Airplane: Does it take off?

[Glen Miyashiro]

Re: Airplane: Does it take off?

The key difference between an airplane and a land vehicle (like a car) is in where the power is being applied.

In a car, the power is applied to the wheels, which push against the ground to move the car forward.

In an airplane taxiing on the ground, the power is applied to the jets (or the propellers), which push against the air.

The airplane's wheels don't do any pushing; they are just passive, un-powered rollers. Their purpose is to keep the plane from falling over, and to make the plane's motion against the ground as frictionless as possible. It's easy to see that a plane's wheels are un-powered if you think about planes designed to land on water (they've got pontoons instead of wheels) or on snow (skis instead of wheels).

So it wouldn't matter if the plane was on a conveyor belt, because whether or not it has traction against the ground isn't important in getting the plane to lift upwards off the ground. What's important is the thrust being applied from the jets or propellers, which pushes against the air, not the ground.

[Kungpao]

Re: Airplane: Does it take off?

The key difference between an airplane and a land vehicle (like a car) is in where the power is being applied.

In a car, the power is applied to the wheels, which push against the ground to move the car forward.

In an airplane taxiing on the ground, the power is applied to the jets (or the propellers), which push against the air.

The airplane's wheels don't do any pushing; they are just passive, un-powered rollers. Their purpose is to keep the plane from falling over, and to make the plane's motion against the ground as frictionless as possible. It's easy to see that a plane's wheels are un-powered if you think about planes designed to land on water (they've got pontoons instead of wheels) or on snow (skis instead of wheels).

So it wouldn't matter if the plane was on a conveyor belt, because whether or not it has traction against the ground isn't important in getting the plane to lift upwards off the ground. What's important is the thrust being applied from the jets or propellers, which pushes against the air, not the ground.

You're not taking gravity into account. The weight of the plane displaces force and energy to the ground. The bearings are in the wheels so that the air from the jets can push the plane forward.

[Leo Lakio]

Re: Airplane: Does it take off?

Slightly o-t (but not as much o-t as if it had been in the NFL thread):
watch this video of a jet attempting to land at SeaTac yesterday ... the winds are expected to be even stronger today.

[damontucker]

Re: Airplane: Does it take off?

Slightly o-t (but not as much o-t as if it had been in the NFL thread):
watch this video of a jet attempting to land at SeaTac yesterday ... the winds are expected to be even stronger today.

DANG LEO!!!! Das some scary stuff!

[craigwatanabe]

Re: Airplane: Does it take off?

and could you imagine if there were a conveyor belt?

Regarding Glen's approach on this topic, it doesn't matter where the propulsion occurs from, it's how it's sustained once you leave the ground.

Whether it's provided by the wheels or a jet exhaust, the action of the forward remains the same either way. How the mass being propelled is shaped makes the key difference in what occurs when air mass moving over and under that shape simultaneously.

With a conveyor belt, there is no change in air pressure over the wing of a stationary plane. Move the wing against air to create some form of gas pressure and you create the physics of lift...or move air at a velocity equivalent to rotational speed of an aircraft and you accomplish the same thing...lift.

On a conveyor belt the plane remains stationary, the air surrounding the wing remains calm. The only air pressure created around the stationary aircraft is the turbulence created from the revolving conveyor belt underneath it.

The only way I can see lift occuring on a conveyor belt is to ensure the wind created under the plane's wing is faster than the air above it. Now some increase will be apparent with a conveyor belt creating turbulance however the amount of wind created will have to be substantial so flaps to generate large movements of air will be needed and would also have to be attached to the conveyor belt.

Great in theory but what happens when the lift occurs and the plane moves away from the conveyor belt. Lift stops and the plane drops back down.

Now remember the air mass above the wing remains constant with the mass of the wing itself so when it becomes necessary to light up those jets once lift occurs, the amount of force needed to displace that constant air mass to one equal to the mass being generated beneath the wing would require as much energy as if you were to simply launch from a runway.

The difference would be the length of the take off would be substantially reduced.

An eaiser method would be as I stated before would be to generate hurricane force winds at the plane and letting the wing slice that wind to produce lift. No conveyor belt necessary. This way the air mass is constant with the negative air pressure and positive air pressure at the leading edge of the airfoil making liftoff more energy efficient.

[Pomai]

Re: Airplane: Does it take off?

With the plane stationary and no head wind, there is no air flow around the wings so no lift occurs.

Going back to the original question, it states the aircraft is standing (stationary), then moves...

from the original question: "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?"

So without going into details it says the aircraft does indeed gain forward momentum (for take-off). So we must assume the plane's engines had enough thrust and landing gear enough friction resistance (made by aliens?) to allow the aircraft (forward) movement for take-off from a standstill and overcome the opposing computer-controlled counter-movement (backwards) of the conveyor belt runway in exact relation to the aircraft's speed. Even if the runway is moving at 155 knots east, the Boeing 747 is still traveling at 155 knots west - enough for take-off.

In this odd scenario, it comes down to the mechanical efficiency of the landing gear's wheel bearings (resistance to friction), as opposed to what many iterpret as a spin (no pun intended) on the physics of lift in relation to fixed wing flight, which we all know relies on AIR SPEED, not ground speed. Under calm wind conditions, if the plane were at a standstill (which the question confuses us by at first), the plane won't go up, down, forward or backwards. As stated, it did MOVE, so I say it eventually gained enough AIR SPEED and it took off!

There are important variables that aren't part of the question such as... being a flat surface, it's unlikely the oppositely-moving conveyor belt runway below will create a significant amount of disturbance in airflow at the wing level of the aircraft while it attempts to take off, but with enough texture on the surface, there is a possibility this might disturb air pressure under the wings (lift). There's also elevation, ground wind conditions and aircraft weight that should be considered but aren't.

Imagine if you applied the brakes at 150 knots in this situation? First the tires would blow, then the aircraft would either do a spin-out or simply drag backwards, then the airflow over the back of the flaps and elevators would probably scoop the aircraft up, tumbling it over tail-over-nose. Ouch.

Turn the situation sideways and think of it this way: If someone were to move the dock as you paddled a canoe, would it affect how fast your canoe moved through the water?

Interesting analogy. In this scenario, the water is the friction factor (wheel bearings) AND the dynamics of travel (flight), whereas the dock is the runway tarmac. Unless that moving dock creates enough current in the water to counter the movement of the canoe, the canoe carries on regardless of dock movement.

This question would be cool if applied in an auto lube tv commercial.

[Kungpao]

Re: Airplane: Does it take off?

Slightly o-t (but not as much o-t as if it had been in the NFL thread):
watch this video of a jet attempting to land at SeaTac yesterday ... the winds are expected to be even stronger today.

Wow I think I just soiled myself just watching that. Oh the things I look forward to when I fly. Good thing I saw this before my flight to Chicago on Saturday

Canoe? Water? Dock? Is the Duck trying to take off?

This argument is pretty cool really. I asked the question to the structural engineer at my work and he feels the same way that I do that the plane wouldn't take off but we both can see the other people's point of view.... to an extent lol.

[GeckoGeek]

Re: Airplane: Does it take off?

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).

So of the plane is stationary, the belt is stationary. The plane moves. Period, cut and dried.

As stated the propulsion system works against the air (which is assumed stationary) not the ground (as in a car) so the ability of the plane to move forward is not affected. Gravity is not needed to move, otherwise the plane wouldn't be able to go any distance once it gets in the air.

The end result is that the plane will take off normally, but the landing gear will see twice the normal ground speed until liftoff.

[craigwatanabe]

Re: Airplane: Does it take off?

The plane does have forward momentum relative to the backward momentum of the conveyor belt, and as the engine's thrust increases that forward momentum the spinning conveyor belt is cancelling it with that law of physics we know simply as action vs reaction.

The difference is the friction within the wheel bearings that works against the plane's forward momentum.

All that said there's a lot of energy being spent to go nowhere as both the plane and conveyor belt are cancelling each other's momentum. In order for momentum to occur, there has to be something stationary to bais itself to so a guage or reference can be established and energies worked against.

And the air pressure surrounding the wing is calm relative to the wing that is attempting to create lift.

The wing is the essential lifting part of the equation here not the plane's momentum or the speed of the ground (conveyor belt). Air speed over and under the wing will make an object fly. This is why aircraft carriers point their bow into the wind for take off. This is why you take off on the reef runway or not. Depending on wind direction will determine which runway will be utilized for take off and landing.

In the movie Independence Day when Air Force one barely escaped the fire cloud that was rushing up from behind the plane. The high tailwind would have stalled the plane and it would have dropped like a rock.

Aircraft rely on headwinds to suppliment their lift.

Wind...it's essential for lift. A conveyor belt is simply spinning your wheels and getting nowhere.

[cezanne]

Re: Airplane: Does it take off?

Going back to the original question, it states the aircraft is standing (stationary), then moves...

So without going into details it says the aircraft does indeed gain forward momentum (for take-off).

Sure it moves. But it moves relative to the conveyor belt. The conveyor belt however, is cancelling out any forward momentum of the plane.

A plane needs to have air molecules passing over its wing surfaces at a prescribed speed in order for it to even rotate and fly off the ground.

I still don't see how you can create airspeed over the wings with the plane virtually at a standstill on this conveyor belt set up.

[Pomai]

Re: Airplane: Does it take off?

Sure it moves. But it moves relative to the conveyor belt. The conveyor belt however, is cancelling out any forward momentum of the plane.

A plane needs to have air molecules passing over its wing surfaces at a prescribed speed in order for it to even rotate and fly off the ground.

I still don't see how you can create airspeed over the wings with the plane virtually at a standstill on this conveyor belt set up.

Yeah, whatevahz. Have fun in your plane stuck on the (conveyor belt) tarmac in the air traffic at Honolulu airport.

My plane already took-off and flew to Maui for a weekend vacation.

[pzarquon]

Re: Airplane: Does it take off?

When this first came up a year ago, I was on the "doesn't take off" side of the fence. Having read some well-presented arguments on both sides, I think I've crossed the line over into the "takes off" camp. Which isn't to say I'm confident in being correct, nor sure the other point of view has been discounted. I mean, if people with fancy degrees and letters after their name can't agree, what the heck do I know?

Still, it came down to me envisioning a plane, say a fighter jet, anchored with a freakishly powerful, quarter-mile long chain to some freakishly strong pole, fully extended, then firing up its engines. No forward momentum relative to the ground... does the plane find lift?

I'm beginning to think it could.

A plane needs to have air molecules passing over its wing surfaces at a prescribed speed in order for it to even rotate and fly off the ground. I still don't see how you can create airspeed over the wings with the plane virtually at a standstill on this conveyor belt set up.

Movement of air over or past the wing is important. But lateral movement of the plane through the air isn't the only way to get that. A plane's engines are also moving a lot of air across those wings. Surely you've seen even enormous jumbo jets come in to land at impossibly slow speeds? It certainly isn't the plane's forward momentum holding those wings up.

Secondly, plane engines aren't just pushing air against or past wings. They're pushing against air behind the plane. Simple thrust, there. A rocket doesn't need wings to get off the ground, either.

Basically, I've come to think that contact with the ground, and position relative to the ground, is irrelevant to a plane's lift. Forward momentum, a rolling start, helps, but isn't required to get airborne. Depending, of course, on the freakishly strong power of said plane's engines, but since we're also talking about a conveyor belt that can move at an infinitely high speed, it's hard to quibble over that.

Imagine you tied a balloon to your bedpost and blew it up -- no wings, just one "engine" (the nozzle) there -- and let go. The balloon probably would flail at the end of the string, but it wouldn't drop like a rock, as long as air is moving relative to the surrounding air (and not the ground).

[1stwahine]

Re: Airplane: Does it take off?

heheheh

I can't 'STOP' myself...



Auntie Lynn

[Pomai]

Re: Airplane: Does it take off?

Basically, I've come to think that contact with the ground, and position relative to the ground, is irrelevant to a plane's lift. Forward momentum, a rolling start, helps, but isn't required to get airborne.

Now you're implying the airplane is a ROCKET, which rely strictly on thrust of the engine, not lift from wings for lift.

Fixed wing aircraft rely on lift from FORWARD THRUST (hypothetical point A to Z, hence a runway) over their WING for FLIGHT.

If you took a fully loaded 747 and stood it on its tail like a rocket (or balloon for that matter) and applied maximum thrust to all 4 engines, do you think it would take off like a rocket? Hell no! A 747 needs those HUGE wings to fly. And the only way those huge wings will make that 747 gain altitude for FLIGHT is by airflow over/under those wing foils. Airflow isn't going to happen if the 747 is standing still. It requires 155 knots of AIR SPEED. Sure the plane can stand still, such as in a wind tunnel, but the only realistic way it will gain this is by engine THRUST to move the physical airplane FORWARD into pressurized air molecules.

That runway can roll backward on a conveyor belt all it wants. As long as the engines can roll the aircraft forward into air molecules, that's all that matters.

Those who argue the airplane will not fly are convinced the runway movement is canceling forward thrust of the aircraft, rendering it at a standstill, but it's not! All the computer is doing is calculating the AIR SPEED of the airplane and counter-rotating the conveyor belt runway. The landing gear wheels are just spinning faster then they should, thus they need super-alien-designed wheel bearings. Bottom line. The aircraft is still moving forward from A to Z toward take-off speed.

Hey Cezanne, how's the traffic at HNL? OGG's nice!

[cezanne]

Re: Airplane: Does it take off?

Yeah, whatevahz. Have fun in your plane stuck on the (conveyor belt) tarmac in the air traffic at Honolulu airport.

My plane already took-off and flew to Maui for a weekend vacation.

Make sure you share some of those imaginary KrispyKremes with us when you come back from your imaginary vacation!