As a school project, we had to make an airplane that flew as long as possible without using an electric motor.
Therefore, I came up with the idea of using compressed air to propel a rotorblade.
If you want to make one yourself, you'll need a couple of things:
- Balsa wood sticks:
- 15 x 15 x 1000 mm (2x)
- 10 x 20 x 1000 mm
- Balsa wood plate 10 x 100 x 1000 mm
- Polystyrene modelling foam (blue) (minimum 100 x 400 x 600 mm)
- Polystyreen plate 2 mm thick
- Duct tape
- Wood glue
- Hot Glue
- Old electric motor with small ball bearings (stepper motor)
- 2 Coke Bottles 1,5L (or other strong plastic bottles)
- Steel shaft ( diameter 3 mm , depending on your ball bearings inner diameter)
- PVC pipe (outer diameter 40 mm , depending on the inner diameter of your motor caps)
- 9 inch rotorblades (Slow-fly props would be best)
- Pneumatic tube 9 mm
- Adapter for 9mm tubes
- Reduction valve
- Pipe clamps
- Presta valve ( bicyle tire)
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Step 1: Turbine
- Take your old motor and cut it open with a grinder or dremel. Do this carefully in the joints, where the motor is connected together.
- Take the two sides apart and remove the stator and wires.
- For the rotor of the turbine, you'll use the steel shaft (3 mm ), the original rotor of the motor a piece PVC pipe and some small pieces of polystyrene plate (2 mm).
- First you need to remove the old shaft, if it's to short. Beat the new shaft on the old with a hammer to remove it. The new one will take it's place.
- Glue six polystyrene plate rectangles in a circle on the rotor (superglue). Make sure none are touching the PVC pipe if you align it. But don't make them too small, because air will get through.
- Cut the PVC pipe slightly larger then the width of the rotor (25 mm).
- Drill a hole in it under an angle, so the air will hit the blades almost perpendicular.
- Drill another hole for the outlet, 30 degrees of the other hole (make sure you take the right direction)
- Glue a small piece of 9 mm pipe to the PVC pipe, on the hole. Make sure it fits just right and the air can not escape.
- Take two washers and place them around the shaft on both sides of the rotor, so the friction on the sides is minimal and the shafts axial movement is limited.
- Glue a polystyrene circle on the inside of both caps ( sides of the motor) so no air can escape there.
- Place both caps with ball bearings on both sides of the PVC pipe with rotor inside. Align them carefully so the rotor has as least as possible friction.
- Finally, connect your proppeller on the shaft.
Now the turbine is ready, test it by blowing air in it.
In this short video I tested my turbine with one 1,5L bottle at 5 bar (72.5 psi). More video footage with 2 bottles will be coming soon.
Step 2: Wings and Tailwing
- Cut the wings using the hot wire method:
- Make the shape of your widest point of the wing (at the center) in wood and attach it to the side of the foam
- Span one side of the hot wire in one point, the other one you can move freely.
- Connect a power supply and follow with the end of the wire the wooden shape.
- When its done, you'll have a wing that's narrower at the end.
- Do the same for the other wing
- The middle part of the wing, you can use a normal hot wire cutter because its just a straight extrusion of the wooden shape (width 130 mm) .
- For the tail wing, just cut out its shape with a belt saw or normal saw. Make it not too thick and not too thin ( about 10 - 15 mm). Rounden its front and sharpen its and with a a file or sandpaper.
- When all wings are cut, coat them with duct tape to make them stronger and to prevent braking pieces of your wings.
Step 3: Frame
Now we have to make a frame to attach the wings to.
- Take the two balsa wood sticks and glue them in one end together and one end 240 mm apart with an other stick between them.
- Then take a balsa wood plate and make curved sticks to make the arc at the front.
- Reinforce the front with extra sticks and a foam tip.
- Attach two other sticks across the arc and mount the turbine in a wooden plate with hole.
- Glue all parts together with wood glue ( superglue for metal - wood combination).
- When the glue is dry, you can place the wings on the frame and glue it together with hot glue.
Step 4: Bottle Tanks
Another important part of this airplane are the bottle air-tanks. They are made using two 1,5L plastic bottles and some valves and adapters.
- Drill in one cap a hole to fit the reduction valve. Screw it tight on the cap with a nut and use hot glue to make it extra air-tight.
- Drill in the other cap a smaller hole to fit the 9 mm tube adapters. Same here, use nut and hot glue.
- Drill in both botlles a hole on the bottom side, one bottle to fit a presta valve and one to fit another tube adapter.
- Connect the two bottles with 9 mm pneumatic tubes in series and fit them with pipe clamps on the tube adapters.
- Test multiple times to make sure everything is air-tight, if not adjust with hot glue untill it is.
- Connect the air-tanks to the turbine.
Step 5: Assembly and Testing
Finally, the bottle tanks can be mounted on the frame and with everything connected, you can test it.
I tested it a few times with 5 bar (72.5 psi) and it kept running for 5 - 8 seconds full speed. I couln't go higher because of some minor air leaks. I do think there is potential with this concept, but right now mine isn't optimised. For instance, I used a to heavy reduction valve and adapters, my wings were to small (40 mm in stead of my suggested 60 mm) and my propeller wasn't a slow-fly prop.