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.









Roll over frame, Turtle Deck

                                                   Roll Over Frame

The roll over frame is a very straightforward design of bolted square aluminum tubing. I considered making it out of wood and welded steel but opted for aluminum when weight was considered. The layout of the truss and all of the fittings is shown in the drawings. The fittings themselves are made from .032 2024-T3 aluminum sheet. Each fitting has an opposite hand part. The flat patterns are the same, the difference being the bend direction. I like to lay out a single piece first. I will leave the protective plastic on the aluminum and cut out two identical blanks slightly oversize. Cover one blank with masking tape and lay out the fitting profile and bend center line from the drawings. Cut out a block of wood or MDH board exactly the same size as the raw blanks. Drill the fastener holes thru the blanks and into the backing block, Drill all holes undersized to the final diameter. In the case of these fitting use a .125" drill. After the first hole is drilled, cleco the blanks to the block. Repeat this with all of the other holes. You can now cut the stack up on a band saw and sand the edges on a belt sander. I like to turn the blanks 90 degrees and finish the sanding parallel to the edge.
                                                                           
Roll over frame during assembly

. When completed the roll over frame will also support the Ballistic parachute


Example of one of the fittings. If the layout is followed exactly per the print, both fittings will align regardless of the compound angle. that's the beauty of designing with a 3D cad system . all of these complicated areas can be worked out ahead of time and simplified back to a 2D drawing.







                                       Rear Turtle deck

The rear Turtle deck  is built around the roll over frame. The purpose of the Turtle deck is to aerodynamically fair the pilots width and height. The main design criteria on this structure is fabric tension. That can be quite severe. Because of this, most ultralights are built with 1 oz /yd sq Dacron fabric and are not fully shrunk
partially completed turtle deck
The bulkheads are made with 2 lb/cu ft extruded Styrofoam sheet, Dow Corning Pink Panther to be exact!!!
They are first laid out on on a sheet of poster board , I only lay out 1/2 of a pattern at a time. The pattern is then flipped and the opposite side is made. This ensures consistency and speeds up the lay out process.

The bond between the foam and the wood initially is made with 5 minute epoxy. This is not sufficient however, and more bond shear area needs to be added. the solution is to add a Micro balloon fillet as shown in the following picture. The micro is mixed with pre-mixed Epoxy resin 
Microballoon Fillet
to a consistency of peanut butter. It is important that the Micro retain a shiny glossy surface. This indicates a resin rich condition and aids in the adhesion to the wood and foam. A wiper stick with a 1/2" radius is used to sweep the Micro.


The bulkhead attaches to the roll over frame and must be beveled to match the final surface of the turtle deck. The pattern of the bulkhead is laid out at the rear edge, so when sanding a bevel, that edge is left intact.
Turtle deck with beveled bulkhead and fairing strips
The fwd face of the bulkhead will get one ply of 120 glass. There are two reasons for this, the bulkhead will see abuse from the canopy opening and closing and the fabric will put a great deal of  bending load on this bulkhead. The final step in the completion of the turtle deck will be to add the plywood fabric transition edge. This is mostly cosmetic. The surface of the fabric needs to transition to the curved edge of the bulkhead. The fabric will be a straight line segment between the fairing strips, so a smooth transition needs to occur at the ends. I first saw this technique on Neal LaFrance's Culver STF and his V-Witt racer.
The fabric transition fairing starts by making a template with poster board. The drawing gives coarse dimensions regarding the width and inside radius, but does not attempt to layout an actual full size flat pattern. There are just too many build variations to be able to do this. Make a full size template for this part. The fairing strip is made from .032 Birch plywood. The fairing strips are made from .125" x 1.25" straight grain Popular, Fir or Pine. I chose not to use Aircraft spruce because of the cost and the fact that these strips exceed 109" and will have to be spliced.
                                                                                   
View looking aft
Careful selection at a good  lumber Get good results.

As shown in this view, the roll over frame also becomes the support for the seat head rest.















Completed turtle deck. Total weight of Deck and frame is 2.1 lbs

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