
The physics of floating screwdrivers [video] - lisper
https://www.youtube.com/watch?v=jAYP6pWrdkc
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icegreentea
Whats super interesting to me is how this system becomes dynamically stable.
In the pure thick shaft version, it looks like the jet is centered on about
1/4 of the way down the shaft - so it should be imparting torque on the whole
thing, but the fluid dynamics are enough to keep it at what appears to be a
pretty constant angle.

Also, as a note about the golf ball comment (this seems to happen quite a
lot), nothing he said was wrong, but there's slightly a bit more nuance going
on. For a sphere (since we're talking golf balls) in a given flow, there will
be a point along the ball will the flow will detach and form eddies and the
mentioned low pressure zone behind the ball causing additional drag. By adding
dimples, you introduce turbulence into the boundary flow that basically gives
it enough momentum to keep up with the ball for longer, letting it stick the
ball and therefore separate later - therefore lesser losses through that
pressure drag.

The reason why we don't dimple everything flying through the air is that
adding turbulence to the boundary flow also increases the amount of drag on
the object (so called skin friction drag). The act of adding dimples may
either increase or decrease total net drag depending on the exact parameters
of the object and the flow.

It just happens that for golf balls flying through atmosphere, typical drive
velocities are right in the range where adding dimples causes noticeable net
reduction in drag.

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lisper
It works for cars too:

[https://www.youtube.com/watch?v=eR5SlwNf4K0](https://www.youtube.com/watch?v=eR5SlwNf4K0)

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iamcreasy
Then why the fuselage of an airplane is smooth?

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platz
[http://physics.stackexchange.com/questions/109395/why-
arent-...](http://physics.stackexchange.com/questions/109395/why-arent-
airplanes-like-golf-balls)

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joering2
Very interesting, consider how heavy that screwdriver is.

What would be a ratio of a cost of electricty (to blow an air) to the weight
of an object it can raise?

Could this be used as initial propeler to raise a rocket high enough to use
this technique as a "first stage engine" in flying object out into space?

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josai
> Could this be used as initial propeler to raise a rocket high enough to use
> this technique as a "first stage engine" in flying object out into space?

I don't really see how. You'd need some kind of gigantic and fairly stable
platform to shoot the jet from - if you have that, it would be easier to just
launch the rocket from that.

And you're forgetting the fact that we seldom just launch stuff "into space"
\- we want it in orbit. Only about 20% of the delta-v budget of a typical
launch to orbit is the "vertical" component, the large majority is spent on
attaining angular ("sideways") velocity. So you'd be doing an awful lot of
work just to cut down that 20% by, say, a quarter - it's simply not worth it.

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mjcohen
A question and a comment:

1\. How much of the lift is caused just by the air pushing up?

2\. The turbulence caused by a square back is (I think) why rear windows
should be closed in a pickup - prevents the exhaust from coming in.

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ThrustVectoring
1\. All of it. A difference in air pressure is just an abstraction for talking
about forces on air particles. The description of what's going on with the
screwdriver would be equivalent physically and much more intuitive if air
pressure wasn't mentioned at all.

What's going on is the curved back pulls air around it, which deflects it
downward, and that downward force on the air implies an upward force on the
screwdriver. With the sharp edge back, this detaches the air flow instead,
meaning there's much less deflection, and thus less force.

The air-deflection model is a much better intuition pump than the air pressure
model. I think the historical dominance of the air pressure model is because
it's easier to measure - just stick some air pressure gauges on surfaces, and
you can calculate what that means for lift.

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avmich
I guess it differs between people, what is easier to grasp :) . Pressure of
air makes a real physical force (actually, pressure is defined as that total
force divided by area). At the same time deflection downward, which implies an
upward force, is correct but doesn't show the mechanism how actually that
implied force is born. It is born out of the difference of pressure between
top of the screwdriver and the bottom - at the bottom you have more or less
undisturbed ambient pressure, at the top the flow reduces the pressure.

What was interesting to me is Ben's experiment with long enough cone which
failed to get lift. Shouldn't be so, I think. The problem could be that his
air jet is relatively narrow - so with a long cone, which makes an angle with
the stream not the whole cone gets flown over, and not the whole area is under
the reduced pressure. It would be very interesting to see if a wider air jet
would still fail to lift the long tapered cone.

