[Phys-L] Re: the plane truth

[John Denker <jsd@AV8N.COM>]

Scott Goelzer wrote:

> Why does a wind tunnel test have to be in a tunnel?

It doesn't.

> Finally I had a student try putting a card board collimator in front
> of a box fan

Sounds reasonable; see details below.

>  The tunnel causes nothing but problems in
> making any device that measures lift or drag.

These are hard experiments.  There's a loooong list of things to
do.  Just making the support that permits controlling the model
and measuring the forces and moments is a big job.

Compared to all the hard things you need to do, building four
walls should be almost the least of your troubles.

Of course it also depends on your goals.  If the only goal is
to get some gee-whiz qualitative observation, then you should
build the simplest apparatus you possibly can.

One situation where walls actually help is if you're testing an
airfoil _section_.  Then you just make a constant-chord constant-
section wing that runs from wall to wall.  By the method of
images, you can see this is equivalent to an infinitely long
wing, i.e. D=2.  Also this allows you to support the wing right
at the walls.  It's all very clean.  D=2 aerodynamics is much
simpler than D=3 aerodynamics.

More generally, no matter what you do, there is going to be
*some* boundary condition.  Walls at least allow you to know
what the boundary condition is.  It might not be quite the
boundary condition you want, but suboptimal and _known_ is
generally preferable to unknown.  I can make theoretical
corrections for "ground effect" and similar interference
effects, provided I know how far away the ground is.

>  Assuming the region of laminar air
> flow is larger than the object being tested, I cannot see why I need
> a big tube.

It depends on how much larger.

Fact: Ground effect is quite noticeable for real aircraft when they
are near the ground.

Generality:  The flow field set up by a wing is significantly different
from ambient out to distances on a scale set by the _wingspan_, not the
chord or any smaller distance.  This sets the scale for ground effect
and lots of other things.
   http://www.av8n.com/how/htm/airfoils.html#sec-circulation-vortices
   http://www.av8n.com/fly/vortex.htm

Do not underestimate how much the "far field" contributes to what is
actually happening.  Controlling what happens just near the wing is
definitely not sufficient.  Beware the _bullet fallacy_.
   http://www.av8n.com/how/htm/airfoils.html#sec-fluid

If you have a small enough model and a big enough box fan and diffuser
(i.e. enough to encompass several wingspans in every direction) then the
presence or absence of walls shouldn't matter.

Also:

Lots of designs for aircraft and other aerodynamic objects (e.g.
sailboats) have been tested by mounting a model atop a car and
driving across a playa, or driving along an isolated road.  This is
already too ambitious for most HS students, but for a larger-scale
project it should be seriously considered.  It's easier than
trying to construct a super-huge fan.
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