Pietenpol-List: Prop - Kevlar Tipping and Fiberglass Wrap

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Original Posted By: Rcaprd(at)aol.com
Subject: Pietenpol-List: Prop - Kevlar Tipping and Fiberglass WrapIn a message dated 10/6/04 6:58:47 AM Central Daylight Time, steve(at)byu.edu writes:>Steve and all,This is another one of my typically long posts, so fair warning !! Kevlar is the most abrasion resistant of the composite materials...that's why bullet proof vests are made from it. Using Kevlar for the leading edge was my idea. I purchased the unidirectional Kevlar from AS&S. It comes 12" wide, buy it by the foot, but you need a 4 foot piece. It has all single flatstrands of Kevlar, held together with little fiberglass strands every couple ofinches. It is some tough stuff, that will quickly dull a pair of good scissors if you cut a lot of it. After all the carving is done, and the prop is balanced, the wood must be cleaned thoroughly to remove all dust and skin oil, with alcohol, and allow an extended period of time for all the alcohol to evaporate. Don't touch the wood with bare hands after that. Measure the length on the blade you need to cover the outer 2/3 length of the blades leading edge, and up half way around the rounded tip. The length of Kevlar must be identical for both blades, for balance reasons. Wearing surgical gloves, lay out a 3/4" width of Kevlar for each blade, on a clean flat surface, and cut to length. It's kind of difficultto get the Kevlar to stay where you put it, because it always wants to curl up, and go crooked. I use scotch tape about ever couple of inches to make it obey. Leaving the tape on the Kevlar strands helps control the fibers, as wellas allowing you to stow the piece of material while you apply the Polyester Resin to the area on the leading edge of the blade. Kevlar does not readily soak up the resin, so you have to use plenty of resin, and kind of work it in after the Kevlar is applied to the resin. The scotch tape now holds the Kevlarfibers stay where you put them, as long as there is no resin where the tape sticks to the wood. Patience is order of the day, here. You can only do one blade at a time, with the prop secure and the leading edge you are working on pointing up. Mask off each face of the prop, leaving at least 1/2" of wood showing wider than the 3/4" wide Kevlar, to catch the inevitable runs in the resin. Have everything ready, and a plan in your head, because when you add those drops in the polyester resin and mix it up, you have but minutes to get the Kevlar on and in place. I never use quite the full amount of hardener, to giveme a few extra precious minutes. If you don't get the 3/4" wide piece on straight the first time, just wait till after it kicks, then pull the piece ofKevlar back off and discard it, and start over. You'll have plenty of Kevlar material for several re - do's. Now you have the Kevlar on both blades, remove the scotch tape, sand and feather the edge of the resin down, but don't get into the strands of Kevlar. Don't use too coarse of sand paper, or scratches will show in the final product. Make sure the prop is clean, and the work area is clean, and lay theprop down on the flange surface on top of a block, to space it up off the table, to prepare for the lightweight fiberglass application. Cut four pieces of fiberglass big enough to cover and overlap the leading and trailing edge, and thetip. Lay newspaper under the blades to control the resin that is going to drip off the blades. The fiberglass should go from the root, almost to the hub,and extend the full length of the blade. This protects the wood, stabalizes the moisture content in the wood, and keeps the Kevlar from coming off with centrifugal force. Do one blade face at a time, and make sure you have 100% saturation, and have a smooth surface. Once the fiberglass is saturated, the finecloth completely disappears, and the only thing you see is the yellowish green color of the Kevlar strands. Again, speed is the key, once the hardeneris added to the resin. Keep it straight by a dabbing action with a one inch paint brush with half the bristles cut off. Use cheap bristle brushes, and discard the brush after each application...it's just not worth trying to cleanit. Allow the cloth to overlap the edges, and trim them off with a razor blade after it sets. Finish sand to blend the edge of the fiberglass. Flip the propover and repeat the process. After the resin cures for a few days, give it a beautiful glossy shine with a couple of coats of Ace Spar Varnish, gloss #16375. Use the varnish step to attain a final balance, by adding an extra coatto the light blade. To build the 3 props that I have done so far, my main instruction was talking with experienced folks in the area of building propellers. I also usedthe drawing of the prop for the Model A engine, that Orrin Hoopman drew up dated 3-3-33. It is 76 X 48, but I think it certainly has too much pitch for theol' Model A engine. I think Ken Perkins runs a 76 X 42, which allows the engine to run up to the rpm where there is quite a bit more power. My other maininstruction is referenced on this drawing to the book by Eric Clutton called 'Propeller Making for the Amature', available through the EAA. The Hoopman drawing, and the book by Clutton, show how to use an X Y chart to determine theblade angle at each 6" station. His book is the only reference I've ever seen to a True Scimitar Prop design, which I plan on building. Although I stillhaven't found anyone who actually built and tested one, and the theory sounds good, the challenge would be in the design location of the mass. With the blade installed and torqued, the blade track must be well within 1/8". I try to get mine within 1/16". That's how I finish off my bug whackers !! Questions ?Chuck GantzerNX770CGweekend weather is looking like the back side of a front - beautiful flying weather !!________________________________________________________________________________

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Original Posted By: "Steve Eldredge"

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Original Posted By: Galen Hutcheson

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Original Posted By: Rcaprd(at)aol.com
Subject: Re: Pietenpol-List: Prop - Kevlar Tipping and Fiberglass WrapIn a message dated 10/6/04 11:09:04 AM Central Daylight Time, steve(at)byu.edu writes:>Steve,The Kevlar is just a very thin one ply of fibers, and I just laid it right over the leading edge of the blade, and made it about 3/4" wide. I suppose youcould sand away a very slight amount of wood, and make it back up with the Kevlar and Fiberglass. Lay it even on the front and back of the leading edge, where the blades get the most abuse. Blend it in before you put the fiberglasson the front and back, and the transition of the edges of the Kevlar blend right in, and you can't even feel the edges. Chuck G.________________________________________________________________________________