Brain Teaser

Site Subscriber Joined: 28 Jan 2005 Posts: 3547

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?

Forum Moderator Joined: 18 Feb 2004 Posts: 7432

Yes, it will take off no problem. How is this a brain teaser?

Site Subscriber Joined: 11 Feb 2005 Posts: 2929

how can it? technicaly it's not moving so wind the pressures required for take off wouldn't occur?

Forum Moderator Joined: 18 Feb 2004 Posts: 7432

why is it not moving?

It will still accelerate and take off at the same airspeed, but fair enough, the wheels will be be spinning twice as fast when it does.

Site Subscriber Joined: 11 Feb 2005 Posts: 2929

I don't get your reasoning?

for a plane to take off, (my understanding is that) air moves over and under the wings to make a negative pressure above the wing, thus allowing it to take off.

If the plane is not moving then no matter how fast the engines are going it wont take off.

Forum Moderator Joined: 18 Feb 2004 Posts: 7432

OK, a seaplane. No wheels. Do they take off? why? How can they accelerate with no wheels?

How about a seaplane on a river flowing the other way, can it take off?

Level 8 User Joined: 14 Apr 2004 Posts: 278

Neal is right.

It moves because thrust is providing the movement, rather than via the wheels.

Think of it like a car on a rolling road, verses a jet fighter on a rolling road.
The speed the jet fighter's wheels are turning at is irrelevant as it isn't using them to propel itself.

Site Subscriber Joined: 11 Feb 2005 Posts: 2929

Neal";p="83503 wrote:

OK, a seaplane. No wheels. Do they take off? why? How can they accelerate with no wheels?

How about a seaplane on a river flowing the other way, can it take off?

yes a sea plane can take off in a river flowing the other way, Why? because at some point the plane will travel faster than the river and start to move forward.

in this scenario that is not the case, is it? the ground will always move at the same speed as the plane so it wont go anywhere.

think about it this way.
If you go running you feel the wind on your face. but if you go running on a treadmill you don't because your not moving.

Forum Moderator Joined: 18 Feb 2004 Posts: 7432

The plane's wheels are free-wheeling. It doesn't use them to accelerate. It pushes against the air.

Site Subscriber Joined: 11 Feb 2005 Posts: 2929

Damn it! your rite.

the plane will still move forward! FFS I hate brain teasers! Evil or Very Mad

Gay Rights Activist Joined: 07 Apr 2004 Posts: 1268

yes it would have to reach a certain grounspeed in order for the aerodynamics that provide lift to function. however, it simply means that the wheels on the plane would be travelling twice as fast as if it wasnt on the conveyor. this doesnt affect its ability to take off since the force/motion is provided by the engine...not the wheels.

imagine this....

you are on a skateboard, on this conveyor. you are at a standstill til i push you. you will still move forwards, but the conveyor will rotate the wheels of the skateboard faster than they would if i pushed u along the pavement, since i am providing the moving force...not the wheels.

its about relative speed/movement

Gay Rights Activist Joined: 07 Apr 2004 Posts: 1268

well edited docherty!

Site Subscriber Joined: 11 Feb 2005 Posts: 2929

done it whilst you were writing you first bit. lol