Has anyone built a homemade Wind Tunner ?

I need to test some aerodynamic shapes for adsolute minimum drag. I am looking for absolute minimum wind resistance. Information on the military stealth bomber looks good. What I need is to test some model shapes but I am not sure how to build a good wind tunnel. I found a model wind tunner online it was built from a bunch of 4" square box fans. The Wright Brothers had a wind tunner I can't find much infor on that. Anyone know how to build a good wind tunnel so I don't waste a lot of time trying to invent something that has already been invented?

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Check this out.
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Aerodynamic Drag consists primarily of three aspects: Surface Character, Frontal Surface Area and Shape.

Surface Character: This is the texture and pattern of the surface. For example, the hair on a tennis ball or the dimples on a golf ball. Companies are starting to experiment more with how to use surface to improve aerodynamics, especially at lower speed.

Frontal Surface Area: As a vehicle is propelled forward, the front profile of that vehicle is what breaks through the wind first. Therefore, the amount of mass or surface area that hits the wind first greatly shields and effects that which is located behind it. For this reason, minimizing frontal surface area is an excellent step towards minimizing overall drag.

Shape: The vehicle’s overall shape drastically effects its aerodynamic efficiency. Shape is not the same as mass, not even close. You can have a small spherical shape and it can be far less aerodynamic than a much larger elliptical shape. This is a big reason why a football can be thrown further and with more control than a volleyball. The shape of an object effects the proportion of skin friction to pressure drag. Skin and pressure what?

Total Drag is a combination of skin friction (“good” drag) and pressure drag (“bad” drag). The proportion of skin friction to pressure drag are directly determined by the frontal surface area and shape of the object.

Pressure Drag is most easily defined as turbulence. The less of it the better. Pressure drag is the disturbed air that spins off an object when air hits it. Pressure drag slows a vehicle down more as turbulent air is the least controlled and most random form the air can be in and acts like an out of control barrier. Blocky or round objects will have more pressure drag than oval or elliptical objects. Air can flow around more elliptical objects smoother where as it is more likely to bounce off turbulently around blocky or round objects. There are specific angles that we touch on below that have been found that minimize pressure drag.

Skin Friction is actually good drag. Skin friction is a layer of deflected air that hovers right at the surface of an object. Think of it as a coat that adds a little bulk, but that protects the layer underneath it and thus helps it go faster. Skin friction flows smoothly around an object. It is good because it can create an isolation layer around an object that can keep pressure drag (“bad” drag) from forming.

Laminar Flow: Undisturbed, smooth air. Air is in laminar flow before it hits an object and eventually returns to laminar flow after an object passes through it. Laminar flow is the most efficient form the air can be in, as it is undisturbed. The quicker that air becomes laminar after going around an object, the less drag it will have. Skin friction drag returns to laminar flow far before Pressure drag does.

Aspect Ratio: Aspect ratio is not just a term used in aerodynamics. Aspect ratio is a proportionate relationship between length and width of an object. If we have a 4” long object that is 1” wide, its aspect ratio is 4:1. If it is 1” long and 4” wide the aspect ratio is a horrid for flying 1:4. Aspect ratio helps to explain why a football flies so well when it is thrown length wise through the air, but acts like a wounded duck when thrown height wise. You get the point… Aerodynamically, NACA (the aerodynamics research predecessor to NASA) studies showed that an aerodynamic aspect ratio of around 3:1 minimized drag.

Shape Taper/Angle: Directly related to the 3:1 aspect ratio is the taper and angle of the object’s surface. Aerodynamically, non-round leading edge with a 14° taper leading back from to the widest point creates an object with a good aspect ratio and an aerodynamic profile.

Boundary Layers: Boundary layers are layers of air created in the space between objects as the object passes through the air. Boundary layers occur between 2 or more objects.



OK, here is what I am doing but first read this. When I was in college 1970 I found a book in the college library. It was blue prints and data and research from the German NAZI V1 pulse jet engine and flying bomb. Everything you need to know detailed blue prints to build each and every part of the german pulse jet engine and flying bomb including the guideance system, air tanks, fuel system, it was all there. I was only interested in the engine so I read all the information about the engine. After WWII the USA did a lot of research on the engine with the hope it would be useful. The engines had a maximum speed of about 400 mph. The book listed all the research the USA did and told all the information on how to make the engines go faster. A tiny change to the engine an it would go 50 mph faster. A tiny little invention added to the air intake and it doubled the speed from 450 to 900 mph. I have a Dyna jet engine that I have been experementing with. I built a test stand it is basically a spinning pole with the Dyna jet engine attached to the end with a counter weight on the other end. I have an electronic speed-0-meter attached to the spinning engines. One by one I made changes to the Dyna jet engine the exact same changes the USA did to the German pulse jet engine and the Dyna jet engine runs faster and faster and faster. The Dyna jet engine is now running over 300 mph on the test stand. Now I need to build an airplane that has absolute minumum drag. From what I have read razor sharp LE and TE stream lines contour LE and TE shape is what I want. Every part of the airplane needs to be shaped for low drag sharp LE and TE with low Aspect Ratio. Not sure I will ever actually fly this thing maybe with 200 ft lines in a 40 acre field. I might never build an airplane either but it sure is fun to experement.

I don't waste a lot of time trying to invent something that has already been invented?


I would apply that statement to the data you are looking for, there are books of data on that stuff no need to regenerate it with less accuracy.
NACA airfoil books "Theory of Airfoil Sections" have lots of data on airfoils and shapes.There are some good older Aero text books that are easy to read, "Airplane Aerodynamics" by Domasch comes to mind, if you want to learn how to apply that data.
If your still bent on building a Wind Tunnel there is a handbook by Alan Pope that is the bible on WT testing. Check Amazon. Beware the calibration of a WT and the development of correction factors to get usable data is not a trivial task.

On second thought, I think gathering real time data on models would be as useful and probably more fun. This is a hobby. There are small recording packages http://www.eagletreesystems.com/UAV/uav.html you can purchase that will store flight information from which you can perform some comparative studies.

>On second thought, I think gathering real time data on models
>would be as useful and probably more fun. This is a hobby.
>There are small recording packages
>http://www.eagletreesystems.com/UAV/uav.html you can
>purchase that will store flight information from which you can
>perform some comparative studies.

Records a lot of information ;except for speed(how does circular flight effect this) and RPM, most of it is useless to us. which of the following do we need about a stunt plane?

Altitude unless the measurement is in inches
Movementsfor 4 servos
Servo glitches
Receiver voltage

Gary:
What Range of Reynolds # do you want and what level of accuracy? Accuracy will cost money. Don't know the correlation between cost and low RN's which we need.

This is really easy: Go to your local grocer; buy a big-ass salmon; copy the shape; enjoy the salmon barbecued.

One can have his cake (or fish) and eat it, too...

You laugh! The current world speed record holder in the streamlined motorcycle class did very much the same thing in designing and building his new 'liner a couple years ago.

Dan

Check out the link I posted it measures airspeed , rpm , alt , temp and more.

>This is really easy: Go to your local grocer; buy a big-ass
>salmon; copy the shape; enjoy the salmon barbecued.
>
>One can have his cake (or fish) and eat it, too...
>
>You laugh! The current world speed record holder in the
>streamlined motorcycle class did very much the same thing in
>designing and building his new 'liner a couple years ago.
>
>Dan

Would a Hammerhead Shark copy be considered a "Canard"?

Use a car. Measure force, maybe with a digital scale, and divide by dynamic pressure. You gotta do the latter, or you can't make a valid comparison between shapes, although the Wright Bros. had an even more clever way of getting valid comparisons. Dynamic pressure is .5 x true airspeed squared x air density. Do a Web search to see how to calculate density from pressure and temperature.

Fluid Dynamic Drag by Hoerner is a catalog of the drag of different shapes.


Howard Rush
Bellevue, WA

Gary-

That's a very interesting investigation you are engaged in, and I'm sure you already know some of this. Just for your considersation though...

At 200 mph on 70' .020" dia. lines (I don't know where AMA's dia is set now for 200 mph, but it's large), the line drag would be several times as great as your total aircraft drag; the line drag itself would consume about 2.9 hp (Edit 2: that's about 5.4 lb of computed line drag), based on the "Line II" program. I don't know your projected model weight, but if it were about 22 oz, your line tension would be 52 lb. For 52' lines it would be well over 90 lb., and line drag would be about four times the drag of the entire model (based on Gene Hempel's calculations - M.A., 7/83). With longer lines, the percent line drag goes up, as the tension reduces. You could "fly" this plane though without wings at 200 mph on 70' lines at a line angle of only about 1.5 degrees below horizontal on line tension alone.

The main point probably is that fairing the lines with an asymmetrical (long inboard) wing would probably make a bigger difference than any other aerodynamic refinement you can make. I think I calculated once from the article by Gene Hempel that the drag of the last two feet of lines equalled the total wing drag of an asymmetrical, 24" FAI speed model at 163 mph on 52' lines. FWIW.

Edit: If you are just interested in measuring performance of your modified jet, you really don't need the plane at all; you can have just the engine and connector and/or stabilizer drag and "fly" on line tension alone - or just aim it at a scale? Be sure no one's in danger though. Good luck.

SK

Serge,
The problem with your statement about being able to fly the mosdel on line tension alone is that it does not take into account that period of acceleration up to the speed in which it makes the line tension. I think the wing really does help in this real life case.

Agreed - to the extent that a wing will help it clear the ground at a more reasonable speed. If the handle is held 4' above ground, then more of the line tension is used for "lift" before lift-off. That is, the vertical component of line tension is a greater fraction of the tension. As the wingless "plane" accelerates from rest, the centripetal pull exerted by the pilot through the lines increases from zero, until, at 135 mph, the "lift" component equals the plane's weight, and it lifts off. Wings would help tame that.

SK

Hi,

sorry about this late reply to the title question. As a bit of oldtimer freak I was a very frequent guest at the Plan Page, See http://www.theplanpage.com/

In one of the scanned publications fron the late 30:ties or so there is two parts describing a home made windtunnel.

I tried to find this actual document, but it SHOULD be there somewhere.


Cheers // Ingvar

On pages 202 & 203 of Frank Zaic's 1964-65 Model Aeronautic Yearbook there is a description and photos of a wind tunnel with enough detail to point one in the right direction for constructing such a device. I think these books can be purchased from the AMA store. As an aside, on plate 135 is a three view of a Nordic glider I built about 44 years ago!
Don

Gary-

'sorry, I posted this by mistake of aging mind in another thread. Anyway, here is my FWIW attempt at a response to your original question:

1) Ritz, Gerald, Jr.; "Science Project Number 2"; Model Airplane News; 6/61; pp. 21-23,50 (This is only a 2-D tunnel using smoke to observe airflow - no force measurements; includes plan drawings)

2) Testa, Al J.; "Homebuilt Wind Tunnel Research"; Sport Aviation, 4/74; pp. 25-30. (This one uses smoke and balances to measure lift, drag, and moments, used for his Mignet 'Flying Flea' research; photos, inexact drawings)

3) Pregler, William and Guignard, Thierry; "Aerodynamic Test System"; Hang Gliding; 3/81; pp. 34-37 (full-sized HG test system mounted above van; simplified schematics and photos)

I haven't filed any more recent material, but I'd guess there are some newer, better ones. Going the "outdoor" or "Car top" route could be beneficial, and who better to ask than Al Rabe? Otherwise, a perusal of the holdings of a major city's or University Library might result in more refined or up-to-date material.

I once tried to make a simple/quick "Poor Man's" wind tunnel to show my general science class how airplane control surfaces work. Well, I was HALF right in that description. I even whittled out and glued the dihedralled profile mustang model victim of this experiment in the theater during the wifely one's musical show rehearsals. 'should have been active in model aviation at the time. Ha!

My guess is that getting good data is not as easy as it might appear. NACA found that tunnel-wall interference and turbulence and other interference from mounting mechanisms really distorted their data, especially regarding what they came to call "effective Reynolds numbers," which biased and confused some findings.

SK

This is really easy: Go to your local grocer; buy a big-ass salmon; copy the shape; enjoy the salmon barbecued.

One can have his cake (or fish) and eat it, too...

You laugh! The current world speed record holder in the streamlined motorcycle class did very much the same thing in designing and building his new 'liner a couple years ago.

Dan

When writing the above I forgot to mention that the team owner of the world record-holding streamlined motorcycle not only came up with the idea of copying the shape of salmon after reading about them and seeing their performance in rushing streams, he also likes the fact that they exhibit a low coefishent of friction.

Dan

>This is really easy: Go to your local grocer; buy a big-ass
>salmon; copy the shape; enjoy the salmon barbecued.
>
>One can have his cake (or fish) and eat it, too...
>
>You laugh! The current world speed record holder in the
>streamlined motorcycle class did very much the same thing in
>designing and building his new 'liner a couple years ago.
>
>Dan
>
>When writing the above I forgot to mention that the team owner
>of the world record-holding streamlined motorcycle not only
>came up with the idea of copying the shape of salmon after
>reading about them and seeing their performance in rushing
>streams, he also likes the fact that they exhibit a low
>coefishent of friction.
>
>Dan
>


Ron B.
F4Fguy

But, you still have to apply a scale factor.

Ron B.

>
>When writing the above I forgot to mention that the team owner
>of the world record-holding streamlined motorcycle not only
>came up with the idea of copying the shape of salmon after
>reading about them and seeing their performance in rushing
>streams, he also likes the fact that they exhibit a low
>coefishent of friction.
>
>Dan
>
However Dan,, you must realize that the coeffishnet of catchion is rather high, at least based upon the nets along the columbia gorge during the salmon run each fall..

Gary
Here are a couple of sites to check out. The Difficult part of making a wind tunnel is not moving the air, the critical part is the balance system to measure the resultant forces and moments, accurately and repeatably.

The Baals tunnel has been built numerous times.
http://www.ceeo.tufts.edu/robolabatceeo/college/windtunnel/baals.shtml
http://www.fi.edu/flights/first/makebigger/index.html
http://www.grc.nasa.gov/WWW/K-12/gallery/ryans_tunnel/tun1.html
http://www.everystockphoto.com/photo.php?imageId=2039928

others,
http://www.aerorocket.com/offer.html

.... and everything you wanted to know about wind tunnels.
http://www.hq.nasa.gov/office/pao/History/SP-440/contents.htm

Gary clearly just wants to build a Wind Tunner! Get a grip! That's a whole different kettle o'fish... Steve

Steve Helmick
Renton, Washington
USA