Test Flights

Test Flights

Once on the pole, it became immediately apparent that the tail boom was too short as there was insufficient torque to swing the machine into the wind. The tail-vane tended to flap and spill air before turning the machine into the wind. This improved as the greased pole yaw-bearing started to wear in but there is definitely room for improvement here.

Once pointing in the right direction, the blades seemed to turn easily enough and the machine seemed well balanced. It does seem that more wind is needed to start the blades turning that the 12 watt machine, but they also seem to have more momentum and take longer to stop. Before I could do anything useful, I needed to filx the tail boom.

The machine was brought down and the tail boom extended by 300mm (Using more angle iron and a couple of bolts). Usual story - threadlock and paint, then the tower was raised again. This was an immediate improvement, the machine swung into the wind easily so now it was possible to see what it could do.

During low winds, the machine will start rotating and happily tick over at about 15 rpm. Unfortunately, this does not generate any useful power. At about 80 rpm, the phase voltage measured was about 8.5v rms.

When things pick up a little, the blades were rotating at about 200rpm and I was seeing 15V rms across the phases. The machine starts to furl at this speed which may be a little early. I could fix this bu adding weight to the bottom of the tail vane but for now it suits me fine. This is a large and heavy machine and I'm quite keen to keep it's rotational speed down until I am happy everything is in order. So if anything, the machine may cut-in RPM may be a bit low and the machine furl RPM may also be a bit low. Once the rectifier is put together and the system attached to the battery bank, I will have a better idea of what needs to be adjusted.

After several days of rather variable wind, my conclusions are that the output from the turbine is very disappointing. When rotating at 200rpm, I see over 30v p/p and if rectified and pushed into the battery bank, it peaked at 4 amps before thr regulator cut in. I believe the alternator is ok, it is the blades that are a problem. ALthough they are light and well balanced, they require a lot of wind to start turning. I believe they are just are just plain in-efficient. The lack of pitch is probably the main culprit and I suspect their large width is giving lots of torque at the expense of speed. Tweaking with the original design paramerters seems to not have been a good idea !

This is disappointing. My time is very limited and building/painting the blades took longer than building the rest of the machine. What is needed is a fast way to experiment with blade sizes/angles before committing to something more permananet. I believe PVC pipe may be the answer here although my previous experience was not good. I did a bit more research and have found that all PVC is not the same. The black waste-pipe I was previously using is made from weaker (cheaper) polimers that the orange larger PVC pipes you see used during roadworks. Some PVC is not UV-resistant so will become brittle in sunlight and a coat of paint will prevent that.

A few observations:

• Long blades give more torque
• Long blades rotate slower than short blades
• Narrow blades rotate faster than fat blades
• More blade have more torque, less speed
• The end of the blade moves much faster so should be thinner

All kind of obvious, but it seems the more of the blade that takes up the swept circle, the slower the rotational speed, but the more torque is available. So my next task is to arm myself with various PVC pipes and do some experiments with various blade sizes and shapes.

PVC Blades Revisited

After obtaining a 1 metre length of PVC, pipe I set to work again. The orange PVC is much thicker than the black stuff I had earlier; it also looks to be much stronger, less flexible but perhaps more brittle. I made some 75mm blades in a similar manner to before. I used a smaller hub and shaped the hub-ends differently to prevent the weak-point apparent in the last design. My daughter managed to delete all the photos from my camera but there was nothing really new in here.

I painted the blades white, balanced them and put them on the machine. From start to finish it took about 6 hours, better than the 6 months required for the wooden ones! Results were immediate, the blades turned the machine in much lower windspeeds and was generally much more responsive. The output was pushed through a 3-phase rectifier and I made sure there was a fully charged battery at the end to ensure the electrical brake would kick in, if things got a bit too windy out there - at least until I could ssee how they stood up to a bit of misuse.