I KNOW I KNOW, it has been munched over a thousand times, but I would like to hear your opinion on this.
For all who don’t know what I’m talking about, here’s 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?”
Of course not! Ground speed is no factor. it’s airspeed and only airspeed, and in this scenario the airspeed is only however fast the wind is blowing, because the aircraft is not moving through the air.
[quote]bushidobadboy wrote:
This was done on mythbusters.
I can’t remember the exact outcome but it goes something like this.
Air moving over the wings creates lift. If the plane is stationary relative to the air, then no lift is generated. However if the plane is propeller driven, then some air will be driven over the wings, generating some lift, even if the plane is essentially stationary.
The plane will move forward relative to the ground - and thus generate life and fly - regardless of the speed of the conveyer backwards because the force generating the forward acceleration of the plane is the engine or propeller that has no relation whatsoever to the wheels that are in contact with the ground. The only limitation would be if the conveyer could accelerate at the same rate as the plane until the point where the planes wheels couldn’t spin any faster (due to frictional limitations). Think of it this way, on solid concrete the plane moves forward with a certain acceleration and the tires rotate at an angular velocity of “m”, if a conveyer is moving backwards under the plane in an attempt to keep the plane stationary, the wheels would simply spin at 2m and the plane would still continue forward.
i drew a little picture to help you out.
A car moves because the engine rotates the axle of the wheels which is attached to the car. As the wheel spins, the car attached inches forward. That means that anything that subjects the wheels to a force will directly affect the car. If the road starts moving towards the back of the car (as on treadmill), then the wheels will continue spinning and the car will remain stationary because movement of the car DEPENDS ON THE WHEEL’S ATTACHMENT TO THE ROAD.
The car’s speed is related to how fast the wheels can turn and inch forward. This means the wheels are actively spun by the engine. So if the wheels are the reason for movement and the road moves backwards and the wheels depend on the road, then the car will not be moving.
Wheels turn twice, the car is moved forward a few feet, but since the road goes back two feet, the car will remain stationary.
The airplane does not move thanks to the wheels. The airplane’s movement is dependent on the turbines which propel it. That means that it does not matter how fast the wheels spin or how fast the treadmill spins. Since the wheels are not actively spun by the plane’s engine, it is irrelevant what happens to the wheels.
The forces of motion are on two different planes (mathematics term) and not connected in anyway.
Since the wheel can spin somewhat freely and is not fixed to the airplane (not the same way as a car is), the plane will be able to move however it sees fit.
In the real world, the wheels would be spinning ridiculously fast and would probably melt or break off but we ignore that.
The simply answer is a plane is not a car, the engine does not drive the wheels.
Tie a rope to a wall, put on roller skates and jump on a moving treadmill. You can still pull yourself toward the front of the treadmill even though the wheels are moving in the opposite direction. Your arms drive you forward just like the plane’s engine or propeller does…kinda
[quote]ucallthatbass wrote:
The simply answer is a plane is not a car, the engine does not drive the wheels.
Tie a rope to a wall, put on roller skates and jump on a moving treadmill. You can still pull yourself toward the front of the treadmill even though the wheels are moving in the opposite direction. Your arms drive you forward just like the plane’s engine or propeller does…kinda [/quote]
That’s a wrong assumption, and it’s not the same with the rope, tree and rollerskates
[quote]matko5 wrote:
ucallthatbass wrote:
The simply answer is a plane is not a car, the engine does not drive the wheels.
Tie a rope to a wall, put on roller skates and jump on a moving treadmill. You can still pull yourself toward the front of the treadmill even though the wheels are moving in the opposite direction. Your arms drive you forward just like the plane’s engine or propeller does…kinda
That’s a wrong assumption, and it’s not the same with the rope, tree and rollerskates ;)[/quote]
Actually this is a really good analogy, your arms pulling on the rope is the equivalent of the airplanes propeller or engine pushing/pulling on the air in order to drive the plane forward.
[quote]JLu wrote:
matko5 wrote:
ucallthatbass wrote:
The simply answer is a plane is not a car, the engine does not drive the wheels.
Tie a rope to a wall, put on roller skates and jump on a moving treadmill. You can still pull yourself toward the front of the treadmill even though the wheels are moving in the opposite direction. Your arms drive you forward just like the plane’s engine or propeller does…kinda
That’s a wrong assumption, and it’s not the same with the rope, tree and rollerskates
Actually this is a really good analogy, your arms pulling on the rope is the equivalent of the airplanes propeller or engine pushing/pulling on the air in order to drive the plane forward.[/quote]
Well, is a jet engine pushing the air back or the gases from the engine are coming out? I think it’s not the same. Propeller engine pushes back air, yes, but air is not a rope you can push or pull to get movement, it’s not homonogeus (a solid object if you will)
A propeller works on the same basis as a wing, Bernoulli’s principle, but a propeller generates “lift” in the horizontal plane, propelling the plane forward which in turn generates lift in the vertical plane on the wings by drawing air over them, allowing the plane to fly. So yes, the planes propeller or turbine CAN in fact push/pull the air in much the same way our arms can pull on a rope.
Well, is a jet engine pushing the air back or the gases from the engine are coming out? I think it’s not the same. Propeller engine pushes back air, yes, but air is not a rope you can push or pull to get movement, it’s not homonogeus (a solid object if you will)[/quote]
Wrong. It creates a high/low pressure zone in front of and behind the prop.
