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.









New Landing gear cont.

I have finished the gear fitting installation to the fuselage. The first step was to locate the fittings using the gear as a guide. I loosely bolted the gear to the fittings and then located the fittings to the fuselage. Once located I back drilled the through fitting through the fuselage bearing blocks. I made a makeshift drill jig by drilling a 1/4" hole through a 2.5" thick block of aluminum. It was very critical to drill these holes perpendicular to the outer flange so the holes would line up with the inside flange. Normally the inside flange would be left un drilled or undersize so the final hole would open up both flanges. I was betting that I could drill a hole accurately enough so the drill would align with the inside flange.
homemade drill guide
I used to work in Wichita at Boeing Commercial aircraft, they had the most wonderful surplus yard I have ever seen. I bought a ton of forged aluminum block and plate stock. This is just a small little piece. There are commercial drill jigs, most notably the "Egg Cup " design. all of these will work, but I didnt have a 1/4" drill bushing for my Egg Cup. 


After the 1/4" holes were drilled, I chased then through with a 1/2" twist drill. Opening the holes up to just slightly under 1/4" (-.003) I then fabricated eight 1/2" diameter aluminum bushings. The 1/2" aluminum stock is slightly larger than 1/2" (+.002) this makes for a nice tight transition fit between the wood and the bushings. I sanded the outside of the bushings and then cleaned them with MEK.
cleaned bushing and epoxy adhesive
once cleaned, I coated the bushings with epoxy adhesive and tapped them into place. The bushings were made with the center hole undersize (.223") so that on final assembly they will be opened up again to .250" The adhesive is used to stop the bushing from rotating when it is drilled out.










installed bushings
the bolt loads are such that they needed to bear into the wood with greater area, this is the reason there are 1/2" diameter bushings.




The landing gear fitting is square and parallel to the landing gear. The fuselage has a slight amount of contour . The load path from the gear to the fuselage is through a mechanism called "Heel and Toe" The design condition is a 1G breaking load applied to the tangent edge of the tires. This creates a large moment that needs to be reacted by the two outer bolts on the gear fitting. Since the fitting is wider than the bolt pattern, the reaction will actually  be between the edge of the fitting (heel) and the opposite bolt (Toe) . Therefore the gap between the fitting and the fuselage must be filled with a material capable of withstanding compressive forces. The fitting is "Bedded" with a mixture of epoxy and cotton fiber (Flox) . Flox is mixed to a consistency of dough. I spread it under the footprint of the fitting and then installed the fitting. The inside of the gear fitting was coated with mold release wax so the fitting could eventually be released if needed.

flox bedding

once the fitting is installed, the excess bedding is scraped away.

When I installed the gear for a trial fit I saw that the clamp block was deflecting and bridging over the gear. To fix this I designed 4 more bushing that were exactly .003" taller than the thickness of the gear. They would crush to equal the gear thickness after installation






clamp bushings
after the clamp up bushings were installed, the bridging stopped and a more even clamp pressure was developed.




























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