Cowl Plaster Splash cont:

When I left off I had just cast poured the nose of the Fiberglass splash. The purpose of this was to make two inlet features that will be added back into the fiberglass splash so a male plaster splash can be taken. The final lay up tool will then be made off of that male plaster splash.

Nose cast

Here is the nose cast. I made a template that gave me the inlet area and shape that I was looking for and traced it on both halves.

cutting the cast

you can see the inlet outline in Red. I knew this would trash my band saw blade, but they are expendable!!!

cut core with draft

I tilted my band saw table 10 degrees so I could get some draft on this cut. After I finished cutting, I sanded the edges smooth. I repeated this for the opposite side.

Cores installed

I placed both cores back in their original position. they self located due to the contour . I used hot glue to bond them in. I then took some modelling clay and formed a 1/2" radius all around the the part. Before I installed the cores, I coated them with a 50/50 mixture of shellac and alcohol. This seals the plaster and allows for easier release. I coated the inside of the splash with a few layers of carnuba  wax. I purposely did not buff out this wax, I wanted it to have a matte finish. If it were polished, the plaster would run off of it in the next step.

surface coat

the next step is to coat the interior with a 'Milk" coat of plaster. Mixing the plaster is critical at this step. there is only one way to do it. you add plaster to the water and wait for all air bubbles to subside. Then you can mix it. This way there are no dry pockets in this coat. The method you use  is to pour the plaster in and then slowly roll the fiberglass splash around coating all of the surfaces. This takes a while because thickness buildup isn't possible until just before the plaster starts to kick. I did this step twice to ensure a nice thick surface skin.

Susan laying the Hemp backing

The surface skin is backed up with a layer of Hemp mat soaked in plaster. My Helper Susan Lassan here is demonstrating this messy step. Susan is an Engineering Student at The University of Texas@ Arlington. Her Father Laszlo works with me at Triumph. Susan is going to be an excellent engineer, because she is trying to understand all aspects of aircraft construction. When she isn't helping me, she is helping another friend Michelle Craig, rebuild her 1946 Taylorcraft wing in my upper shop. I bought two bales of this hemp from surplus at Triumph. Got both bales for $5. The retail cost is around $140 because hemp is a controlled substance.

Released Male Master

        It is chemically very close to Marijuana. I never tried smoking the stuff, but I understand its not the "good stuff"  

Here is what it looks like after its released. I will chisel out the inlet cores and then do a ton of clean up before I start the final fiberglass molds.                                                                                                                                                      

Welcome EAA Experimenter readers  

Unfortunately the editors could not use the lower resolution pictures I originally sent them for the article. These pictures related to the story being told. If you go to this earlier post, you can see the original wing tip master model process.

https://www.blogger.com/blogger.g?blogID=25566109025005725#editor/target=post;postID=4859920072808022827;onPublishedMenu=overviewstats;onClosedMenu=overviewstats;postNum=72;src=postname

Mark

Cowl continued

After the divider plane was constructed, the master was waxed with 7 coats of carnuba wax. Never use silicon based anything around fiberglass. Years ago I destroyed a $10,000 high temp fuselage mold by using silicon wax for a release agent. When you use carnuba, water harden or "spit shine" between each coat. sprinkle a little water and buff out the wax. the idea is to build up multiple release layers. In addition to Carnuba mold release wax, I shot the last wax coat with PVA (Poly vinyl acetate) film. this is a water soluble release film that 100% guarantees a final release. I have months of sanding in this master and the last thing I need is to destroy it with a stuck splash.

PVA release applied

The hot weather here allowed the PVA to fully dry in less than 10 minutes. Thats a record for me. Its really tough to dry this stuff in high humidity and cold temps. This is a fail safe release, if it becomes stuck, all you have to do is crack open the edge and squirt some water in the gap. Capillary action will suck the water to the rest of the mold eventually. I have
 never had a tool stick using PVA.





After the PVA cured I sprayed a coat of gel coat. I spent some money years ago and bought a commercial gel coat gun. Its the only way to go. It uses a disposable paper cup that speeds up clean up. I allowed the gel coat to almost fully cure and then I wiped a bondo fillet radius in the sharp corner at the divider. I need a radius here because the fiberglass mat will not conform to a zero inside radius.

gel coat and bondo fillet

The most important thing to remember when making a mold like this is to ensure the thin gel coat face is supported by the backing fiberglass mat. This is not the final tool, so I can live with small areas where the gel coat may break open. But if this were a final tool, I would immediately lay up a layer of finer glass cloth once the gel coats tacks off.  I would use a open weave like a 285 Crowfoot that complies to compound contours. I would take my time and using a paint brush stipple out the first layer to ensure no air bubbles. But in this case, a single layer of mat is all I need.
The next step is to lay up  the fiberglass mat. I'm using Polyester resin, because its cheap and cures fast. Its suitable for this intermediate splash and my final tool but it must be laid up in at least 3 layers for the final tool. Polyester will continue to creep and move around after cure. Its best to move swiftly through this intermediate steps before it deforms too much. To keep the resin from kicking off to quickly, transfer it to an open pan after mixing. this reduces the exothermic reaction. I like to pour at least 1/2 the resin immediately on the tool. I will mix the resin in batches. In this case it took two batches of 28oz . do not use the same mixing stick when you stir succeeding batches, also use new brushes. the older resin can force the new resin into a premature cure.

lower cowl fiberglass layer

When you use mat you need to use a serrated roller to work out the air bubbles. They work remarkably well too. I let this cure and then the next day using ropes, a come-along and hand winches I flipped the Robin over. I'm sure she was getting dizzy sitting on her head for these last 6 months.

Robin right side up

Its been a while since I have seen her like this. I like the way the spinner will blend into the new cowl. I used the old spinner I built for the old motor. The side profile was designed to blend into that shape so why ruin a good thing?
The same process is again repeated on the upper surface. I will fly thru the next few picture without much explanation

high solids primer

I needed to do some minor sanding on the upper surface. especially in the area of the spinner. when I cut off the old spinner I damaged some of the original surface. The key to success is sand, sand and more sanding. when you get tired of sanding, sand some more!!

Wax and PVA added

 Same as the bottom. Wax and PVA were added.

See the light area in the blend radius. When I looked at the left hand vs the right hand surface I noticed the tangent line of the blend radius wasn't even. This is usually referred to as the highlight line in studio surfacing. So I reduced the blend radius with my flexible sanding rod. I applied wax and PVA then shot gel coat.

Susan and Zach  cleaning the released splash

About 2 hours later the splash was sufficiently cured to release both the upper and the lower cowl. In this picture two young enthusiastic friends of mine Susan Lassan and Zach Strout are cleaning the released cowls. Because we used PVA everything eventually comes off with water.

Before I released both splashes. I trimmed the edges with a Milwaukee saw. I then drilled a bunch of 1/4" holes thru both flanges so they could be reassembled with fasteners.

Zach holding both splashes

here is Zack holding up both splashes. At this point I can remove the old foam master and clean up the engine area.

outside surface

Here is the finished splash.

Casler 1/2 VW

Finally the engine is exposed to the air!!!















Now this is the beginning of the next step to make the air inlets. I am starting this out by taking a plaster splash of the inside of the fiberglass splash. I am using US gypsum Ultracal 30 molding plaster. this id the best stuff to use for a plaster mold. it has a long work life and if you run into a problem, it can be rejuvinated by adding additional water and stirring it up. When you mix plaster, always add plaster to the water and let all air bubble subside. this will ensure you do not have dry spots inside the plaster.

plaster being mixed.

here is the plaster being mixed. I bought 30 90 lb bags of this plaster at the surplus store at my company Triumph Vought a few years ago. I got each bag for 50 cents. they sell for $23 to $30 normally.


















I reassembled the splash leveled the spinner plane and poured plaster into the nose. I will let this cure, split the fiberglass splash and pull out the cast. I will lay out two inlet shapes and cut them out on my band saw. I will set the table with 7 degrees of bevel so the shapes come out tapered. They will be used to develop the  inlet for the next steps.

plaster splash

this is about 1.5 inches thick. this is the thickness of the air inlet




















more in the next blog update!!!

LOWER COWL

When I left off on the cowl, I had flipped the Robin over and  supported her with a false spar attached to the wing spar attach bulkhead. The fiber glass surface of the cowl  was sanded down to knockoff the high spots. A coat of automotive bondo was then swept over the entire surface. The first goal of the final sanding is to eliminate large waves in the surface. Later the goal will be to fill small pin holes and divots.

initial rough sanding

I will sweep in the junction between the cheek cowl and the cowl side. In order to do this I want both surfaces to be relatively wave free. I added additional bondo on both of these surfaces and used my air file board to level them.

new circular drag

I made a larger Circular drag for the lower surfaces. The method I use is to sweep the fillet in 3 stages.

1st sweep

After the first sweep, I let the bondo cure and knock off high spots with a tubular sander with 36 grit paper stuck to it. There is no sense in using anything finer at the initial stages.

2nd sweep

Again, I  let every thing cure and then knocked down the high points with a tubular sanding block. I used flat sanders on the flat areas and tubular sanders for inside fillets and compound contour areas. Never mix the two because you will leave streaks in the bondo.

third sweep

As you can see the fillet is fully formed. I swept a thinned coat of  Bondo over the entire surface. I thinned the Bondo with a product called resin honey. Its actually Styrene monomer. It thins out the Bondo and will fully cure once it is catalyzed. After the entire surface is swept with thinned Bondo, I drop down to 80 grit and continue leveling the surface. I am now paying attention to small depressions and pin holes.

blocked out with 80 grit

I'm getting pretty close to a wave free surface. At this point its time to remove the spinner. The actual cowl needs to close up the area around the crankshaft. The spinner was added initially to aid in the fairing of the foam blocks.

spinner disc installed

I cut a disk of aluminum the same diameter as the spinner base. I also cut a 2" hole in the center that matched the crankshaft prop hub flange. This centers the disk. I used Bondo to bond the disk to the exposed foam in the cowl. I then added Bondo around the edge of the disk to fill in gaps between the disk and the cowl surface.

finished disk

More filling and sanding and the cowl is now ready for its last fill and sand step. I will use a catalyzed spot putty.

spot putty

Spot putty is like bondo but much thinner and consists entirely of hollow micro balloon sphere filler. This makes it much easier to sand and allows me to now drop to 180 grit sanding paper.

spot putty coat

The entire surface is coated and then blocked out using 180 grit paper.

sand able primer

 After sanding the spot putty, I sprayed a coat of high solids filling primer. This is super easy sand-able paint and is used to fill sanding marks. The paint is blocked out with 220 grit paper. After the sanding is finished I used a LED flash light to inspect the complete surface looking for minor imperfections and pin holes. I circled every defect with a sharpie and re coated locally with spot putty

inspected surface

I also struck a level line in preparation for the next step I will add a split plane that will be used to form a flange on the fiber glass splash. I am not making the actual cowl mold here. Normally this step would he done with Ultra-cal Plaster, but that would add over 200 lbs to the fuselage which is already nose heavy. What I am making here is a lift of the OML (Outside Mold Line) surface of the cowl. I will still need to add inlets. That will come after I make the OML splashes.

split plane

This is the beginning of the fabrication of the splash split plane. I was aided by 15 year old Eloi Mathieu. Eloi was a very enthusiastic helper. He is the Brother of Perrine Mathieu who once worked for me at Triumph Aerostructures. Eloi had his first opportunity to run a band saw. It took a few tries but he rapidly got the hang of it and cut out boards with very good precision. Its still not 100% possible to fill the small gaps between the board and cowl surface so the gap is filled in with modelling clay.

modelling clay

This shows the precision of Eloi's cut vs mine. His board is on the left. The lower half of the cowl is now ready for 3 coats of PVA  (Poly Vinyl Acetate) release agent. I need to be able to shoot gel coat within 12 hours of spraying PVA. In high humidity, the PVA will absorb water and swell.

Thats the subject of my next blog entry