, fuse into a solid lump, and dig into the ground, they have little effect on stopping the craft. If our aircraft on skies can take off, so can the aircraft on a treadmill. Once our skies start to dig in, is about the same point a wheel would start to slow the aircraft.

So the question is not "can an aircraft take off from a treadmill" but "can an aircraft take off with broken wheels". That depends on each aircraft, some produce enough thrust, most do not (see carrier launched craft as an example. :P ).

PS, breaks are not wheels. As much as parachutes are not the ground. Breaks work by increasing friction. Our treadmill does not, it can move 1mph or 1million mph, the friction stays (roughly) the same. Though I do agree at some point friction will overtake it... at which point you sheer off the wheels and get to the point of "broken aircraft" trying to take off again. :P

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It comes down to friction, really. The actual answer is that there are 3 types of planes in this regard.

1: Planes that can't take off, from the conveyor belt AND level ground. Imagine a seaplane without secondary wheels on the pontoons. The propeller's thrust might tip the plane over, but it will not be able to gain enough speed to take off, even if the ground under it were not moving.

2: Planes that can take of, from the conveyor belt AND level ground. Any regular plane with regular wheels will be able to take off from level ground with ease, and there isn't much a conveyor can add to this.

Most importantly to this discussion, 3: Planes that can take off from level ground... but can't take off from a conveyor belt This will heavily depend on the design and mass of the plane, but think of something like a plane intended for use in northern areas, where snow and ice are everywhere. It will have skids instead of wheels. Now, the wonderful thing about friction is that it exerts force, proportional to the friction coefficient and the relative velocity at which the two objects are moving. In most normal circumstances, a plane will either be unable to take off from level ground, or able to take off from anywhere.

But take a plane with skids and a powerful enough engine that it can barely gain enough speed to take off from level ground and, if you put it on a conveyor belt that doubles its effective velocity relative to the ground, it will fail to reach enough speed to take off, due to increased friction, as well as friction not being reduced by the lift it would have gotten from actually moving forward against level ground.

Entertaining enough? :P

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Yep. Wheels tend to be good at reducing friction. So it's effectively a treadmill covered in ice or oil. Without the wheels, or with wheels that have high friction, then yes, the aircraft gets through backwards and all that the result with give. (crash?!)

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Wheels don't make the plane move, they allow the plane to move. The engine, whether propeller driven or turbine driven makes the plane move. Whether the plane is on a conveyor belt or not is irrelevant.

Here's some examples of what happens in different situations:

If the plane is on a fixed runway, and the required airspeed for flight is 60kts, the headwind/tailwind is 0kts, and you were to put a speedometer on the wheels, the speedometer would indicate 60kts, the airspeed would indicate 60kts, and the plane would lift off.

If the plane is on a conveyor belt with all of the same parameters above except the conveyor belt is moving the opposite direction of the aircraft's thrust at say 20kts, the aircraft will move forward until the airspeed is 60kts, the speedometer will read 80kts, and the plane will lift off. Same if the conveyor would moving backwards at 300kts. Airspeed would indicate 60, speedometer would indicate 360, the plane still takes off.

If the plane is on a fixed runway, and the required airspeed for flight is 60kts, the headwind 60kts, and you were put a speedometer on the wheels, the speedometer would indicate 0kts, the airspeed would indicate 60kts, and the plane would lift off.

If that's too difficult to understand, think about this:

Imagine you are wearing a pair of rollerblades/rollerskates, or on a skate board. You hold a rope attached to a winch, and you are on a normal floor. The winch begins to turn, and pulls you. You will be pulled across the floor - at say 5 knots. If you had a speedometer on your wheels, it would indicate 5kts. Same situation except on a airport people mover. The winch is located off the end of the people mover which is moving 'backwards' at 5kts. You hold onto the rope, and your wheels spin beneath you - but you don't move backwards. The speedometer on the wheels indicate 5kts. Then the winch begins to turn, and you are pulled along the people mover. The winch pulls you at the exact same RPM as before, and so you are now moving forward at 5kts, the speedometer on the wheels would indicate 10kts, and if you had an airspeed indicator it would indicate 5kts.

The engine is the same as the winch/rope; completely independent of what is going on with the free turning wheels and so just as you move forward, the plane moves forward.

Now, this is assuming the bearings in all the wheels have 0 friction. In the real world there would be some friction in the bearings, and so you would need to have ever so slightly more power to attain that 60kts of airspeed.

Signed,

Me

Certified Flight Instructor and owner of two airplanes.

Edited by EdFred

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Quick question: Is the plane a VTOL?

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Smarty-pants. However, I'm confident that the internet will overlook your wisdom.

But... I tried really hard to take the bait, it's just that the physics were stronger than trolling :( Sorry

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Airflow from props OVER wings makes a difference in lift.

Behold: http://www.cafefoundation.org/v2/pdf_tech/High.Lift/AIAA.2002.High.Lift.ChannelW2.pdf

Saw one at the Redding Air Museum once and thought WTH is that!?! with it's wild wings, but it turns out it works! It's also covered in a book titled "Fly(ing) the wing" which is a book about flying for pilots where it talks about how prop airliners have an advantage not only in the fact that their engines respond almost instantly to throttle up, but also in that when throttled up the airflow over the wing is increased even though the forward speed of the plane may not be yet. Where as with a jet, there is NO increase in airspeed unless/until the whole aircraft increases it's forward velocity.

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How is this a game? It's in forum games

No longer.

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I'd like to add this to help people visualize the situation. :)

1838b145gffppjpg.jpg

No, the frame will clip the wings. :wink:

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How do you propose to increase the velocity of air passing over the lift surfaces without forward movement?

Wheels count.

This is exactly what I mean. People make all kinds of fundamentally faulty assumptions. Wheels do not count, as they are not powered. The propeller or the jet engine provides the forward motion so that the wings can provide enough lift to fly. Wheels have nothing to do with that. If you want to nitpick you could say the wheels reduce friction, and are therefore relevant, but they do not provide any of the speed that gets an aircraft into the air. Planes are not cars. Wheels are not powered on them. Wheels are just passive things.

Air speed is what counts, not ground speed. This is what I mentioned earlier. Slow craft, like gliders, can actually fly backwards in relation to the ground. However, they can never do the same in relation to the air, as that would be falling, not flying. They need to maintain proper airspeed, while ground speed is totally irrelevant. That is why taking off into the wind is easier and taking off with the wind harder. You already have some of the relevant speed you need in the first case, and need extra in the second.

If that still is not enough for you: aircraft can fly while tethered to the ground in strong winds. It is the reason flaps and spoilers need to be set in such a way that it pushes itself to the ground. If ground speed mattered, that would be impossible, yet it is a very real concern in airfields when there are strong winds.

Edited by Camacha

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