Help with son's science fair project (airfoil designs)

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The son wants to comparative study of airfoil designs as a science fair project and he (and I) need a little help.
His plan is to make blades out of balsa, put them on a small remote controlled helo he got a few years ago, place helo on a scale (small enough so the majority of the thrust doesn't fall on the scale) then see which design gives the best lift.
He has found some plans for different designs but I think he could use others. Any websites out there for this?
Also I have NO experience in working with wood models or caving and he has even less. How would you suggest he goes about carving the foils? I was thinking a dremal type tool would work but what kind of blade/head would be needed?
My biggest question is how in the world do you make sure that the shape in the wood is the shape you want?
Any and all help would be great and quickly because I'm sure he's going to be asking me and I'd like to impress him on my vast knowledge of woodworking and airfoil construction. ;)
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I can't help you with the design as I'm clueless when it comes to much other than drilling holes...
But I can tell you that asking the same question in a model airplane group may get you better results than anywhere else. Those folks are sometimes beyond fanatical about their knowledge of such things.
Regards, Joe Agro, Jr. (800) 871-5022 01.908.542.0244 Automatic / Pneumatic Drills: http://www.AutoDrill.com Multiple Spindle Drills: http://www.Multi-Drill.com
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no spam wrote:

I did something similar back in high school.

My technique was to build a wind tunnel using a standard house fan and simply put the wing on a post stuck through a hole in the bottom, resting on a digital scale. You can then graph angle of attack vs lift for each airfoil type.
A better technique would be to hang the wing from some thread front and back. This would allow you to graph angle of attack vs both lift and drag for each airfoil type.
If you want to use the scale, you could flip it around so that the force is downwards. This would solve the problem of the helicopter flying away, and would also mean that the wind produced by the blades would be going up, and thus not affecting the scale nearly as much.

There are some at: http://www.gliders.dk/airfoils.htm
The Kline-Fogleman airfoil is kind of interesting. It almost looks like it has a chunk taken out of it.

A knife works fine, with sandpaper to finish. If you know someone with a tablesaw you could kerf it at intervals to give you a baseline. A bandsaw would allow you to hog off most of the excess.

Kerfing as mentioned above helps. Otherwise draw the shape on the ends, cut away everything that isn't the shape.
Chris
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Chris Friesen wrote:

Go with the wind tunnel. Mounting new blades on a model helicopter isn't a repeatable process. Changing blades will change the load on the engine, causing it to run faster or slower, createing more or less lift. Just changing the size or angle of attack of the rotor blades, while keeping the airfoil the same will change the performance of the helicopter. Wind tunnel doesn't have to be very long. A square or rectangular cross section works fine and is easier to make than a circular cross section. Have the fan suck air out of the wind tunnel rather than blow into it. The fan blades make the air turbulent which reduces lift and adds vibration to make your instrument readings jiggle. Make one side of the wind tunnel clear plastic so you can watch the action. Arrange a protractor some how to let you measure (and set) angle of attack. Angle of attack is very important, a change of a few degrees will change the measured lift greatly. One way of measuring lift might be to make a beam balance scale, the airfoil under test (pointed down) goes onto one arm of the balance, and you add weights to the other arm until the airfoil balances. I'd go with balsa wood 'cause it's light and easy to carve to shape. If the kid is new to carving, make sure he cuts AWAY from the fingers holding the wood. You can get more consistant results by making templates of the desired air foil out of file folder stock and using them to check the shape as carving progresses. Surface finish affects performance. Couple of coats of shellac, sand between coats, will increase performance.
A really sophisticated tunnel would measure drag as well as lift, the true measure of airfoil performance is the lift/drag ratio. The ideal airfoil would create pure lift, no drag. Practical airfoils don't do that well.
You might check with the boy and make sure that he really wants to do a wind tunnel as opposed to flying the helicopter. He could do a science project by measuring the performance of the helicopter, best altitude, speed, range, payload, fuel consumption. Make the measurements on a hot day, a cold day, a high pressure day, and a low pressure day. See if temperature or barometric pressure makes a measureable change in performance.
David Starr
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David Starr wrote:

"Best altitude" could be an interesting project in itself depending on how much the helicopter can lift and how high it can really fly.
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OT reply:
Just remember watching an old cable TV show about wind-tunnel and aircraft design---AKA late 1930's
The engineers mounted the model wing airfoil design upside down. Attached thin wires to a weight scale above (zeroed out for gravity and mass of model). As wind tunnel air moved across inverted airfoil, the forces would want to move the wing down, thus registering a greater weight of the wing on the large dial of the scale.
For the 'suits' in management, they had to turn the camera upside down when they filmed the wind tunnel results because any movement of the model would then see on the film as the wing moving "UP". Mustn't upset the thinking of the 'suits' you know.
Phil
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IMHO your best bet is to grind or cut a custom scraper for each profile you want, and use that to cut the balsa to the right profile. That way, the results are predictable and consistent - just scrape until the scraper hits the table (holding it vertically) and you know the profile is the same all down each blade.
Note that balsa is a porous wood; you should use a wood filler at least, and paint perhaps, to get a smooth surface. Otherwise you have to account for turbulence too. Hey! Something else to measure.
You could use the dremel tool to grind the scrapers.
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The problem with using a propeller is that not only is it curved it's twisted as well with the angle of attack changing as you move from the center of the blade out to the tip.
I agree that testing a wing rather than a propellar would be easier to deal with and display.
Setting up a fan with a thin foam baffle in front of it might simplify the project, but if I remember the Wright brother's tunnel was actually an elongated circle like a race track with the fan on the back side away from the testing area. The curves also served to stablize the air flow somewhat.
Then he can set up the wing to be tested on a stand on the scale in the tunnel. Using threads on the wings he can show the airflow over and under the wing.
By changing the angle of attack he can show how the lift and airflow change that way.
Graphing the data for comparison would show a lot too. I would look for wings with known flight characteristics
I'm wandering though. Using a wing rather than a propeller he only has to have one pattern for each wing, where the only way I can think to carve a propeller correctly would be with a CNC machine. There are a LOT of balance issues to look at with propellers.

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My daughter did the same thing to me several years ago. My solution to get the project done, was to show her how to build the fuselages from the expanding foam in a can, (Great Stuff).
We made the basic shape using toilet paper rolls and card board cones. Once the basic foam shape was created the foam carved easily into the proper shape for the fuselage, and surprisingly the foam pores were quite small so it looked good when finished.
While we used balsa wood for the wings and stabilizer, you could build the airfoil shape of the wing the same foam.
no spam wrote:

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"no spam" wrote:

<snip>
> wood is the shape you want?
You take me back to the days of my misspent youth where a lot of time was spent building stick balsa and tissue paper model airplanes.
The same technology works for air foils.
Use 1/8" sheet balsa to make profiles along the air foil.
Lay out and cut to size with an Xacto knife.
These profiles are glued to a spar, say 1/8 x 1/2 located in the middle of the profile on equal spaces.
Form leading and trailing edges from balsa as required, then glue in place.
Cover with tissue paper that has been glued in place, then stretched by sprinkling with water and allowing to shrink while drying.
Finally, paint with model airplane dope and you have an airfoil.
Tools needed:
Xacto knife, model airplane glue, straight pins, sand paper and some paint brushes for models.
Try to find an old time hobby shop and kibitz with some old farts, if you can.
Have fun, both of you.
Lew
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But it is going to have to be tougher because of blade spin. Some like like your old Hillers were solid wood with a spar. Other like some of your first Bells were a spar with the spacers he mentioned and sheet of thin plywood (balsa sheets with grain in the direction of blade length in your case) with fiberglass or urethene coat. Hell..... Bell in the early days built a frame on the lines of his 47 in model format with a plugin electric motor to test his ideas.And various blades out of wood for his prototype helicopter......used to have some old movies of that in his driveway testing it out.Guess he was one of the original 60/90 series model helicopter guys. Wood blade like wood props are fragile compared to other materials........ Have done them for a Whistler autogyro out of solid balsa (basswood is best for beginners more stable but less reponsive) You need a razor plane and a long sanding block..... You also need to add reenforcing plys on the top and bottom of the root attached with medium cyanoarcrylate. CA is used around the mounting holes for reenforcement as well. Mount blades and sand ...heavy blade to balance. After you got them balanced cover with MonoKote Then final balance ...cover... light blade tip with a different color covering. On these 90series blades the travel adjustment was 5" max.vertical. AT 4 degrees negative incident . I take it outside to check it.... holding up the rotor mast in a 5 to 7mph wind with a quick hand spin . It should spool on up to about 700rpms and give you a 5 pound tug for lift on 39" disk. This is with asymetric blades.....roughly a 8-h-12 profile A lot of helicopters use NACA0012 profiles(If your son has that one, he has the most popular used on helicopters) That outer 1/3 of the blade is giving the lift. some modelers add a 10-15 degree twist just like you see on props to extend the lift along the leading edge. That would have the outer half of the blade giving lift. Since the root is moving a lot slower than the tip at a given rpm. You would need a jig and copying dremel to carve that one...... Beyond anything I have done,so far on wood model blades.........
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How big are the blades? I'm not sure that the helo is the best way to demonstrate the lift compared to a fixed wing in a makeshift wind tunnel.
There are a few ways to build wings. Spar and ribs are the traditional method, but you can also hot wire foam board, add a spar and cover it with Monokote, an iron on covering. http://www.monokote.com /
I'd do a comparison of at least three styles of airfoil, you can see some profiles here http://www.allstar.fiu.edu/aerojava/Wing31.htm
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You will note that since you are using a helicopter that the stall line will be different on a synmetrical airfoil like actually used on most helicopters and supersonic airplanes than on a asynmetrical like used on autogyros and subsonic airplanes.The stall line actually walking off the airfoil more at large attack angles. Should be able to get airfoil profiles for you to use by googling that. I would use the blue form with one coat of fiberglass(lightest cloth like in an model shop) over that. with feather lite to smooth it up.For hard points to bolt blades on you can use hardwood dowels glassed in the form,then drilled for the bolt.
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no spam wrote:

The Bible on wing sections is "Theory of Wing Sections" by Abbot & von Doenhoff. 15 bucks from Amazon.

Knife, maybe a cabinet scraper. I'm not going to try to describe a sanding spline but if you google that term you should get some good descriptions.

Make a template or set of templates out of cardboard or brass or aluminum or thin plywood or whatever else floats your boat--when the wing matches the templates you've got it right,.

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Styrofoam (house insulation) and a hot knife might work well. For the hot knife, you can use a couple of feet of steel wire (guitar string; can't remember which one, but just a single wire) and an auto battery charger. If he makes a couple of templates (cardboard works fine) in the shape of the airfoil he wants, then sticks them to each end of a block of Styrofoam, then hold the wire against both templates at the same time as he moves it around the templates, he can make smooth, light weight airfoil sections. Kerry
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Kerry Montgomery wrote:

Careful with the selection of foam though--the beaded stuff won't cut smoothly that way and polyurethane gives off toxic vapors when hot-wired. If you're got non-beade polystyrene though the technique works a treat--that's basically the procedure that Burt Rutan specified for cutting the wing cores for the VariEze and LongEze.
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John, Yep, that's why I suggested Styrofoam (house insulation). Kerry
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Kerry Montgomery wrote:

FWIW, both beaded and non-beaded foam cuts well with a hot wire, but possibly at different temperatures.
I've formed lots of radio control aircraft using white (beaded), as well as pink and blue foam. I often used the cheaper beaded stuff on combat aircraft, as they were so frequently destroyed.
The OP should ask on rec.models.rc.air. Those folks literally do this stuff every day.
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http://www.vatsaas.org/rtv/construction/hotwirecutter.aspx
http://www.terragenesis.co.uk/infopages/page171.html
http://en.wikipedia.org/wiki/Hot-wire_foam_cutter
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Kerry Montgomery wrote:

Styrofoam is a brand name for a variety of products, some of which can be hot-wired safely and others that can't. The blue Styrofoam at the local Home Depot is polyurethane, not styrene.
Read the label, know what it's made out of, don't just assume because it's "house insulation" that it's safe to hot-wire.
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