Wind Tunnel Testing

[smed]

HI I know this is off topic, but I am a high school student and I was hoping to build a slow speed wind tunnel this summer to test indoor model propellers at low reynolds numbers. I was hoping to gather some data that would be applicable to UAVs (unmanned aerial vehicles) such as the Helios, and have been told that generally wind tunnels cannot give performance readings at such low speeds because they don't have sensitivve enough instrumentation. I have plans from a quite large 1930s wind tunnel designed to run at these low speeds but I was thinking that my wind tunnel would be around 18" by 18" in size. Does anyone here know about wind tunnels, how to build them, or of previous testing, or know someone who does?
Thanks, Adam

[Ollie]

See:
http://vonkarman.stanford.edu/tsd/pbstuff/tunnel/

[Quacker]

Whenever I think about building a wind tunnel for propellers, I get to thinking about getting the air flow smooth, getting the tunnel large enough to avoid fringing effects and drag and turbulance from the walls, etc. Then I wonder if the whole thing would be easier if the thing was done in free space by using a large wheel which could be made from plywood and bike parts, etc., with the propeller mounted tangent to the perimeter. Free stream speed could be controlled by the wheel RPM. Only reading of onboard instruments could be a bit tricky. For this part, I think a small video transmitter like the X10 could be used to view instruments rotating with the wheel. Something to think about.

Q

[BMatthews]

In Soartech 8 Dr Selig devoted a whole chapter to wind tunnel factors, control of turbulence and instrumentation. It's quite a read and very daunting. I'm afraid that unless you're possesed by this as a true passion and are willing to devote untold months of detailed efforts, not to mention serious money, to this then it's better just to lie down until the feeling goes away. Or do what indoor modelers have been doing for decades. Fly and record your findings. A closed room, a model and a stopwatch is a better indicator of efficiency than any windtunnel could be in this case. Make a test model that uses a shorter motor so the flight times are not crazy long.

But you should be aware that the Reynolds numbers between your indoor props and the types used on the Helios UAV's are a lot different. It would be very difficult to take what you learn for indoor props and apply then to a UAV type environment.

But here again direct flight testing could come to the rescue. A properley telemetered outdoor test bed model could return data on watts (I'm thinking electric here as it's possible to measure the power input more closely), rpm and climb rate. Testing could include time vs power to altitude for different all up weights and minimum power required to maintain level flight. These tests are a very good indicator of propellor efficiency. And done in mild to calm conditions using different props within a limited time frame to avoid air density shifts you could amass a great degree of data that would reduce the thermal influence.

Not to mention that flying is more fun than sitting in front of a box of wind.... :D

[smed]

Hi thanks for our help.

I am hoping to compete in science competitions next year with my school, and I think that in order for my research to be taken seriously, it must have applications to things besides model planes. And I think that UAVs are the clear application. Unfortunately, I would like to do flying tests, but I think this will be like reinventing the wheel, and there will be so many variables. Plus there is the nonlinear torque band from rubber.
BMatthews, why will my work be so hard to apply to UAVs? I found the Re of an f1d prop and it is about 6000-8000. Once I find the coefficent of viscosity of air at 30,000m, and the rpm of a UAV prop, I will be able to do a calculation of Re for UAVs. And I have a feeling that they will be similar. Also, I think it will be interesting to find data on how mach number affects the prop performance.
I looked into Selig's work and Mueller's at Notre Dame, and they told me that they only went as low as 60,000 Re. They mentioned that I might get some imput from aerodynamics of small birds and insects, which has been somewhat explored.
Additionally in the 1986 NFFS I found an article with plans for a 1930s indoor model wind tunnel which coul take readings of .001 oz! Unfortunately, the wind tunnel took more than a year to build and I have about 1/4 of that time. Right now I am in contact with the owner of the Swiss slowflyer co. DIDEL, and he just told me that he has a friend in the US who has a slow speed wind tunnel.
We'll see how this goes.

[BMatthews]

I must admit that it was mostly a shot from the hip based on the fact that indoor model props and the larger electric model and low power UAV and human flight props I've seen have very little in common shape wise and yet I think we can say that both design extremes have been largely optimized for their specific purposes.

Also as I understand it from reading Dr Selig's findings and having once gone off on a tangent and reading all I could find about wind tunnels myself I understand that the air turbulence becomes more critical for some phases of the studies as the speed goes down. You may actually have more luck making a tunnel more for a speed range around 50 to 80 kph than for a very slow speed type suited to the indoor model applicaton.

But then it all depends on what sort of research you wish to do.

And if it's for a science fair competitions then it's pretty hard to bring your outside research indoors I would imagine.

Anyway good luck with your efforts. If you persue this work please post back with the results. It would be fascinating to see.

[JonB]

Dear SMED:
When I was a young lad in Junior High School (1962)! I too built a functioning wind tunnel for Science fair competion. I worked from plans that I purchased from "Model Airplane News", It worked very well! I won! It was built from 1/2" plywood and plexiglass for the viewing area. It was powered by a vacuum cleaner motor. Smoke was generated by electrically heating a wick - DT fuse! (using a step down transformer from 110v to 6v, commonly called "Bell Transformers"). The wick was immersed partially in Mineral oil which was contained a glass jar. The jar having an outlet and hose that went to the smoke jets, which were 1/8 dia brass tubing. The smoke jet device was simply a symetrical airfoil shape with many -maybe 20-25 of theses tubes extending from the TRAILING EDGE for about 1.0" length. A sample shape was inserted into the approx 1.0" wide chamber This device when placed in the chamber and back lit via several light bulbs, air flow started, provided beatiful straight lines of smoke. i could vary the air flow speed and the angle of attack of the sample airfoil from 0 deg to about 55 deg. it was amazing to watch the lines of laminar flow separate at high angles and low airflow! Any way Contact M.A.N. they may be able to dig through there archives for a reprint of the article and plans.
Sincerely,
Jon B. Shereshaw
PS. A funny storey...At the time of Judging by "Pillars" of local industry, I had to run the tunnel for about an hour and a half straight...This performance produced a noticable "HAZE" in the auditorium! Some good citizen then proceeded to call the Fire Dept! By the time they arrived, the judging was over and I had shut down the tunnel. The source of the "HAZE / SMOKE" was never found!:D

[edwinde@cs.com]

You may wish to look at www.aerorocket.com: There is information on that site about a relatively small, relatively inexpensive (if there is such a thing) wind tunnel that has been used by at least one AIAA chapter to encourage interest in aeronautics of younger groups. This site offers commercial testing for small model rockets etc.
Hope someone finds this of interest.
Ed

[smed]

Thanks everyone for the help. JonB- I will definitely look into the Model Airplane News tunnel design. That's a cute liitle story- I never thought a vacuum cleaner motor could do that... Question-Did you make instrumentation for your wind tunnel to measure forces?
Incidentally, I think that found someone who has a wind tunnel designed for indoor models. His name is Art Ellis, and he runs a Museum near New Haven, Connecticut. Perhaps I can modify the tunnel for my purposes. I am getting info from Selig and also getting the book "Low Speed Wind Tunnel Testing" by Alan Pope which will get me familiar with terminology and with setting up a wind tunnel experiment.

[Majortomski]

Smed, give the folks at Parks College of St Louis University a call. The profs at the school designed thier own low speed 3 x 3 tunnel back in the early 70's, followed by a 2 x 2 water tunnel and then a 3" x 3" supersonic tunnel (most insects dont hold up past 300 mph:rolleyes: ) They still might have some usefull advice

Tom

[antani]

Smed, I don't know where you are located, but if you are near Washington, DC, the folks at Catholic University (mechanical engineering) have a low-speed wind tunnel that hasn't been used much recently, and they also have a lift/drag force balance just waiting to be installed

[Sparky Paul]

Originally posted by antani
Smed, I don't know where you are located, but if you are near Washington, DC, the folks at Catholic University (mechanical engineering) have a low-speed wind tunnel that hasn't been used much recently, and they also have a lift/drag force balance just waiting to be installed
.
I used that wind tunnel in 1960! for my Senior aero thesis.
The tunnel dates way back from there even.
.
Modern electronic instrumentation in the form of wave rakes and strain gauges are probably commercially available. The lift-drag balance instrumentation was new in 1960.
The way it was set up in 1960 the engineers couldn't touch the hardware. There was a grizzled old tech that did all the mechanical work. He might still be there. :)

[jrb]

Doing a wind tunnel is not all that difficult; here’s a good resource:

http://www.lerc.nasa.gov/WWW/K-12/windtunnel.html

Some other of interest:

http://www.lerc.nasa.gov/WWW/K-12/airplane/index.html

http://www.lerc.nasa.gov/WWW/K-12/airplane/bga.html

Here’s where I worked while on active duty; the worlds largest collection of winds tunnels, space chamber, propulsion test facilities, and ballistic ranges: sea level to 1000 miles and up to Mach 23!

http://www.arnold.af.mil/

Got my MSAE here, while there:

http://www.utsi.edu/

[antani]

Originally posted by Sparky Paul
.
The lift-drag balance instrumentation was new in 1960.

Actually, I am talking about new instrumentation I bought a couple of years ago.
The tunnel is old, but I bet it beats a home-built one ...

[Sparky Paul]

Originally posted by antani
Actually, I am talking about new instrumentation I bought a couple of years ago.
The tunnel is old, but I bet it beats a home-built one ...
.
My aero prof, Dr. Max M. Munk (of the Munk triplane theory, and the Munk prop, and the M-series of NACA airfoils) was quite proud of the balance we used in '59-'60. It was a work of art.
.
The Munk prop BTW was a one-piece variable pitch wooden prop. The laminations permitted pitch changes under load. I never saw one, but I recall they were quite sought after way back when.

[smed]

Hi thanks everyone for the suggestions. Actually I was ecstatic yesterday when a professer offered to let me conduct testing at their wind tunnel, which is already set up for testing props! This is a relief. Now the trouble is making the props...
I will post my plans for the project in the "Modelling Science" room later today.