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.

Cowling access doors

once the cowling was fitted together , I swept a bit of Micro-Balloon and epoxy into any of the gaps. The upper cowl was waxed so the Micro only stuck to the lower cowl. The final hole diameters were opened up and countersunk washers were bonded on. The washers were first sandblasted on the bond surface. The cowl was sanded and then cleaned with isopropyl alcohol. The washers were bonded with T-88 structural adhesive.
Sand Blasted Washers
These washers are available from Aircraft Spruce. They solve the problem of the fastener sawing thru the fiberglass. They sit slightly proud. I taped up every one so the final finish will be polished stainless steel.

When I put the cowling halves together, I needed an accurate way to trim the inlet flange. I wanted a nice planer trim. As I mentioned before the neatest way to trim fiberglass and graphite laminate is to use a grout cutter and a multipurpose vibrating tool. I set up a MDH wood surface by bonding it to the inside of the cowl with Bondo. I then set the tool flat on this surface and cut away the inside. It was perfect! Nice and even and planar.

MDH trim planes

This shows the cowl with the MDH plates bonded in . The grout cutter sits flat on the MDH surface.

Final Cowl inlet trim

The final trim.

access door layout

The next step was to make two access doors for the oil dip stick and the oil filler cap. This broke my heart, having to cut out these panels on a surface I worked so hard to get perfect. But it beats the hell out of pulling the cowl every preflight to check the oil. The section of the cowl I cut out will become the access door. Before I do that I need to transfer these outlines to the inner surface. Once I marked the inner surface, I applied 5 coats of Carnauba mold release wax to the inner surface (IML Inside Mold Line) . I then cut 4 ply's of glass and laid them up centered on the outline.

door doublers laid up

After 2 days of curing (its getting cooler in Texas) the doublers were ready to be released. They were separated and set aside. Later they will be marked for trim after the doors are cut out of the upper cowl.

beginning of the cut

I used a .030dia drill to start the cut. Once I connected the holes, I was able to insert a small air operated scroll saw. I took my time, and cut out both doors. Once the doors were free, I sanded the edges even and trimmed back the hinge side for clearance.

raw cut

when I laid out the door, I purposely set the hinge side on a contour element line (straight line) so there would be no binding of the hinge. I used a MS aircraft hinge and using countersunk Cherry N rivets, I attached the hinge temporarily using Clecos.

Door with Hinge

The next step is to mark the backing doublers for trim.

Bonder door doubler

the doublers were trimmed and the edge beveled 20:1. This reduces the peel forces on the edge off the doubler. I used t-88 adhesive and Cleco clamps and bonded it together.

fairing in the doors
The final step was to get the surface back to the original smoothness. I waxed the door and then assembled them I dragged a coat of micro into the small gap between the cowl and the door. This micro will fill any gaps and remain with the cowl and doubler.

I have made more progress that I am saving for the next update. I am currently setting up the engine for its first run. I need to install a pressure gage and oil pressure sender. I need to rewire everything and set up new throttle and choke cables. I just ordered a new Prop from CSG in Canada.

Cowling Baffles and engine installation

I continued working on the Baffles. I finally finished those up.
This shows the detail at the L/H cylinder. I used .020 7075-T6 aluminum sheet for all of these baffles. That's not a magic alloy, its what I had laying around. It does tend to crack pretty easy, but I guess that's what a stop drill is for.

Support is where you find it!! I made a bracket that picked up one of the case bolts and used it to support the upper baffle. I used clip nuts here so the baffles could be removable.

This shows the detail on the R/H cylinder. Note the cut outs for the exhaust. All around the edge of these baffle plates a blade of silicon rubber will be added. this will make a flexible seal between the baffle and the inside surface of the cowl.

This picture shows the silicon seal added to the baffles. I needed to make a retainer plate for the silicon. This served two purposes. It clamped up the silicon seal and it created a stiffener that keeps the baffles from bending away. I made the retainer by bending some aluminum over a form block.

This is the MDF Form block. note the bend relief cuts all along the edge near an outside curve.  When the metal is formed over it compresses along the edge and causes a wrinkle, the slits allow the metal to move and makes it much less prove to wrinkling.

After I finished with the metal work, I made a template for the final trim of the cowl. There is a fly weight that is attached to the aft face of the prop flange that needs to have clearance to the cowl. The old trim was just a rough cut any way.

finally I added countersunk washers to the main cowl fasteners. These will spread the fastener load out over a greater area and allow for a flush countersunk head. I covered the show face in tape and then sand blasted the back side. I countersunk the cowl fiberglass, sanded a small area under the washer and then bonded the washers to the fiberglass. I'm using a cleco with a small washer to clamp the washer during cure.

Engine exhaust and cowl internal baffles.

The next step in getting this engine ready was to fabricate new exhaust pipes. The twin pipes that came with my Casler engine were 90 degree pipes designed to stick out the bottom of the engine. That's pretty crude in my mind and a 1.5' diameter tube creates a lot of drag sitting 6 inches behind the prop in the prop wash. I needed these pipes to snake around inside the cowl and exit on the outlets of the side cheek cowls. I started the project by ordering some pre bent tubes from Columbia Mandrel bending. They did a really good job. I ordered some 180, 90 and 45 degree sections along with a straight section of tubing.

raw tubing from Columbia Mandrel bending
this took some time and a lot of cutting and fitting. the pipes were a 3 dimensional bend. I had to route the pipes under the push rod covers and along side the block. This required multiple bends.

Here are pictures of the L/H and R/H pipes

Left hand Pipe

Right Hand Pipe

the cowl was buttoned up and the clearances checked once again.

nice tight fit. I have good clearance to the fiberglass cowl. I will wrap these pipes in heat resistant insulating tape.

After the pipes were fabricated, I started n the internal cowl baffles. The inlet air has to be forced thru the cooling fins. I started fabrication with a cardboard mock up.

This was time consuming. the shapes are just approximate at this stage of the mock up. have a needle contour gage that is very handy in creating offset curves. The baffle plates will be made from .020" 7075-T6 aluminum sheet. the seal edges will be lined with silicon rubber baffle seal. That will be pop riveted to the aluminum sheet.

here is the lower baffle with the cut out mocked up for the exhaust pipes. you can see the small bent flanges that seal the bottom of the cylinder fins.

I made the first one and them promptly bent the flanges in the wrong direction. Oh well practice makes perfect. I should have the baffles all wrapped up by this coming week end. After that, I can do the final trim and finish on the cowl, I will also install the final nut plates and then prime the cowl once more. The next step will be to finish the engine installation and then get a prop and try and start it!!!


I had just laid up the Spinner and Spinner backing plate. I had to wait 72 hours for my resin system to cure sufficiently to be able to trim and sand it. That is very important!! I mounted the spinner backing plate on my Lathe. I then made a self centering coupler for the tip of the Spinner. I used this coupling to mount the spinner to the arbor. After a little tweaking I was able to dial the spinner in with less than .004" of run out.

cutting prop hub hole
this is a live shot of the running lathe. I am cutting the prop hub centering hole. I mounted the backing plate between two wooden plates. The hole in the plate was larger than the center shaft. This allowed me to adjust the outside run out with a dial indicator. once it was perfect, I drilled 4 holes and bolted the plate to the wood backing plate. this locked it in for good. Once the backing plate was spinning true, I set the spinner in position.

mounted on engine
Unfortunately I don't have any pictures of this procedure. but the idea was to get it true and mounted then drill 4 holes for a 3/16" cleco and locate the parts. Here its shown mounted on the engine.

side view
 here is a side view. I couldn't locate the cowl until I had the spinner mounted. The top surface needs to fair cleanly into the spinner loft.

3/4 view
this is my favorite view. The inlet is still rough trimmed, and the cowl has not been sanded. You can see the witness line where the upper cowl mold was attached to the lower cowl mold.

that's it for now.


I prepped the lower cowl for layup by applying 7 coats of Carnauba mold release wax. Followed by a buildup of Rexall Partall 10 PVA release film. Since the lower cowl needs a surface extension into the upper cowl, that mold was also recoated.

upper and lower molds joined

After both molds are joined together I shot more primer onto the surfaces. You want to be ready to do the layup immediately after the primer flashes off. This will give you the best mechanical and chemical bond to the resin.
The seam between the molds is stuffed with modeling clay.

modeling clay
The clay is pushed into the seam and then gently scraped off with a sharpened tongue depressor. nothing will stick to the clay because its an oil base clay. This will keep the laminate near this joint from becoming porous and it makes a smooth transition between the mold halves.

final layup

As I mentioned earlier in this blog, I need to make an inner flange that will be secondarily bonded to the lower cowl and form a flange that the upper cowl will sit on. In order to do this, I need to lay up the lower cowl at least 2 inches beyond its normal trim. Remember the actual parting line between both cowls is etched into the upper cowl surface. This is the reason I needed to join the upper cowl.  Here is an image of the initial lay up. the surface is covered with polyester fabric, the same fabric I cover the plane with. In this case its functioning as Peel ply. I like to lay a final layer of Peel ply on my wet lay-ups especially at any ply splices. The peel ply beautifully transitions these ply's together. The other reason is in the area of the overlap flange, I want a smooth surface with no rough ply edges.

Its HOT today in Texas, humid too. after I finished the lay up, my tee shirt was soaked clean thru. I actually wrung a cup of sweat out of this thing!!! I can hardly wait for fall. The high temp keeps dropping one degree per day. By mid October, it will be wonderful again in Texas!!!

multi tool trimmer

I released the upper cowl and here is what is left. Even in this high heat, I needed to let the laminate cure for 48 hours. Its important that the resin become fully cured almost to the point of being brittle. The tool I like using for trimming is a vibrating multi tool. I use a grout cleaner as a cutter. There is carbide grit bonded to a steel disk. This will wear thru beautifully and with very good control. The actual final trim of the cowl is one inch below the trim that you see.

inner flange being released
Here you can see the secondary flange lay up I did on top of the original cowl lay up. I laid 7 layers of wax on the inside of the cowl before I laid up the inner flange. Here I am parting the inner flange from the cowl. once I have released the inner flange, I will re trim the cowl an inch lower at the scribed trim line.

more inner flange

here is another view of the inner flange. You can see that it will lock in and "key" perfectly to the inside surface (IML) of the cowl.

bonding the inner flange

Here is the inner flange being bonded to the IML of the cowl. you want to make sure you use an actual structural adhesive here and not thickened laminating resin. . Structural adhesives all have an elastomeric polymer added to the matrix. This dramatically increases the peel strength of the bond. This will keep the flange from peeling away from the cowl. I used T-88 structural adhesive.

Trial Fit
Time for a trial fit up. I joined both halves and temporarily attached them with Clecos. Eventually I will replace each Clecos with a bonded nut plate. Before I can actually set the position of the cowl, I need to mount the spinner and backing plate. The cowl must sit with a 3/16" gap between the outer surfaces and the spinner. the surfaces must all be true and faired. So before I can finish mounting the cowl, I had to lay up a new spinner and prop backing plate. Fortunately I already have the molds.

Another view
The cowl I about 6 inches too long, this is why I made the extension surfaces on the master model. I wanted to be sure I had more than enough excess.

Backing plate mold
here is the backing plate mold. This will get 3 ply's of .014" thick Graphite. The Lathe will be used to true it up and trim it.

Spinner mold laid up

here is the spinner laid up in its mold. the white fabric is peel ply. This is a very hard mold to release because you cant start a edge peel. But I coated the mold with Wax and PVA. Once cured, I will submerge this mold overnight in water and hopefully it will float out after the PVA dissolves.

Backing plate being trimmed

I made two wooden backing plates and clamped the backing plate between them. The hole in the backing plate is much larger than the center bolt. This is so I can use a dial run out gage and adjust the position of the backing plate until it runs true.

More in a few days or so!!!!

First part

I finished laying up the upper cowl. I have a neat trick that works very well that I will share. The process of contact wet lay up, that is so familiar with the home built airplane crowd, is by its nature, very porous. If you were to lay up directly on a mold, and then shoot a coat of primer, the surface would be covered with thousands of pin holes. These are the result of the paint being wicked into the laminate due to capillary action. your next task would be to spend hours filling and fairing with either more primer or with spot putty. Very time consuming. So the way you avoid this is to shoot the mold with your primer BEFORE you do the lay up. Here are some pictures of the lay up steps.

Adding trim scribe
In this picture I am scribing the trim line. The trim line has reversed during the molding process from a low to a high. I need to scribe it into the mold surface before I wet sand the mold to finish the final surface. If I didn't scribe it, I would lose it in that step. The mold was wet sanded with 200 grit paper to remove all surface imperfections. The mold was then seasoned with 7 coats of carnauba paste wax.

PVA being shot
In this picture I am again shooting Partall 10 PVA. (Poly Vinyl Acetate)  This is a water soluble release film. I have never lost a part when I use Partall 10. I really swear by this stuff. To properly apply it you must build up thickness gradually. In the case of this mold and in this weather (90 degrees F) it took 6 light coats. The PVA is allowed to fully dry between coats. In this weather it only took about 5 minutes per coat. Do not get anxious and try to shoot a single thick coat. You will get runs and the coat will be ruined. The next step after the PVA cures is to shoot the high solids primer. Because the PVA is water soluble. it needs to be applied just prior to lay up. if you let is set overnight in any kind of humidity, it will start to bubble up. I shot the primer until the surface was fully coated. I shot 4 light coats, again building the thickness slowly. Once the primer had dried (about 20 minutes) I laid up 3 ply's of Rutan 285 BID cloth.  It took 20 hours to cure sufficiently to the point where I could release it.

Upper cowl untrimmed
Here is the final result. The part released with a full film of PVA. I few minutes under the hose and the surface was totally clean. This is why I have never had a part stick with PVA film. If a part appeared to be "Stuck" all you have to do is peel back an edge and pour some water into the opening. eventually the water will wick completely through the PVA. at that point nothing is holding it on. The next step is to lay up the lower half. That will be a little more difficult because I need to attach the upper cowl mold to the lower cowl mold. The lower cowl trims to the same line as the upper. I also need to lay up an overlap flange. I have a good idea for this and I will cover that and a slick method of trimming fiberglass that completely eliminates the possibility of edge damage and delamination. I will cover that next

Finished Mold

The mold is finally finished. I know I'm going through a lot of extra work, but who knows, I might want to sell some cowlings some day. I waited until the following Saturday to start the opposite side of the mold. I needed about 5 uninterrupted hours to make the opposite side. Once the Gel coat was shot, there is about a  1.5 hour wait until it cures sufficiently to finish the lay up.
Duy Tran
My friend Duy Tran came out on Saturday to give me a hand. I met Duy when he came to Triumph as an Engineering Intern from the University of Texas@ Arlington. Duy worked for me as an Intern and later as a graduate engineer. He is one hell of a kid. He is very enthusiastic and always has a smile on his face, as is evident in this picture. He has a desire to learn all aspects of aircraft design and construction and I really appreciate all the help he has given me.

The first step in completing the back side is to coat the plaster with release agent. I am using Partall 10 by Rexor Corp. This is a water based PVA agent that will dissolve with water. I have never had a trapped part when I used Partall 10. I swear by it.

5 PVA coats
the old header boards were removed and the tool shot with PVA. there are 5 coats on this tool. I usually shoot light coats and build up the film slowly. If you shoot too heavy a coat the PVA will rum and its ruined. You have to wait until it dries completely and the peel off the film and start all over.

The next step before lay up is to create indexing pins that will align the tool halves together so the seam can be minimized. I do this by using a 1/2" drill and cutting only the beveled tip.

Registration pins
If I had thought a little more about this step, I would have laid up 4 additional small mat pads in each location. As it was, some of the areas were so thin I broke thru and required a repair patch of Bondo on the back side. Later after the second side s laid up, I will drill 1/4" holes between these pins to clamp up the tool. These holes are coated with Carnauba wax before the gel coat is shot. after the gel coat is shot and mostly cured, they will be filled with Bondo before they mat is laid up

Registration Pins
Here is what they look like before Gel coat is shot.

Time to shoot Gel coat. I am using a Gel Coat applicator gun. This is a special gun with a disposable paper cup. It was well worth the money. There really is no other way to apply this thick gel coat resin.

Shooting Gel Coat
so the tool gets shot with Gel Coat. I shot 28 oz. on this side. Based on previous experience in this 100 degree weather, I modified the mixing ration slightly by reducing it. The last thing I need is to have it cure in my gun. It can be cleaned, but it will take hours. I shot a nice even coat. I will turn a fan on after I shoot to blow the vapor out my main door. I need t let this sit for about 1.5 hours until its almost fully cured. It was noon about this time, so Duy and I ran out to get some Vietnamese sandwiches ( Bahn Mi). Duy says this means "Elbow Sandwiches" in Vietnamese. They are made on an excellent light French Baguette. They kind of look like your fore arm with an elbow.

Final Gel Coat layer
Here is what it looks like fully covered. when it hardens in 1.5 hours, I will form a bondo fillet along the surface of the old header board and fill in all of the registration pin holes.

now the lay up begins. we are nice and content with a belly full of Elbow sandwiches!! I did something on this cowl a little different this time.

I decided to fill the inlet with Bondo so I would not have to lay up inside the lip. This made life much easier. Duy's job at this point is to mix the resin as I need it. We limit the mixture to 28 oz. the rule is a new container, mixing stick and brush each time. I don't want to risk previously mixed resin setting off later batches. Again with lessons learned from the previous cowlings, I cut back on the hardener ratio. The mat is cut into 12" x 12" squares. they are laid up with a 1.5" overlap. The reason I do this is to limit the amount of movement I will need to push the fibers around. Mat is made from non direction fiberglass roving held together with a starch binder. The mat needs to be wetted out before the starch will dissolve. until it does, it cannot be pushed to contour. I purposely limit how much contour the mat sees. if the contour is  great, I cut down the size of the mat. Its important to work fast at this stage. I don't want mat that isn't formed and rolled to cure in place. I use industrial Polyester Resin. My supplier is one who services the counter top and pool spa manufactures, The industrial resin does not have any cobalt additive. A lot of commercial resin available at auto parts stores are loaded with Cobalt. this gives the resin a purple tint. The Cobalt is a cure accelerator, and the last thing I need at this point.

Final Lay up
the technique I use to lay this mat is to first wet the surface under the Mat. This helps hold it in place when I add further resin to the top surface. I will place about 4 sheets and then come back to the first sheet. I then use a serrated roller to work out the wrinkles and the air. I let Duy try his hand and he quickly got the hang of it. Its important to get all of the air out of the mat and not have any unsupported gel coat  that will crack. I laid 3 layers of mat on this tool.

bolt hole drilled and edge trimmed
In this heat it took about 2 hours to fully cure. I got out my Milwaukee Sawzall and trimmed the flange back. I have about 2" of overlap left. I then drilled a series of 1/4"holes to lock both parts together. I decided that the plaster was going to get trashed after this, so I didn't care if I damaged it removing the tools. One tool is sufficient to lay up the lower cowl, but the second tool will have to be added to the first lay up the upper cowl.

Finished tool
The tool is released and then cleaned with water to remove traces of the PVA. The surface is wet sanded with 320 grit paper until perfectly smooth. The constant reversal of surfaces in this process means that the final surface finish gets better and better. Lows become highs and highs become lows. sanding ridges are easily removed at this stage. The only line I want to keep is the trim line that is now a resin ridge. I will carefully rescribe that line back into the mold. Next step is the cowl lay up!!