Greetings all. A buddy of mine here in Corpus has asked me a couple of times about wind tunnels, and their use for designing models. Knowing nothing about the topic, I figured i would openit up to y'all.

Has anyone here used a makeshift windtunnel to help perfect a design. It sounds usefull to me. Maybe to determine trim, lift, and basic flight characteristics.

What do you guys think. Usefull, of a waste of time?


Randy (Digger) Birt

Waste of time for much other than fairly gross testing.

You should read the Soartech 8 book of Michel Selig. In there he went into the requirements of the wind tunnel and the need for low turbulence values. It's quite the eye opener. Believe me, just sticking a honk'in great fan at the end of some heating ducting isn't going to tell you much at all.

You'd be better off with test models that you hand launch.

To elaborate on the test model idea.

With a small scale all balsa model you can determine basic stability, spriral stability, optimum CG location and, when it's all done, you've got a neato little decoration to hang above your computer....

For airfoils it would be possible to compare basic drag and lift by using the distance traveled and comparing that to the time taken to land from a common height. Of course to get something meaningful you'd have to do a LOT of test glides and then average out the results. Some sort of test rig for launching with the same energy each time could prove useful for this aspect.

This is a good question. I've moved this from the 1/2A forum to Aerodynamics where it'll get some replies from people that have forgotten more than I'll ever know about this sort of thing.

From what I've seen you could build a good wind tunnel if it was you and Bill Gates. The tunnel BMatthews wrote about Micheal Selig doing his test in was own by Princeton U. very hi-tech and he still had to do all his testing at night so thermal generated winds wouldn't mess up the test, each one ran for some 6 hours if I remember correctly. Micheal's tunnel was about 3'x3' and it was only good for testing a small wing section. I made a section for the tunnel, (FX60-100), and modified one, (E-214mod), worked really hard to get them exact.....but they were not exact. I think the vacuum bagged sections from Calif. where the best, It's a lot of work for something that's not going to fly.

I think you get good results from full models with sailplanes, but not with power planes. I tested my Saggitta in dead air, 5am, and could get 3 minutes 25~30 seconds repeatedly, my Windsong 5 minutes 20~25 seconds repeatedly. I knew the Windsong was better, but if you put a motor on them and the Saggitta got better performance, was it the motor or the plane.

Dave Thornburg wrote an article I'll always remember, it was titled "Sharks fly better." What he pointed out is look at the beautiful fillets on the fins, the slow changes in cross-section. If you make ever effort to make your plane a "Shark" it will be a good flyer. Take a boxy airplane and "Shark" it up, it will always fly better.

I remember that article well. I believe it was in Model Builder.

Dave certainly had a way with words. I've never seen his old buzzard's soaring book but I'll bet it's a page turner too.

Micheal Selig was working with really fine differences and I would guess that getting a wind tunnel smooth enough to do it was a real chore.

The big tunnels we used at McDonnell Douglas (I spent half of the best years of my life testing models in them) probably have higher turbulence but also were bigger and have much wider ranges of velocities.

I have often thought that it would be interesting to put a pattern model in one and look at the data but after several thoughts more realized that our models are so simple that it really isn't necessary.

Complex configurations with slats, flaps, differential tail and ailerons, external weapons or tanks make theoretical calculations suspect. That is where the tunnels are worth the time and effort.

Like BMatthews I tend to use little foam models to determine CG, etc for airplanes that are off the beaten track. Luckily most airplanes that fly nicely have a lot of characteristics in common and it makes the job of estimating what they can do easier.

A couple of years ago, Les Garber and I used the U of Minnesota Hydraulics Lab wind tunnel to evaluate a number of parameters governing the performance of model autogyro rotors. The tunnel at the U has a test cross section about 6' X 6' and can produce velocities from about 10mph to over 80mph.

Results are summarized in an article in RC Modeler, August 2001. The most remarkable finding was that one of Michael Selig's airfoils, the SG6042 produced significant great lift than almost any other airfoil tested. Further, the SG6042 is only slightly different than the good old Clark-Y.

Another interesting finding was the effect of "trips" placed at the 20% chord point. These were .005" thick, .125 wide automotive striping tapes. With trips the rotors would spin up quicker and produce larger lift at lower airspeeds.

For me this indicates that subtle differences in shape can make big differences in the performance of our models.

Bill

It probably also indicates that for unflapped, unslatted, fixed configuration airfoils at low subsonic flight conditions that there is just one good shape. The designer of the Clark Y had some good intutitive vibes going on. I am impressed with both airfoils and the work that went into them.

The trips we used in the McDonnell Douglas (now Boeing) wind tunnels (a Polysonic 4X4 ft and Low Speed, a 12 ft cross section) were made from a strip of fine grit like sparse sandpaper at the 5% aft of the leading edge location.

We would mask off a 1/8 inch wide stripe, spray on some clear Krylon and sprinkle the grit onto the wet paint. It was felt that this was more subtle/finesse way of triping the boundary layer. The grit drag was in the noise of our ability to measure the force.

I often wonder why the same thing is not used by modelers some times? Especially in the glider duration type of contest.

Ben, thanks for this tip about the grit. I've used trip turbulators to very good effect on two of my sailplanes so far. But being the automotive striping tape types I did find they slowed the upper end a trifle.

I'll try your grit turbulators on my next one.

I did a test once using automotive striping tape on a 100" sailplane with a E-205 airfoil.

I first put a strip on the right wing at 25% then launched. The sailplane went straight up on tow, but after coming off tow it slightly turned left.

Next I put one strip at 10% on the left wing. On tow the plane pull slightly to the right, then it turned slightly to the left after coming off tow.

I then put a strip at 10% on the right wing. It launched straight, then turn slightly left after coming off tow.

I then add a stripe at 25% on the left wing. The plane launched straight and flew straight.

I concluded that the 10% strip was working at High CL, and the 25% was working at lower CLs.

I am curious to find out:

What is the capacity of the various wind tunnels (located in the USA), and where are they located?

I am trying to determine the maximum size of a model that can be be analyzed in the testing area of each tunnel.

Regards,

Al Parry
[email protected]

Al...
The wind tunnel that Les Garber and I used to test autogyro blades and rotors is located at the U of Minnesota. St. Anthony Falls Laboratory, Dept. of Civil Engineering, Mississippi River at Third Avenue S.E. Minneapolis, MN 55414-2196. Phone: 612 627 4010.

It has a test section that is approximately 7.5 X 7.5 feet square. Air velocities are variable between 10mph-80mph. Cost per day (in 2000) was $850. There are photos of our set up in RCM August 2001, pp 70-76.

Bill

Al,
NASA Ames Research Center has some pretty big tunnels. Does 80 x 120 feet sound big enough? Expect to pay many dollars per second for tunnel time!

Tom

You don't have to be Bill Gates. A few years back I built a small In-draft (sucks air thru) tunnel, about the size of the Wright Brothers tunnel, to investigate a number of different things. Total cost (for the tunnel, not instrumentation) was around $200. Test section dimensions are 20"Lx9"Hx14"W, max velocity in test section to date is 120 feet per second. If you can build your average RC balsa model you can build a wind tunnel [its more cabinet making than model building].

Dr. Selig's work requires very precise data because of the nature of his work [comparing very small differences in Lift/Drag data for airfoil shapes].

The tunnel I built was originally designed for studying model stability [you can actually fly the model in pitch and yaw in the test section].

The top image is a view looking down the throat [12" span Katana aerobatic model], the bottom image [is not out of focus] the model CG is at a critical location [in this case about 40% of the mean aerodynamic chord] were the model becomes divergent in pitch. Meaning your out of control, in this case the model would oscillate wildly in pitch and eventually slam into the ceiling or floor of the test section.

The difficult part of wind tunnel work is data gathering. For a little money and a LOT of time I built a Lift/Drag balance using two very simple but very sensitive load cells. The first thing I did was to perform some Lift/Drag tests on well documented shapes; Sphere, Square cornered Flat Plate, Cone etc. Once things were well calibrated and I had confidence in my test section velocity measurements I was able to get REPEATABLE results within 1-2% of other published tunnel results.

Cornell Univ. has a couple of wind tunnels. The one I have used is a 4' x 5' capable of wind speeds of 100+ mph. It has to be run at night because the blast of the turbines against the wall of the building in the basement and the noise disturbs the occupants.

Elson