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
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. ;)
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.
Joe Agro, Jr.
Automatic / Pneumatic Drills: http://www.AutoDrill.com
Multiple Spindle Drills: http://www.Multi-Drill.com
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:
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.
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
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
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
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.
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.
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.
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.
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:
> 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
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.
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
Have fun, both of you.
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
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.........
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
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
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.
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.
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.
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.
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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