Welcome to my Robin Blog.

It was suggested to me that I start a Blog on my ultralight project the "Robin". I have been working on this project for 4 years. On one of my first days at Vought aircraft, a stress man and future friend named Kenny Andersen walked up to me and said, "Aren't you the Mark Calder that designed the Wren Ultralight" Why yes I am I said. "well what have you done lately?" That was the genesis of the Robin design. The first 2.5 have been spent in the design phase. Actual construction started 1.5 years ago and has actually progressed smoothly. There have been a number of changes from the onset, but for the most part it is following my original concept. I will eventually sell plans for the Robin and make available all molded parts, fittings and welded assemblies. The Robin is designed to FAA part 103 and as such requires no pilots license to fly, although I think its a good idea to actually learn how to fly!! The actual name "Robin" was my Daughter Jamie's idea, I asked her to name the design based on my "cute little bird" theme (Wren)

Every good aircraft design has a "Mission" in mind before the actual design is started. A good designer will refer back to this mission every time a design decision must be made. Good design after all is just a series of good design decisions. On my first Ultralight design the Wren, the mission was to design a high performance low powered aircraft. The reduction of drag was the prime concern. I had been flying powered Hang gliders prior to this and because of this experience, I placed a high priority on climb performance. While most designers chose bigger engines, I chose lower drag and high aspect ratio (low span loading) wings. The Wren could out climb conventional Ultralight with up to 65 hp. The Robin follows this philosophy, but tries to improve on the performance of the Wren. Ultralight are not built by "rich" people, they offer an inexpensive means to enjoy one of the greatest experiences of my life, low speed soaring and flying.

Design Concept

The cost of an aircraft is directly proportional to its weight. , if low drag can be achieved then lighter and cheaper engines can be used. The Robin expands on the design mission of the Wren by using a longer span (40') wing and using a low speed laminar flow airfoil, (Wortmann FX 170) The leading edge of the wing on the prototype is molded fiber glass. The spar has been placed at 33% of the wing chord because the chosen airfoil is laminar over the first 32%. The aft covering is light weight Dacron Fabric. The leading edge of this fabric is purposely pinked and placed at the 32% chord point to facilitate laminar transition and elimination of separation bubbles. The main difference between the original design of the Robin and the current final design is the elimination of the single mono wheel retractable landing gear. Part 103 does not allow for a retractable landing gear. Which is really unfortunate because I spent a long time designing a really neat mechanism!!

In the course of the 4 years I have worked on the Robin, the structural design concept has evolved radically. Originally I was going to draw on the design of the Wren and use essential the same construction concepts. The original design of the Wren was heavily influenced by my Friend Steve Wood's Sky Pup design. I lived in Wichita Kansas and worked at Cessna Aircraft along with Steve. I watched his progress on the Pup and was very impressed with his concepts. I adapted the concept of using Styrofoam sheeting as the shear panels for the fuselage and the wing ribs. I did not however use the foam for the shear webs of the wing as Steve did. I originally wanted to build the fuselage of the Robin in a similar manner. Weight and the desire to not use foam for the basic structure due to the danger of fuel leaking eventually drove me to a all wood fuselage design. The wings were designed to take advantage of the Graphlite carbon pultruded material pioneered for the experimental aircraft by Jim Marske. I was familiar with this product from my experience at Bell Helicopter where it was considered in the construction of the V-22 wing.

More Cowl. Cowl mold.

I cannot believe how many Russian subscribers I now have!!!  Welcome!
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!!

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.

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.


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!!!

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.



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!!!


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