Physics Question for you.....

[Tom Miller]

Here's another point to ponder: say the airplane, on a normal runway, can reach a max speed of 250 mph. When the engines are up to full speed, and the conveyor belt is spinning to keep the plane motionless, how fast do you think the conveyor belt is moving? Do you think you have enough info to make a guess?

No, I don't think you do. The force that's countering the jet engines is mostly the friction force in the bearings of the landing gear. I think that conveyor belt would really have to be humming to build up enough friction force to counter the engines.

Regards,
Tom
p.s. Did I mention that I don't think the airplane will fly?

[LarryG]

Lee is right. That the engines are not powering the wheels is immaterial. Think it through:

1. The conveyor is stopped, the plane's engines aren't running. Neither is moving. The plane's wings are stationary relative to the body of air surrounding them. Obviously, the plane cannot fly (assuming a normal aircraft, not a helo or jump jet).

2. The conveyor starts running (backwards, relative to the way the plane is pointed) at 1 MPH. Since the plane is sitting on the conveyor, it too moves backwards at 1 MPH. The wings are moving backwards through the air surrounding the plane at the same 1 MPH speed.

3. The pilot starts the engines and applies enough thrust to counter the speed of the conveyor. The conveyor is running backwards at 1 MPH; the plane is moving forward (rolling on its wheels) at 1 MPH. The wings are now back to being stationary relative to the body of air surrounding them.

4. The plane speeds up to 10 MPH, then 20 MPH, then 50 mph, then 100 MPH. As the OP stipulated, the conveyor has a mechanism that tracks the plane's speed and increases the conveyor belt speed to compensate. Which means that no matter how fast the plane moves forward, the conveyor belt will be running backwards at exactly the same speed, canceling out the plane's forward motion.

Result: the wings are not moving through the body of air surrounding the plane. No motion, no lift, no fly.

To take it a little further ... suppose the plane had a take-off speed of 200 MPH, and the conveyor was running at only 10 MPH. The plane would be moving forward at 190 MPH relative to the surrounding air, but assuming an infinitely long conveyor belt, it would still never fly because it would always be going 10 MPH too slow to achieve lift-off.

[Stytooner]

Yes it will. The engine isn't powering the wheels. it will eventually move off the conveyor. more of a trick question than a physicas question.

Well maybe with a Harbor Freight conveyor the plane would reach the end of the belt, but he said "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 opposite direction). " I take that to mean if the was a pole beside the plane when it started, the pole would still be right beside the plane with the engines at full throttle. That is assuming it's an aviation quality conveyor


If you are on a concourse in an airport and step onto one of these high tech conveyors and the operator wanted to mess with your head, he would wait till you get to the middle of the conveyor and then speed it up to keep pace with you. You get miffed and start to trot. It's now troting just as fast but the other way. You try to sprint, but this operator is very good and the belt matches your speed flawlessly keeping you in the middle. How many miles will it take you to run to the end of the conveyor?
Better yet, say you are wearing a beanie cap with a propellor on top. How fast is it turning when you are giving it all you have?

[onedash]

That's what a helocopter does too. The blades are realy wings that are moved through the air at speeds to cause lift on each wing/blade. The wing/blades then lift the helocopter because they're attached via the rotating shaft.

aka...Rotary wing aircraft

[onedash]

Yes it will. The engine isn't powering the wheels. it will eventually move off the conveyor. more of a trick question than a physicas question.

[Stytooner]

Again if this were possible, they'd be using it on Aircraft Carriers.

If the engines actually moved the air over and under the wings, all the wings, then perhaps it would be possible. However the engines only move air past themselves. This action is enough when the plane is allowed to be tugged along by the force of this air and will eventually have enough airflow over and under the wings to create the lift. Much greater air force is running past the engines to maintain air under and over the whole plane.

With the runway moving in an equally opposite direction, the air around the wings will essentially not be moving anywhere. Same scenario is when you lock the wheels and rev up the engines. If this were possible, you could acheive takeoff that way too.
Nope, it ain't happening.

Given one other variable though, it may be possible. If you have a Cat 5 hurricane blowing straight toward the cockpit, you won't need the runway and you won't even need the engines to fly until you change direction. It would lift off right by itself.

[TheRic]

NO: Niki is correct in how a plane flies, and what keeps it up. Kistofor is correct in the over all effect.

Lets say the runway has a post off to the side of the runway. Remember the wheels do not more the plane, force against the air moves the plane. As the plane revs it's engines the runway will start to move. The plane relative to the post will move forward, the runway relative to the post will move backwards, the wheels will spin at twice the speed. The plane will move forward (relative to the post) at a greater and greater speed. The runway relative to the post will increase at a greater and greater speed. The wheels will spin twice as fast. Once the air speed (relative the the post, not the ground speed) is great enough the plane will lift off.

Assuming no friction, no wind, perfect world, etc, the plane will take off at the same point if the ground was moving, stationary, or moving WITH the plane.

[Kristofor]

It's going to fly.

Airplane engines don't power the wheels, they power the props push air back or propel mass out the back of the jet.

The speed of the conveyor only matters in that it adds some additional friction to the system so it will not accelerate quite as fast as normal, but I would guess this is a small impact at the <1xx mph speeds needed to take off.

It's straight forward mechanics question.

Force = Mass x Acceleration

The engines put out the same amount of force regardless of the motion of the plane (I'm guessing this isn't really true, but for slow speeds we'll pretend)

The mass of the plane doesn't change when it's in motion vs. standing still (hush you relativity fans, we're talking very low fractions of the speed of light for lift off)

So the plane is forced to accelerate at the same rate.

Wind resistance and friction from the wheels slow down that acceleration some, but so long as they don't impose so much resistance as to bring the acceleration to 0, and the plane itself can lift its own load, eventually the plane will take off.


Kristofor.

[leehljp]

It's not a Physics question. It is a common sense question.
Everyone knows that if there is no airflow over the top of the wing then the little bitty bernoullis fall off and the plane crashes!

[niki]

First, all those that answered NO, are correct.

The force that elevates and keeps the airplane in the air is called "Lift"
The Lift (L) depends on the density of the air (the Greek letter Ro), the square of the Velocity (V²), the wing area, (S) and the wing characters like the shape of the profile and the angle of attack that are summed in the term "Lift Coefficient" or CL.

The Lift is measured in Pounds and when the Lift is greater that the airplane weight, the airplane will Liftoff or climb.

If we put all those into equation it will look:

L = CL ½Ro V² S

The air density is not the same all over the world and will be different with High pressure system, Low pressure system, the airport elevation above sea level or the flight altitude and it's a function of the air pressure and temperature (the colder, the denser).

The Velocity, is simply the speed of the airplane (the wing) relatively to the air around the airplane and is measured in Knots (Nautical mile/hour. 1 Knot = 1.15 mile/hour).
As you can see, the Velocity has great effect on the lift V² (and on head on accidents).

The wing profile: the greater the upper camber the greater acceleration of the air above the wing. when air (or fluid) is accelerated, the pressure drops (and the temperature increases).
When the pressure drops above the wing, the pressure difference between the lower and upper sides of the wing increases and an upward force is created - Lift.

That is the reason that on slow speed flying airplane the wing is very "thick", to create the maximum possible lift at slow speeds.
But this thick wing creates also a lot of Drag that is slowing down the airplane and costs more fuel ( we need more engine power to overcome this drag).
That's the reason that on the high speed airplanes the wing is more "thin" or less cambered.

Another problem is: when the airplane is flying near the "Speed of sound" speed (Mach).
The airplane may be flying at 80% of the speed of sound (or 0.8 Mach), but the air over the wing is accelerated to a speed greater than the speed of sound that creates a "Shock wave" (something like a wall) over the wing and drastical loss of lift (and all the airplane is shaking like vibrator).
You can see the fighters wings, very thin and very low camber.

The layer that the air velocity is increased from zero (on the surface) to the free stream velocity is called the Boundary layer, depends on the airspeed, this layer is normally turbulent and does not create any lift.

BTW, that's the reason that I increased the Shop Vac hose diameter, to have more "free stream" area.

So, next time if somebody will ask you "what keeps the airplane in the air", you answer: "The Captains salary", believe me, I know.

niki

[LCHIEN]

It's going to fly.
The airplane propellor or turbine or jet creates thrust against the air behind it, not by rolling the wheels like a car.

remember that the air will be still, relative to the ground (assuming wind = 0).

The airplane will move forward at some velocity V (relative to the ground and hence, air). The belt will have a velocity of -V and the wheels will spin like crazy at 2 x V. The air right at at the surface of the belt will actually be moving at 2 x V (due to the boundary surface drag of the belt) relative to the airplane but the air just below the wings will be moving at just over V respective to the plane and more importantly the air over the wings will be moving at V. So when V = takeoff velocty the plane will lift, no problem.

There of course will be extra wear and tear on the conveyor belt and the landing gear, so you don't want to do this too often.

[gjat]

Plane wing is profiled so that when airflow is present, an elevated pressure zone is formed under the wing, and near-vacuum above it. This produces an upward thrust (antiwings on racing cars act int the opposite direction, pressing the car to ground for more traction) If plane is not moving relative to the air around it, no upward thrust is generated, and plane will not lift off.

Real close, but not quite.

It's not 'thrust', but SUCTION and THRUST that lifts a plane up. As the wind travels over and under the wing, the air that has to go over has to travel farther because of the shape of the wing. This decreases the density of the air above the wing, sucking the wing up because the density of the air below the wing is higher. Suck and thrust, so to speak. Everything is relative to wind speed above and below the wing that causes the air density differential.

The entire plane is pushed by jets or propellors in order to move the wings through the air at a certain speed to create the 'lift' (short for thrust and suck) effect on the wings.

That's what a helocopter does too. The blades are realy wings that are moved through the air at speeds to cause lift on each wing/blade. The wing/blades then lift the helocopter because they're attached via the rotating shaft.

And my physics teacher thought I wasn't paying attention back in high school.

(But nobody's convinced me that the Low Oil light isn't the same as the Low Fuel light.)

[BigguyZ]

But what if the conveyer is very long and the friction between the conveyer and the air around it creates a boundary layer? Thus the conveyer moves the air enough so that the air is pushed into the plane at 100MPH?????

OK, I know I'm reaching....

[Stytooner]

I think a Harrier might pull it off. If this were really possible, don't you think we'd have conveyor belts instead of catapults on an aircraft carrier?

[mmgross144]

plane on a conveyor belt

Since the plane will be essentially standing still, there will be no "wind beneath its wings". No airflow, no lift, no fly.

Got it dead on...no airflow, no lift, no fly...and by the way, I understand lift off speed to be 188 mph for a modern passenger jet.