Fiberglass cowl mock up

I just completed the lay up and fitting of the fiberglass mock up cowls, spinner and spinner backing plate. I decided to make some scrap fiberglass parts first so I could set the final trim lines and locate the attach fasteners. I used fiberglass because i could see thru it and measure from the edge of the mating structure underneath the cowl. The final parts will be 2 ply's of 7 oz Graphite cloth. I don't want to scrap those parts!! the material is $32/yd compared to $4.90/yd for glass.

scrap parts

As you can see from this picture.The cowling is translucent and the sub structure was visible. All in all the cowling fit very well. Even so, I will still not drill mating holes in the lower graphite cowl. I will match drill those holes from the upper cowl. The cowl will be fastened with countersunk screws into a nut plate riveted to the lower cowl.

view looking aft

this is a good view of the asymmetrical cowling. i chose to do this because I will flow the air over the single cylinder. The inlet will have some foam blocks added to the inlet lip that will act as a flow diffuser.  The idea is to slow the air down before it turns the corner. Heat added when it flows across the head will recover the pressure loss. its very critical that this cowl be fully sealed. a series of aluminum and silicon baffle plates need to be fabricates. as does an aluminum heat shield for the exhaust. The next step will be to cut out the out let around the tuned pipe.

another view without the spinner

The cowl will be pretty light. The final parts should weigh in at just over a pound.

graphite backing plate

This is the graphite spinner backing plate. although I only used 3 ply's of 7 oz graphite, it is half the weight of the 7 ply fiberglass mock up, yet it is at least 10 times stiffer. I will wet sand the resin line that is visible. this is the joint between the two mold half's. I will chuck this part in my lathe in the 3 jaw chuck to register the center, This will be a check to see if I located the center true enough in the mold.


On another unrelated note, I am now back in Design at my company Triumph/Vought. I am working on a manned/unmanned program based on the Berkut Home built. We are designing a larger version. I had the pleasure last week of meeting Dave Ronnenberg, the designer and builder of the Berkut. I was invited late this week to come out to Santa Monica and fly the Berkut by Dave. I'm sure we will also talk about some structural issues!!!I leave tomorrow morning for Santa Monica via San Diego. My friend Mike and Neal La France are going to pick me up at the Santa Monica airport in a Luscombe Sedan  and fly us back to Gillespie field in San Diego. Neal and Mike are old friends from my early Wichita days. its been years since I have seen them.

Lower Engine cowl, spinner backing plate molds and parts.

Well my traveling is finally over and I am now able to make real progress on my Robin. I have completed the upper cowl mold and at this time I have laid up and pulled a fiberglass mock up part. I need to fit the cowls together and locate the attaching fastener locations and layout the final trims so i can transfer this information back to the cowl molds in the form of scribe lines. I am using one ply of 8oz graphite fabric for each of the upper and lower cowls. There will be a thicker build up at the edge band for greater bearing on the attach fasteners and to match the .080 offset gap at the overlap. At the same time I laid up the mock up cowl, I also made a mold for the spinner backing plate. Again this first part was fiberglass and for the same reasons.

The happy builder

The photo to the left has been doctored for the more sensitive blog readers (I have a T shirt on) normally I work out in my shop in just shorts and shoes. Its been over 100 degrees here every day for 2 weeks. in this picture I was wet sanding the molds with 1200 grit wet paper. This removes the small lines, waves and imperfections that are left from the parting agent. The smoother the mold the easier the parts will release.






Both molds were wet sanded and trimmed. The molds need to be "seasoned" before they are ready for lay up. This means coat after coat of Carnuba based mold release wax. In between every coat the surface is water hardened (spit shined). The reason for this is to create multiple shear planes for the release wax. I will   coat each mold at least 9 times before I lay up in them, because wax and elbow grease is  hell of a lot cheaper than making a new mold.

upper and lower cowl molds

 


Its always a good idea to plan for excess on the mold surface and to incorporate a 90 degree flange like the ones shown. this allows the mold to keep its dimensional stability. I will make a foam supporting frame that will be fiberglassed to the mold shell. This aids in the actual lay up and also adds to mold stability.

There needs to be a spinner backing plate that sits between the prop flange and the prop. Since I want to use countersunk fasteners and nut plates to attach the spinner, I need to build the spinner with about 1/8" edge flange thickness. This means that the backing plate must sit 1/8" inside the inside mold line (inside surface0 of the spinner. I decided to modify the original spinner master model because I could then ensure that the spinner plate would run true and concentric. Nothing is more embarrassing that a wobbling spinner!! So I set the mold back into my lathe. I dialed it in concentric so I has less than .004" run out. I then cut a .125" parallel offset. 

backing plate mold construction

The backing plate mold started by making a centering plug out of Teflon. You can just make out the small white triangle in the center of the mold. A masonite divider plate was fabricated to span the mold. A small v groove was filed so that the centering nib would be exactly at the center line of the parting plane. Gel coat was sprayed on and non directional roving mat was used with polyester tooling resin for the lay up. After the resin cured I removed the divider and then drilled 4 partial holes into the 1st half of the mold. These will act as centering and alignment pins for the opposite half.

Backing plate mold

Here is the final Backing plate mold. I will keep the mold bolted together so the gap never warps or creeps. I made a circular cutting pattern for the fiberglass. I also made a smaller circular pattern out of Masonite that I used as a lay up aid. I set the masonite onto the round cloth and then folded the excess on top of it. I held it in place with another Masonite plate. the idea here is to keep the flange material folded away until I could unfold them and into the mold. This worked quite well. The final graphite backing plate will only be 3 ply's.

Spinner, prop hub and upper cowl

All of these parts are for trial fit up, as you can see the fiberglass is opaque and this allows me to locate the fasteners with correct edge margin from the underlying structure. The hub by the way is for a ground adjustable prop. I plan on using this to dial in the correct pitch needed. I will then fabricate a laminated spruce prop to save the weight.

View thru the inlet

So the cooling method I will use will be to bring the air into a diffuser inlet to slow it down. I will divert a small portion to a sealed filter box for the carb. The rest of the air will slow down and turn 90 degrees to the left. a series of baffles will ensure that this air only passes across the cylinder head and fins. The outlet will be on the opposite side of the cowl thru a hole cut in the r/h cheek cowl. In this exit area I will place 90% of the tuned pipe.
By the way, this blog allows comments from the readers, i would be interested in any feed back. 

Engine cowl part 8 (Mold).

Well I finally got back to work on my poor Robin!!  This work stuff keeps getting in the way!! I had originally been scheduled to travel to Charleston South Carolina for 3 weeks. I am (or was) an MRB engineer specializing on the Boeing 787. My company (Triumph/Vought Aerosystems) designed the aft portion of the 787. They call these sections 47 and 48. 47 is the last pressurized section and 48 is the first unpressurized section. during the design I was the lead engineer for all metal structure in section 48. Separating the sections is a huge composite pressure dome ironically built and designed by EADS the parent company of Airbus. I went to Charleston to help with an issue relating to some detail parts. I was working with a young woman MRB engineer on this same project. We were a day into the job when she got up from my desk and excused herself for a few minutes. She walked around the corner past a partition. I did not see her, but I hear a sound that sounded exactly like someone smashing a pumpkin. The next thing I heard were shouts for an ambulance and when I stood up, I saw her face down on the floor. She had had a mini maul seizure and fell directly and unprotected on to the concrete floor. The poor thing fractured her skull and had to have immediate emergency surgery to stop internal bleeding. Because of this unfortunate incident, I agreed to extend my trip an extra week to cover for Tracy.

Anyway, once I got back I started immediately on the lower  cowl mold. before I left I had the plaster all prepared and waxed in preparation for the fiberglass tool. The surface of the plaster was sprayed with a mixture of 50/50 shellac and alcohol. This mixture will soak into the surface and seal the porous surface of the plaster. after the shellac was applied, I put 7 coats of Carnuba mold release wax. machined buffed between each coat. The final release agent is a product called Partall 10. This is a polyvinyl acetate water soluble film. because it is water soluble and hygroscopic (absorbs moisture) it is applied no more than 24 hours before the application of gel coat.

gel coat gun

This is my newest toy, this is a commercial gel coat gun. The plastic cup is replaceable and is used to mix the gel coat and hardener. It clamps up to the handle assembly and dispenses gel coat through a huge nozzle (.080" dia.) The gun is held upright when it not spraying. Because the cup is disposable, clean up is a snap, although I am reusing my cup.

lower Master with Gel Coat

This is what the master looks like coated with Gel Coat. The gel coat is laid on really heavy, I shoot a surface that is almost too thick to not run. I like a thick gel coat surface because it allows a lot of clean up sanding in areas off the tool surface.

fiberglass mat

After the Gel coat is shot, you wait about 30 minutes until its very tacky. at this point raw resin is brushed onto the surface to ensure a good bond between the gel coat and the fiberglass and mat. I pre cut the mat into9' x 9" squares. The mat is set into the raw resin and then allowed to soak up. additional resin in brushed into the mat and allowed to soak up. I usually work in sections while the mat is soaking up resin. Non direction uni roving mat is a product produced in sheets. They use a starch binder that will dissolve eventually in the laminating resin. after the mat has wetted out, a mat roller is used to drive out the air bubbles.

another view of the curing mold

After resin cures, the mold is released. Becaue the Partall 10 is water soluable, a small crease is created with a plastic wedge and then water is sprayed into the mold. its just a mater of minutes before the Partall dissolves and the mold releases

Engine Cowl Part 6 (Final Plaster Work)

I had hoped to be farther along at this point in the project, we have had some bad weather down here that has caused a bit of a disruption in the build. Either a tornado or straight line winds of 95 mph hit our area and damaged some of my fencing. I had to use some of my free time to straighten and fix this. Then I was sent on another trip to Charleston and thats actually where I am at right now. Before I left last Monday I was able to complete both the upper and lower final splashes. I am using the plaster splash method of making the final mold masters for my cowling. This process is needed because of the desire to build a lower cowl that joggles under the upper cowl. This means that the lower cowl OML (Outside Mold Line) will transition to a negative .080 offset from the master OML surface. In order to do this a female plaster splash of the foam master had to be completed. This is where I left off on part 5. The foam master had the actual trim lines scribed into it so the location of the joggled flange could be determined.

Upper cowl splash

The first splash taken from the female was for the upper cowl. By reversing these molds, low spots become high spots and pin holes become "pimples" That means that the surface quality actually keeps improving as I get closer to the final lay up tool.

This is the upper splash. The next step will be to make the upper cowl lay up mold.


The main reason for doing all of this additional work was to make lay up mold with the flange joggled. Here is a picture of the female splash with the pattern wax laid in to simulate the joggled area.

Female Splash with pattern wax


This is a standard product called pattern wax. It comes in various thicknesses. This particular thickness is .080" I cut two inch wide strips of this wax and set them in such that they extended upward past the lower cowl trim line.









another view of pattern wax

 
This is another view of the pattern wax showing it as it transitions thru the L/H inlet area. Pattern wax can be bought with an adhesive backing or bare. The adhesive has about the same "Tackiness" as a Post it Note, so I sprayed some 3M 777 adhesive in addition to the supplied adhesive.

The pink area on the female splash is a repaired area using bondo.



Lower Splash mounted on MDH Board


This is the lower cowl mounted on a piece of MDH board and ready for the lay up of the actual mold. The splash was mounted with  plaster and an extension was added with plaster to the flange to ensure enough excess tool margin. The raw plaster will be coated with a mixture of Shellac thinned with alcohol . 5 coats of Carnuba mold wax will be applied and then a coat of PVA water soluable release coat.

I will update this blog when I return from Charleston in early June.

Engine cowl Part 5 (plaster work)

The next step in the production of the cowl tooling is to create the master plaster splash. There is only one plaster to use when you make a splash and that is US Gypsum Ultracal 30 molding plaster. I was pretty fortunate to have been able to buy 70 bags or 6300 lbs of plaster at an auction at my companies surplus sales. normally a single bag of plaster is $40/bag. I was able to get all of this plaster and a bale of reinforcing hemp for $160. I learned how to make plaster molds from a old timer at Cessna Aircraft in Wichita Kansas. I would sit out in the factory every lunch hour and watch this craftsman. I eventually came up with an idea to make some vacuum formed covers for accelerometers we were bonding to the outside of the wing of the Citation 3. I was an instrumentation technician back then and it was found that the accelerometers were causing a Mach shock wave  buzz on the ailerons. The solution was to make a little canopy like cover that streamlined the accelerometer. I was able to talk my self into a small apprenticeship with the plaster shop and I made up all of these covers on my lunch hour. It was time well spent. I learned quite a bit. There is a real trick to mixing this plaster, I use a large plastic bowl that doubled as our trick or treat candy bowl.

Ultracal 30 plaster

 An amount of plaster is first scooped into the bowl and then water is added to completely cover the plaster. The plaster will then start bubbling and the internal plaster will start to wet out. more water is added if the surface dries up. Do not stir the plaster until the water stops bubbling. This is very critical for the first layer or "Milk coat" This ensures there will be no dry spots and the coat will be consistent.

mixing bowl

this shows the plaster wetting out.

Foam master

The foam master was shot with 4 coats of automotive urethane top coat over my Friend Eds House in his spray booth. The top coat was then block sanded with an eventual grit of 2500 grit. This removed all traces of any orange peel. I decided to break up the master splash into 3 sections. The upper half would be pulled straight up, while the two side wings on the aft portion of the cowl would be pulled outboard. The reason for this was the undercut area of the L/H air inlet. 

The inlet on this master was purposely made excessively deep. The reason was to allow me the option to fill the cavity with plaster so as to allow a clean back surface and sharp break line.

cowl inlet lip being leveled

The first step in pouring the plaster back wall was to level the cowl inlet lip. This way the back wall will be parallel to the inlet lip. 

inlet lip back wall plaster pour

This is a good image of the plaster back wall after it cured. also notice that the cowl part line has been scribed on the foam master. This method created a nice clean break and limited the depth of the inlet lip in the final lay up tool. After the plaster cures 4 complete coats of carnuba mold wax was applied to the complete surface of the foam master.

divider plate

The next step was to fabricate the upper and lower mold divider. The point that I chose to divide the mold was just above a point that was scribed with a steel square set atop the mold. This point would represent the point the draft reverses and would lock in the top plaster against the foam master. a sheet of coated and tempered 1/8' masonite sheet was used. I used a contour gauge to make the coarse cut and then just continued to sand with my hand sander until the contour was close within an 1/8"

divider plate with masonite gussets

after the divider plate was trimmed, I made three 90 degree gussets. I used some Bondo to attach these to the bottom surface of the plate. The plate was then bonded to the foam master using more Bondo. Because the mold was coated with carnuba mold wax, this bond will not be too strong and will eventually beak off and clean up

Modeling clay

As I said earlier, it is only necessary to get within an 1/8" of the foam master surface. Modeling clay is pressed into the void and a sharp wooden scraper is used to remove the excess.this is a trick that all of the poured counter top guys use, this is why there are no visible part lines in their final part.

Milk Coat being added

This shows the next step, although this is a milk coat being added to the lower part of the cowl, the process is the same for the upper half. I used a sauce ladle and gently poured it on. It takes a number of tries to completely coat the surface,. but a full coat is added and allowed to almost completely cure. One of the huge advantages of using Ultracal plaster is the ability to rejuvenate the plaster if it starts to cure. This I did with the excess plaster that was left over from the Milk Coat. I placed the bowl under the mold to capture any run off, but its still a messy operation. This is why I coated my shop floor with epoxy floor paint and 5 coats of industrial wax. it makes clean up from bondo, epoxy and plaster spills a piece of cake. I will let the stuff cure and then just pop it off with a floor scraper. After the milk coat cures, plaster is mixed and clumps of Hemp are saturated and laid on top of the milk coat. Before I do that however, I will coat the drying Milk coat with a layer of wet plaster to ensure there will be no voids under the milk coat.

Hemp

 Yep, this is Hemp, closely related to the stuff you once smoked in College!!! Its actually a controlled substance that has Federal paperwork tracking it. This is the same stuff George Washington grew and all of the Hippies use to weave clothing. I am using it for reinforcement of the plaster. You work with a single handful at a time and wet it out in the plaster mixing bowl. Hemp is not cheap, a single bale sells for around $160. I got this one at the surplus auction along with the plaster for $168 total!!! I beat out a guy who wanted it for an archery target!!!  Straw works good for that and its a hell of a lot cheaper. In the past I have used Straw, dried Alfalfa and cut and dried grass, they all will work, but not as nicely as hemp.

Index pin holes being added

After the upper splash cured, the next step is to remove the divider plate and drill some indexing pin holes. I started the holes with a spade bit, but then finished them out with a tapered grinding stone. That gave the walls some draft angle. This exposed surface is allowed to dry overnight and then the plaster is coated with 4 coats of carnuba mold wax. Especially the index pin holes. The process is repeated on the lower half and a full splash is made.

attach bolt holes being added

I used a Milwaukee Sawzall  to cut the excess flange off. I then drilled a 1/8" pilot hole through both flanges. I used the pilot hole as a guide to spot face each flange with a 3/4" spade bit. I then drilled a 5/16th through hole. These are for the three bolts that will hold the lower wings on. The opposite site was completed in a similar manner and then both wings extensions were released.

Wing extensions after being released.

These came out perfectly. The surface is very shiny and extremely smooth. The excess material will eventually be cut off and the edge beveled back with a sanding wheel. On the left wing in the picture, I went a little too deep with the counter bore, no big deal. I will use clamps to hold it together during the internal splash.

Main Splash

Here is the main splash and wings assembled. I had to ask my Friend Ed to come by and help me lift off the main splash. It weighs about 175 lbs. It was important to lift straight up because that was the designed release direction. At this stage, there are a few small pin holes in the main splash and one slightly damaged area. Its not a concern. The damaged area will be fixed with bondo and the pin holes will become bumps on the next splashes. They are then easily scraped off. The next step is to add the pattern wax for the lower cowl splash and then lift the lower and upper splash. I will do the upper first and then do the lower splash. From these final splashes I will finally make the fiberglass lay up mold. That's the subject of the next blog entry.

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Engine cowl part 4 (Master Model).

After the Cheek Cowl foam masters were bonded in place, a single ply of 8 oz fiberglass was laid up over the foam cores. light "Milk"coats of Bondo (Automotive Body Filler) are spread over the fiberglass. I use the term 'Milk Coat" for this body filler layer. This refers to a thinned out coat of filler. MEK or Acetone is premixed with the raw filler to a consistency of heavy pancake batter. This flows very well and fills the surface cleanly. here is a little trick I use when mixing Bondo, this is a good excuse not to throw away all of those free phone books. 

Phone Book mixing Palette

Every time you mix bondo you just tear the old page out. An excellent model maker at Johnson Controls in Holland Michigan showed me this trick. So far on the Robin project I have used 1.5 phone books. If you are thinning out the bondo, you make a pool in the middle just like you are spooning Gravy at Thanksgiving!!!

I use professional grade Bondo available only at Automotive supply houses, I stay away from the popular Bondo brand available at places like Auto Zone.

As the surface is sanded smooth, a final coat of Lacquer based 'Spot" putty is squeegeed on to the surface. This is a lower viscosity, air drying product designed to fill coarse sanding scratches and minor depressions. there is a methodology used when filling and sanding the surfaces. The cheek cowl was first coated and sanded flush and flat. Long flexible sanding boards are used for this step. These products are all available at professional Auto body supply houses. My favorite is Sherwin Williams automotive paint stores. Once the cowl and cheek cowl surfaces were sanded flat and wave free, I made a "drag" or radius sweep from scrap plywood with .75" radius. This is also the radius of my foam circular sanding board. Bondo was swept between the two surfaces. A small chisel shaped wood scraper was used after the bondo started to cure to remove away the excess that escaped from the sides. Get to know your Bondo product, you will be able to judge the correct time to sand, scape or surform plane. As I finished sanding the surfaces I would sweep on the spot putty. I usually let this cure for 24 hours before I actually finish sand the putty. As you progress from step to step finer and finer sand paper is used.

Front view of cowl during shaping

Another view at this stage. The L/H cowl has been roughed in and the inlet lip is still square shaped.

Inlet lip "Drag"

This is the technique I used to form the leading edge inlet lip. The tape was added as a guide . Bondo was mixed and swept into position. This is a long and laborious process. It took well over 5 hours of sanding and re sweeping to get this just right. Thank God for Sirius radio in my shop. This is a lot of work, I will admit this, but done correctly the results are beautiful. The anticipation of the final installed cowl, is really what keeps me going. This is truly a labor of love. Thankfully these cowls will be premolded for the plans builder. Not saying you can't do this yourself, but you will spend more money for Bondo and supplies than what I will eventually sell the cowls for.

Inlet during forming

add bondo, sand Bondo...and so on and so on. Until its perfect!!!!

The Master Spot Putty was sanded down with 180 grit. Then it was removed from the fuselage. This was easier than it sounds. It took a while to finally get the master off. I was able to get it removed with very minor damage.The next step was to construct a holding fixture that would hold the master vertical and horizontal. I want the master vertical so I can pour a plaster insert into the L/H cowl inlet. An insert is needed because any "splash" or cast of the master would be die locked in the area of the inlet. There are 4 more steps involved before the final layup mold is built. Because the cowl is designed to split through the cheek cowls, I want to have a joggled flange or "rebate" set into the surface of the lower cowl so the upper cowl sits flush. In order to do this, a master splash will need to be taken of the entire cowl . After the main splash is made, pattern wax of the desired thickness is set onto the master along the predetermined trim lines and another splash is made that reverses the contour. A second splash for both the upper and lower cowl will have to be made, From these splashes, the final lay up mold will be made.

holding fixture

This is the holding fixture I designed and built. The two perpendicular surfaces are designed so I can rotate the master 90 degrees. The top surface of the fixture will bond to the foam inside the master. I used generous scoops of Bondo for this

Fixture rotated 90 degrees.

its important to plan the work height when building fixtures. I will spend hours sanding this tool and econometrics are really important. 

Master installed on frame

The front of the master will need to be closed up. As you recall I set a 1/4" foam spacer behind the spinner master, this was so I could add the 1/16th inch spinner backing plate to close this off and decrease the gap to 1/8th inch between the spinner and the cowl

The next step was to shoot the master with a high solids filling primer. This is a catalyzed product that cures in about 3 hours. Really thick build ups can be accomplished with this product. The main splash will start after this. That is the subject of the next blog.

Engine cowl part 3 (Cheek cowls).

The next step in the construction of the master model was fabrication of the cheek cowl cores. The Robin is a loose copy of the Fournier RF4D Motor glider. This plane was originally powered by a  flat 4 cyl. VW Limbach conversion. as such it had two side cheek cowls that streamlined the cylinders. On the Robin, I am using an upright single cylinder 2 stroke. I decided to retain the cheek cowls because they serve two purposes, they will streamline the carburetor and the air filter and streamline the exhaust. At this point in the construction of the engine cowl, I have completed the main body of the cowl and fared the firewall to the spinner. The main cowl is shaped, a layer of fiberglass was added and a light milk coat of body filler was added. I began construction of the Cheek cowl by laying out the surface loft on my 3D cad system. I plotted the plan view and section cuts space exactly 3" apart. I transferred the loft to some Masonite and cut out all of the templates.      

R/H Cheek cowl Patterns.

        
1/8" Masonite was used for the templates. The original pattern was plotted on printer paper and 3M 777 spray adhesive was used to affix them. do not use water based adhesive for this step, the paper will swell and distort. at some point the contour reverses and the same template is used for the fore and aft position.

Cowl template in position

I trial fitted the template at this point to ensure I had enough excessive flange area. As shown in this view, more tape had to be added above and below the cheek cowl. The actual part will only be a 1 inch flange beyond the cheek cowl, but excessive edge is needed for the building of the lay up tool and plaster splashes.

foam being sanded flush on the belt sander

All of the templates were lightly tacked to the foam with 5 minute epoxy of bondo. just a small dab is all that is needed, because the template is removed later. The foam is rough cut on the band saw and then flush sanded on the belt sander .

templates being bonded

the foam had to be pieced together for the larger templates. a small bond was all that was needed and it was placed such that it was buried inside the final contour.

foam blocks in place on master template

The foam was bonded with a very small dab of bondo to the master template. I want to remove the foam later after it has been shaped. in this view, the plan view off the foam was sanded flush to the template.

Top surface was sanded tangent to the foam blocks

In this view the sanding stick was used with 36 grit paper to sand the top surface tangent to the foam. The surface is sanded until the edge becomes a line.

corners sanded tangent

the same sanding stick was used to sand the corners flush. At this point the foam can be final shaped using a block of scrap foam as the sanding block

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as you can see the results are excellent!!!

Foam shown during final shaping.

By sanding the foam with the foam, both pieces abrade each other. the result is a beautiful spherical interface. This surface is wave free and readily finishes after a layer of fiberglass is applied.

Another view of the final surface

After the foam was shaped I was able to peel the main template off without damaging the foam.

36 grit sandpaper tacked to the cowl

After the cowl was formed, it was necessary yo contour the base so it would fit flush to the engine cowl. I hit on the idea of sanding the contour into the foam cheek cowl cores. It worked like a charm!!!

cheek cowl bonded into position

After the cheek cowl was fit to the main cowl, it was bonded in place. The next step was to lay one layer of 8 oz fiber glass on the foam so I could complete the finishing. this is urethane foam, so any suitable laminating resin can be used. Because of cost and time considerations I used polyester resin. The cure was complete in 45 minutes.

L/H cheek cowl/inlet

The L/H cheek cowl was fabricated similar to the R/H cheek. The main difference is the addition of a cooling air inlet.

the cowl was also sanded into position and later attached.

L/H cowl in position

As I write this update i have completely coated and block sanded the cowls. I have added blend fillets between the cheeks and the main cowls.




The final finish and removal of the cowls is the subject of the next Blog.