A Wind Tunnel You Can Make and Operate
April 1957 American Modeler
computer software has replaced much of the simulation and experimentation
that used to be the sole domain of wind and smoke tunnels. The mathematical
equations are so complex for high resolution, 3-dimensional calculations
that very powerful computers are required to run even relatively simple
simulations. While there are programs that can be purchased for about
$1,000 that do a good job for uncomplicated shapes, large, university
and corporation scale computers are needed for "serious" work like designing
commercial and military aircraft, passenger cars, competition sailboats,
and many other applications. Even so, NASA, the ESA, and other large
organizations still operate tunnels for testing prototypes that have
emerged from simulations. This article provides plans and instructions
for building your own smoke tunnel that uses components easily obtained
at the hardware store. If you have ever thought about building such
a device but haven't found an easy way to do it, this might be just
what you've been looking for.
A Wind Tunnel You Can Make and Operate
Okay, so be technical and label it a "smoke" tunnel. But that won't
lessen the value of your findings one bit!
By Paul J. Palanek
is an instrument of scientific proportions, not just a novelty. The
serious minded experimenter can probe many secrets relative to the movement
of air and bodies placed in its stream. Knowledge reaped from smoke
tunnel studies can lead the model builder, particularly the free flight
enthusiast, to design better models.
Prior to settling down
to the serious task of building the instrument, let's delve into a brief
history of the wind tunnel - a close cousin to the smoke tunnel.
The world's first wind tunnel was built at Greenwich, England in
1871. Its designer, F. H. Wenham, proposed it to the then recently organized
Aeronautical Society of Great Britain, as an instrument to obtain data
on which a true science of Aeronautics could be founded.
1901 Wilbur and Orville Wright built a small wind tunnel at Dayton and
with it tested some 100 different wing models at various angles of attack.
In the year of the Wrights' flight, Crocco built a wind tunnel near
Rome; Prandtl built a large one at Gottingen in 1908. Eiffel built one
of unique design with an air tight testing chamber at Paris in 1909.
The British government constructed its first large tunnel at the National
Physical Laboratories in London, in 1910.
There are today about
200 tunnels in these United States. Half are for subsonic work, both
commercial and in schools; the remainder for testing at high speed.
They vary from low speed, low density types to tubes, developing 18,000
mph speeds by hot expanding gases - in reality an explosion.
There are two basic types. The first, called the open circuit (or "Eiffel"
or NPL) tunnel, has no guided return of the air. After the air leaves
the diffuser, it circulates by devious paths back to the intake. If
the tunnel draws its air directly from the atmosphere, entirely fresh
air is used.
One of the most simple approaches to a "do-it-yourself" tunnel
we've ever encountered. This makes a terrific project for a
science class, model club, or a school science exposition entrant.
Smoke system and specimen details
The second type, called a closed circuit, "Prandtl,"
"Gottingen," or "return flow" tunnel, has, as the name implies, a continuous
path for the air.
Our unit is a two dimensional smoke tunnel
devoted exclusively to section testing. For this type of airfoil research
the test specimen is a flat cross section that spans the shorter axis
from one wall to the other. The tunnel is constructed of common materials;
what must be purchased, can be had for a few cents. A good deal of the
material came from the scrap, heap and waste bin. The most expensive
component was the lucite observation window. Glass could have been substituted,
but would have been more difficult to handle and fit.
copper tubing smoke manifold appears most difficult part, let's get
along with this. A piece of 1/4" or 3/8" diameter copper tubing is used.
Drill eleven holes enabling a press fit for the 1/8 " manifold reducers.
Solder these tubes in parallel. Be certain that the solder remains clear
of the I.D. of the tubes. Exit nozzles are soldered to the 1/8" reducers
Tunnel uses 1/2" bass or pine top and bottom, with 3/16" tempered
Masonite for the walls. Layout the contour lines and shape accordingly.
Make the window cut-out cleanly since it carries the sealing Lucite
Assemble tunnel by first inserting the smoke manifold,
then with brads secure the sides to the upper and lower walls. Seal
off the exit end of the tunnel with a piece of 1/2" hardwood. A wood
baffle spans the minor axis of the tunnel; fasten with brads in the
shape of a "V" as indicated. This baffle prevents the smoke streams
from terminating sharply.
At this point, seal all seams with
model cement. This will strengthen the assembly noticeably, When the
cement has dried, spray the tunnel both inside and out a dull black.
Mount the tunnel on its three 1" diameter dowel legs fastened
to the baseboard. This will hold the instrument in good position for
working. Note the location of the securing bolts. With the tunnel positioned,
secure the flanged pivot bushing in the center of the vertical axis
and in the center of the front face cut-out. Secure to the aft end of
the tunnel a vacuum cleaner flange, one to fit the vacuum cleaner used.
Keep this as air tight as possible for obvious reasons. Cut from the
full size plans the calibrated scale and cement to the face of a 1/16"
plywood panel, then fasten to baseboard, locating so zero line is directly
below center line of the pivot bushing, Use 1/4" sq. hardwood strip
to mount the dial plate properly.
You will note that the entry
end of the tunnel has a grating 1" sq. something like an egg crate separator.
Build this of 1/16" smooth balsa sheet. When complete, the unit should
fit snugly in place. This grid work straightens the air as it is drawn
into the tunnel.
The smoke generator is a simple affair. It can be constructed from equal
diameter fruit cans or from machined brass rod. The wood cartridge carries
five cavities for five burning cigarettes. This assembly is fastened
to snap-on clips for loading and unloading.
Here's the reason you don't see any supersonic balloons flying
around these days!
This should interest every model builder who notes air flow
at the wing tips.
Now we're getting down to cases. Quickly, Henry, hand me
the Clark "Y" section.
Since a control
of some sort is necessary to meter the air to the smoke generator, a
simple wood clamp is mounted to the rear of the unit on the baseboard.
Rubber tubing, 1/4" I.D. is used for all piping.
To pivot the
test specimens, a brass or bronze flanged bushing is fastened to the
back face of the tunnel, fitting flush to the inside. The hole through
the bushing is of little importance - 1/8" to 3/16" diameter should
Test specimens are outlined; these should be made accordingly of hardwood
or balsa. The pieces should be painted black with a narrow white border.
This simplifies photography if used. Tunnel is sprayed a dull black
to provide good photographic background.
Cigarette smoke source.
Complete the face window.
The Lucite fits snug into the tunnel opening with the framing fastened
to the window by #10 sheet metal screws. This assembly should fit flush
to the inside walls. It may be necessary (depending upon fit) to clamp
the window assembly in place to avoid any seepage of air.
create the decreased air pressure in the chamber, a vacuum cleaner of
standard make was used, along with an inner tube as shown in the pix
to activate the smoke flow.
We strongly suggest photography
to permanently record each experiment. This pertinent data can be referred
to at a later date and further tests run with varied sequences. The
versatility of the instrument is up to the experimenter.
and fertile field of research almost totally neglected by the wind-tunnel
engineer is that covering non-aeronautical experiments.
are seriously affected by drag and turn over angle. In a number of vessels,
the modern "decorations" on the stacks are the direct result of tunnel
As road speeds have increased, more and more attention
has been given to streamlining of automobiles. Race cars, particularly
those designed for attempts at the world speed records, must of necessity
pay much attention to wind tunnel tests.
A vast field also exists
for commercial tunnel work such as air conditioning outlets, rail-shielded
inlets, automobile manifolds, drying setups, anemometer calibrations,
wind-driven power plants, and a host of air flow devices.
editor will welcome comments on this tunnel-building feature. If you
find it of special interest, why not drop him a note. It would be very'
much appreciated if photos of completed smoke tunnels - especially in
operation - were sent along, too, to "American Modeler."
Wind Tunnel Plans
Bill of Material
1 pc. 3/16" x 12" x 12" Masonite for window
frame; (2) 3/16" x 12" x %4" Masonite for walls; (1) 1/2" x 4" x 8'0"
white pine for top and bottom, pedestal base, deflector, tunnel end;
(1) 1" dia. x 12" dowel for pedestal risers; (4) 1/16" x 1" x 36" balsa
sheet for inlet mesh; 1 box 1" long brads to assemble tunnel; (1) pc.
1/4" O.D. x 14" brass tubing for smoke manifold; (1) 1/8" O.D. x 36"
brass tubing for manifold reducer; (1) 1/16" O.D. x 36" brass tubing
for exit nozzles; (1) 1/4" dia. rubber tubing for smoke and air couplings;
(1) flange, standard vacuum cleaner coupling for Aft end of tunnel Air
coupling; (2) metal fruit cans for smoke generator; (1) 1/16" x 2" x
2" sheet brass for manifold reinforcement; (3) 1/4"-20 x 4" nuts and
bolts for pedestal mounting; 1 can rubber cement for tunnel sealer;
1 can Krylon black spray for tunnel blackener; 1 pc. 3/16" Lucite x
10" x 13" for specimen window; 1 doz. 10-32 bolts 3/8" long for window
fasteners; 1 pc. 1/16" x 3" x 10" plywood for angular setting; 1 pc.
1/8" dia. x 12" steel rod for segment pointer. Misc.: Cement; solder
and paste; white cardboard; small brads; inner tube for smoke generator
Alternate Smoke Generating Devices for Classroom
Type A: Ammonia Chloride Smoke
needed: 2 wide mouth bottles; 2 large two-hole stoppers to fit; glass
tubing; ammonia hydroxide; hydrochloric acid; notes on setting up:
The apparatus may be set-up as indicated in the drawings. Air
from a compressor, a hand pump. or an old inner tube previously filled
at a service station is most satisfactory. If the smoke is not dense
enough, try the use of a more concentrated solution. Too concentrated
solutions, however. are likely to produce particles that will clog the
By the use of "T" or "Y" tubes three or four jets may
be used at the outlet to give smoke over a wider area. All experiments
should be supervised by a trained leader.
Type B: Dry-Ice Smoke
Material needed: 1 wide mouth bottle; 1 large Two-hole stopper
to fit; glass tubing; Dry Ice.
Dry ice may be obtained from
such sources as dairies, ice cream factories, air products companies,
and drug stores. It costs a few cents per pound. One pound will produce
a good deal of "smoke." It must be used within a few hours.
Warm water will change the "Ice" into vapor more quickly and produce
a denser "smoke" than using cold water.
Notes: Suggestions for
When used with a wind tunnel or in another
air stream the smoke producers will have to be placed so that the smoke
will be carried along by the air stream. The rate of production will
have to be adjusted to the velocity of the particular air stream (see
text for detail. about metering damp). If the velocity is high, the
smoke may be too "thin" and thus not visible to the eye or a photographic
The air velocity is from 2 to 10 mph depending upon the
vacuum used. This can be varied at the exhaust end of the vacuum cleaner.
Full-size construction plans for smoke tunnel are on Plan
#457 by Hobby Helpers, 770 Hunts Point Ave., New York 59, N. Y. (50c).
Posted April 4, 2011