In the past, wind powered devices did useful, directly mechanical, work such as grinding corn or pumping water. Wind powered pumps are still to be seen on some Scottish farms today. Nowadays the emphasis tends to be on the potential of wind power for the generation of electricity. We can easily model such devices in the classroom. In our models we will of course be constrained by those propellers we can buy or have in the bits and pieces box. Now most propellers are meant to be just that, a means of propulsion. Will they be equally good on a wind generator?
Constructional details for a wooden framed buggy (so called, and fairly wee-kent, 'Jinks method' using 10 mm2 section softwood).
Can we devise a test to discover if one that is good at propulsion is as good on a wind generator? The graphic above shows one way. Different propellers are used to drive a buggy and it is timed over a measured distance. The chassis of this buggy is a simple wood and paper frame. It makes use of 10 mm square wood strip with the addition of A4 card. Fold the piece of A4 card length-wise through the centre, then fold again.
Cut the card through the folds into four pieces (keep the spare pieces). Using the long side of the card measure and cut two pieces of 10 mm wood. Now measure and cut the two short ends. Remember they will fit inside the two long strips. Glue each strip to the outside edge of the card. Try to keep the frame as square as possible. Now, on the spare piece of card, mark off two 30 mm squares, draw a diagonal through each square and cut into triangles. Glue these card triangles to the frame corners as shown in the diagram. With the remainder of the card mark, out eight equal rectangles each 30mm x 20mm, these will be used as axle bearings.
The hole size for the bearing will depend on the material chosen for the axle and the wheels. Make sure the block of wood is deep enough for the prop blades to clear the floor or table. The proportions of the finished shape of the axle bearings are shown in the graphic above.
Tower for mounting and testing a model wind generator. Note that the output is almost certain to be insufficient to light a conventional torch bulb - an LED should be OK. Use hair drier with care.
The propeller and motor that have been chosen as the "best" by investigation can be tried in a wind generator. What you will need is a tower on which to mount it. This tower will have to be stable and strong to withstand wind pressure. The propeller can now be made to turn with the help of a hair dryer, on the cold setting. The low voltage produced by the generator can be read from a voltmeter or multimeter should the school have one. Failing that you can use the output of the generator to light an LED (light emitting diode or as one recent client charmingly called it a "little electrical device"). For this investigation an LED can be used on it's own, in other circuits it usually needs a current limiting resistor in series. LEDs in different colours (and advice on their use) as well as stocks of propellers, motors, wheels etc. are available from SSERC.