The Baals Wind Tunnel
Click on the area of the picture below you want to learn more about:
This is the Baals Wind Tunnel. It can be built for under $200 and can
get about 30 mph in the test section (where the plane is). The complete
plans for building one can be downloaded on to your computer by going to
our Download Page. If you would
like to read about how to build the tunnel, read on.
Other People that have built the tunnel:
Savannah AIAA section
Note - the following only makes sense if you have the downloaded
plans. This text is included in the downloaded file.
· Wood Frames
To build the diffuser section, construct a wood frame for the upstream end
according to the Diffuser Frame plan and cut a 3/4 inch plywood sheet for
the Diffuser Exit according to the Diffuser Exit plan.
· The Triangles
Cut the four triangular top, bottom, and side pieces from flat 1/4 inch
plywood according to the Diffuser top and bottom Dimensions template. Using
the small ends of these triangular pieces as a guide, cut out the four notches
in the 3/4 inch Diffuser Exit piece. Now fit the 4 triangular pieces into
the wood frame, holding them temporarily in place with masking tape. Make
sure the inside of the plywood pieces fit flush with the inside of the frame.
Lay the 3/4 inch Diffuser Exit on a flat surface, notch side up. Making
sure the top of the wood frame is lined up with the top of the Diffuser
exit and with the triangular pieces hanging from the frame, fit these triangular
sides, top, and bottom into the notches in the Diffuser Exit. When everything
looks okay, add epoxy to all joints and reassemble, holding everything together
with small brads until the epoxy sets.
· The Curved Surfaces
The curved surfaces between the top and side triangular pieces are formed
with tag board available from art supply stores. Cut this tag board about
1/8 inch larger than the opening. Cut three more pieces of tag board this
same size since all openings are the same. Temporarily tape the tag board
on the inside of the diffuser using 1/2 inch masking tape. From the outside,
run a fillet of epoxy or polyester resin around the perimeter of the tag
opening. Cut four pieces of 7 oz. fiberglass cloth the same size as the
exposed tag board. Brush a coat of polyester resin through the fiberglass
cloth to saturate both the glass and tag board. If more stiffness is desired,
a second layer of fiberglass can be added.
|THE ENTRANCE CONE
Construct a simple wood frame for the front of the Entrance Cone according
to the specifications for the Entrance Cone frame. The back of the Entrance
Cone (where it joins the test section) is a wooden frame constructed just
like the Diffuser Frame.
Cut the Entrance Cone top, bottom, and sides using the coordinates from
the "Entrance Cone Template" drawings (Top Panel) (Side Panel). Cut the curved surfaces
from thin poster board available at art supply stores. Pre-bend the curvature
of the poster board top, bottom, and sides to their approximate finished
shape, then glue each piece to its corresponding notch cut in the large
entrance cone's frame. Temporarily tape together the edges of the entrance
cone's curved sides. Slip the entrance cone exit frame over the exit end
of the poster board curved surfaces. Permanently glue the edges of the board
together with a fillet of waterproof adhesive such as epoxy. After the glue
has set, remove the tape and set the entrance cone face down on a flat surface.
Measure from the exit of the entrance cone to the flat surface to make sure
that the exit is parallel to the entrance. Trim the exit if necessary. Now
glue the exit frame to the poster board exit using masking tape to temporarily
hold the poster board to the exit.
· Hardening The Posterboard
Carefully lay a layer of 7 oz. fiberglass cloth on one side of the entrance
cone. Cut the cloth such that each end just touches the wood frames. Let
the sides extend past the edges 1/2 to 1 inch. Brush a coat of polyester
resin through the fiberglass to saturate both the glass and poster board.
Let the resin partially harden, then use a razor blade to trim the edges
flush with the sides. If more stiffness is desired , a second layer of fiberglass
cloth can be added.
· The Flow Straightener
The ideal flow straightener for the 7 X 10 inch wind tunnel is a 1/4 inch
cell by 1 inch thick phenolic or aluminum honeycomb. If this cannot be found,
two layers of "egg crate" florescent light diffuser that have
been glued back to back give adequate results. Use model airplane glue or
Super glue to bond both halves together, being careful to align the halves
as best you can. Saw the "egg crate" to a size slightly larger
than 14 X 20 inches, then carefully trim the edges such that it fits snugly
into the front part of the entrance cone. When fitted properly, it will
secure itself in place by friction only.
· The Plexiglas Pieces
To construct the test section, cut all test section pieces from a 12 x 36
inch piece of 1/4 inch Plexiglas according to the Test Section plan. Cut
the top, bottom and sides to exactly fit into the wooden frame at each end
of the test section. With the Entrance Cone facing down (large end down)
place the Plexiglas top, bottom, and sides in the Entrance Cone frame, temporarily
holding the pieces together with tape. Mark each piece of Plexiglas, identifying
each piece and its orientation so that there will be no confusion as to
what piece goes where during assembly. Drill through the existing holes
in the wood frame with a #32 drill bit and into the Plexiglas only about
1/32 of an inch. After each of the four pieces of Plexiglas have been "spotted"
in this manner, remove the Plexiglas from the frame and drill each "spot"
with a #43 drill and then tap each hole with a 4-40 tap. Lay out and drill
the 4-40 clearance holes (#32 Drill) along the bottom edges of the side
pieces and also at each end of the top piece. Countersink the outside of
these drilled holes (not the tapped holes) with a 1/4 inch 82° countersink.
Reassemble the Plexiglas pieces into the Entrance Cone frame using 4-40
x 3/4 flat head machine screws. "Spot" the six (6) holes in the
bottom Plexiglas piece using a #32 drill through the holes that were just
countersunk. Also "spot" the two holes in the top edge of the
side pieces. Disassemble once again and then drill the "spotted"
holes 1/2 inch deep using a #43 drill. (Wrap a small piece of masking tape
1/2 inch from the tip of the drill to act as a drill depth gauge.) Tap there
drilled holes for a 4-40 machine screw. Lay out a precise longitudinal centerline
on both the Plexiglas bottom and top door pieces. Carefully scribe both
centerlines. These lines are used as a reference for setting tunnel models
at zero degrees angle of attack prior to testing. Assemble the four pieces
of Plexiglas test section using 4-40 x 1/2 flat head machine screws where
appropriate. Set the Plexiglas test section on the diffuser frame and "spot"
the eight (8) screw holes as before then drill (#43) and tap the holes.
Screw the test section to the Diffuser and Entrance Cone using 4-40 x 3/4
flat head screws. Now carefully trim the Door on the top of the test section
such that its sides are flush with the test section sides and there is no
gap between the top and the door. attach the door to the top using two 3/4
inch brass hinges, and 4-40 machine screws.
The Fan Screen with the motor mounting slots must be cut down in diameter
to a dimension somewhere between 16 and 17 inches. The Motor should be 1/4
HP, 1625 RPM, 2.7A, 110V, 60 Hz, AC 3-speed and should mount directly to
the outside of this screen with four 10-32 nuts and washers.
· The Fan
The fan should be mounted close to the screen, maintaining about 1/4 inch
clearance from the screen. The Fan Blade is mounted such that the blade
hub flange is facing upstream. When mounting the Fan Blade, make sure that
the set screw is tight. Add a drop of Lucite on the set screw threads for
added insurance against vibration loosening the Fan Blade. Further security
can be attained by drilling a slight dimple on the motor shaft where the
set screw contacts the shaft.
Set the finished diffuser section with the exit facing up. The Fan/Motor/Screen
assembly is then placed on top of the exit. Carefully position the Fan assembly
so that the blade is centered in the opening and you have at least 1/16
inch blade tip clearance. Fasten the screen to the 3/4 inch plywood with
four metal screen chips screwed to the wood with #8x1 sheet metal screws.
Place a 3/4 inch by 1/8 thick foam pad under each Screen Clip.
As for the motor hookup, the motor starting capacitor is attached to the
motor with a strap. The motor controls are housed in a 4 inch duplex box
mounted on the outside of the 3/4 inch plywood Diffuser exit. This box houses
the three speed fan switch as well as variable speed motor control that
is connected in series with the high speed motor winding wire. Verify that
the motor rotation is counterclockwise when facing upstream. Protect all
wires and connections between the motor and the control box with a flexible
The Centerbody's function is to divert the air around the fan hub in a controlled
manner so as to preclude and flow separation in the diffuser section. Since
the aerodynamic loads on the centerbody are quite low, it can be made from
very light weight materials such as Styrofoam. Cut the desired profile shape
in block form on a band saw, then round off the square corners with a knife.
Cut a 6 1/2 inch diameter disc of 1/4 inch plywood with a 2 inch hole bored
in the center. Securely bond this disc to the flat end of the Styrofoam
with a slurry of epoxy and micro balloons. The final round cross section
can be achieved by sanding with coarse sandpaper. A strong, hard surface
finish can be achieved by applying a thin layer of fiberglass cloth using
epoxy. (Do not use polyester resin because it will destroy the Styrofoam).
If a model safety screen is to be used (see below), it is important to add
this layer of fiberglass so as to ensure a secure bond between the Styrofoam
and the 1/4 inch plywood. A small 1 inch diameter by 2 inch deep cavity
must be hollowed out of the foam to provide clearance for the fan hub. Mount
the Centerbody on the tunnel centerline with about 1/8 inch clearance from
the fan blades to the Centerbody.
|CENTERBODY AND SCREEN
To mount the Centerbody, use two 1/4 inch wood rods. Drill four 1/4 inch
holes horizontally through the diffuser sides such that each rod will pass
through one diffuser side, through the Centerbody, and into the far diffuser
side. Epoxy the rods to the diffuser sides and Centerbody. If the wind tunnel
is to be used in a classroom environment, or you are worried about the possibility
of a model going through the fan (with catastrophic results), a model safety
screen can be added between the fan blades and the Centerbody. You will
pay a slight penalty in maximum air speed, but the tunnel will be much safer
in the event a model ever breaks loose. Cut the screen approximately 14
inches in diameter and securely attach it to the Centerbody with four #10
sheet metal screws and wood washers screwed into the 1/4 inch plywood.
To measure the "lift" and "drag" of the models in the
testing section, we used two Force probes from Vernier Instruments (503)
297 5317. These probes are mounted horizontally and held by two clamps which
fit vertically through a 1/2" thick Plexiglas base. The clamps we used
were simply three fingered test tube holders which can be ordered from a
chemical supply company. The base also holds a stepper motor that turns
the model mount in 0.5o increments both clockwise and counterclockwise.
The clamps should be mounted close to 5" from the motor shaft forming
a 90° angle from each other. They can be held in place using thumbscrews
twisted through a threaded Plexiglas sheath which is attached to the base.
Legs should be added to the base to elevate the motor so that it doesn't
vibrate. Simple plex rods can be glued in place for this purpose.
|FORCE PROBES AND BASE