More Cowl. Cowl mold.

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On Saturday I swapped labor with my friend Michelle. She needed a new Radiator installed in her Subaru, I needed help doing the lay up on the cowl mold. Michelle used to work for me at Triumph Vought. I was her lead engineer on the Embraer 190.195/175 Rudder and Elevator Program. We both were the first two people assigned to that program. We spent quite a few months together in San Jose Dos Compos Brazil. The home of Embraer and Santos Dumont, the early Brazilian aviation pioneer. Michelle and I did all of the preliminary design and lay out of these control surfaces. I met Michelle when we were both studying to get our Pilots license. I am quite satisfied with my private Pilots license, but Michelle had a much higher calling than me. She wants to be a Mission Pilot for MAF or JAARS. In order to do this she needs a commercial pilots license and an A&P license. She needs tail dragger time and about 500 hours. So at my suggestion she bought a damaged 1946 Taylorcraft and has been working this past year on its restoration. Michelle is one ambitious woman!!

Michelle

This is right at the start of the cowl lay up. We had just finished swapping out her radiator. It was over 100 degrees on Saturday so we started early so we could be in the shade of the shop.


















The tool had been previously prepped for the Gel coat with a coat of Partall 10 PVA release agent. This is a water soluble release agent, and like I have mentioned before, I have never had a part stick when I used PVA

ready for mixing

Michelle's main task was to keep me supplied with mixed resin. Because of the heat, I cannot mix a large batch. It would quickly go into an exothermic reaction and turn to jelly. The mold required 4 28 oz. cups of resin and one 28 oz. cup of gel coat. Gel coat is polyester resin mixed with Calcium Sulfate. This creates a thixotropic mixture that has superior surface hardness. I have a special gun that is designed to shoot gel coat.

shooting Gel Coat

here you can see me and the gel coat gun. I have a fan blowing all the overspray out the main door. the idea here is to get an even but thick coat. Its not too critical if the gel coat starts to run, the surface of the tool is already covered.

initial layer

Once the Gel Coat started to tack off to the point it was about 3/4 cured, I wiped a Bondo filler radius all along the corner between the master and the header board. The following layers of mat and fiberglass cannot lay up into a sharp inside corner. This step is required to ensure that the gel coat has adequate support and will not crack. The first layer down on the fresh gel coat was a five 1.5 oz. layer of cloth. I cut a yard of cloth up into small 6" x 6" squares so they would be easier to handle. I like to use a ply of thin fiberglass against a gel coat surface. this again ensures that the gel coat surface is being supported. After the initial layer is added, I follow up with fiberglass mat. The mat has been held together with a starch binder, before you can move the fibers around, the mat needs to be wetted out. I like to have resin on the surface that will draw up into the mat, but I usually brush on additional and let it absorb the resin. once its wetted out, the mat is rolled with lay up roller to move the fiber and eliminate air bubbles. The rollers work very well.

Finished

I added 3 layers in total. It only takes about 2 hours before the tool is completely cured. I have modified my plan slightly. I was going to release this mold and then move the boards 3 inches to the opposite side, but when I tried to release the tool, the master started to crack on the good side. rather than risk any damage to the master, I have decided to lay up the next tool to abut this one.

opposite side ready to go

I removed the tooling boards and shot the surface with PVA. Its all ready to go. I need to get a helper to do this side. I will update the Blog when I finish the other side.

Cowling continued

I have finished extending the cowl surfaces to the table. This was a ton of mostly useless work. The plaster master was built slightly beyond the trim line, but the cheek cowls may or may not have been made long enough. I figured that I would just do a tangent extension of all the surfaces and then there wouldn't be any question if  the mold was big enough.

Extended surfaces

The surfaces were extended with Bondo. Like I said, a ton of useless work.




The next step was to seal the plaster. A couple of coats of Shellac thinned with Alcohol are brushed onto the surface. The dry plaster absorbs this like a sponge. Its important to seal the plaster so the release agents do not soak into the surface. My friend Duy came out Saturday to help. After the shellac dries a number of coats of Carnauba mold release wax is applied. Between each coat water is sprayed on and power buffed out. This is known as "spit shining" in the military. In reality its called water hardening. It creates a tougher surface finish for the wax and a release plane. I put 4 coats on the mold and hardened each of them. After the wax was applied, a fillet of Bondo was laid at the table base.

Duy adding Shellac

Bondo fillet

This is added so there will not be  any voids in the tight corner. The fiberglass mat will not lay up in a sharp 90 degree corner. The gel coat needs to have a solid backing or the surface will crack out. A void at the surface will cause gel coat cracking. After the fillet was laid, I scribed the master at the cowling trim line. Normally a mold is made such that both halves part at the same plane, but in this case, I want the upper and the lower mold to overlap each other by 1 1/2 " Both molds will have the same transferred scribe line for the final trim. I need to make an overlap flange for the lower cowl. In order to do this I want to lay up the actual part in the lower cowl mold. I will let that cure then I will put down a layer of silicon based peel ply and then lay up the overlap flange on the inside of the actual cowl. Since the cowl mold overlap the trim by 1 1/2" the overlay flange will be in the correct position. I will then trim the lower cowl and then bond on the overlap flange. This saves me 4 more intermediate tooling steps.
The next step is to construct the parting plane. I will cover this step with a little more detail because the clay trick is really neat and I want to show the detail of how to do it.

This is the beginning of the mold plane. I am using 1/8" tempered Masonite. I bought a sheet on Saturday and was planning to leave it in the back of my truck until I needed it on Sunday. On Sunday afternoon I went to retrieve it and it was gone!! I thought I was losing my mind too!!! Then I realized it probably vibrated out of the back of the camper shell on my truck. So I went to buy another one and sure enough, I found my original sheet busted up alongside the road out in the boonies. Because its not nice to "Mess with Texas" I cleaned it up and added it to my burn pile. Tempered Masonite has a nice smooth finish. The surface is still porous, so it too needs to be wax sealed. I rough cut the Masonite and you can see a few gaps between the Master and the template These are easily filled with modeling clay. the Template is attached to the master with small daps of Bondo. This is why the master was waxed with Carnauba wax.

Modeling clay

You can get this clay at any crafts store of fiberglass supply shop. being very generous, roll up some pieces and stuff it into the seal all along the edges of the template boards. Overfill all of the Gaps.




In order for the clay to stay in the gap, the back side of the board must have a tongue depressor hot glued to cover the gap.

Gaps covered

here you can see the tongue depressor covering a gap and the daps of Bondo holding on the board.












After the gaps are filled make a wooden scraper from a tongue depressor. sand the edge square and then add a sharp bevel.

Beveled tongue depressor

you can buy Tongue depressors by the carton from Aircraft spruce. Its cheaper that stealing them from the doctors office given the co-pays these days!!!
















After the gaps are filled its time to scrape away the excess clay. Take your time and slowly work into the clay lump. Use the template to keep the scraper flat and square to the surface, The results are very   clean and work quite well.

Before and After

This picture shows the clay before (upper) and after scraping (lower)

filled gap

this shows how well a large gap can be filled and why its not critical that the fit up be exact between the master and the template. The next step is to wax the template with Carnauba and then spray PVA release agent on the template and the plaster. I will cover that step and the mold lay up in the next update. I have to order the PVA so it may be a few days.

More Cowl Plaster master

When I left off I had just released the Plaster male master from the female plastic splash. The male master had inlet inserts incorporated into it. I removed the inserts and exposed the inlet cavity. This really worked quite well, but I did not like the inside junction near the spinner. So I modified the master by sweeping bondo with a larger circular drag. After a ton of sanding, this is what the inlet now looks like.

inlet

You can see the larger circular fillets I added on the inside with the blue colored Bondo. I set the master on top of my welding table. It has a 1/4" thick flat steel plate for a surface. I coated this surface with carnuba wax and then made a wooden support and using Bondo, I temporarily attached the plaster to the table. I set the spinner plane level, I need to set up a set of waterlines and stations that I can use to scribe the trim line into the plaster. This will transfer to the tool as a positive resin line, which will then transfer to the final part as a negative scribe mark. 

The surface finish of the plaster is perfect. But if it gets scratched, its not the end of the world, scratches will become small resin ridges in the lay up mold, I usually wet sand the molds with 400 grit paper, this will knock off any resin ridges. 

There is quite a gap between the master model and the table. I want to use this space to extend the surfaces. Its always a good idea to have excess tool surfaces that are set beyond the final trim line. In a production environment these would be used for the vacuum bag seal surface. 

Here is a side profile with the old spinner in place. I like the lines!!!  This should end up looking pretty good. The spinner was designed originally for the MZ-34 engine. This engine was a much more compact design and placed the spinner much closer to the firewall. Since I didn't want to remake the spinner and the backing plate lay up tools, I elected to stay with the original. That presented a real challenge when I reset the upper surface contour. It turned out ok, but it basically determined the center line of the crankshaft for this new engine mount position. The real problem was the lower lines blending into the existing fuselage lines. If I had stayed with the original fuselage line, the cowl would have had an inverted curve and looked terrible. So I re-faired the fuselage by creating two false longeron lines and increasing the depth of the cowl. The results however speak for themselves. 

foam block extension surfaces

So here is where I'm at right now. I blocked up the gap at the base with my floral foam.  I will saw and sand the foam until the surface is at or slightly below the extension surface. I will then sweep some plaster over the foam to stabilize it and then follow up with a final sweep of Bondo to finish the surface. Its pretty tough right now getting a lot of work done, in the late afternoon its been over 95 degrees and the heat index is over 106. So I probably wont get this next step finished until this week end when I can get out to the shop early before the heat builds. Texas is a wonderful place to live, 9 months out of the year!!!  From late June until early September however, it's hotter than hell!!!