Robin Ultralight

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.

Wing Walk and some test photos.

I started modifying the L/H wing to reinforce the leading edge so I could add a wing walk. The redesign was pretty straightforward. I removed the existing wing skin with my multi-tool. That thing zipped right through the fiberglass. After the skin was removed, I reinforced the foam ribs with a ply of 8 OZ fiberglass on each face of the exposed ribs.

lateral stiffeners

I then slotted the ribs every 2 inches and inserted a wooden stringer. the Stringer was sized to react 250 lbs between the farthest span (6") so basically each stringer can support the weight of one step. In reality the load will be distributed between at least 3 members. The stringers were potted in with micro balloon and epoxy. Using the leading edge lay up mold, I laid up a 2 ply skin of .012 thick carbon fiber.

leading edge walkway skin

2 ply's of graphite and a single lightweight ply of glass comprise the walkway skin. The leading edge was taper sanded to allow blending into the existing skin.

Right now the skin is curing after having been bonded 8 hours ago. Tomorrow I will be able to flip the wing over and finish sanding the skin in preparation for covering. I should be able to finish the covering tomorrow.

rib stitching

My airfoil is a Worttman 170 low speed laminar flow airfoil. It has a reflexed trailing edge on the upper and lower surface. Because of this, the fabric tension after shrinking will try to pop off the reverse curve surface. So I elected to rib stitch the trailing edge where the reflex is. Everywhere else on the wing, the fabric is bonded.

wing re test

The original wing test spar has been sitting out in the weather for over 2 years. The ultra violet radiation has attached the foam and the fiber glass. Periodically I retest the wing. The original wing was rebuilt after initial destruction testing and re mounted to the back side of my test stand. A 2 x 10 wood extension was added to increase the applied moment such that it equals the actual moment on the wing. This is a little conservative because the wood does not deflect similar to the wing. Because of this the stress is concentrated more toward the root, there is no relief that would normally occur it the complete wing was allowed to deflect.

anyway, this is a picture of the wing at just a little over limit load. There is 1200 lbs of force being applied. In the previous picture you can see my load cell that I designed. I made a piston with exactly 1 inch of area. I then bored a cylinder out of some scrap aluminum.I drilled and tapped into the cylinder and installed a 3000 psi pressure gauge. I filled the void with transmission fluid. The actual load now is a one to one read out. I calibrated the load cell at work on a universal testing machine one day at lunch. There is a little hysteresis error, but that was under 100 lbs. Close enough!!!

Welcome to my Robin Blog.

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