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I'm thinking this question had to been asked before, but I did a search and couldn't find anything on it. It's really bugging me, because I never could find out the right answer.
Suppose you have a jet airplane sitting on a runway. But this runway acts like a treadmill. The jet starts it's thrusters and for every mile an hour the Jet's wheels move forward, the treadmill moves a mile an hour in the opposite direction.
Will this jet eventually take off? Thanks.
Rob
No.
It's all about relative wind. If there is no wind flowing over the wings, there is no lift.
That's why carrier based aircraft do so well. First they generally have a good headwind and then they have the speed of the carrier added to this. They could be doing 60 knots before they even release brakes.
Thanks Old_Fart, but...Quote:
Originally Posted by Old_Fart
We're talking about a jet which uses thrust. Theoretically, wouldn't the thrust propel the plane forward (even though the treadmill is moving in the opposite direction), and eventually take off?
We're talking about how the plane gets up into the air. Not how it's propelled.Quote:
Originally Posted by Old_Fart
**edit** Forces on an Airplane
There was a very long thread here about that same subject a while ago. I'll try to dig it up.
Opps I was wrong, the conversation was on Digg. Plenty of discussion about this topic everywhere: plane treadmill take off - Google Search
As a pilot - the answer is no. Simple physics - there is no air passing under the wing to provide liftQuote:
Originally Posted by lobrobster
Thanks NeedKarma! That's what I was looking for! We had a HUGE debate at work about this awhile back that lasted for almost a month. I didn't know this was such a popular question.Quote:
Originally Posted by NeedKarma
On second thought, the tires would spin very fast but if the thrust was sufficient to move the craft against the still air, who cares how fast the tires have to turn?
Since there's no air flow over the wings, they are essentially useless in this scenario. So they may as well not be there. Now the question may be easier to comprehend - would a plane fuselage with jet engines but no wings ever lift off? Obvously not (unless it's a Harrier or some other VTOL design).
Are you sure? Jet planes are not axel driven. So why wouldn't the thrust propel the plane forward?Quote:
Originally Posted by MarkRealEstateConsultant
Btw- I don't know the answer. I'm just confused.
I've been thinking some more about this, and I want to revise my earlier answer. It's never too late to see the light! I had been thinking that the premise of the riddle was that the treadmill is designed to move at a speed that keeps the plane stationary. But what the riddle says "for every mile an hour the Jet's wheels move forward, the treadmill moves a mile an hour in the opposite direction" - thus if the plane (and its wheels) moves forward at 100 MPH, the treadmill moves the opposite direction at 100 MPH, which means the tires are spinning at the equivalent of 200 MPH. Hence the plane does move forward, and can take off. The tires will have had to spin at twice the rate as on a normal runway, but that's really the only difference.
ebaines, remember that the jet's engines are pushing against stationary air, irrespective of the tarmac, thereby producing thrust. The speed of the wheels on the landing gear don't enter into this equation.
Agreed. Previously I thought the premise of the riddle is that the treadmill moves fast enough to keep the plane stationary, and the only way that could be is if the friction between tires and treadmill would exactly counteract the thrust. Thus the treadmill would have to move at high enough speed to generate enough friction to counteract the jet's thrust. I now realize I had misinterpreted the wording of the riddle.Quote:
Originally Posted by CaptainRich
Would everyone agree that a car on the same treadmill/tarmac would never get above zero mph?
Depends...
What car are we testing and what is the top speed of the treadmill?
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If the aircraft isn't moving forward into the wind, the wings will not receive airflow and the plane will not take off. The only way this is theoretically possible is with a severe headwind equal to the takeoff speed of the aircraft. But in this case, I would not want to be in the aircraft.Quote:
Originally Posted by lobrobster
A plane flys by a force called lift, lift is created by the air going past the wing at a specific angle ( this angle is adjusted by the flaps)
But the plan will have to have a forward motion to get lift, no lift no flying
You're losing me. Any vehicle that is propelled by a drive train (i.e. it is driven by the wheels moving as opposed to thrust), theoretically, should remain stationary relative to someon standing next to the treadmill, correct?Quote:
Originally Posted by CaptainRich
This whole issue is one of thrust. Since a plane is not powered by it's wheels, they are in a sense, irrelevant. Technically, the plane should still move forward.
Picture a skateboard on the same treadmill with you at the opposite end (off the treadmill), holding a rope tied to the skateboard. Would you be able to pull the skateboard towards you?
Most treadmills are only capable of certain set speeds... does this version free-wheel?
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