Robin Ultralight: November 2016

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.

once the cowling was fitted together , I swept a bit of Micro-Balloon and epoxy into any of the gaps. The upper cowl was waxed so the Micro only stuck to the lower cowl. The final hole diameters were opened up and countersunk washers were bonded on. The washers were first sandblasted on the bond surface. The cowl was sanded and then cleaned with isopropyl alcohol. The washers were bonded with T-88 structural adhesive.

Sand Blasted Washers

These washers are available from Aircraft Spruce. They solve the problem of the fastener sawing thru the fiberglass. They sit slightly proud. I taped up every one so the final finish will be polished stainless steel.

When I put the cowling halves together, I needed an accurate way to trim the inlet flange. I wanted a nice planer trim. As I mentioned before the neatest way to trim fiberglass and graphite laminate is to use a grout cutter and a multipurpose vibrating tool. I set up a MDH wood surface by bonding it to the inside of the cowl with Bondo. I then set the tool flat on this surface and cut away the inside. It was perfect! Nice and even and planar.

MDH trim planes

This shows the cowl with the MDH plates bonded in . The grout cutter sits flat on the MDH surface.

Final Cowl inlet trim

The final trim.

access door layout

The next step was to make two access doors for the oil dip stick and the oil filler cap. This broke my heart, having to cut out these panels on a surface I worked so hard to get perfect. But it beats the hell out of pulling the cowl every preflight to check the oil. The section of the cowl I cut out will become the access door. Before I do that I need to transfer these outlines to the inner surface. Once I marked the inner surface, I applied 5 coats of Carnauba mold release wax to the inner surface (IML Inside Mold Line) . I then cut 4 ply's of glass and laid them up centered on the outline.

door doublers laid up

After 2 days of curing (its getting cooler in Texas) the doublers were ready to be released. They were separated and set aside. Later they will be marked for trim after the doors are cut out of the upper cowl.

beginning of the cut

I used a .030dia drill to start the cut. Once I connected the holes, I was able to insert a small air operated scroll saw. I took my time, and cut out both doors. Once the doors were free, I sanded the edges even and trimmed back the hinge side for clearance.

raw cut

when I laid out the door, I purposely set the hinge side on a contour element line (straight line) so there would be no binding of the hinge. I used a MS aircraft hinge and using countersunk Cherry N rivets, I attached the hinge temporarily using Clecos.

Door with Hinge

The next step is to mark the backing doublers for trim.

Bonder door doubler

the doublers were trimmed and the edge beveled 20:1. This reduces the peel forces on the edge off the doubler. I used t-88 adhesive and Cleco clamps and bonded it together.

fairing in the doors

The final step was to get the surface back to the original smoothness. I waxed the door and then assembled them I dragged a coat of micro into the small gap between the cowl and the door. This micro will fill any gaps and remain with the cowl and doubler.

I have made more progress that I am saving for the next update. I am currently setting up the engine for its first run. I need to install a pressure gage and oil pressure sender. I need to rewire everything and set up new throttle and choke cables. I just ordered a new Prop from CSG in Canada.

Welcome to my Robin Blog.

Cowling access doors