The Naked Scientists

The Naked Scientists Forum

Author Topic: Take off ?  (Read 2774 times)

wolram

  • Full Member
  • ***
  • Posts: 103
    • View Profile
Take off ?
« on: 10/03/2006 14:17:59 »

This may be old to some.

An aeroplane is placed on a conveyor, the plane is started up, as soon
as the wheels start to move the conveyor moves at the same speed in
the opposit direction, no matter what the pilot does with the planes
throttle the conveyor keeps track in the opposite direction.

Will the plane take off ?

A born optomist

daveshorts

  • Moderator
  • Hero Member
  • *****
  • Posts: 2602
  • Physics, Experiments
    • View Profile
    • http://www.chaosscience.org.uk
Re: Take off ?
« Reply #1 on: 10/03/2006 16:49:55 »
A plane's wheels are just free running so unless you burn out the wheel bearings what is happening to the ground has no bearing on what the plane does. So yes it will.

rosy

  • Hero Member
  • *****
  • Posts: 1028
  • Chemistry
    • View Profile
Re: Take off ?
« Reply #2 on: 10/03/2006 16:58:46 »
Really? Surely the wings aren't moving relative to the air? Much? I guess it depends on the size of the track and how much air it's pulling with it?

wolram

  • Full Member
  • ***
  • Posts: 103
    • View Profile
Re: Take off ?
« Reply #3 on: 10/03/2006 17:33:20 »

My first thought was, the plane has no (air speed), i could stand
beside it and it would not move in relation to me, so if the plane
has no (air Speed) the wings can not generate lift, try running on
a tread mill, you will not feel a wind on your face :)

A born optomist

another_someone

  • Guest
Re: Take off ?
« Reply #4 on: 10/03/2006 17:48:01 »
quote:
Originally posted by wolram


My first thought was, the plane has no (air speed), i could stand
beside it and it would not move in relation to me, so if the plane
has no (air Speed) the wings can not generate lift, try running on
a tread mill, you will not feel a wind on your face :)

A born optomist



As others have said, there is no correlation between airspeed and land speed (in an extreme case, a small microlight could lift off in a gust of wind while not having any power applied to it whatsoever, and in a hurricane, even light aircraft can be lifted off the ground).

In fact, if you forget your conveyor belt, which is a total irrelevance since, again as others have said, the wheels are freely rotating and no power is applied through the wheels; if you have a string enough tail wind (for a conventional fixed wing aircraft), then that alone could prevent lift off, which is why pilots try, if it is possible, to take off into the wind rather than with the wind behind them.

And then there are planes, and there are planes.

In a helicopter, the wings move separately from the body (the wings rotate above the fuselage, rather than being fixed to it), so while the body remains stationary, the wings are still in rapid motion.

Then there are VSTOL jet aircraft (such as the Harrier) which can lift off vertically under the thrust of its engines alone without any aerodynamic airflow over its wings.



George

daveshorts

  • Moderator
  • Hero Member
  • *****
  • Posts: 2602
  • Physics, Experiments
    • View Profile
    • http://www.chaosscience.org.uk
Re: Take off ?
« Reply #5 on: 10/03/2006 17:55:28 »
Yeah as the wheels are free running if the plane is going left at 'v' the conveyor will be going right at 'v' so the only effect will be that the wheels are turning at '2v' rather than just 'v'.

wolram

  • Full Member
  • ***
  • Posts: 103
    • View Profile
Re: Take off ?
« Reply #6 on: 10/03/2006 19:07:46 »

I think it is the same as,
an aeroplane that can take of (no wind) at 40 mph, encounters a tail
wind of 40 mph, it will not take off.
Where is the difference ? :)

A born optomist

ukmicky

  • Moderator
  • Hero Member
  • *****
  • Posts: 3012
    • View Profile
    • http://www.space-talk.com/
Re: Take off ?
« Reply #7 on: 10/03/2006 20:35:08 »
quote:
Originally posted by wolram


This may be old to some.

Will the plane take off ?

A born optimist


yes this is an old one and I'm quite surprised this hasn't been posted on this forum before.
yes the plane will take off as the wheels are freewheeling and have no control over how fast the plane moves forward unless the brakes are applied of course

AN


Michael
« Last Edit: 10/03/2006 20:35:46 by ukmicky »

wolram

  • Full Member
  • ***
  • Posts: 103
    • View Profile
Re: Take off ?
« Reply #8 on: 10/03/2006 20:42:07 »
I am quite sure this problem has nothing to do with the wheels, it is the wings that have to
generate lift by moving in relation to the air, if the aeroplane is not moving in relation to
me, a static observer, then no aerodynamic lift can occure

A born optomist
« Last Edit: 10/03/2006 20:45:39 by wolram »

ukmicky

  • Moderator
  • Hero Member
  • *****
  • Posts: 3012
    • View Profile
    • http://www.space-talk.com/
Re: Take off ?
« Reply #9 on: 10/03/2006 21:10:19 »
The wheels are freewheeling which means when you turn on the engines and apply forward thrust the plane will move forward no matter how fast the conveyor is moving in the opposite direction. all the engines have to overcome to provide forward motion is the friction from the wheel bearings which is miniscule compared to the power of your average jet engine.

Even if the conveyer was travelling at 20.000 miles and hour the level of engine power required to prevent the plane from moving back with the conveyor would still be tiny.
Michael
« Last Edit: 10/03/2006 21:22:15 by ukmicky »

wolram

  • Full Member
  • ***
  • Posts: 103
    • View Profile
Re: Take off ?
« Reply #10 on: 10/03/2006 22:03:16 »

Yep that seems to cover it, thank you all.

A born optomist

another_someone

  • Guest
Re: Take off ?
« Reply #11 on: 10/03/2006 22:04:25 »
quote:
Originally posted by wolram

I am quite sure this problem has nothing to do with the wheels, it is the wings that have to
generate lift by moving in relation to the air, if the aeroplane is not moving in relation to
me, a static observer, then no aerodynamic lift can occure




Agreed, if the aircraft is static in relation to you, and you are a static observer, then the aircraft will not take off.

The assumption you are making is that you are a stationary observer, and that the aircraft is stationary relative to you.  You have not demonstrated either to be true.

That the conveyor belt is moving backwards does not make the aircraft stationary, either relative to the air or relative to the ground, because the forward thrust if pushing back through the air, not through the ground.

For a motor car, because it is being pushed forward by effort applied to the wheels, thus if the wheels are rolling on a moving conveyor belt,m then that is sufficient to stop the motor car.  The same is not true for an aeroplane, because the effort that causes forward motion is not transmitted through the wheels.



George

 

SMF 2.0 | SMF © 2011, Simple Machines