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Would the plane take off?

Discussion in 'General Chat' started by imported_Nigma, Mar 21, 2006.

  1. imported_Nigma
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    imported_Nigma Guest

    [Mar 21, 2006]
    Here is kind of a brain teaser... Its not a trick question, comes down to the basic laws of physics.

    A plane is standing on a runway that is made of a large conveyor belt. The plane fires up its engines, but as it moves forward, the conveyor belt senses the speed of the plane's wheels and moves at exactly the same speed in the opposite direction. Can the plane take off?

    Anand
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  3. RichA3Turbo
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    RichA3Turbo ...Watching you! Staff Member Moderator

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    [Mar 21, 2006]
    [ QUOTE ]
    Here is kind of a brain teaser... Its not a trick question, comes down to the basic laws of physics.

    A plane is standing on a runway that is made of a large conveyor belt. The plane fires up its engines, but as it moves forward, the conveyor belt senses the speed of the plane's wheels and moves at exactly the same speed in the opposite direction. Can the plane take off?

    Anand

    [/ QUOTE ]

    Course it cant...no airflow over wings.
    #2
  4. robthehungrymonkey
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    robthehungrymonkey Member

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    [Mar 21, 2006]
    no, no matter how fast the "runway" is moving, the air won't be flowing over the wing fast enough to generate the required lift. but...

    with the engines on, the plane would be going forwards at the same rate as it would be with a static runway, with only the added drag from the wheels moving twice as fast. So it would take off.

    I think!
    #3
  5. robthehungrymonkey
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    robthehungrymonkey Member

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    [Mar 21, 2006]
    To make my point a bit clearer, i think because it's not driven by the wheels, but by jet engines it would take off. As ground speed is irrelevant, only air speed.
    #4
  6. RichA3Turbo
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    RichA3Turbo ...Watching you! Staff Member Moderator

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    [Mar 21, 2006]
    [ QUOTE ]
    To make my point a bit clearer, i think because it's not driven by the wheels, but by jet engines it would take off. As ground speed is irrelevant, only air speed.

    [/ QUOTE ]

    Exactly...and if the belt is moving in the opposite direction, the speed cancel each other out, to the plane will be static on the runway, and with no airspeed, theres no lift, so no take off.

    RIch
    #5
  7. benk
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    benk Member

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    [Mar 21, 2006]
    The question is "Can the plane take off?".
    The answer is probably "yes", assuming it's an airworthy plane.

    The fact that someone is cocking about with runways doesn't mean the place can't take off, just that it won't when placed on such a nobbish runway.
    #6
  8. benk
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    [Mar 21, 2006]
    Ok, seems this "teaser" has been bandied about a fair bit, with the argument that a plane's wheels are free-rolling, so the thrust of the plane should override the low friction of the wheels, and mean it still travels forwards, regardless of what is going on with the wheels, and consequently can take off.
    #7
  9. RichA3Turbo
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    RichA3Turbo ...Watching you! Staff Member Moderator

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    [Mar 21, 2006]
    Actually...it definitely will take off... As rob said, the wheels although moving twice as fast, have no bearing on the speed of the plane as they are not driven... The planes engine provide the power, and even if the ground is moving, the air isnt, so airspeed will increase and the speed of the wheels will double.

    Rich
    #8
  10. imported_Nigma
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    imported_Nigma Guest

    [Mar 21, 2006]
    The best way to visualise this...

    Imagine a toy car, with free running wheels (not so hard to imagine).
    Now place this car on a conveyer belt. You're hand is the equivalent of the planes engines.

    No matter how fast the belt is moving, you can easily push the car forward with very little effort. What this shows is that you pusing the car forward, has nothing to do with whats going on at the wheels. = plane will fly
    #9
  11. Buzz
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    Buzz Full Throttle

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    [Mar 21, 2006]
    All the planes engines have to do is overcome the rolling resistance of the tyres and friction of the wheels bearings. The planes engines should easily be able to overcome this and as previously stated the plane should fly.
    #10
  12. robthehungrymonkey
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    robthehungrymonkey Member

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    [Mar 21, 2006]
    Good question. Got any more? These sort of things just help to keep my brain ticking over during a long day!
    #11
  13. benk
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    benk Member

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    [Mar 21, 2006]
    This thread has a danger of getting very very long.

    We just need to wait for the debate about the wording of the question (is the conveyor belt matching [actual plane speed] or speed of [plane's wheels], etc).
    #12
  14. robthehungrymonkey
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    robthehungrymonkey Member

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    [Mar 21, 2006]
    [ QUOTE ]
    This thread has a danger of getting very very long.

    We just need to wait for the debate about the wording of the question (is the conveyor belt matching [actual plane speed] or speed of [plane's wheels], etc).

    [/ QUOTE ]

    That still won't make any difference, as the speed of the plane is dependant on the thrust, not the speed the of the wheels. Even if the conveyor was going faster, it would still take off.
    #13
  15. CJ A4
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    CJ A4 Active Member

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    [Mar 21, 2006]
    No it wont fly unless its a harrier jump jet!
    #14
  16. imported_Nigma
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    imported_Nigma Guest

    [Mar 21, 2006]
    [ QUOTE ]

    We just need to wait for the debate about the wording of the question (is the conveyor belt matching [actual plane speed] or speed of [plane's wheels], etc).

    [/ QUOTE ]


    "the conveyor belt senses the speed of the plane's wheels and moves at exactly the same speed in the opposite direction"

    Think that means the belt matches the speed of the wheels.
    #15
  17. robthehungrymonkey
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    [Mar 21, 2006]
    [ QUOTE ]
    No it wont fly unless its a harrier jump jet!

    [/ QUOTE ]

    Yes it will! /ubbthreads/images/graemlins/soap.gif
    #16
  18. god_thats_quick
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    god_thats_quick Numptie of the highest order

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    [Mar 21, 2006]
    I'm not sure about this one, there would be enough thrust to get a plan off the ground normally, but that thrust is effectivly just acting against the belt and as mentioned earlier there will be no air moving over the wings so I'm not convinced it'll take off as I can't see how it'll generate lift whilst being stationary even with all the thrust in the world - I could go look at the original thread where this was brought up on another forum, but I can't be arsed!
    #17
  19. Buzz
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    Buzz Full Throttle

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    [Mar 21, 2006]
    The only thing stopping the plane moving forward is the friction of the wheel bearings. No matter how fast the belt is moving the thrust of the engines will move the plane forward providing sufficient air flow and lift to make the plane fly.
    #18
  20. madvw
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    [Mar 21, 2006]
    It's my opinion that the plane wont fly.

    assuming that any forward motion at all is negated by the equal and opposite motion of the conveyor belt.

    If the physics law of flight state that the "lift" required to keep the plane in flight requires there to be a certain amount of air passing over the entire wingspan, then this is not going to be satisfied by the planes engines.

    The engines will just draw air from the front of the plane, and expel it from the rear of the engine. In effect, no air will be drawn over the wings, so it wouldn't generate any lift.
    #19
  21. god_thats_quick
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    god_thats_quick Numptie of the highest order

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    [Mar 21, 2006]
    [ QUOTE ]
    the engines will move the plane forward

    [/ QUOTE ]

    That's where your wrong because the plane won't move other than over the belt as it says "speed sensing" so there will be no airflow over the plane.
    #20
  22. Buzz
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    [Mar 21, 2006]
    [ QUOTE ]
    [ QUOTE ]
    the engines will move the plane forward

    [/ QUOTE ]

    That's where your wrong because the plane won't move other than over the belt as it says "speed sensing" so there will be no airflow over the plane.

    [/ QUOTE ]

    The original question states the belt ''senses the speed of the plane's <u>wheels</u> ' and not the plane. Therefore IMO for your argument to be true, the belt would have to move at near infinite speed to cause sufficient friction at the planes wheels to counteract the thrust produced by the engines.

    I think?
    #21
  23. hop2407
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    [Mar 21, 2006]
    Wont take off....... (IMO).... the clue is in the words... "but as it moves forward, the conveyor belt senses the speed of the plane's wheels and moves at exactly the same speed in the opposite direction"

    No air flow over wings.... Imagine you are the plane and your feet are the wheels.... when you start to walk (ok you might move a little bit) but the conveyor belt detects your forward motion, so counteracts this forward motion by starting to move in the opposit direction - the quicker you walk (or the quicker the planes wheels go round) - the quicker the conveyor belt moves in the opposite direction, thus no physical forward movement ....

    Hope that makes sense and just my opinion....
    #22
  24. jojo
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    jojo S3 Drift King! Staff Member Moderator

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    [Mar 21, 2006]
    My answer is YES, the plane can take off, the thrust or power would be coming from the jet engines, and not directly from the wheels, so regardless of how much the conveyor belt effects the wheels on the plane, it is thrust thats pushing the plane forwards. So long as the plane can reach minimum flying speed, it will take off.....
    #23
  25. bainsyboy
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    [Mar 21, 2006]
    I AINT BOTHERED, FACE BOTHERED NO, IM GOING BY F--KING BOAT TO THE ISLE OF WIGHT SO I AINT BOTHERED
    #24
  26. imported_Nigma
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    imported_Nigma Guest

    [Mar 21, 2006]
    [ QUOTE ]
    No air flow over wings.... Imagine you are the plane and your feet are the wheels.... when you start to walk (ok you might move a little bit) but the conveyor belt detects your forward motion, so counteracts this forward motion by starting to move in the opposit direction - the quicker you walk (or the quicker the planes wheels go round) - the quicker the conveyor belt moves in the opposite direction, thus no physical forward movement ....

    [/ QUOTE ]

    You cannot use this analoagy, the plane and the wheels are effectivly independant of each other.
    The wheels of the plane do not drive the plane forward, its the thrust from the engine. The wheels are there simply to keep the plane off the ground. The better analogy is the one i used above with the toy car.

    An other analogy....
    The plane is not a plane, its a hovercraft. The wheels represent the air below the hovercraft.
    Turn the fan on, the hovercraft moves forward, The belt moves in the opposit direction at the same speed. But will have no effect on the hovercrafts forward speed.

    mathematically
    The thrust causes the plane to move forward at 100kph. So now the wheels are moving at the same rate. The belt moves in the opposit direction at 100kph. Therefore the wheels are moving at a speed of 200kph relative to the ground. So the forward motion of the plane is still 100kph.
    #25
  27. bainsyboy
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    [Mar 21, 2006]
    I AINT BOTHERED /ubbthreads/images/graemlins/sleep.gif
    #26
  28. robthehungrymonkey
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    [Mar 21, 2006]
    [ QUOTE ]
    It's my opinion that the plane wont fly.

    assuming that any forward motion at all is negated by the equal and opposite motion of the conveyor belt.

    If the physics law of flight state that the "lift" required to keep the plane in flight requires there to be a certain amount of air passing over the entire wingspan, then this is not going to be satisfied by the planes engines.


    The engines will just draw air from the front of the plane, and expel it from the rear of the engine. In effect, no air will be drawn over the wings, so it wouldn't generate any lift.

    [/ QUOTE ]

    By expelling air from the rear of the engine, forward thrust will be generated. pushing the plane forward. This will still happen no matter what the wheels are doing. The fact that the wheels are moving are irrelevant, the only force they exert on the plane is the friction of the wheel bearings, which is fairly insubstantial compared to the thrust generated from the engines.

    If this was to happen, it would see the plane taking off at normal speed (Say 90mph), but the wheels rotating at a speed relative to 180mph, as the "runway" would be going backwards at 90mph.

    Is anyone reading any of the other posts, or just answering the question at the top?

    I can't see anything explaining how i'm wrong!? And I was the first poster!
    #27
  29. benk
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    [Mar 21, 2006]
    #28
  30. The Slug
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    The Slug Active Member VCDS Map User

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    [Mar 21, 2006]
    On take-off dont the wings actually flex upwards to help/due to the airspeed wind, so i dont think it will work, no airspeed friction, no takeoff, lol.

    My brain hurts.......
    #29
  31. TQS
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    [Mar 21, 2006]
    Try sticking a set of wings on a Bugatti Veyron, remove downforce and then have a go on a rolling road.... will it fly? Doubt it. The only things that are moving are the wheels.
    #30
  32. RichA3Turbo
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    [Mar 21, 2006]
    [ QUOTE ]
    Try sticking a set of wings on a Bugatti Veyron, remove downforce and then have a go on a rolling road.... will it fly? Doubt it. The only things that are moving are the wheels.

    [/ QUOTE ]

    We are forgetting one thing here. In a car, the power comes from teh driven wheels. If the wheelas are doing 100mph &gt; and the converyor belt 100mph &lt; they cancel each other out and the car will not move. In the plane, the wheels are never driven, so all the engines have to do is take the strain of the aircraft i.e product enough thrust to cancel out the aircraft weight... At this point, further increase in thrust will mean the aircraft will start moving. The wheels will now be moving at the cancelled speed (say 100mph) plus the increasing speed of the aircraft. You can not compare aircraft to car. If you put a 747 on a rolling road, you'd get 0bhp... however, it would have sucked in the rolling road, the workshop, tools, operators, the building and would be half way round the world! lol.

    Rich
    #31
  33. [Mar 21, 2006]
    It WILL take off, no doubt about it.

    "the conveyor belt senses the speed of the plane's wheels and moves at exactly the same speed in the opposite direction"

    So the plane continues to move forward, just as for any normal take off - the conveyor belt is moving same speed in opposite direction, so the (freewheeling) wheels will rotate twice as fast as they normally would, but the plane still moves forwards and takes off as the appropriate airspeed is reached!

    the question itself doesnt work unless the plane is moving forward ("the conveyor belt senses the speed of the plane's wheels", hence they are turning as it moves along), and if it is moving forward then air is passing over the wings just as normal. The propulsive force is from the jets not the wheels, so the groundspeed is irrelevant to the takeoff - the jets are pushing it forward through the air.

    The conveyor belt is not exerting a braking force, it is merely moving in the opposite direction to the wheels.

    If the motive power came from the wheels like a car, then the speed could be matched by the belt, like a rolling road - however....

    a seaplane can take off against an opposite current of water - probably similar friction to that of freewheeling wheels and bearings against an opposing speed belt..

    Am I right?? I think so!!! /ubbthreads/images/graemlins/woohoo.gif
    #32
  34. TDI-line
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    TDI-line Uber Post Whore

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    [Mar 21, 2006]
    Armand, your out there man. /ubbthreads/images/graemlins/bang.gif
    #33
  35. Mart
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    Mart Active Member

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    [Mar 21, 2006]
    Where the pilots & passengers drinking redbull??
    #34
  36. CJ A4
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    [Mar 21, 2006]
    This thread still going?? /ubbthreads/images/graemlins/bang.gif

    ok i changed my mind...the plane will take off it just means that the wheels will be roatating at twice the speed at which they normally would be during take off! Still reverting to my first statement that if you had a harrier jump jet it wouldn't matter either way! /ubbthreads/images/graemlins/lol.gif
    #35
  37. fallmonk
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    fallmonk Turbo Sport

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    [Mar 22, 2006]
    I say it would take of , having been to many a rolling road , where the rollers match the speed of the car why do they still strap the car down????? it still trys to jump out the rollers .
    Plus i also agree the jet engines drag the AIR in and thrust it out so its technical nothing to do with road speed ! just my oppinion /ubbthreads/images/graemlins/book.gif
    #36
  38. Charlies604
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    [Mar 22, 2006]
    Bugger all to do with road speed.... its an AEROPLANE, Not a CAR!!

    Its wheels are just there to stop it digging into the ground.
    They in effect, remove the plane from the runway.
    Does a plane need wheels in the air? NO
    Does a plane use the wheels for anything other than removing friction?
    NO (Well if it dint have them landin would be hard! But nowt to do with it speed)


    It may not fly at full speed, as it would with wheels up due to the small amounts of friction produced by these wheels tho.
    These wheels would spin twice as fast as the other two objects tho.


    As for this rolling road stuff..... If the rollers go one way, yours cars wheels the other.... the car stays still.

    Now...
    Put it in neutral, try and power the rollers like the conveyor belt would be (We no they actually try not to spin and measure that using load cells but here me out!)

    Now the wheels and rollers are traveling 2getha...
    The thrust is provided by you and your mates pushing the car by the sills, boot etc.
    You can push the car off the rollers...
    Add wings... and more mates (For greater thrust) And u have flight!

    The wheels and the rollers have a relative velocity of 0mph...

    The only question in this is can thrust overcome the friction of the wheel bearings!

    Or if u look at it like BK... it only asks what the plane can do! Therefore sod the silly runway and away u go!

    Rant over...
    Charlie revell
    BEng Mechanical Engineering and Compostie Technologies! hehehe
    #37
  39. NEiLS3LK51
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    [Mar 22, 2006]
    No,

    Relativity!

    The thrust produced by the engines does not produce lift, it pulls the plane forward in the
    air/ground by a lateral change in air pressure.

    For a wing to produce lift it must be accelerated forwards to a sufficient speed relative the the
    air around it and create a low pressure above the upper surface of the wing.

    If the forward acceleration of the plane's wings is opposed by a negative accelertion of the
    conveyer belt then there will be no sufficent forward movement of the wing relative to the air
    around it and hence it will not produce lift.

    This is why aircraft carriers are BIG, a conveyer belt system described does not work.

    If the belt could were replaced with a big fan you could generate lift short term before stalling.
    I seem to remember a model plane tethered in front of a fan at the Science museum many
    years ago where you could vary the fan speed and hence the lift of the model plane.
    #38
  40. FactionOne
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    [Mar 22, 2006]
    I'm with NEiLS3LK51,

    I've come across this teaser a few times in the past, both in aviation-circles (discussed it with engineers, pilots and others involved in simulation), and otherwise...

    As Neil has hit on, it's all about the transition between being a vehicle on the ground and a vehicle in the air (and that's the reason a lot of the answers I've heard/seen are incorrect, no matter how credible the source might seem in cases)...

    As prescribed by Neil and Bernoulli's principle, a conventional wing needs air passing above and below it at different speeds to cause an offset in pressure resulting in lift. This means that to generate lift, the wing has to be moved through the air, or the air has to be moved past the wing. The speeds required to generate lift are dependant upon a number of factors, but we'll keep it simple for now...

    These aren't _strict_ aerospace numbers and practices, but it'll make it easier to see what I'm driving at...

    Imagine that I build a conventional, fixed-wing aircraft and outfit it with wings which will provide lift for it above 150 knots (in these examples we'll assume that air-pressure and temperature are constant, and there's no wind)...

    So, to get my aircraft to be an aircraft, and not just an odd looking car, I need to have a closing-speed between wing and air of 150 knots.

    I could acheive this by pushing the aircraft through the air until I reach that speed, or I could even move the air across the wing at that speed - YES! If my aircraft was parked in a smooth-headwind of anything above 150 knots, it could theoretically be a vertical take-off aircraft too! That's unlikely though, as winds that speed aren't exactly smooth, so you'd probably roll it or flip it into the ground at the first anomaly. So we revert to plan (a), moving my aircraft through the air until it reaches 150 knots and the lift is present to get it airborne.

    Now imagine a car on a rolling road once again, lets take a de-restricted Veyron for instance... Now lets say I come along and while you're not looking, weld-on some wings I've carefully designed to lift the mass of the Veyron (and the wing, driver, fuel etc.) at 150 knots (in the same atmospheric conditions as my plane)...

    OK, we're ready to go... Get on the gas! Your Veyron will take off once it's moving 172.62 mph through the air... WE'RE GOING TO SEE A VEYRON FLY!!!

    ...Alas no, it may take up to a full tank of juice to realise, but we're not going to get your winged-Veyron moving through the air at over 170 mph until we take it off the rollers...

    EDIT: And to just quickly further deal with any point regarding the wheels of the aircraft not providing the power, it's irrelevant where the power comes from until you've gained the magic 150 knots airspeed. Lets imagine that my wing as I mentioned, generates lift at 150 knots, but that's only when you have some extension of flaps, if the flaps are retracted, we need 180 knots.

    Now I put some clever-engine technology on my aircraft, meaning the wheels can drive it up to oh, 180 knots. My clever engine-technology also means I can cut the jets in at 100% thrust in an instant.

    I now align my aircraft to the threshold of a non-conveyor-belt runway; with the flaps retracted and apply full power to the wheels... I roll to 170 knots and extend the flaps. My aircraft now has sufficient airpseed for the wing's configuration to provide lift and I take-off. At the same moment, I apply full power to my turbofans and up I'll stay. Any method of propulsion can do it, but you still need a method of getting the aircraft from 0-150 knots before the magic of the wing's interaction with the air will stop you having to worry about such trivial things as the ground... (Perhaps a BIG catapult on a LOOOOOONG (and not very short roller-equipped) Aircraft Carrier as Neil said...

    Regards,

    Rob.

    PS&gt; Glad to be back with the audi-sport.net bunch again, after another little gap, all will be explained soon...
    #39
  41. rickparmar
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    rickparmar va va voom

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    [Mar 22, 2006]
    Am going to send it to brainiac and let them try it lol
    #40

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