From: "Nick Ugolini" Subject: RE: COZY: Ground test: Prop errosion Date: Thu, 18 Jan 2001 18:39:05 -0500 Hi Al, I too built a prop a-la-Nigel. It works really well and has been on my plane for 1000+ hrs with no problems. Let me tell you the Al tape DOES not work. I have tried just about everything, from stainless steel, tapes, copper, foils, etc and I have really put it to the test with lots of rain flying. Only two items I would recommend to you. 1. Is Leading Edge protection from 3M I'll be happy to send you a piece to put on your prop. You only need about 1/4" on the bottom side (flat side) and about 1/2" on the top side getting wider as you move toward the hub. I cut the strips about 12" long, 3/4" on one end and about 1.25 in on the other. Plus: easy to install when prop is built. Minus: Reduces efficiency (not much) and wears out at about 350 hrs with erosion, but is easy to replace. 2. THE BEST ... you almost need to build this into the prop. I sanded the leading edge of the prop back about 1/4 - 1/2 in in the area where the solid carbon graphite is and made some dams and applied Devcon metalized epoxy to the area (you also use a product such as JB weld.) Let it harden, then file the product down to match the airfoil shape. It is VERY tuff, seems to be impervious to rain and is EZ to repair (such as having you prop damaged from the plane tipping over!!). All my next props will be: 1. leading edge protection will be Devcon from tip to about 1/2 way up the prop. 2. Not painted. Pain in the ass. Take the prop, fill and get the airfoil perfect then put a layer of glass on the prop (sacrificial) the just put clear epoxy on it and sand and buff until smooth. Let others look at your handy work. I have toyed with the idea of ... fill/finish like you would for painting, then put a thin veneer of wood on top for UV protection. Very similar to what they do with the WEST boat building system. It would look great and would not need paint. You could even paint the hub (which rarely needs refinishing) and veneer the blades.... The next time I refinish my prop the paint is out of here... I am planning on building a prop for Jack Wilhelmson. It will be carbon graphite with foam cores, and NOT painted. Just the beauty of carbon cloth.... Have fun . Glad to hear we have another prop builder in the club..... Nick LongEZ N29TM Cozy 3 under construction -----Original Message----- From: owner-cozy_builders@betaweb.com [mailto:owner-cozy_builders@betaweb.com]On Behalf Of alwick@juno.com Found erosion of my prop tips, leading edge. Only area w/o alum tape at leading edge. I'll re-flox and tape full length from now on. Love my prop. Can't believe something you built from scratch works so well (so far). From: PLMJohnson@aol.com Date: Wed, 24 Oct 2001 13:07:00 EDT Subject: Re: COZY: Prop diameter In a message dated Tue, 23 Oct 2001 10:41:00 AM Eastern Daylight Time, "John Slade" writes: Builders, What's the optimum diameter for a Cozy prop? I see 70 mentioned often, but occasionally 72. Comments? John, The optimum diameter for your Mazda powerd aircraft will not be the same as the optimum for the Lycoming powered aircraft. As a rule of thumb you want the tips to have the highest possible speed without causing significant compressibility issues. To this end the magic number seems to be a tip velocity of 0.85 Mach. In calculating this number you need to find the vector sum of the tip speed and the aircraft speed. For example, if we took Nat's maximum documented speed of 216 mph @ 8500ft and calculated the tip velocity for various propeller diameters (@ 2700 rpm) then the mach numbers will be as follows: RPM = 2700 Dia Mach @ MSL Mach @8500ft 72 0.82 0.94 70 0.80 0.91 68 0.78 0.89 66 0.76 0.87 64 0.74 0.84 RPM = 2800 Dia Mach @ MSL Mach @8500ft 72 0.85 0.97 70 0.83 0.94 68 0.81 0.92 66 0.79 0.89 64 0.77 0.87 As you can see, for cruising at altitude with a Lycoming at 2700 rpm the optimum prop size is around 66 inches. If your Mazda spins a little faster then even smaller may be the solution. You will pay the price in the efficiency equation since the smaller prop is less efficient but only significantly in the climb regime. If you look at my web pages I have a discussion on efficiency. I hope this helps. Phillip Johnson WWW.geocities.com/plmjohnson