Hey Toni,

 

I'll be glad to...

 

My status... a lot less than I thought I'd be... mainly because I can't seem to find much time lately.  I have just shy of 30 hours into it.  I have the lee hull partially  assembled.  It has the bottom and sides attached with stringers in the corners.  I've also attached the fiberglass strands for the pick-up points of the diagonal beams.  I have the blanks for the rudders completed and one has been filament wound.

 

Design brief of the rudders.  Once my first one cures, I'll be stress testing it.  You might want to wait for those results before using any of this.  Some of this is me experimenting.  The proof will be in the test.

 

Anyway...

(1) I'm using JavaFoil to find the section per Rick suggestions.  I started out using the NACA-07310 that he suggested and found that depending on Reynolds number and aspect ratio some other sections were slightly better.  Only because of the differences in input values assumed and frankly, I'm not sure my fabrication expertise is good enough to build the differences.

 

(2) Using the Cl from JavaFoil, I was able to determine the rudder dimensions I needed to supply the required lateral load (600 lbs) at the minimal drag (13.7 lbs).  The dimensions for MLM are 24"x6"x0.6" using a NACA-0410 foil that has been mirrored fore/aft.

(3) I happen to have a spool of fiberglass tow (50 lbs) so I'm using that... well... because when you have a hammer everything looks like a nail.  (Hope that translates well into other cultures).

 

(4) I made a trivial spread sheet.  I'll put it in my files folder.  All it does is allow you to estimate the amount of tow required.  You can enter some dimensions of the rudder and other properties (these are in blue).  All other fields in black shouldn't be changed and are doing some beam bending and safety factor calculations.  If you have some other tow like carbon lying around, you'll need to put those properties in instead.  I've left the values in the spread sheet as they pertain to my MLM rudders.  The analysis does not account for any stiffness and strength contribution of the wood... its just the glass.  After it cures, we'll see how good the estimates are

 

(5) Filament winding.  Well its not a household word.  But, you may have seen air bottles, propane bottles and such that are made of tow.  The tow (string) is wrapped around a male mold usually in a well controlled pattern to support the expected loads.  Pressure vessels are the most common use of filament winding, but not the only.  Anyway... what I'm doing is about a crude and simple as it comes.  But it has one important advantage... the fibers are far straighter than can be achieve by hand placement and they'll stay that way even after pushing them around and even through consolidation (vacuum bagging). 

 

(6) The rudder blank consist of three layers of 5mm ply bonded together for a total thickness of 0.59 inches.  The two outer plys have a gap in the center (1.35 inches) wide.  The pictures show this gap already covered up with fiberglass. 

 

(7) The filament winding is totally manual (nothing exotic as the name implies).  You'll see that there is a 2x4 frame attached to my workbench.  The rudder blank has a board on each edge to help keep the rudder blank from bending.  Two screws are running through holes in the frame and work bench and screwed into the edges of the rudder blank at its center point.  The rudder blank can spin about these screws.  In the 0004 picture, you can see it in the frame with the rudder blank standing vertical.  You tie the tow on and wind it up.  Simple as that.

 

(8)  I lay the tow on dry and I usually do ten wraps across the width and tie it off with the clamp and epoxy both sides using a brush.  Then do the next set of ten laps. 

 

(9) Picture 0001 shows the rudder blank with fiberglass tow taken out of the workbench frame.  I forgot to mark the time, but I'm guessing it took about an hour. 

 

Summary - Surprisingly, it came out better than I thought it would.  And considering my WAG on some things like percent fiber volume, I surprised how close it came.  I actually got 140 wraps.  Which indicated I'm getting a percent fiber volume slightly better than 60%... which equates to a fiber weight percentage of 78% which seems highly dubious. I'll have to take some samples and measure them. 

 

Also, I did not use any off axis plies.  Rob's engineer pointed out this deficiency.  He is definitely right... its almost always advisable to have off axis plies.  And most things absolutely require it.  If I try to scale up this same technique to my MLM cross beams (12 feet) it will definitely require off axis plies.  However there are some companies making pultruded fiber tubes and bars that have no off axis plies.  Also, I've designed some tubes for satellites with extremely high percentages of unidirectional.  Unfortunately, both those cases had far more expert fabricators than me...  I hope, my testing doesn't make me eat crow! 

 

Dennis