Emergency rudder laminate schedule; question

[AlanH]

OK, after attending the E-Rudder seminar I decided to bite the proverbial bullet and make a laminated emergency rudder with cassette for Ankle Biter (Santa cruz 27)

I ordered one of those nifty NACA foil blanks from the company that Brian Boschma mentioned. For $40 I got 5 feet 6 inches of symmetrical foil (in two lengths), hot-wire cut out of what looks basically like styrofoam. The foils are *perfect*...I am impressed. The chord length from leading edge to trailing edge is about 11 inches, by the time you count the overlapped fiberglass that sticks out beyond where the foam ends at the trailing edge, the actual rudders chord will be about 12.5 inches. The chord is constant throughout the length of the rudder. I'd say the rudder is about 2/3rd the size of Brians, if you were at the E-Rudder seminar. It's gonna go into a plywood and 'glass cassette. The top of the rudder will have a layer of mat and a layer of roving glassed on it, to add thickness beyond that of the bottom 3/4 of the rudder. That way the bottom of the rudder should slip easily through the cassette, which will hopefully form fit the top 1/4.

I've now epoxy glassed-on two layers of 23 ounce knytex. Knytex is two layers of unidirectional glass fibers, aligned at right angles to one another....both stitched to a layer of mat so that you can apply the whole thing in one go-round. Man, does it soak up the epoxy! crikey. The glass fibers are continuous from one end of the rudder to the other, both layers are single pieces of Knytex wrapped around the leading edge.

ANY-way, I was gonna do two layers of knytex and then two layers of Unidirectional carbon (on each side). The second layer of carbon would go on under wax paper (poor mans peel-ply).

What'cha all think? Is that gonna be beefy enough or should I stick down another layer of knytex before I epoxy on the carbon?

 

[tiger beetle]

I've now epoxy glassed-on two layers of 23 ounce knytex. Knytex is two layers of unidirectional glass fibers, aligned at right angles to one another....both stitched to a layer of mat so that you can apply the whole thing in one go-round. Man, does it soak up the epoxy! crikey. The glass fibers are continuous from one end of the rudder to the other, both layers are single pieces of Knytex wrapped around the leading edge.

ANY-way, I was gonna do two layers of knytex and then two layers of Unidirectional carbon (on each side). The second layer of carbon would go on under wax paper (poor mans peel-ply).

What'cha all think? Is that gonna be beefy enough or should I stick down another layer of knytex before I epoxy on the carbon?

For the peel-ply, get real peel-ply from Svenden's - it's inexpensive, won't leave wax in the layup, and most importantly will stretch and twist to conform with the shape - something wax paper will not do (and then you'll be left with little lumps of and lines of epoxy to sand off).

Talk with Jim Antrim and ask him to prepare a laminate schedule for the blade - he has his own software that can quickly produce the schedule for you. I do not know what he would charge for this work.

The key to the laminate design is the orientation and weight of the fibers. Putting the carbon on the outside of the DBM is good as the additional thickness of the knytex increases the distance from the carbon to the center of the rudder, which significantly increases overall strength of the laminate. Ultimately you are likely to want most of the glass fibers running in the 0 (zero) direction vertically up the blade and some glass set at +-30 or +-45 degrees. If you plan to do any fairing on the final shape (e.g., sanding to clean up variations in the final blade surface), then wrap a layer of 10oz boat cloth over the carbon so you're sanding the boat cloth, not the carbon.

- rob/beetle

 

[AlanH]

For the peel-ply, get real peel-ply from Svenden's -.............. ).

....... Putting the carbon on the outside of the DBM is good as the additional thickness of the knytex increases the distance from the carbon to the center of the rudder, which significantly increases overall strength of the laminate.......... Ultimately you are likely to want most of the glass fibers running in the 0 (zero) direction vertically up the blade and some glass set at +-30 or +-45 degrees. If you plan to do any fairing on the final shape (e.g., sanding to clean up variations in the final blade surface), then wrap a layer of 10oz boat cloth over the carbon so you're sanding the boat cloth, not the carbon.

- rob/beetle

I had enough knytex for a third layer so I went ahead and just did it. I'm gonna go ahead and put on the two layers of uni-carbon. The multi-axial is just too much $$$ at $40 a yard!! However, I *will* go get the peel-ply if it's really cheap...Thanks for the heads-up Rob. I thought it cost a lot. That'll have to wait until next weekend, though.

This thing is turning out heaver than I'd hoped, but with this third layer of knytex, it's really strong.

 

[Jeff Lebesch]

Warning about mixing fiber types! The strength of glass and carbon fibers running in the same direction DO NOT add up. The carbon, being less stretch than glass, will bear most of the load until it breaks, then transfering all of the load to the glass, which will presumably then be overloaded and break, too. Probably best at this point to just use the glass fiber and save your money on the carbon.

 

[AlanH]

Warning about mixing fiber types! The strength of glass and carbon fibers running in the same direction DO NOT add up. The carbon, being less stretch than glass, will bear most of the load until it breaks, then transfering all of the load to the glass, which will presumably then be overloaded and break, too. Probably best at this point to just use the glass fiber and save your money on the carbon.

Well, I had bought the carbon already, and I can't take it back and I don't have any other use for it, so I went ahead and put it on.

The rudder blade is essentially done, but for painting. I think I won't bother putting a layer of 10 oz mat on top and sanding it super-smooth. The thing is really strong, and reasonably fair and a nice fat foil shape. Good enough.

Now on to the cassette, which will be 3 layers of epoxy-soaked doorskins with glass mat in between and formed to the upper part of the rudder foil with a mess of 10 pound dumbell weights.

I gotta finish this thing and move on to other projects, too.

I just bought an etrex, which means that my 1996 vintage 3-channel Garmin 38 GPS handheld moves down to "desperate backup" status. The GPS 12 acquires satellites and fixes a position in less than 3 minutes almost every time but the old 38 sometimes needs ten minutes or more.