"Homebuilt" wind tunnel

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addaon

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Feb 24, 2008
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San Jose, CA
I've heard lots of talk about folks doing the "homebuilt" wind tunnel by attaching a model to the back of their pickup. I'm at about the point I'm ready for this. My goals would be:

1) To figure out approximate CLmax, with and without various wing fences.
2) To figure out the pitch moment of "zero moment" flaps and try to get "zero moment rudders" figured out.
3) To confirm that elevon deflection is zero at the expected point; that twist is correct.
4) To very roughly estimate drag of the configuration.

I know Reynolds number effects are always a factor for this sort of thing, but at least for the stall speed operations I'm not too worried. My vehicle will handle twice my stall speed if I take it to the salt flats, so I'd be at 2/3 actual reynolds number with a 1/3 scale model.

The two open questions in my mind (currently... there'll be more) are about structure and measurement.

1) If I just CNC mill the whole darn thing at 1/3 scale out of a piece of cheapish hardwood, will I be structurally okay? I'd much prefer to make this a cheap and easy experiment (and, importantly, easy to redo with a changed wing) that make it a structural exercise too.

2) What mounts do people use for this sort of thing? I'd imagine I need at least lift and drag load cells; is this plus a ball mount (so pitch and yaw can be set by eye during the test by servo controlled surfaces) the way to go, or should a higher-degree-of-freedom mount be used? Anything commercially available?
 

jlknolla

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Jul 9, 2009
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311
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San Diego CA, USA
Can't be much help for the actual design but in terms of providing some general ideas for you, I recall there are some great photos of Rutan's famous car top windtunnel setup in a book I have back at my RV, I'll see if I can find them and scan them in for you this evening.
 

kolibri282

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Aug 1, 2009
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3
Location
Germany
The mount would probably have to compensate inertia forces created by the car riding on a not so perfect road plus with any measurements filtering the results afterwards is a point to keep in mind. Just my two cents....
 

HumanPoweredDesigner

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Sep 6, 2009
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1,030
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Arizona
I prefer to stick wings out the side window, attached to a stick and fish weighing scale. You get less disturbed air that way.

As for the structure, yes, a 1/3 scale will be much stronger. Or rather it will deflect much less. That is actually a bad thing. It will deflect much less and give you a false belief that your full scale will also deflect that little and get the same lift.
 

wsimpso1

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Oct 18, 2003
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6,730
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Saline Michigan
The Scaled Composites rig I have seen is actually a rigid gadget hung on the front of the vehicle with sprung wheels. This gets it in undisturbed air. Yeah, don't even think about taking it on public roads that way...

Billski
 

HumanPoweredDesigner

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Joined
Sep 6, 2009
Messages
1,030
Location
Arizona
1) To figure out approximate CLmax, with and without various wing fences.

4) To very roughly estimate drag of the configuration.


2) What mounts do people use for this sort of thing? I'd imagine I need at least lift and drag load cells; is this plus a ball mount (so pitch and yaw can be set by eye during the test by servo controlled surfaces) the way to go, or should a higher-degree-of-freedom mount be used? Anything commercially available?
I'd sit in back and have a streamlined tall pole with the bottom on a pivot and hook a fish scal or something to see how hard it pulls backwards. Record that. Then add the model on top and repeat. Subtract. There will be interfereance drag too, but estimate it at 5% maybe?

As for the pole, it probably should be a pyramid on dual pivots so it is strong. Better yet, scratch the fish scale. Put the back leg of the pyramid on a weighing scale flat in the bed. Weigh before and after. Divide the difference by the ratio of the height to base of the pyramid. That will give you the drag of the airplane. Butthe error in this set up is that the lift of the plane will want to pull the pyramid back up straight, which will give you a lower drag measurment.

Well, I did not find out what to do, but at least I warned you what not to do.
 
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