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brain teaser.

5K views 255 replies 56 participants last post by  SloRoll 
#1 ·
got this off another bike forum. i was amazed at the awsers. please give reason for your answer.

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
 
#10 ·
ChiefBD said:
I would think yes, similar to launches on an aircraft carrier.
I thought carriers use a design closer to a slingshot. Anyways, I'd say no, because if there is no air moving around the wings, it can't generate lift.
The addition/deletion of wind doesn't really matter.
 
#15 ·
i don't think it will fly - even with jet engines. if the take off is designed to match the wheel speed, the plane will be standing in the same spot with all the noise it can get. but, to take off, it needs an oncoming AIR, to create lift force. without it planes do not fly...

i think.
 
#16 ·
on the other hand, if that take off in addition to the wheel speed matching had another little feature, namely HUGE FAN, to blow in the direction of the plane, than the plane would take off and fly right above the take off without moving ahead.

that would be cool too. but USELESS ;)
 
#17 ·
pitbull76 said:
Yes. The wheels don't drive the plane, the jet engines do.
Asumption : We are not talking about VTOL here.

Why would that even make an ounce of difference? Flight is based on Bernoulli's principle of flight. It would make no difference how the airplane is powered. In order for lift to be created, air has to be traveling over the wings. Basically the curviture of an airplane's wings forces air to flow faster on the topside of the wings and slower on the bottom side. As a result, lift is created.

Can we agree that because the treadmille is set to matchthe airplane's speed but in reverse, so the airplane is basically running to stand still. With no air passing over it's wings, lift can not created thus no lift off.

This is exactly why air craft carriers run into the wind when planes take off. Running at 35 nauts or so into the wind gives the plane a head start.
 
#18 ·
maoisn said:
Asumption : We are not talking about VTOL here.

Why would that even make an ounce of difference? Flight is based on Bernoulli's principle of flight. It would make no difference how the airplane is powered. In order for lift to be created, air has to be traveling over the wings. Basically the curviture of an airplane's wings forces air to flow faster on the topside of the wings and slower on the bottom side. As a result, lift is created.

Can we agree that because the treadmille is set to matchthe airplane's speed but in reverse, so the airplane is basically running to stand still. With no air passing over it's wings, lift can not created thus no lift off.

This is exactly why air craft carriers run into the wind when planes take off. Running at 35 nauts or so into the wind gives the plane a head start.
exactly... the engines will be pushing the plane forward, against a treadmille that is set to keep the plane standing exactly still... which as a result the plane never moves, and because the air around a wing is what causes flight, the plane will just waste gas...
 
#19 ·
maoisn said:
Can we agree that because the treadmille is set to matchthe airplane's speed but in reverse, so the airplane is basically running to stand still. With no air passing over it's wings, lift can not created thus no lift off.

This is exactly why air craft carriers run into the wind when planes take off. Running at 35 nauts or so into the wind gives the plane a head start.
It has nothing to do with wind or the airplane being on a treadmill. The wheels are there for directional control on takeoff. The force from the engine will propel the plane forward. All sitting a plane on a treadmill would do is make the wheels spin really fast.

The reason aircraft carriers turn into the wind is to help shorten the takeoff roll and raise the margin of safety. A catapult can only accelerate an aircraft so fast and the added wind raises the airspeed of the plane in question.
 
#20 ·
Deuces said:
It has nothing to do with wind or the airplane being on a treadmill. The wheels are there for directional control on takeoff. The force from the engine will propel the plane forward. All sitting a plane on a treadmill would do is make the wheels spin really fast.

The reason aircraft carriers turn into the wind is to help shorten the takeoff roll and raise the margin of safety. A catapult can only accelerate an aircraft so fast and the added wind raises the airspeed of the plane in question.
So are you saying even with the plane sitting on a treadmill in matched reverse speed, it can still propell itself forward?
 
#21 ·
Yes, The wheels are like your car in neutral. If you have thrust from the engine it would be like pushing your car so regardless of what the wheels are doing, there is still the force pushing the aircraft forward.
 
#22 · (Edited)
Deuces said:
Yes, The wheels are like your car in neutral. If you have thrust from the engine it would be like pushing your car so regardless of what the wheels are doing, there is still the force pushing the aircraft forward.
:confused Can your car move forward, if the wheels cannot move forward? Better yet, when I run on the treadmill, I actually stay stationary, and I do not move forward. Why don't I end up in Chicago, when I run 18 miles on the treadmill in Naperville?
 
#23 ·
Brian said:
:confused Can your car move forward, if the wheels cannot move forward? Better yet, when I run on the treadmill, I actually stay stationary, and I do not move forward.
But on the treadmill there is nothing pushing you forward. your traction comes from your feet. Now if you were on rollerskates on a treadmill and someone pushed you from behind, you would move forward regardless what the belt was doing.
 
#24 ·
Think about this, you have a wheel with an axle going through the middle of it, and you are standing to one side of the conveyor. The axle is in the hands of one guy on either side of the conveyor, and the wheel is on the conveyor. The two men are acting like the jet-engines, producing forward force on the axle. As the men begin to walk forward, the treadmill accelerates in the opposite direction. However, the wheel will still make forward movement, because the men arent on the conveyor, just as the conveyor wouldnt affect jet propulsion.

The only thing the conveyor will do, is necessitate twice the normal wheel speed for a given ground speed.

FYI - If a car was on it, no forward movement would be possible because it relies on friction and drive from the WHEELS, not the air to move forward by.

Jake
 
#25 ·
Brian said:
:confused Can your car move forward, if the wheels cannot move forward? Better yet, when I run on the treadmill, I actually stay stationary, and I do not move forward. Why don't I end up in Chicago, when I run 18 miles on the treadmill in Naperville?
Ahhh, but what happens if you grab the hand rails on the side of the treadmill and push yourself forward? The engines are pushing against the air, not the treadmill.
 
#26 ·
Yes.

The planes wings develop lift from AIRSPEED.

The wheels on the treadmill only affect GROUNDSPEED.

The engines of the plane would propel it regardless of the treadmill to
an AIRSPEED that was sufficent to create enough lift to take off.


Remedial Physics Anyone ? :doh


Tom
 
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