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Vacuum Bagging Cowl Parts
Rear Engine Top Cowl
Note: You wouldn’t know it by subscribing to this blog, but I have actually been continually working on things in the Garaggio. My frustrations over the failures I was experiencing with vacuum bagging and how demotivating that was made it difficult enough to keep working on projects much less write volumes of blogs over failed attempts. Mixed in over this time, we had a condition inspection on Betty, a few workshop organization projects, finishing my welding table, and small bits of future Legacy projects. So rest assured I am still very actively working on the Legacy, Betty, and mostly Sid (Brian’s Defiant).
Why vacuum bag if it has been such a hurtle?
The light weight, lofted, high quality, molded carbon airframe of Brian’s Defiant was a major reason I wanted to become its caretaker. Those traits made me excited about the project and inspired to continue in the same vein. The vacuum bagging process was integral to achieving the quality and light weight results that became a hallmark of Brian’s work.
Putting a part in a bag and sucking the air out of it seems like it should be straight forward and relatively simple. For me, it has been a huge, humbling hurtle.
Being strong willed (Mom doesn’t like me to say stubborn because it always begets the question, ‘from whom I got that trait?’ and lest I get in trouble, remember ‘whom’ is both singular and plural), I wasn’t willing to give up and just stick with wet layup parts. I figure, Brian’s Defiant should have better.
Vacuum bagging is a skill I want in my repertoire. It is a superior way to make composite parts. I have spent a significant amount of time on youtube, consulting with experts who have been generous with their time, looking over catalogues and technical data sheets, and reading all I can about vacuum bagging. With all this, I consider myself to have more than average technical competentence with the theory. But significantly lacking in practical experience.
If you didn’t buy any of that, I started down the vacuum bagging path with this project and it kicked my butt. There is no way my strong-willed self is going to let vacuum bagging get the better of me.
Just how many failures does it take?
I’m reminded of TV commercials from my childhood with an animated owl asking,
“How many licks does it take to get to the center of a Tootsie Roll Pop? 1, 2, 10. The world will never know. “
It probably would have been better for me not to know. But with my (im)patience, I was counting. I had 13 failures before I had what I would call success. I won’t list them all off, but the picture below shows 4 of the eventual 9 identical induction inlets I made. All of which were failures. Some of them are useable parts, with enough finishing work. But a vacuum-bagged, molded part should have a finish much, much better than this.
What was the cause of the failures?
It took me a while to figure it out. While each had their own nuance, the root cause of all the failures was one thing, poor quality molds. My molds were virtually incapable of sealing without further work. Even if I did a perfect job of applying a leak-free bag to the mold, I was going to have leaks through the mold. Leaks, that in some cases, the vacuum pumps couldn’t overcome. Sure, I also had the occasional poor technique when sealing a bag that caused a massive leak. The majority of leaks I experienced were in the molds themselves.
The aft engine lower cowl mold was made in two pieces and has a seam down the middle, that requires sealing in preparation for a vacuum bag. That one is easy to understand having a massive leak. But all the molds, even my tiny little inlet mold, leaked. Why?
- I made all of these molds in the hottest part of summer. Usually single-handed, I was working against a quickly curing pot of epoxy in 100+ degree temperatures.
- To add insult to poor ideas, I also used a minimal number of plies. Because of how much work and time making a mold requires, I wanted to use only the amount of material that was absolutely required to make what I figured would be an effective, single-use mold.
- Additionally, I didn’t have the proper tools. I should have had bubble busters (serrated rollers) to remove air bubbles from chopped strand mat that was a large percentage of the thickness of the laminate.
- The little inlet mold had epoxy-sealed plywood as the sealing surface. Even with saturating it with epoxy, plywood is porous and a poor choice of material for a mold intended for vacuum bagging.
If you are a composite person, you are grinning at my folly and could have predicted the outcomes. End result is I ended up with molds made of thin laminates, tons of pin holes, and didn’t set myself up for a high probability of success.
How do we avoid or fix these problems?
The answer to that is somewhat simple. Make the best quality molds you can, no matter if they are one-use molds or intended to pull 100 parts. That may not be as actionable of advice as you want, but it is basic and good advice. Don’t do a mold layup in 110 degrees, with epoxy, using minimal mold thickness. Maybe plan to do your mold making in a multiple layup process, or in an environmentally controlled space so you can lower temperature. Whatever you do, try to ensure that you plan a mold that minimizes the possibility of leaks. This could be,
- using tooling gel coat
- making sure you have a thicker mold laminate that minimizes the possibility of pin hole leaks
- having the proper tools so that you can minimize air bubbles in thicker plies such as chopped strand mat
- Using more modern materials such as tooling dough if you are laying up a mold
If you end up with a mold you suspect has leaks, there are ways to possibly fix or work around that. They are band-aids, but they may salvage the mold you made. They did for me.
- You can always add layups to the exterior of the mold. A secondary layup may seal pin holes.
- You can possibly put the entire mold “inside the bag.” This essentially eliminates the requirement for the laminate surface of the mold to be air tight. The challenge for me with this was not only size of the mold, but also that many mold features would likely tear a bag.
- Paint the exterior of the mold with multiple coats of flex-seal. This is what I ended up doing.
My first success.
I have paraphrased quite a bit of the last few months of vacuum bagging trials and tribulations. This has been quite the learning curve for me. But it has been worth it. My first vacuum bagging success was the fiberglass tool cowl from the aft engine upper cowl mold. Far from perfect, but the best I’d done to this point.
The exuberance I felt when we achieved vacuum on this part, literally had me hopping and hollering. I felt validated for not giving up on the process. When I saw that we were able to draw 25″ of vacuum on this part and that I was reasonably certain that we were going to end up with the expected results, I was elated.
I want to give credit to Greg and Sebastian who were integral parts of doing this layup. It took the three of us 3.5 hours to wet out the fiberglass, put on the bagging consumables, and draw the vacuum. We worked out a very comfortable and predictable method and set of roles to make this layup and vacuum bag a success. I was pumped.
The next day we pulled the part from the mold and I was even more excited. We had fantastic surface quality, good laminate compaction, and what was to be expected from a vacuum bagged part.
Seems over-zealous to be so excited about a sacrificial part meant only to be a template or a tool. But it did allow us to fit the cowl to the airplane, trim mating edges, and come up with trim lines using lasers, sharpies, and chalk lines that will represent the aft edge of the cowl. Additionally, we were able to locate an oil check and servicing door as well as cleco holes that will allow us to precisely locate future copies of the cowl to the fuselage with minimal fuss.
Once we had the tool cowl representative of what we wanted the final version to be, we put the cowl back in the mold and used a sharpened scribe to trace the perimeter and cleco holes into the mold. These scribe lines will transfer to subsequent parts and make any copies of this mold repeatable. The oil check door location made a reference for us to apply sheet wax that made a joggle in the mold. This joggle will become the sealing surface and flange for the oil check door which will be a separate part.
Greg, Sebastian, and I did a rinse and repeat of the molding and vacuum bagging process. The fiberglass tool cowl was a great dress rehearsal that we just needed to mimic. This layup we used the carbon fiber that we were planning on using for the final part.
If I was elated with the tool cowl, but when we pulled this carbon flying part from the mold I was jumping for joy. Literally. I was literally jumping up and down and pumping my fists. We achieved vacuum. We controlled the pull down of the vacuum to have good consumable placement. We did the layup in good time.
When we pulled the part from the mold, the surface was as expected, not perfect but very good. We had excellent compaction of the layers. We had a good oil servicing door joggle. The scribe lines transferred.The cowl was appropriately stiff. And surprisingly, it was amazingly light. I knew it would be light, but it was surprsing how light it was. I haven’t weighed it yet, but I can’t wait to see where it comes in.
Has it been worth it?
Unequivocally, yes. It certainly is. Now. I may not have said that 6 weeks ago, but I think it has been time well spent. I put the above word success in italics, if you noticed, because I didn’t achieve leak free vacuum and there is improvement that can be made through further learning. But we have been able to achieve at least the 95% result of a perfect vacuum bagged cowl. With my experience level with this method, that is more than I had hoped for.
November 2nd was the fifth anniversary of Betty’s first flight. So in celebration we had to take Betty flying, of course. As luck would have it, Jon was here on a layover to celebrate. Jon helped me on Betty all the way back to the early days, I think even the first year I was working on Betty. He was quite the appropriate passenger for the sunset flight.
In addition, I did put Jon to work, you know… for old times sake. We trimmed the front cowl around the new exhaust pipes and started mocking up what the new fairings around the exhaust and augmenter will look like. More to come on that.
I ended up a bit behind on blogging while Greg was here. By the time we were done with the day’s activities and I ensured my slave labor, I mean volunteer help, was well fed, I just didn’t have it in me to write up a post. So in an effort to document what we got done and catch you all up on the status of the Defiant, here is a quick summary and most importantly some pictures.
- Canard Mounting
- Elevator Actuation
- Betty Check Out
You’ve seen the explanation of what the exhaust scheme is for each engine. Front engine has a 2 pipe exhaust system with an augmenter. The rear engine has a 4 into 1 exhaust system with augmenter. I called in my custom exhaust guy to do his magic. It took him a full two days to get both engines done, and it is exactly what I wanted.
After the exhaust was done, Greg and I moved on to mounting the canard. With the exception of the canard lift tabs being an extremely tight fit between the firewall and the canard mounting bulkhead, it was a mostly straight forward task. The only real challenge was that Brian had already drilled the lift tabs so we had to match drill the fuselage side of those. If they would have been line bored as an assembly it would have been much easier. Of course we measured sweep, vertical position, incidence, and roll many many times before drilling. With digital levels, lasers, and tape measure we were able to get things to within .1 degree on the digital level and approximately 1/16″ for sweep and vertical position. Surely accurate enough for an airplane designed in the bubble level days.
Once we got the canard mounted we also started working on the elevator actuation. As I got them from Brian, they were built long and needed to be trimmed to fit. That was the first task. Once we could install the elevators we started working on actuation. This included carving an arc shaped hole for the elevator actuation to pass through the side of the fuselage. Once we had that done, we could mock up the elevator components and check to ensure we got full travel.
Getting full travel of the elevators proved to be a challenge. There were two factors, or adjustments, that made it challenging. The defiant elevator torque tubes have a disc installed at the root end which locates the pushrod attachment. The rotational orientation of this disc is not easily measured to a high degree of accuracy on the plans nor the actual elevator. A small angle change of that disc seemed to have a large impact on the elevator travel. So before we drilled and riveted it in, we 5 minute epoxied it into the tube to test travels. I am glad we did as we ended up changing its orientation multiple times.
The other adjustment that was elusive until we dug deeper into the plans and arm-rests was the attachment of the push tube on the stick in the cockpit. There are 3 holes to choose from on the stick and 3 on the elevator torque tube bellcrank. Since we didn’t build the stick, Greg and I spent some time figuring out how to easily disassemble and move the torque tube to the lowest hole on the stick for the most linear displacement. Lastly, we also had to remove the stick grips as they were also limiting travel.
Little by little, we found each of these challenges and adjustments and eventually we were able to get full displacement of the elevators. It was kind of tedious and frustrating at times, but the elevators are essentially actuated now. I do have to order proper sized hardware and remake 2 of the push tubes as they ended up slightly too short. But with it mocked up, there shouldn’t be any surprises there.
When Greg offered to come and help, our big goal was to get the Canard mounted. To get the elevators actuated as well was a big plus.
Lastly, its taken 5 years to do it, but we finally got Greg to fly Betty. He has gone for rides in the back, but he finally was able to fly as PIC in Betty. He has tons of Long Ez time, so it really was a non-event for him, but Greg is the first person I’ve let fly Betty. We did a flight with me in the back so I could answer any avionics related questions etc, then he went out and did a solo flight. After his many years of helping me with projects, Betty especially, I am happy he is now checked out and can fly Betty anytime he wants to.
On short notice, I took a week off of working on the Defiant project. A friend of mine was coming to Phoenix to do a commercial glider add-on, so I invited myself along on the adventure. It was about a week of really fun flying. I highly recommend it.
After my soaring sabbatical, it was time to get back to work on Brian’s Defiant. As luck would have it, I had a friend decide to come to Phoenix to help work on the airplane. For those of you who followed along with Betty’s build, you will recognize Greg.
Greg is a friend of mine from St. Paul, MN and a Long Ez builder/owner/pilot. His Long EZ has been flying for 32 years. For the last several years of building Betty, Greg came over at least one day a week and many times two or three times a week to help me get Betty flying. His building experience and dedication are a big part of the reason why Betty is flying.
On the Defiant, we left off with the lower cowl mold halves laid up. When Greg got here, we started by building a wood structure around the mold halves. This not only gave it extra rigidity, but also allowed us a couple bolt holes to repeatably re-align the two halves together.
Then it was time to release the molds from the plug. Unfortunately, this didn’t go as smoothly as I would have liked. We ended up pulling quite a bit of the primer off the plug when they separated. Good news is there was a good coat of PVA on the plug so the primer came off the mold easily. I am not quite sure why the primer came off, but at this point no harm, no foul.
After a bit of clean up, we did the usual wax and then clay along the center seam. In addition to that, there was one area where we put 1/16″ sheet wax. The sheet wax is there to create a joggle, or offset. This will allow the top cowl to come down over top of the aft flanges of the bottom cowl and will make an integral mounting flange in the initial layup. It is hard to see in the photos, but it is the rectangular yellow opaque area on the aft vertical surface. Then it was 5 coats of PVA.
Since I am still trying to learn resin infusion and vacuum bagging, we decided to try resin infusion with the first part from this mold which will be a fiberglass tool cowl. Greg and I took our time cutting 1 ply of deck cloth and 4 plies of BID. These were arranged dry into the mold. Then it was all of the infusion consumables including peel ply, flow media, and then bagging film.
Unfortunately, the best we could get after hours of trying to fix our setup was 10″ of HG vacuum. We had major leaks. We tried everything we could think of. We went around the bag perimeter multiple times looking for leaks. We painted the outside of the mold with flex seal thinking there may be a dry spot with pin holes. We trimmed the centerline flange for access and bagged the exterior of the centerline flange/mold split. We put tacky tape on suspect areas on the outside. We put tacky tape along the sharp edges of the flow media. We added bags on top of the bag. We got a stethoscope out and listened for leaks. We blew smoke at the mold looking to see if we could find where the leak was.
None of that was successful. So we took the bag off the mold. Our plan was then to do a wet layup and try one new bag over top of that as a wet layup vacuum bag. We didn’t expect it to perform much better than the first bag (unless it was in fact the flow media that punctured the bag), but 10″ was at least some kind of vacuum and would give some of the benefits of bagging to the part. But this round of bagging went sideways too. The seal was non-existent and didn’t pull any vacuum on the part.
To make matters worse, I was frustrated enough with the time spent and lack of fixing the problem that I convinced myself to take a shortcut with the wet layup. The mold was already packed with our 5 plies very nicely. I was concerned about getting the plies out and back in the mold without distorting the fabric to the point where we would end up short of material in some dimension. So I reasoned if I could wet out a 5 ply stack on the table reasonably well, we could do that in the mold. (BTW, never do this with a flying part… it is very bad technique, heavy, and you cannot ensure the each layer of the laminate is wet out appropriately.)
As all of you have done composites work know, this was a major mistake. It was not only probably more time consuming in the end, but ended up producing an abysmal part. There are dry areas. There are inter-laminate bubbles. There are places where the cloth didn’t lay against the mold and it is not representative of the shape. etc.
There are 3 good things to come out of that mistake. The first is that the perimeter of the part is wet out sufficiently well to use the tool cowl as intended. It will allow me to precisely trim to fit the firewall and wing roots, etc and transfer these features to the mold. The second good thing is it will give us a part to use to ensure things like the exhaust are located inside the cowl with plenty of spacing, etc. Lastly, I was able to remove the part from the mold without splitting the two halves. The fiberglass tool cowl is a bit more flexible than the final carbon cowl will be, but I think I could get away with bonding the two halves of the mold together and still get a part out. This will remove one major area that is likely the major cause of our leaks.
All in all there was a lot of learning that went on with this part, mostly what not to do. I will end up with a good part out of this mold eventually, but for now we are going to switch gears to something else.
The lower cowl geometry doesn’t lend itself well to making it a single piece mold. The centerline scoop/tunnel/boat tail has geometry that would “capture” the plug, and subsequent parts, in the mold if it was laid up as a single piece. You can still do a single piece mold, but there are some draw backs.
If I did a single piece mold, I would have to break apart the plug to get it out and will no longer have it useable if we wanted to redo the mold or modify our shape. Also, you don’t know how the mold surface turned out until the plug is out. So you are destroying the plug and committing to making repairs on the mold surface if there are problems with the mold. Additionally, you are relying upon the final part that is laid up in the mold to be flexible enough that you can get it out of the mold without having to cut it into pieces or destroy the mold. For these reasons, it is a safer approach to make the lower cowl mold in two pieces.
In order to accomplish this, the only thing I needed to do was add a tooling flange down the centerline of the part. Of course, that means jigging the flange to our fully prepared surface. In order to protect the finish, I laid down a layer of vinyl tape first, then bondoed the tool flange on. You could also use hot glue for this.
From here, the rest of the molding process is mostly the same as what you’ve already seen me describe in previous posts. Tooling flanges, wax, PVA, and layup. Once you have one half of the mold done and cured, you can remove the centerline tooling flange backer board form. The completed mold half is left in place, and the centerline flange on the existing mold becomes the tooling flange form for the second side. This way they exactly match.
On the second half of the mold, I want a joggle or channel to facilitate sealing the centerline of the mold when assembled. This channel will allow me to put a rubber weather strip in between the halves and hopefully get an air tight seal. I wanted a 1/8″ deep channel. Turns out that my tacky tape was the perfect easy thing on hand to make the form for the sealing channel. I adhered tacky tape to the centerline flange, then I put vinyl tape over the top of the tacky tape as a releasable surface. This should give me a 1/8″ channel or depression in the second side of the mold to facilitate sealing the molds to each other. Again, the mold making process was the same as for the first half. Tooling flange forms, waxing, PVA and layup.
One thing you will notice is that one of the mold halves appears white, the other black. A few weeks ago, Dave Anders told me about a mold making technique he learned from Dave Ronnenburg, the designer of the Berkut. Dave uses essentially an “aircraft structure” approach to making molds where he has laminate-core-laminate. Dave uses micro for his core. This makes for very light, inexpensive mold structure. So on one side I tried that approach (white micro visible), the other side I did the woven cloth and chopped strand mat (see through the glass to black primer). I did this to see if I like micro core better and check if it saves time.
It turns out that both mold halves took me within 15 minutes of each other. The micro core mold took a large amount of micro-balloons, but was much less epoxy than the chopped strand mat required. Because of my temperatures, I didn’t dare mix up a large batch of micro, so I did it in 5 or 6 smaller batches. This certainly added some time to the process. I also have learned some better technique for chopped strand mat and how much epoxy it requires.
On these mold halves, I don’t see a clear winner between the two methods. However, if I could do a single mix (or maybe 2) of micro to get what I need to for the whole mold, I think it would be significantly faster and lighter to do a micro core. I can’t wait to get them off the plug and see if one is stiffer than the other.
Vacuum bagging is a skill I have yet to master, as was proven when we attempted bagging the old design upper tool cowl. so why not take it one step further and try to learn vacuum bagging and vacuum resin infusion all at the same time? I like to take big bites of the elephant, so why not.
There are some advantages to resin infusion. 1) it’s less messy and sticky. 2) it is more repeatable. 3) you have all the time in the world to arrange, cut, and straighten the plies of the layup schedule before going on the clock when you mix the epoxy. 4) you test the vacuum bag before mixing epoxy and if it doesn’t seal, you can replace the bag or start over with little lost but some bagging film. 5) you don’t have to wet each layer out individually.
There are also some drawbacks, the biggest of which to me is the steeper learning curve. Infusion also requires an infusion resin which is very watery and not suitable for a wet layup. So you will need another resin system/kit. Also you need some additional consumables like flow media to make it work.
Those pros and cons being said, I am hoping to infuse my final cowl parts and get a really good surface finish. Therefore I need to learn this new technique. I also was getting a little bored of working on the cowl plugs so I decided to spend the morning yesterday taking my first shot at resin infusion with my induction inlet mold.
Prep was the same as for any layup. Five coats of wax, five coats of PVA, cut cloth, cut consumables the consumables were peel ply, flow media, and bagging film.
For the cloth, I wanted one ply glass, two carbon, and a ply of glass. With the circular nature of the part, it is hard to get flat cloth to lay into the mold nicely. So I ordered some glass and carbon fiber sleeve. It is really neat stuff, almost like a Chinese finger trap. It is woven in such a way that you can expand or shrink its diameter within certain ranges. It really makes slick work of tubular or circular parts like this. You can find it at composites supply shops online such as Composite Envisions.
I then stuffed the mold with the glass, carbon, glass, peel ply, and flow media. From there it was tacky tape around the perimeter and make, pleat, and seal the bagging film. On one side there is a vacuum line, and on the other is the resin supply line clamped off.
To my surprise, it actually pulled pretty good vacuum. About 26” HG. I was pretty satisfied. So we mixed the Proset INF resin. We dropped the supply tube into the cup of epoxy and opened the valve. Low and behold the resin started traveling up and infusing the part. It was very fun to see.
Of course about that time we also started having leaks in the stretchelon bagging film. I think the cut ends of the flow media was puncturing the bag. It took me about an hour, and half a roll of tacky tape to find the leaks, but eventually I got the bag to seal again. It looked horrendous and I don’t suggest resealing this way.
By the end of my workshop session, the resin was cured and I de-bagged and released the part. Which wasn’t easy, especially with all the tacky tape on the bag itself.
There is certainly room for improvement in the part and my technique. The surface finish is not very good with one small area where the cloth bridged and many pin holes. I know it can be better, and I will keep practicing until I get it there. This part would be useable with a few hours sanding time and some finishing work. It is a strong and light part. But I should be able to make them where they come out of the mold with little to no requirement to do any secondary processing to use them. If I am going to infuse the cowls, I certainly need to be capable of better surface finish too.
All in all, it was a very educational day in the Garaggio, and taking the first steps in the infusion learning curve will get us that much closer to being able to use it as part of our regular bag of tricks.
The last few days, when I have been looking at the rear cowl, I have thought that the trailing edge shape reminded me of something, but I couldn’t figure out what. Turns out, it is the character Sid from the cartoon movie, Ice Age. While I am fairly certain this destines Brian’s Defiant to henceforth be named Sid, I couldn’t help but share my revelation. It gave me a pretty good chuckle.
The last few days in the Garaggio have been pretty exciting. Since Jodi’s working visit, I did add one more tooling flange just forward of the cowl to firewall intersection. On the first version of the cowl, it was challenging getting the plies of glass to end reasonably well and have the bag sealing tacky tape all in a 2″ area. The 4″ wide flange will be great to separate the plies of cloth and bagging materials and give more room to work. It will also make the mold dimensionally stable.
From there, on Wednesday and Thursday morning it was all the usual steps to ensure the mold releases from the plug. I rubbed it down with 5 coats of Part All mold release wax. I may or may not have mentioned it before, but I really hate waxing. It really isn’t that hard, and it certainly signifies that we are ready to make a major step in progress. But somehow, it always takes me way longer to do than it should, and its a lack of enthusiasm causing it. Never the less, a full 5 coats was put on the plug and tooling flanges. Spraying PVA Thursday morning was a breeze and pleasure compared to the waxing. With both done, the plug was ready for a mold layup.
This time, I was a bit smarter than last. I waited until Kevin had the time to come help. Good thing I did as this layup took a good 4 hours with the two of us. We did make things a bit easier on ourselves by using the “extra slow” hardener. I used West System Epoxy as it is cheap, I had a ton of it on hand, and it is available locally. A lot of you will ask again why not polyester, and I haven’t slowed down long enough on this project to give it a try and learn how to use it. I will eventually as I have some.
The layup schedule for the mold was:
- 1 ply deck cloth
- 2 plies uni/bid
- 1 ply chopped strand mat
- 1 ply deck cloth
Usually, I would use bid for the whole second and third plies. However, I had a ton of “scrap” uni laying around that I wanted to use up. My scrap pieces were 30 and 15 inches wide and 5 feet or so long. These pieces were folded and old, so not suitable for a flying part or anything truly structural. They worked fine, but bid is easier to work with. The bid wets out easier too.
Of course, we had some of the same issues with wetting out the chopped strand mat. It is a pain in the butt to work with, takes a lot of time to wet out with Epoxy, and is prone to air bubbles that are difficult to get out. Even so, it did build up the thickness, and knowing how thirsty it was, I could have Kevin mix up more epoxy each round than I did in the first top cowl mold.
I did make an error of omission during the layup. I forgot one of the plies of BID/UNI from the original layup schedule. So the mold ended up a bit thinner than desired. Surprisingly it is actually pretty rigid with the whole perimeter tooling flange. If this were a production mold that I was intending on pulling many parts from, I would go back and add more stiffness by adding plies. However this mold will pull just a few parts, so it will be fine as is.
Today, I released the mold from the plug. It was again surprisingly easy to do. I am so impressed with the mold release wax and PVA combination. It is a bit more labor intensive than I would like, but it sure does a great job of releasing. It took no more than 30 minutes to have the mold off.
From there I cleaned it up. That involved getting rid of stray clay and a thorough wash to get rid of the PVA. Cleaned, I then started on the path of polishing the surface. It was pretty good as it came off the plug, but I did do a round of each 400, 800, and 1000 grit sand paper on the DA. I think that should do it.
I likely will add a wooden structure around the mold for two reasons. One, it will help with a bit of rigidity since I missed a ply in the original layup schedule. The other thing is the wood structure will give me feet for the mold. Right now to put this mold on saw horses, I have to rest the mold on areas that will be a part of the final cowl. This would slightly deform the cowl shape, which is obviously not right. A wood structure will allow the shape to be unaffected. Other than that, the top cowl mold is ready to be used.
Early next week I will be prepping to make a top tool cowl out of fiberglass. I debated going right to the final carbon cowl, but there are a few advantages to making one out of fiberglass first. I should also be able to get a mold made for the lower cowl. It is significantly smaller than the top cowl, and should be simpler to make.
Some of the materials used to make the mold described in this Post:
The below images are amazon links, click on them to learn more.
On Monday, I got to the point in contour sanding and surfacing on the cowl where I was struggling to find flaws that needed to be fixed. Don’t get me wrong, there were still minor flaws, but most of them small enough that I would be liable to go backwards instead of making improvements if I added filler. With the cowl all in black primer and polished to 800 grit, it looks pretty darn good.
Next up is preparing to make the mold. To my fortune, and her chagrin, one of our Reno ‘September Family’, Jodi, got her flight cancelled in Phoenix. That means she got to spend some time in the Garaggio, and I got a workshop helper.
This mold making prep is the same as what we did for the first version of the cowl mold. Though this time a touch easier. We made poster board templates of the tooling flanges and cut them out of backer board. The backer board was jigged to the airplane using bondo blobs on the back side. Then the gaps were filled with clay and a nice radius formed in the corners to facilitate the glass flowing from cowl surface to tooling flange. Pretty simple, though takes a little time to get a nice fit on the tooling flanges and nice radius with the clay. Was nice having Jodi to help where more than 2 hands was nice, and to get the filleting done quickly. Thanks Jodi.
Last week Thursday, before our trip to Kanab, I got the layup on the lower aft cowl done. What an exercise in frustration. I have done quite a few layups in my building history, but this one made me feel like it was the first time I had ever seen epoxy or cloth.
The plan was to break the lower cowl into four sections, each with their own plies of glass. There would be left and right above the wing & left and right below the wing. Each of these sections would have some overlap. The above the wing pieces are easy. No so much with below the wing. Long story short, this may have worked had I had 6 pairs of hands, but not by myself.
Trying to get pre-wetout fiberglass plies on plastic to stick to the bottom surface of the cowl plug, all the wile working against gravity is a loosing battle with only 2 hands. I was using vacuum bagging film as my plastic backer for wetting out the glass on the table, which made things worse too. It is really nice smooth plastic, but it is heavier than you would think. So the additional weight of this plastic made each ply heavy enough it was impossible to get it to stick to the cowl plug. Oh yea, it is also 100 plus degrees out and the epoxy is kicking off quick enough that each piece was sticky to the point of difficult to manipulate before I could get a ply in place.
I tried everything I could think of to get it to work, and eventually came to the conclusion that I had to not only cut the plies into smaller pieces, but also get rid of the plastic to get it to stick in place. This made it really difficult to get the plies on in any kind of organized manner or with any concern for ply orientation, fiber straightness, etc. It was certainly NOT an aerospace grade layup. Luckily, this only needs to be a tool.
I eventually succumbed to the fact that the layup wasn’t going the way I wanted and it was best to piecemeal it. So I took about 1 foot square pieces of cloth, stuck them in place with overlap, and stippled on epoxy. It was messy, sticky, time consuming, and aggravating. It was not pretty, but it got the job done. And a fair bit of epoxy all over me.
By the time that layup was done, I was glad to be heading to Kanab for the weekend. I needed the time away from glass work, some R and R, and some cool flying. We know how that turned out. Instead I got more glass work, some R and R (just not the R and R I wanted), and a little bit of the cool flying.
Since getting home from Kanab, it has been more of the same process you saw for the top cowl on the rest of the aft cowl surface. Contour sand the fiberglass. Add filler. Sand. Repeat. Once it is to the stage where you think you have the shape, start spraying with primer. Sand. Find flaws, pinholes, etc. Fill Sand repeat. And then get Joe-ed.
While I was sanding on the lower cowl, I was thinking about how I was going to make a mold with a wing root into it etc. Any way I sliced the problem, I figured it was going to be a complex mold with multiple pieces. You certainly can do multi-piece molds, but it adds all kinds of challenges for vacuum bagging etc. So I took a step back and asked, why am I parting the cowl where I am? The simple answer is, thats where the original cowl parting line was. I was just doing the same thing Brian did.
Turns out that there would be significant advantages to moving the cowl seam to the plane of the top surface of the wing. One, there is a significant improvement in serviceability of the engine just with the top cowl off. You can get at just about anything you need. Two, it makes the molds easier. Top cowl can be one mold, so can bottom cowl. Three, it moves the cowl seam to a less obtrusive location for improved aesthetics. The downside is I have to make a new mold for the top cowl. But If the lower cowl was going to be 3 pieces, I am actually making 1 less mold than with the original plan.
So this week I glued top TCSN2 (tool cowl serial number 2) to the bottom TCSN1 and jigged it back on the airplane. I have the full shape now in primer, and for the first time looking very representative of the final parts. I am liking it more and more. A little more polishing and pin hole filling and we will be ready for tooling flanges and making molds.
To me, this cowl project has taken a long time. I am ready to move on to other parts of the project. I am ready to be able to claim victory over a part. In actuality, at just a bit over 5 weeks, its moved at a pretty good clip. I just can’t wait to have the satisfaction and MVP (major visual progress) of having actual finished cowl parts. That will be a lot of fun to see.