Robin Ultralight: covering part 1 (Tail Feathers)

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.

I started covering the tail surfaces. The rudder was first. This is the first time I am using the Stewart system Eco Bond adhesive. As usual there is a learning curve to go down. I will eventually re-cover the rudder, but I learned a few good lessons. The Eco bond is really good stuff, but it needs to be applied with a foam brush. The natural fiber paint brush I was using does not apply the adhesive evenly. This will make for a lumpy covering job. In the Stewart manual, they give a lot of good tips, one of them was the use of this disposable cup method.

Chinette plate and Dixie cup holder

this is a holder for the Eco bond adhesive. In the middle is a cut down Dixie cup that has been bonded with 5 minute Epoxy to the center of a Heavy duty Chinette paper plate. A fresh Dixie cup is used for the actual adhesive. Its placed in the Dixie Cup holder. This way it wont spill on you. about 1/3 a cup is all the adhesive you work with at a time. That's the length of out time you have available.
The Eco Bond is water soluble, but once it cures, its set for good.

pre coating the structure

The method I finally settled on is to pre-coat the entire structure and let it fully dry. The Eco Bond is heat activated. The fabric is placed into position and then ironed onto the frame with a iron set and calibrated to 200 degrees. The heat of the irons is extremely important. Its very important to use a good IR thermometer when you calibrate the Irons. The normal main shrink temp is 250 degrees, on a certified plane with heavier structure, the shrink is accomplished in three steps, 250 degrees, 300 and finally 350 degrees. that is way too much for an ultralight airframe. This is the main reason I am recovering the rudder. I used the higher temp and I bent the trailing edge. Its slightly scalloped. All ultralights stop the shrink at 300 degrees.

Rudder

I am using a single wrap piece of cloth for the rudderThe first wrap will attach to the trailing edge The cloth will be wrapped over the trailing edge. The opposite side will wrap over the railing edge and attach to the cloth only. All ribs are also bonded,

Ribs being bonded

once the adhesive dries, the fabric is tacked on using a heated iron, after everything is in place, additional Eco bond is brushed into the surface to adhere to the previous coat. Once this sets, its really on there.

beginning of the Horizontal

The horizontal came out really well. I used two separate pieces to cover this surface. Both pieced will over wrap the leading edge.

Finished Horizontal

So far the covering process added only 2.5 ounces, the real weigh gain will be the finish. Tomorrow I will finish the elevator and the ailerons. My buddy Ed will come by tomorrow and we will pull the wings. I can cover the R/H wing immediately, the L/H wing needs some repair on a plywood piece that was damaged during storage. But I should have both wings covered by Saturday. Then its on to the fuselage. after I cover the fuselage and finish the plane, I need to re assemble the wings and then build a root fairing. I decided it would be easier once I figure out where the fuselage fabric is placed.

Welcome to my Robin Blog.

covering part 1 (Tail Feathers)

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