I am making a wing for an F-5(s)D. So far I am + or - .005". I used a Blitz1 (?) airfoil kindly given to me by Troy. This , I printed out on a laser printer. I pre-heated my aluminum template stock (.063" 6061-T6 ) to 375 F. I then took a hot iron and transfered the ink to my template stock. I figured if you could do it to balsa, why not aluminum? It works VERY well!

end part 1

.Transfering the laser printed paper to the aluminum worked well. Unfortunately, the printed lines were about .010" thick and so even by splitting the lines when cutting and filing the templates, I figure I am only about + or - .005" of a true airfoil, assuming the printer was accurate. This is about the best I can do and so now there's nothing left to do but set up the templates and do a plaster sweep.

end part 2

I then drilled locating holes and threaded the templates to all-thread rod to locate them. This is/was a standard technique used to make plaster masters in the aerospace industry for decades until CAD/CAM came along. I learned it in a past life working for McDonnell Douglas.

I am now going to attach them to my surface plate ( 1" thick, blanchard ground aluminum tooling plate, surplus from an old Xwing helicopter program from years ago, Bell I believe..) I will then sweep the contour with B-11 tooling plaster. This I will seal with laquer or polyurethane and then vacuum bag my wings right to the released plaster surface.

end part 3

As my surface plate is only + or - .002" and my templates are + or - .005" that stacks the tolerances up to + or - .007". We'll see how much I remember about sweeping plaster, but even on my worst day I'll keep + or - 0.010" O.M.L. over the entire 39"-40" wing span. I would have been laughed out of the tooling department if I did anything that sloppy, but, hey, this is a toy airplane and no one is paying me to take the time to finess it in any closer. This is, after all, 5 sheets of paper worth of thickness.


end part 4

The wing is set up at :
0 dihedral
0 sweep, fore or aft
0 twist

root rib is 128mm and the tip rib is 77mm. The same as a D-99.

Troy. should I put in any twist?

See attached picture of current progress.

try again

Originally posted by Bob K
I will then sweep the contour with B-11 tooling plaster. This I will seal with laquer or polyurethane and then vacuum bag my wings right to the released plaster surface.
end part 3

Very cool technique you have going on there. Ive seen a similar method employed but using male templates & tooling epoxy resin from which female molds were taken. Im interested to see your progress.

What is B-11 tooling plaster? What sort of shrinkage charactersitics does it have relative to mold making urethanes & epoxies. Do you pour it in stages or are there exotherm issues? Are you going to have some sort of wiper arm mechanism to sweep accross the templates or how is the fine tolerance finishing achieved? Have you done molds like this for a composite application? How will you index the u/l mold surfaces usingthis method? Whats a Blitz1 airfoil?

Originally posted by Bob K
Troy. should I put in any twist?

Looking good! I am new to this process but I get the jist. Please take pics of how the process is done when you sweep the plaster, that should be interesting for us all.
I would say no twist. When Bernard did cores for my planes, he did some aerodynamic twists with airfoil tweaking but it had more to do with lift distribution than washout. I've always built my slope wings and previous pylon wings as straight as I could and the only time I've ever had a problem with snapping in a turn was when I had a warped wing, another issue all together.
Ptxman,
The Blitz airfoil is designed by a buddy of mine, Bernard (Francois on the Ezone). He's an understudy of Dr. Mark Drela of MIT and currently an engineer at a place I use to work at doing composites. It's a 6.4% profile thickness with 1.34% camber. Designed for our reynolds numbers and camber mixing.

OK. zero it is.

The B-11 is a US Gypsum product. Look them up on the web and all your plaster questions will be answered.

Surface finish will not be glossy. I am going to just paint any ways. The gloss on the surface will be determined by the sealer coat(s), not the plaster. Yes , it exotherms and I believe it might actually expand a bit. Once againg US Gypsum has the specs.

Bob K

PS. I'm in the impound booth 10-12AM at the MWE on Sunday.

I got some Hydrocal B-11 from Squires-Belt Industries in San Diego. This is what a $29 bag of B-11 looks like. ( 100 pounds)

This is what the right amount of plaster to water looks like. What you do is, to sprinkle the plaster in the water until it sticks up out of the water. You then let is set for about 4 minutes and then mix it up by hand. The official mix ratio is 44 parts of B-11 to 100 parts of water, by weight.

My first layer of plaster had some hemp fibers in it to strengthen the tools up a bit. You can just use some hemp rope and fluff it up and it will work also. Other wise you have to buy a whole bale of hemp. You can also use chopped glass fibers I would think. Heck, you could probably just use the hemp rope straight like re-bar . It's just something to give the plaster some tensile strength.
There's nothing magic to it. Kinda like adding straw to mud and making adobe.

This is the second layer. It's called a scratch coat. I just took a spline ( a screed, a thin straight edge ) that was set to about.050" undersize and screeded some straight plaster onto the tool. Then I took an old saw blade and scratched the surface so the final coat of plaster will stick well. Each one of these layers takes about 25 minutes to set up. This is enough time, but, you have to work at a commercial pace!

Here is the final coat. This is about .050" thick and is screeded with a thin straight edge that I double stick taped to a thicker piece of metal to keep it rigid. The screed has to be no more than about .010" to .020" thick or else it will drag on the plaster and no give you a nice finish. Kinda like a good squeegee has a thin edge but has a back up structure to give it rigidity. Is the final layer. I will now let it set up a day and then start sealing it up with some left over Minwax Polycrylic. ( Because thats what I have! ) Regular old laquer sanding sealer is the standard sealer used in the industry, but, lots of finishes can be used as long as they tolerate some moisture. Other wise you have to really dry out the tool before you can proceed. Now, I am going to lightly sand and dress down the surface a bit and make it "smooth as a baby's butt".

Wow, looks good! You make it look easy. I can barely spline in a nice even coat of Bondo