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.









Front spinner, mold and cowl

I am just starting to build the front cowl master model. The process stated with the construction of the Spinner master model. The spinner surface was designed to allow the cowling loft lines to blend in the side profile and the top or max half breadth profile to the rest of the fuselage. consequently its oversize from the typical spinner one usually sees on an ultralight or conventional airplane. I took the surface loft directly from my 3D model and intersected it with planes spaces 3/4" apart. I then plotted all of them out together and made a plot of the combined stack up. I also plotted a 1" diameter hole in the center that was common to each section. This was to allow all of the sections to be aligned on a 1" diameter tube. I cut all of the sections out from a sheet of MDH board. I stack glued them together and then mounted the stack up on my lathe. 
MDH sections bonded and mounted on a face plate
                                                                            MDH board is a good material to use for this master model, it is basically pressed sawdust and because of this it has no grain direction. It will smooth and work evenly in all directions. The stack up was mounted to an aluminum face plate to allow me to chuck it onto the lathe. I stated turning the stack up. The actual loft contour line is at the bottom of each circle.


spinner master being turned down

I set up a tool fence on my metal lathe similar to that used on a wood lathe. I worked the stack up using conventional wood lathe chisels. This was a relatively quick job. It wasn't long before the final shape started to emerge.



Master after sanding and before a coat of Bondo.
 At this point the surface had to be coated with a light coat of Automotive body filler or "bondo" to fill in all of the surface pores and to allow for the final smooth finish.






Master with a surface coat of Bondo
 After the Bondo was applied, it was sanded down smooth. Subsequent coats were added and each time a finer and finer grit of sand paper was used. Eventually the surface was painted with sand able primer and then further filled with spot putty. The final surface was sanded with 400 grit sand paper and a top coat of urethane automotive paint was applied. From this "plug" or Master model I then made a female mold. At the same time I built the Spinner mold I also made the Horizontal/Elevator tip cap mold. It was more efficient to shoot the Gel coat when I had a second mold to produce.

Spinner and Horizontal Tip cap molds.


Both of these molds will be wet sanded with 600 grit paper and then "seasoned" with up to 12 coats of carnuba mold release wax.









As I mentioned earlier I made the original Spinner plug with 1" diameter holes to use for alignment. The secondary purpose of this was to allow the original plug to be temporally mounted to the engine prop flange So I can start the shaping process of the  cowl master model. I will again use my floral foam method to shape the cowl surface. There will be a sub frame built to support the foam blocks while they are shaped and to eventually allow the foam master to be slipped off the fuselage so a plaster splash can be taken. Here is a picture of the spinner in place before the foam is added. 
Spinner Master attached to prop flange

















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