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.









Seat Construction

I was trying to figure out what would be the most comfortable seat position for a pilot in a reclined position.  My Goal was to reduce the height of the fuselage to reduce the projected frontal area. The other problem was the carry through of the main wing spar. I was aided in this problem because I decided early on to place the spar at 33%, this would place the spar aft of the center of mass of the pilot. In the design of an ultralight, because the Pilot weighs as much as the airplane (At least in my case!!) its a good idea to place the pilot in the center of the CG range of the plane so as to avoid the need for ballast.. One day I was reading my latest addition of Soaring magazine and I saw an ad for Jantar Sailplanes. In the ad they were boasting that their seat was the most comfortable in the industry. They had a inboard side profile of the cockpit and seat. Well if it is good enough for Jantar, then it was good enough for me!!  I scanned the ad and did an underlay trace on my Cad System (Rhino 3D) and I then had the seat curve. I orientated it to clear the spar and place the pilot on CG and then designed the structure around it.


Seat back up structure being installed.

The seat back up structure is designed to the bottom contour of the seat. One of the design criteria I adopted was to ensure that the seat did not deflect under max G loading such that it interfered with the aileron control torque tube that runs under the seat itself. In this picture temporary jigging is fixed to align the 3 seat supports. Most of the secondary seat support structure is made from Spruce Scrap and or Birch plywood. The contours and shapes are laid out from dimensioned drawings on the plans. For ease of build in areas like this, I plan on having laser cut card board templates made.

Seat center support intercostals
 There are two parallel seat support intercostals that not only serve to keep the pilots behind from deflecting the seat and jamming the controls, but these also provide the mounting provisions for the aileron torque tube. The Intercostals are built up of 1/32nd plywood and 1/2" spruce square stock. All glue joints used T-88.








seat supports completely installed
 The center intercostals were mounted to the fwd face of the main wing box. External stiffeners tie the intercostals to the wing box, like wise on the landing gear frame. In the final plans I will clean up this area a bit. I had to add plywood fillers that were not planned for originally. I will modify the gussets of the landing gear frame to extend them such that they act as fillers. I will also redesign the truss members to eliminate some redundancy . The intercostal stiffeners could become a part of the wing box internal truss, likewise for the landing gear frame. It will probably save 5 to 6 ounces.

One of the reasons I love to design ultralight airplanes, is the discipline it gives you in the area of light weight and tight design. I remember when I hired on at Beech Craft in Wichita, I was making a bit of a name for myself locally with my Wren Design, and I started receiving calls with job offers. When I was interviewing with the various engineering groups, the guy running the design group for the Starbarge (Star Ship) looked down his nose at me and said, "we don't hire Ultralight designers on the Starship" That was a fact!!!  It was 5000 lbs over weight and out of the class. Beech petitioned the FAA for a special category so the lead sled didn't have to meet Airline design criteria. I guess they had all the "Ultraweight Designers" they needed. I ended up doing manufacturing R&D, but that's another really good story!!!

another view of the seat supports.
 a short intercostal that spans the gap between the center intercostals will be added later to support the torque tube. There is a cutout for a Delrin Shoulder bushing.













Seat Flat pattern being trial fitted
I did not want to actually mold the seat. So I hit on the idea of laying out a seat flat pattern from 4.5 oz PVC foam . I pieced together the cores bonding them with a mixture of Micro balloon and Epoxy. The actual pattern was taped together Poster board cardboard. I have seen this forming technique used to make ECS ducting on commercial private jets. I filed this technique away for future use. The future is NOW!

Only one side of the foam is actually laid up, and that is the surface that will be in tension. Now the seat curve reverses and because of this, you do not lay a layer of glass the full length on the bottom surface. The glass stops where the panel curvature reverse. The seat is really nothing but a wide thin beam. The fiberglass laminates on the faces alternately resist tension and compression forces. If only one surface (Tension side ) is laid up and the seat is deflected, the foam itself experiences compression. And since the foam is relatively weak and is of low modulus (Stiffness) it readily forms to the curve of the underlying seat structure. This is why its important that all of the seat structure be in align and level.

Seat being pushed into position.
 In this picture the seat is pushed to the support structure. After a trial fit, I removed the seat and taped up all of the seat support structure and the side truss. The seat was then put back into position and hot glue was used to temporally hot the seat into position.









seat being held while the hot glue cools.

The front of the seat lower surface was left bare and not laminated. I used 1 single ply of 120 style glass for the bottom surface. Since this surface was in tension, there was no need to have a thicker laminate because thin section stability is not an issue . I have experimented quite a bit with foam and glass construction, its against intuitive thought, but the lightest laminates I have made were with 4.5 lb/cu/ft PVC foam. PVC foam is a closed cell foam, this is important because wet resin will not continue to wick into the core, and in the process draw in air. This of course will add extra weight. Laminates over foam usually start with a slurry coat of micro balloon and resin, usually Epoxy, that is trowelled onto the surface. The idea here is to fill the exposed cells with a lighter weight material than the raw resin. When the foam is cut at the factory, the cut cells are exposed, the lighter the foam, the larger the cells. I have found that 3 lb density foam has too large a cell structure, this is why the 4.5 lb density is lighter.


Another view of the fwd edge of the seat

 I elected to form the fwd reverse curve with a heat gun. I will not do it this way in the future, I would tape the lower surface with aluminum duct tape and then hot glue the foam to the supports.
once the seat core is formed into position, a single ply of 8 oz 285 weave style glass was laid up. You could also use a  8 oz plain weave because of the absence of compound curvature. This would be a little cheaper.

Seat after final trim
This is the seat after it cures and the edges are trimmed. The edge treatment is to lightly sand into the exposed foam and a mixture of Micro balloon and resin mixed to a consistency of bread dough is pushed into the edge. After it cures the edges as sanded smooth. The final seat weighs 15 oz. The seat needs to be removable to gain access to the wing attach pins, so it is held in place with twing strips of Velcro tape
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