Wednesday, December 3, 2008


Can an airplane on a treadmill take off?

Apparently it's a well-known question. Much discussed. Sort of a will a man walking down the up escalator get anywhere thing. Except that it's not the wheels that make a plane fly, nor its relative position to the surface of the earth. It's wind velocity, whatever the technical term, and thrust. It's not speed relative to the ground, but relative to the air. A car on a treadmill wouldn't get anywhere, because it is a land vehicle. Will a plane? -- an air vehicle?

The answer I get comes from reframing the question. Will a stationary "plane" in a wind tunnel take off? Manifestly yes. It's just a matter of wind velocity kicking the laws of aerodynamics into play. The plane must somehow be "thrust" through the air, creating wind, or wind must race over the wings. That's what's needed -- wheels and ground have nothing to do with it.

Lord I'm smart.

As with so many almost-imponderables, confusion lies in insufficient facts, or in faulty perspective, or in incorrect assumptions. Bush lied, so we should hate him. Indeed, if he did lie, evilly, to get us into an evil war, for his evil purposes, he is evil. But he didn't lie. Wrong about some of the facts. An error, then, of fact, exploited by those who are in error in their facts, perspectives and assumptions.

Terrorism. Allah is God and all who don't abase themselves before him are wicked. And should be killed. By any means necessary. Terrorism. An error in perspective. Allah is not God, and if he were, and good, he wouldn't condone terrorism, but rather only war. The islamist terrorists look at the issue from only one angle. Their religion then, such as it is, is an optical illusion. What looks straight to them, is actually bent -- what seems to connect, doesn't.

Errors of assumption. Any of Zeno's paradoxes, say. Before we can arrive at a destination, we must go half the distance; but before we go half, we must go half of that half; and half of the quarter, and half of the eighth, and half of the sixteenth, and so on -- infinitely halving, so that not only can we never arrive at our goal, but we can't move at all. Yet we can. An error of assumption. That space is infinitely divisible. But it's not. There comes a point where you can't get any smaller. You are either here, or there, but there is no in-between. The same holds for time. Quantum leaps. It happens, literally, everywhere, all the time. How odd. Just on such a small level we don't notice it.

We don't notice our wrong assumptions. Maybe that's why we don't get where we want to be.

So. I leave you with this.
A picture of a seated man and a woman standing to our right, his left; on the man's lap sits a small child wearing a white bonnet and a white blouse.

What. What. You think you see the image of some saint or deity? This is a common problem. The Virgin Mary in tacos and the will of Allah in unspeakably tortured then murdered Jewish Chabad charity workers. Some errors are more damaging than others, it seems. If only something could be done about that.



Will C. said...

"Except that it's not the wheels that make a plane fly, nor its relative position to the surface of the earth. It's wind velocity, whatever the technical term, and thrust. It's not speed relative to the ground, but relative to the air."

If the belt is moving fast enough it could cause airflow through the body of the plane and over the wings (perhaps enough for lift) due to the "no-slip condition" between the moving belt and the air above it as defined in fluid mechanics. But then the higher velocity air would be moving moreso along the lower surface of the wing thus causing I've disproved my assertion.

But wait if you inverted the wing and its say...a nice NACA 2412 profile and ran the belt fast enough you could generate lift. One caveat is that the nose (or front) of the plane would have to be tethered to some distant point over come the friction of the rolling landing gear. But, if the plane is simply dropped into the flow field then you would eliminate the need for the tether.

See what you've gone and done making me think and stuff.

Jack H said...



Jack H said...

You're supposing that the virtually flat surface of a treadmill could generate enough wind via surface friction to lift the mass of a plane? Although my background is more in the humanities, I suggest the belt speed required, if your premise is even correct, would surpass the material specs. Wind generated by a rotating flat surface seems negligible. Blades have an angle, after all. Also, would there be an eddy effect from the lower belt, with wind convecting up and confusing any lift effect? And what about the spinning of the plane wheels? Does any wind generated by them cancel out? Presumably.

I'm still puzzling over whether or not birds flying inside an enclosed container make it weigh more.

Will C. said...

Read up on the no-slip condition. If the flow can be made laminar (low Re number) fields of flow lines will eminate from the fast moving belt surface essentially causing moving air above the surface of the belt. You never said how large the plane had to be (I'm thinking of something on the order if the little basla models that had a metal clip for ballast) and you never said how fast the belt could move nor the surface composition. All could be optimized to create greater flow. This in addition to "ground effect" (a benefit caused by flying low wherein the displaced under-wing air has less room to evacuate resulting in increased lift) should be sufficent to cause lift on the inverted NACA profile.
Any further doubts in my assessment will result in your demotion in the Will C. administration from Cabinet Level to something sucky-er...say, Secretary of State.

"I'm still puzzling over whether or not birds flying inside an enclosed container make it weigh more."

I would estimate that it weighs more. The downward forced air is in equilibrium to the lift the bird is maintaining. This force would be equivalent to the bird's weight and would contribute to the total weight (or downward force F=ma where m = total mass of bird + jar and a = 32.2ft/s^2).

Jack H said...

Re birds, that was my supposition -- I was thinking that given an open container the displaced air would redistribute the downward force outward -- wind is force, after all. Seems like an averages thing -- if you jump on a scale, first it's heavier, then lighter, then it bears your weight again.

As for all that jibberjabber about slip flows and balsawood toys, I just can't follow it. Weren't we talking about PLANES???

Anyways, I resigned in righteous dismay and deepest disgust from the corrupt Will C klepocracy long, long ago. Started my own breakaway republic, governed with benignant authority by a certain here-unnamed genius of untold brilliance and beauty. Can you guess?