Tony Goetz wrote...

>We plan to measure two principal forces - lift and drag. These will be
>measured off a lever attached to the test model and protruding through =
the
>side of the tunnel. I really have no idea how much force to expect at =
the
>speeds we'll be running (maybe up to 40mph), but high precision isn't
>necessary.

At those speeds you might be talking about forces of several pounds, I'd
guess, depending on your model size.  Tough job...

>I did some web searches a few weeks ago but found basically what you =
guys
>seem to have confirmed: these things aren't cheap! I was hoping I just =
hadn't
>been looking in the right places. The thing with using electronic scales=
 vs.
>mechanical springs and weights is that, primarily, the output display =
would
>be easier to read since the project should be usable by people who have =
no
>idea how to use the wind tunnel in the first place. (since it also gives=
 me a
>chance to finally use a Pic in a practical application, why not?)

>What about a piston type arrangement, where the lever pushes on an air =
filled
>piston and the pressure inside is measured? Any thoughts on how complex =
this
>kind of thing might be?

You'd need a piston with very low friction, short throw (i.e., very
small chamber displacement so your model doesn't move a lot) and a very
tight seal, which could be a nightmare.

How about ripping the guts out of an electronic bathroom scale and using
them to measure your forces?  Might take some experimenting around, but
in the end it could be the easiest and cheapest way to get the job done.

I'm working on a general-purpose, el-cheapo force sensor for some hobby
projects of my own, but I'm not going to have it ready any time soon;
too many other projects (like a PID controller for my toaster oven so I
can do some REALLY precise frozen-food work) that are higher up on my
"TO DO" list...

Dave

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