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.









Horizontal/Vert fin fairing

One of the last "big" jobs I need to finish is the fairing between the Vert fin and the horizontal. There is a natural gap between these two parts and passing through this space is the elevator. A slot will be cut in the fairing to allow for full up travel of the elevator. Because I cannot make a lightweight "insitu" fairing out of foam, I decided to make a full mold. The fairing will be laid up in this tool with one 3.2 oz fiberglass surface ply and one layer of 8 oz graphite.
Tape being applied to exposed plywood
 The project started by covering all of the exposed plywood with aluminum duct tape. We used to call this 500 mph tape at Cessna, we used it to attach external instrumentation wires on the Citation 3. The white strips on the middle of the Vert fin leading edge are adhesive backed 3" wide 60 grit sand paper. I will uses these to contour the floral foam blocks that are attached later. I have used this technique to make contoured sanding blocks for automotive body repair. If the car I am working on was too severly damaged in this area, I would find an umdamaged car and make contoured sanding blocks out of styrofoam.
1 lb/c ft Floral foam blocks
The area where the fairing is to be built is stuffed with floral foam blocks. Notice how the sand paper has some foam dust on it. The blocks are rough cut using a wood saw and a hacksaw blade. The first contour cut will be the side profile of the fairing. Before starting the foam blocking, I used a piece of welding rod and taped it to the horizontal and the leading edge. I used two pieces of tape on each end so the wire would have an established tangency. The fairing needs to smoothly blend between these surfaces.I made a tracing on a piece of poster board and I will use this as a guide when I cut the side profile.

Foam after contouring
After the side profile was shaped, the foam was sanded to a pleasing shape using another block of foam. Once I was satisfied with the shape, I laid up 2 light weight plys of fiberglass and let everything cure overnight. The next day, I started coating the glass with Bondo. I have gotten pretty good working with Bondo. I attribute this to having grown up in the Detroit area. As a 16 year old driving around, every spring was spent repairing rust from all the salt on the road.
The technique for using Bondo is to lay on multiple thin layers. Sanding in between each coat.
multiple layers of Bondo

36 grit paper on an air powered file board is used to rough contour the bondo. As the shape gets closer, finer and finer sandpaper is used. The last grit will be 180. There is no need at this stage to use any finer. A word about materials here is in order, do not rely on the chain stores (Auto Zone...) for your materials. Find a body shop supply business. They will have the professional grade body fillers and sand paper. The price is always less than the inferior commercial products.
Final Bondo Coat
After you are satisfied with surface and it is wave and pit free, block sand the surface with 220. The next step is to coat the surface with professional grade Spot putty. Spot putty is a lacquer based product that dries by evaporation. The thicker the coat, the longer it takes to dry. Spot putty is not used to contour the surface, its used to fill the sanding marks an very slight depressions.
Spot putty applied





I usually let this dry for 24 hours .








Final sanding after application of Spot Putty

This will give you an idea about how much of the spot putty is left after sanding. The spot putty is first sanded with 220 grit, and finish sanded with 320. at this point I taped up and papered the rest of the plane and applied 5 coats of carnuba mold release paste wax.


PVA Poly vinyl acetate

On top of the polished wax, 6 coats of PVA were sprayed on. The coats were allowed to thoroughly dry between applications. Once the surface has been coated, the PVA forms a film that will act as the release surface




Texas AutoClave
Between coats, I rolled the fuselage out into the "Texas Autoclave".  85 degrees F here today, in the middle of October. Texas is a wonderful place to live 3/4 of the year. True Texans however are solar powered and don't care a whit about the heat of summer, in fact they say its a natural Yankee barrier!! They curse the day Airconditining was invented!!!
Gel Coat being applied

Using my Gel coat gun, I sprayed 1/2 a pint of Gel coat. the gel coat is allowed to cure until tacky and then raw resin is applied over the gel coat and pieces of non directional mat is applied. It took about 2 hours to cure enough so I could pull the mold off.



Fairing Mold
Tomorrow I will lay up the fairing.













Fairing in position





 Finished fairing!!! 1.0 ounces.

The fairing will be bonded to the horizontal
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