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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


Response to Travelling Dynamometer 


Stuart and others, greeting from NZ......a friend and myself have been playing with a similar thing to what you describe.

A 400 electric motor spinning at 9,000rpm on a ballrace sliding double rail system, then thru a meter reading WATTS , all coupled to an electronic set of scales. Power for this thing is thru a 12 volt car battery. my scanner is currently down but I'll put a photo Stu in the mail to you. Interestingly you and I seem to be on similar tracks. A typical F3d prop produces 324 grammes pull, 162 watts, at 9.000 rpm. I use this machine to allow me to make/ finish F3d props and although not running at 30,000 rpm as typical to an f3d motor certainly making the props the same at 9.000 must go along way to having similar/ same props.

Click to Enlarge

How this little machine came about was because I do not have access to CNC milled prop moulds, and as you know 'repeat props' on Aluminium filled epoxy moulds cannot be guaranteed to be the same, the moulds do move so I decided in conjunction with an electronics friend to put together the little toy that would enable me to 'work propellers' ( read file and sandpaper) that would give me similar results i.e thrust, wattage at a 9,000 RPM. Its purely a system that I can get a comparison of before and after. 

Spinning the motor at 9,000 allows me to read off the wattage and thrust which I can then compare to a 'known working prop'- I've been playing with the HARVEY WESTLAND prop and although only reading to 9,000 RPM on this toy, surprisngly over three flying props the ground RPM was darn near the same 27,800 - 28,300 and the radargun gave results all within 3.5%. Whilst it maybe a crude way of doing things it certainly now has given me a system that will give me props that produce the same trust and wattage draw- the inference to me therefore is that they would then be similar 'behaving' propellers. 

There are possibly better ways of doing things but even after playing with a Prather pitch gauge and Vernier calipers I could never be assured of the props giving the same results.......this has solved the problem! If you have a close look you will see a rubber band in a triangular format over two bobbins/ buttons which is a very satisfactory way of bringing the aluminium sliding bar, which is ball race mounted, back to a zero reading on the electronic scales. The whole thing gets mounted into my bench vice.

I'll ask my electronic mate about electric motor efficiency- he is a bit of a boffin and maybe able to advise on your electric motor problems including 'shaded pole'. Will advise within a couple of days. If you want I can do a drawing of the whole system. 

Trevor Henderson,
Henderson Design Ltd.