by Chappy » Wed Sep 01, 2004 10:35 pm
Well, I can tell you not to build them out of aluminum!
Now, before you you say, "Well, of course not", you should know that back around 1980 several companies did use aluminum prop shafts in their redrives (the Hummer was one). When I designed my redrives in 1980 (for the little Pioneers) I let Frank Allan in Florida, who built the parts for several company's redrives, talk me into using aluminum (2024-T4 IIRC), even though I thought it was a terrible idea. One broke (at a poorly done radius) after only about 15 hours of running, mostly on my test stand, but I had one successful flight on them. Fortunately, I had one back on my test stand when it let loose. I still have the broken prop. I'll try to post a picture here tomorrow.
I rebuilt them with solid steel shafts, and they have run many hundreds of hours. I wish I could remember what alloy I used, but I can't (or more likely never knew). I seem to remember just using some scrap pieces I had in my pile. From the way it machined, I think it was a little better than cold rolled. I knew it wasn't stainless.
Incidentally, my props are 44" pine, three laminates of Murphy and Aims (a local lumberyard - long gone now) #2 better pine glued together with polyester resin - I had run out of Epoxy! They are 23 years old now and doing fine. I painted them with Pactra Formula U model airplane paint, along with my lift struts - and they still look good.
I started out with a 3:1 reduction, but grossly underestimated the amount of prop slippage that would be present (due to such fast turning props in such a slow moving body of air) and built my first props with way too little pitch. So I built a second set of props with more pitch, but it still wasn't enough! I had planned to cut them some if require, too. Because I am not a "wood person", I decided to make new sheaves for the engines and went with a 2.75:1 ratio to speed up my props some, and make up for the under pitch. I believe my first props were 16", the second set 20" pitch. They are non-helical in pitch distribution. Beware - most prop formulas and nomograms will NOT take into account the affects of high prop RPM/slow flying aircraft - and will give you an under pitched prop! BTW, my Pioneer's turn almost 8500 rpm on takeoff! You don't even want to know what the cylinder head temps run!
I probably spent 5 times the work, at least, designing and building my little redrives as I did building my first Lazair. I even signed up for an adult education class to learn how to run a metal turning lathe so I could build my own components. Being able to make many of my own parts is a personal freedom that I now rate right up there with flying my own plane.
There were a few more pictures posted on the Lazair Yahoo forum site of my plane and Pioneer redrives. I guess they are still there.
Chappy (Lazair #25)
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