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.

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

New Casler Cowl Cont:

I am writing this update from my hotel room in San Jose Dos Compos Brazil. This is my last day here. This was a quick trip, 3 days. I came down here for a design review on the EMB 175 elevator. I am the design lead for the Embraer 190/175 Rudder and Elevator. This was our last day and when we got ready to go into Embraer, we got a phone call telling us the place is on strike. This is the 5th time I have been shut out of Embraer. 

Anyway, I made significant progress on the cowl since the last update. I am almost finished with the shaping of the upper half of the cowl. I will start where I left off.

lower template installed
I completed the side view template on the lower portion. I had to disconnect and re-position the lower fabric stringer on the fuselage. Unfortunately, I will also have to build new fabric stringer stand offs at the side of the fuselage. The fuselage lines will not blend with the new engine position. 
The next step is to set the plan view templates. 

plan view template r/h side

plan view template l/h side
Before I could set these these templates I had to add a temporary fabric surface on the fuselage. this is the surface that the cowl will sit on so I needed to mock it up. The  shape is determined by the clearance envelope around the engine. I am holding everything in place with hot glue. 

The shape was set a little long so I could trim the cowl to size the outlet area. 

The next step is to block up the upper cowl with foam blocks. there are two separate shapes that need to be formed, the first is the transition surface between the spinner and the fuselage, the second is the shape of the cheek cowl. The two surfaces will eventually transition to each other with a 2" radius. 

upper blocking

the blocks are fitted as close as possible and attached with as little hot glue as needed. In all cases, I try to attach at the lower surface well away of the final contour. Sometimes that's not possible and the glue has to be dug out before foam shaping.

once the blocks are in place I used a hand saw and butcher knife to rough carve the shape.

rough carved
 you can start to see the first shape appear. I will use a hand sander and sand along the vertical template horizontally to establish the side profile. Once this profile is established, and the foam is rough carved. Then the true magic happens with this method. The foam is used to sand the foam. Both materials will abrade each other at the same rate creating a spherical interface.

this is the result.

foam sanded
 You can now see the smooth transition between the spinner and the fuselage contour. The next step is to block up the cheek cowls. I ran out of the grey foam so I bought a different brand for the cheek cowls. As long as I don't mix and I  use the same foam to sand itself, its not an issue. I will jump to the image of the final foam sanded cheek cowls.

Add caption
now the shape of the upper cowl is apparent. The next step is to lay one ply of light fiberglass cloth on top of the foam. This will allow for the final surface finishing with Automotive Bondo. I have decided that I need to make a full mold for this cowl, Anything less will be heavy and ugly. I will work on the upper half of the surface until its ready to be splashed (initial mold) then I need to flip the whole plane over so I can work on the under side. that will be a real job!!!

with a layer of fiberglass
I put a layer of aluminum tape on the spinner and the fuselage so I could lay the fiberglass on it. I put some mold release wax on that tape so it could be removed later. The fiberglass is draped over the transition nicely, but it still has too small of a transition radius. I used polyester resin for this step so I could move on to the Bondo step later in the afternoon. Before I added the Bondo, I block sanded all high spots and generally roughed up the fiberglass so the Bondo would adhere.

bondo surface and circular drag
This shows the first coat of Bondo and the circular drag I made out of Aluminum to sweep in the transition radius. Now its a matter of sand, sand and more sanding until the surface is perfect. Then I will flip it over.

I will be back to Texas tomorrow Morning and I will have all week end to work on the project.

see you at the next update

                    Engine mount Installation cont:

I completed the in-situ design of the engine mount. At this point in the project, I am designing as I go and not laying out anything on the computer first. I do check loads and stress however!  The addition of another motor other than the original MZ 34 is less than optimum. I never designed for these engines so their installation is not extremely clean and light. I know I could do a lot better and make a cleaner installation from scratch. I increased the area of the 4 longerons  that terminated at the firewall. I attached 4) 4130 steel angles to the ends of these longerons. The main issue with this method of attachment is the lack of a load path for vertical shear. I polled my designers at work and Dave Magerstadt, an excellent designer and former ESA treasurer,  came up with a solution!
shear intercostal
I bonded a 3/8" plywood intercostal to the firewall. Before I did I added 4 fiberglass doublers to the old firewall. I fabricated 4 fittings that picked up the 1/4" engine mount bolt and shear tied into the wood intercostal.this will react the vertical shear that the motor weight  x 6.2 g will induce.

This is another view of the firewall. I have already added the ceramic firewall that will protect the fiberglass firewall bulkhead. Later the ceramic and  the wood intercostal will be coated with Contengo. an ablative latex coating designed to char rather than burn.

iso view of firewall
I fabricated 4 fittings from Aluminum that tie the ends of the intercostal to the single 1/4 engine mount bolt. The loads are low, about 287 lbs per fitting.  The next two pictures show the Contengo coating and a view of the engine mount installed with the shear fittings.

the white coating is Contengo,

shear fitting
this shows clearly the load path I am developing. If I did not add these intercostals, the vertical shear would be reacted by differential bending of the upper and lower fitting. This is a recipe for disaster, an inaccurate load path invites unpredicted failures in the basic joint. Thanks to Dave's Idea, I have a good load path reaction.



engine mount
After I welded the Mount and primed it with Zinc Phosphate I asked my buddy Ed Gardner of MFR Inc to top coat it.


Ed is the Picasso of Pistols, the Rembrandt of Remingtons the Wyeth of Winchesters.... You get the idea. Visit Ed site and you will see some beautiful work!! Some of it is mine too!! I asked Ed to topcoat the mount with Urethane paint. While he was at it, he top coated my friend Michelle Craig s 1946 Talyorcraft wing fittings and the cowl of the 1984 Tiafum Motorglider I am repairing for Dan Stofman.

Unfortunately the engine mount was also a compromise. I had to add two .032 gussets to react the non optimal truss. I would have liked to have terminated the truss member into the the other tubes cluster, but I had to revise the tube angle to clear the intake manifold. This creates a local overturning moment that needs to be reacted by a gusset plate. Not efficient, but again, all the load paths are accounted for.

Engine mounted
Well the good news is the plane didn't fall over on its nose when I released the overhead straps. The plane was tail heavy with the light weight MZ 34. That will be remedied with this engine!! Its 50 lbs heavier and sits a full 12" fwd of the old MZ 34's position. This means that I have to build another new cowl. I have decided not to make a full mold for this cowl since this is a prototype. I do not want to invest the time and money making a mold for a one off cowl. My plan is to sculpt the floral foam and then cover the foam with squares of aluminum foil attached with spray glue. I will then wax the surface and lay up on top of the foil. I will fill, fair and sand the cowl until the finish is acceptable and then remove it from the foam form. I will split it while it is on the mold. To start the process, after work today I stopped at the local Michaels hobby store. I'm usually the only guy in the store and all of the woman think I'm a gay florist when I buy out the floral foam.

all of the foam from the store
This is all the foam from the Mansfield TX. Michaels store. Thats the trunk of my new Scion FRS by the way!!!

I usually confirm the clerks doubts and tell them I have a "Big Wedding"

So the first step in developing a proper cowl shape is to lay out a Masonite template of the side view and plan view or in nautical terms, "Max Half Breadth" My desire is to use the existing spinner and backing plate. I have molds for both of those parts. So I need to develop lines that blend from the spinner to the fuselage lines. This was pretty easy to do on the upper loft.

upper cowl spline template
this is good clean line. Much longer than the original Robin Cowl with the MZ 34, but sleeker. not a bad start. I will semi permanently attach this template on Butt line Zero. The lower template proved to be a bigger problem. The drain sump and the lower case caused a reverse inflection of the lower line. I use a piece of 1/8" welding rod as a spline strip. I had to cut loose the lower fabric stand off on the fuselage to achieve a smoother spline. The following picture shows the final result.

lower spline template
 I still need to develop the plan view templates, but once finished. I will cover the engine in Aluminum foil to protect it and then block in the spaces with floral foam. That is  when the real fun begins and I will get to express my hidden artistic talents!! I will have more as I complete this mold. Stay tuned!!