The old airplane/treadmill revisited

[master53yoda]

that plane is a Zenair STOL CH701 it will lift at 36 mph. air speed. he just wasn't pulling the treadmill fast enough to keep its air speed down. Myth busters blew there test procedures again.
http://www.metacafe.com/watch/yt-59s...h_701_skyjeep/

that clip is what it will do without the tread mill.

[cgallery]

The myth they were busting was that the plane would NOT stand still.

Then they busted the wrong myth.

The real question is whether a plane "standing still" relative to the ground can become airborne.

I agree w/ Art (below), I think they blew the test.

[woodturner]

The planes that are always tested seem to be prop planes that have the propellers at the front of the plane, which would force airflow over the wings, providing lift.

To answer the question, we have to define it a bit more precisely. If you mean that the conveyor is moving such that the plane is stationary to the ground, the airflow over the wings is 0, and the plane does not take off - whether prop or jet is irrelevant.

However, that is not how the problem is normally stated - and if you watch the Mythbusters episode, it's clear that the conveyor is not moving fast enough, so that the plane advances along the conveyor, achieves the required airspeed, and takes off.

Here is a link that explains it a bit more scientifically.
http://mouser.org/log/archives/2006/02/001003.html

[leehljp]

Yes, a jet would take off too. It is a semantics thing that is causes the confusion as to yes and no.

They don't say that the plane won't move forward (either by prop or jet), but only that the mat speed matches the speed of the wheel.

Saying this another way - wheel speed matching with treadmill has NOTHING to do with the airplane speed moving forward. The statements of "matching speed" do not preclude the plane moving forward itself.

The semantics of it all gives the initial impression that there will be NO forward movement of the plane itself. BUT Plane movement relative to the ground IS allowed within the statement, and once air speed/thrust for flight is reached it will lift off.

[Mr__Bill]

Then they busted the wrong myth.

The real question is whether a plane "standing still" relative to the ground can become airborne.

I agree w/ Art (below), I think they blew the test.

Actually that is not an issue at all. The plane takes off when it reaches it's takeoff airspeed. This is why aircraft carriers turn into the wind and speed up, they can add perhaps 50 knots to the airspeed of a plane standing still on the deck. If the aircraft carrier was going fast enough the plane would just float up into the air rather like how small aircraft want to fly in a strong wind storm and have to be tied down to the ground.

The whole conveyor belt thing is just misdirection.

The car in the box van thing:
The car is going say, 50 mph and the tires are spinning around at a rate to make the car go that fast. It enters the ramp of the van that is going only slightly slower than 50 mph. It's not necessary to slow the mass of the car to zero, only to the slightly slower speed of the van. The spinning tires are the only part that needs to be stopped. It takes a lot less energy to stop the tires spinning than to stop the mass of the car. Thus only a short distance is necessary for the stop. Try putting a car on jack stands, start up the car and put it in gear and speed up to 50 mph on the speedometer (note that the ground speed is zero as is the airspeed) Now hit the breaks and see just how quick the wheels stop.

And now a new theory of relativity: We can't go faster than light, or energy as we know it today, because as we accelerate out mass expands to infinity at which point we become light, or pure energy. As we know, nothing can go faster than it's self, ergo we can not go faster than light.

Bill
over here on the way to nowhere

[crokett]

that plane is a Zenair STOL CH701 it will lift at 36 mph. air speed. he just wasn't pulling the treadmill fast enough to keep its air speed down.

I believe this is incorrect. It isn't really possible to move it fast enough to keep the airspeed down. This is because the plane pushes against the AIR to take off, not against the treadmill. So once the plane starts pushing against AIR, the motion of the treadmill becomes irrelevant for takeoff. The wheels on the plane will just spin faster than they would if the ground were standing still. The plane would still move forward and take off.

Think of a seaplane on a river. The flowing water is the treadmill. Do you think the seaplane would have problems taking off heading upstream? Of course not. The prop is pushing against air, not water. A boat in the same current would lose knots heading upstream. The prop is pushing against the water headed downstream. A swamp boat in the same river wouldn't lose speed - relative to an observer on the bank - because the swamp boat has the big prop on the back pushing against air.

It is Newton's Third Law: "For every action there is an equal and opposite reaction." So the plane has to move because the props are pushing on the air to make it move.

Mythbusters is entertainment and nothing more. The science in most of their shows is somewhat faulty.

[master53yoda]

you are probably correct although it did take it quite about longer to take off on the tread mill then that plane will do on its own. they may also not have put the plane on full power in mythbusters. the only opposition that the treadmill would give is wheel friction so you are correct in your thinking. I'd be curious to see what it did with a plane that needs near 100 air speed to lift.

[Bill in Buena Park]

I don't know how I've missed past threads on this, but what fun.

Some salient points:

1. A stationary plane on the ground, with a strong enough headwind, doesn't need the engine running or forward progress to experience lift. Airflow passing over & under the wing provides the lift, and with the flaps down, you get BIG lift. In a large wind tunnel, a plane would lift at 0 groundspeed without engine thrust.

2. A plane will only lift in still air if the wings move forward through the air fast enough to create lift; therefore, in still air at takeoff, groundspeed is the airspeed needed to achieve adequate airflow and lift.

3. The engines on most aircraft are not designed to put sufficient direct airflow over & under the leading edge of a wing to create lift from a stationary position.

4. It might take longer to for the plane to achieve sufficient airspeed (= groundspeed in still air) when contending with negative groundspeed of a conveyor, but the engines pushing against air would ultimately overcome the conveyor issue and engine thrust might get that plane to sufficient airspeed before the bearings overheated and the wheels seized, or the tires exploded from too high RPM.

So the wheels and conveyor are red herrings if you can remove friction from the equation. At some point before takeoff, I believe the wheels first and and then the conveyor would move beyond light speed, giving you have a whole new set of problems.

But seriously - lets remove wheels; glacier planes using skids take off from sheets of ice. And sea planes using pontoons take off from water. Would they not fly if their "ground" element were acting as the conveyor? Ultimately, they would have to overcome the conveyor action as engine thrust pushes the plane forward relative to the air, not the ground medium, and they should slide forward faster than the medium could counteract.

Can I get a fact check on this, please?

[crokett]

I am assuming a frictionless environment for the purposes of this discussion. Obviously the treadmill will have some effect due to friction losses, but the plane will still fly.

.

[chopnhack]

If the plane was put on rollers such that the wheels could not advance forward (just spin in place on the rollers) then the plane would have no forward movement and thus never take off as the wind speed would be zero. The engines could be at full throttle and the wheels could spin until their bearings fail, but no forward speed in relation to the ground equal no work being done Just a small leap forward to a conveyor belt, so long as the planes forward speed never exceeds the conveyor belts speed, there will be no net forward movement and thus no lift. The mythbusters setup was lacking in the sense that the forward motion wasn't matched adequately. My two bits that make sense to me

[master53yoda]

lift is a product of airspeed over the wings. the ground speed has 0 to due with lift. If your taking off into the wind the lift time shortens by the wind speed if you need 45 air speed and the head wind is 15 you would only need 30 ground speed to lift. Although i wouldn't lift that quick in case I hit dead air space.

[cgallery]

The question is stated as:

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?

Stated this way, the conveyor control is preventing the plane from moving forward relative to the ground or the air that surrounds it.

No motion relative to the air = no lift. The plane stays grounded.

[master53yoda]

If the plane was put on rollers such that the wheels could not advance forward (just spin in place on the rollers) then the plane would have no forward movement and thus never take off as the wind speed would be zero. The engines could be at full throttle and the wheels could spin until their bearings fail, but no forward speed in relation to the ground equal no work being done Just a small leap forward to a conveyor belt, so long as the planes forward speed never exceeds the conveyor belts speed, there will be no net forward movement and thus no lift. The mythbusters setup was lacking in the sense that the forward motion wasn't matched adequately. My two bits that make sense to me

sorry to say but your thinking is off. the engine puts no power on the wheels at all. The wheels do offer a restriction due to friction, the engines power is delivered to the air that it moves. Air is about 13 cubic ft per lb so the prop is moving a mass times the pressure difference between the leading edge and the trailing edge of the prop this = engine HP. the only way to keep a plane from moving is to anchor it with the equivalent weight of the air the prop is moving or that the jet engine is thrusting. Most airplanes due not move as much air with the engine as they weigh so they need forward motion. and the brakes are sufficient to keep them stopped at full power. The military planes that will go straight up have more thrust then they weigh.

[master53yoda]

The question is stated is:

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?

Stated this way, the conveyor control is preventing the plane from moving forward relative to the ground or the air that surrounds it.

No motion relative to the air = no lift. The plane stays grounded.

Under the conditions described the conveyor speed would get quite high but you are correct in that it is offering the wheels equal friction to the hp delivered to the air stream. the wheels bearings etc would not survive very long because of the acceleration on the conveyor would equal the delivered HP on the engine. causing major heating.

[JeffG78]

Wow, this is one topic I thought we had completely beat to death a few years ago and would never resurface.

For anybody just seeing the plane vs. conveyor topic for the first time, you might want to go back and find the old thread. It went on for a LONG time and in the end, I think nearly everybody finally agreed that the conveyor has no effect on the plane's ability to take off.

This is true for prop as well as jet planes.