Mini Wind Tunnel w/flowbench?

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I am throwing this thought out as there are many people much wiser than myself on this forum. I was reading in another forum about wind tunnel testing and it was brought up that most air craft manufactures did wind tunnel testing on smaller models of the actual aircraft. I was thinking of making a minature wind tunnel out of some acrylic tubing I have and having my flow bench suck through it. Now let's say I have a square or rectangular block of wood mounted in the middle of the tube and flow air around it at a certain depression and get a CFM reading. Then I work on streamling that block of wood and flowing again at the same depression. If I get an increase in cfm at the same depression am I getting a better CD (coefficient of drag). Am I up in the night or can I make this work?

John

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John, There is a lot more to it than that, but what you suggest will give qualitative answers. For quantative answers, you need to consider a number of things. If you change the cross sectional area of the piece you are testing, the drag could be higher than the original, but the flow can still increase. What you are really trying to improve is the product of Cd (drag coefficient) and Acs (cross sectional area). Lower Cd and constant Acs is lower drag, but so is the same Cd and lower Acs. The equation is Drag=Cd*q*Sref, where q is dynamic pressure or 1/2*rho(air density)*Vair^2. The Sref on an aircraft is usually the wing area, but for most other testing is a reference Acs. For a little read about the tunnel at Texas A&M, try:



This will introduce you to more terms and other considerations. There is a set of plans available (with no dimensional data to tell how big it is) on eBay:

[url=http://cgi.ebay.com/ebaymotors/_Aviation-Parts-Gear__BUILD-YOUR-OWN-WIND-TUNNEL-COMPLETE-PLANS-ON-CD_W0QQitemZ16027676364
3QQadnZAviationQ20PartsQ20Q26Q20GearQQadiZ2877QQcmdZViewItem?hash=item160276763643&_trkparms=39%3A1%7C65%3A1%7C240%3A1318&_trksid=p4506.c0.m245]http://cgi.ebay.com/ebaymot....c0.m245[/url]

Most small tunnels are Gee Whiz! but of little value. We had a 2ft x 3-1/2ft "developmental" tunnel at Rockwell before Boeing bought us. The Union didn't even require their help as they considered it a "toy". The first model we put in it saved about $2 million (and the tunnel only cost $0.5 million) so they regretted that decision. -- Doug

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John, Doug,

I have often thought of doing this slightly bigger to house a Cycle with rider using a small engine to generate say 200MPH wind.

But in all this my thoughts led to using a linear bearing tabel/floor and measuring the actual force applied against the surface aera of the object tested. I/E how much is the wind pushing on the object. Doug is this more in line?

Rick

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Rick, That is much more in line with the usual type of testing. Check out the wind tunnel link above and you can find exactly that type of mounting system. The key there is to avoid measuring the force on the floor along with the force on the "system" being tested. Belen Wagner, who held many small engine records at Bonneville, tested her riding position and bike in the tunnel at Purdue. I believe the CanAm motorcycle team also tested full scale, but not sure if the rider was included. It must have worked as their record was 35 years old when broken last week. (Mine was only 3 years old!) Most of the measuring systems use a 6-component balance: lift, drag, side force, roll, pitch and yaw. When you start figuring out that "small" engine for a full scale consider that the Purdue tunnel used a big Chrysler Hemi! -- Doug

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Doug,
Thanks so much for your input. I did not make it out to Bonneville but it looks like it was a good year. at the SCTA website I checked out the results. In the photos section where they showed all the people in the big group photo, are you in there?

John

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John, Adding a second Short Course made a world of difference! I talked to people who made 3 runs the first day. Last year, with a similar number of entries (457 was the last count I saw for this year, on Wed), some people were in line all day and didn't make it to the front for a single pass. I heard stories of 6 runs before lunch later in the week. I haven't checked for number of records, but we Certified 18 bikes the first day of record runs. I am in the big photo, but a better(?) pic is on Aug 16 page 1, next to the last pic. The Butlers are the folks who broke the CanAm record I mentioned previously. The previous shot is for a rubberband powered vehicle. I don't think it ran, but the owner is serious! Then again, Ron Main is always serious about his racing! The first shot on page 2, Aug 16 is my toy, the former record holder.

Check here for the team from Paris!



They ran as a vintage sidecar and a vintage bike with a 4 man team. With gas and fuel classes in each, they took 4 records home, one for each member. They are planning 2 more years with a different vehicle each year. I'll post their website when I get the last of the gear unpacked and resorted. Don't miss next year! -- Doug

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Thanks Doug,
It is good to put a face with the name. Under general BS I had made the suggestion to update our profiles with our pictures. I actually did but maybe my ugly mug scared everybody else. I would love to build and XO/STR for the salt. I have the engine, just need to get going on it. btw I found an interesting article online from a local newspaper about early Bonneville Salt Flats racing
John

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John, Check out the USFRA World of Speed Sept 17-20. Bar stool racing, VW Beetle racing, go kart racing and more fun stuff plus SCTA-BNI classes. Then there is the SCTA-BNI World Finals in October. Both are smaller venues and motel rooms are easier to get (if you don't drive home at the end of the day).

I am always confused by car classes. XO/STR is an overhead valve conversion on a flathead motor and a street roadster chassis? Is yours an ARDUN or ??? Maybe you can team with Bruce and make an XO bar stool or go kart for USFRA. Bruce, are you listening? :p -- Doug

PS - Thanks for the link about early Bonneville! I have a small "shrine" to the Bluebird in my shop office.

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Doug,
I still do not get half of the classes, but XF is early flathead V8 ford XXF is the ones with aftermarket heads. XO is Vintage inline OHV engines 1955 and older. XXO is with like the GMC with that 12 port head. I have an old 261 Chevrolet engine with stock head that is like the 235 Chevy but a bigger bore.

John