Leaving on a jet plane (or, how smart are you again?)

Turok · Oct 25, 2007

acronym:
>In fact, the plane would take off faster than
>normal, with said induced air flow providing
>a headwind.

You're thinking is correct here, but I'm not sure whether the minimal take-off distance advantage from the treadmill induced headwind will be greater than the friction drag disadvantage of the faster spinning tires. An interesting question in and of itself.

>Edit-as for the control tower scenario, it may be
>possible to blow out the tyres, but thats about it.

Yes. (and you are one of the sharpest in the drawer so far with your quick grasp of the situation).

JB

acronym · Oct 25, 2007

Quote from Turok:

acronym:
>In fact, the plane would take off faster than
>normal, with said induced air flow providing
>a headwind.

You're thinking is correct here, but I'm not sure whether the minimal take-off distance advantage from the treadmill induced headwind will be greater than the friction drag disadvantage of the faster spinning tires. An interesting question in and of itself.

>Edit-as for the control tower scenario, it may be
>possible to blow out the tyres, but thats about it.

Yes. (and you are one of the sharpest in the drawer so far with your quick grasp of the situation).

JB

I suspect it would be the same as a normal headwind, and considering, given a perfectly smooth surface, and the best pilot in the world, theres nothing stopping a plane landing at its full airspeed-the wheel drag, after the initial turning moment, isnt that significant.

But what about a floatplane, taking off upstream of a perfectly smooth river, flowing @50knots?

Again, once the thrust passes a stabilising momentum, (assuming the aircraft isnt carried over some rapids in the meantime) the higher speed of the water should in fact cause the floats (assuming they are well designed, there have been some horror stories there) will pick up a water "plane" faster than they normally would, rendering the floats as efficent as at near take off speeds.

A slightly different scenario to wheels, as there is an inherent efficiency factor involved, actually relating directly to the "speed" of the surface compared to the aircraft itself.

macal425 · Oct 25, 2007

As long as you can create enough thrust to overcome the force of friction between the tires and the runway, theoretically you could take off. Another way to look at this is to imagine that you keep the wheel brakes on, then ramp up the thrust. The wheels will not move, but the plane eventually will. It will skid down the runway before taking off. If enough thrust has been used to overcome that friction, then it doesn't matter whether or not the runway conveyor is moving.

Turok · Oct 25, 2007

Quote from macal425:

As long as you can create enough thrust to overcome the force of friction between the tires and the runway, theoretically you could take off. Another way to look at this is to imagine that you keep the wheel brakes on, then ramp up the thrust. The wheels will not move, but the plane eventually will. It will skid down the runway before taking off. If enough thrust has been used to overcome that friction, then it doesn't matter whether or not the runway conveyor is moving.

If your saying that in this scenario, there is now a *sliding* friction between the conveyer and the wheels, I would disagree -- the wheels are rolling quite freely.

If your just saying that a wheel turning twice as fast creates more *rolling* friction, than I agree -- but comparably, there is not a lot of rolling friction to begin with so doubling it doesn't mean much.

JB

Supermax · Oct 25, 2007

turn the plane around facing it the other direction, turn off the engines, leave belt on. the plane takes off.

now, turn on engines and REVERSE thrust till the speed of plane (wheels) is equal to the belt. as the speed of the plane approaches the belt the plane slows then stands still and cannot take off.

answer to original Q: the plane CANNOT take off

ElCubano · Oct 25, 2007

Quote from marketsurfer:

wrong. the air flowing around the airfoil shaped wings generate lift. the motors merely provide the speed to increase the airflow around the wings, which in turn lift the plane. the plane on the conveyor belt would not take off.

surf

exactly....but hey I aint no scientist

Turok · Oct 25, 2007

Quote from Supermax:

answer to original Q: the plane CANNOT take off

I sure hope the little smiley face means your joking -- otherwise you're just wrong.

JB

Supermax · Oct 25, 2007

Quote from Turok:

I sure hope the little smiley face means your joking -- otherwise you're just wrong.

JB

no, you're wrong

the answer lies in the airspeed to wing

ElCubano · Oct 25, 2007

Quote from Optionspoet:

The wheels of a plane don't do anything. They can go 400mph or 0 mph. Their function is not the same as that of a car. Think about how a HArrier Jet achieves lift-off. It needs no run-way...just goes straight up.

One question to the poster - is there a freakin answer to this? Or is this one of those god damn riddles that ends with half the people on this thread debating physics for 50 yrs?

Forget it. I think I already have the answer to that.

harrier jet thrust its airflow into the ground in order to take off...a conventional jet thrust its airflow back and if the conveyor belt is moving at the same speed as the thrust their can't be enough airlift to take flight....but hey i might be way off...

Turok · Oct 25, 2007

Quote from ElCubano:

exactly....but hey I aint no scientist

Cubano, you and Surf are both correct -- it takes wind moving over the wings to create lift. The part you both are missing is that the plane moves down the runway *just as usual* and the wings create lift *just as usual*.

The key to the whole problem is the realizatin that the conveyor *can't stop* the plane from moving down the runway since the wheels aren't driven but rather are spinning freely.

JB