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.

Latex House Paint Covering method.

Big milestone today. I started finishing the fabric. I decided a while ago after some research that the Latex house paint system was the lightest covering method I could use. The major covering methods are mostly dissolved thermoplastic. All of these processes are heavy. The latex system I believe is also similar to the Stewart covering system, http://www.stewartsystems.aero/default.aspx in fact I am using their adhesive called Eco Bond. There has been a ton of research done on using Latex House paint. A friend of mine in Wichita, Duane Life covered his J3 Kitten and I have always been impressed with the finish. A lot of good work and research has been done on this process, the most impressive is Drew Fidoe's blog http://www.bowersflybaby.com/tech/latex.html, other good sites are the reprint of Charles Threewit's article http://www.lazair.com/housepaint.htm and this article in Ultralight News http://www.ultralightnews.ca/articles/housepaint.htm

Fresh Bloom

The process starts with a good base latex Acrylic latex enamel. I am using Sherwin Williams D-100 gloss exterior enamel The base color of my Robin based on my Friend Bills design is Gun Metal Grey. This may be too dark, so after the base UV coat is applied I may pick another lighter color if Bill decides I need it. The formula for using latex involves adding a viscosity modifying agent called Floetrol. This smooths out all brush and roller marks. Each coat is additionally thinned with standard windshield wiper fluid. The links give the ratios.

The day started nicely with a fresh bloom in the flower box. We had a few 85 degree days last week. Like I said earlier, Texas is wonderful in the spring, winter and fall!!!

I started with the ailerons. I figured they were the easiest to recover if I screwed up.

Single coat on Ailerons

The procedure for finishing goes like this. After the paint is well thinned and mixed with Floetrol. it is rolled on with a foam roller. Right behind the roller you follow up with a foam brush and push the first coat into the open weave. Let that dry and and complete the same process on the opposite side of the part you are working on. The second coat is put on with only the roller and is rolled 45 degrees in the opposite direction of the first coat.

Ed doing the Brushing duties

Its amazing how well this covers. By the end of the day, all of the control surfaces and Horizontal were finished and I had used less than a quart of paint. The beautiful thing about that is its mostly water and it evaporates away.

Ailerons with second coat

Depending if Bill likes this color, the next step will be a spray over coat at Eds house in his spray booth. At this point the fabric on the surfaces sounds like a snare drum when you tap it. Its pretty exciting seeing it at this stage.

Coating the elevator

This is me doing the elevator. I tried to measure the weight gain at this point, but I found that my scales were not sensitive enough to measure the difference. I will know exactly when I reassemble everything an re weigh, but from the looks of it my weight gain will be measured in ounces. My estimate of the paint weight was 12 lbs. Based on that I was going to be 10 lbs over weight.