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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.


Use of PVA release agent 


What seems easy to others always seems tough to me. Nobody I know has a problem with applying PVA release agent to a mould, so its really a waste of time writing this article. But just in case YOU do, here are my experiences.
I have always brushed it on, straight from the bottle. The results have been very consistent: that is, always lousy. For a long time I brushed it on over QZ11 silicon wax. Then one day it decided to fish-eye, and refused ever to lay down on QZ ever again.

No real loss,the QZ was building up in the mould and slowly ruining it.
After a switch to PLP1, things got better and I carried on brushing:
still with lousy results. This became critical when I went to larger moulds, so I asked around. One said buy the most expensive Yak's fur brush from an artists shop and use that; I did, and produced magic Mongolian relief maps on the surface of my props. One said spray, so I did. Wonderful orange peel effects, but I preferred the Gengis Khan results. The PVA seemed to have too much surface tension, so I tried thinning with water. This helped a bit, but evaporation was slow, and there was a strong tendency to get runs. More advice, this time from a real expert; thin with metho. Well, I added some metho and everything turned white, globs sank to the bottom and disillusionment really took hold. I tried fresh metho, fresh PVA, nothing helped. 

Then I asked these experts if they had actually done what they said. Well, no, but it should have worked. They actually wiped it on with a rag, so thin you couldn't see it. That's OK for some, but my moulds are pretty rough and I need some thickness in the PVA. So it was back to brushing.

This time the PVA seemed to crinkle up, like a dust bag had been emptied onto it. After washing this off, I tried again. This time it went on fine, in some places, anyway. Crazy, sheer bloody-minded chemicals. So I asked a chemist, the scientist type. No idea.

Well, an idea came to me, after mixing with water the soluble oil for my milling machine. You must add the oil to the water, not the other way around; a bit like adding water to concentrated nitric acid, you only do it once. So I figured the same might apply to PVA , and came up with this recipe.

I added 15 parts metho to 15 parts water, then stirred it. Then this solution was poured into 70 parts of PVA whilst stirring. Vunderbar, no white precipitate and the solution was nice and runny ! Spraying was now much better, no fish-eyes and quite controllable. Some care was still needed in getting a wet enough spray onto the mould, but the runs were gone and the results great. No dust bags either. Warming the mould helped a bit as well, especially as it was mid-Winter in Perth.

So there it is, hope it works for you.

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