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.









More Cowl Plaster master

When I left off I had just released the Plaster male master from the female plastic splash. The male master had inlet inserts incorporated into it. I removed the inserts and exposed the inlet cavity. This really worked quite well, but I did not like the inside junction near the spinner. So I modified the master by sweeping bondo with a larger circular drag. After a ton of sanding, this is what the inlet now looks like.


inlet

You can see the larger circular fillets I added on the inside with the blue colored Bondo. I set the master on top of my welding table. It has a 1/4" thick flat steel plate for a surface. I coated this surface with carnuba wax and then made a wooden support and using Bondo, I temporarily attached the plaster to the table. I set the spinner plane level, I need to set up a set of waterlines and stations that I can use to scribe the trim line into the plaster. This will transfer to the tool as a positive resin line, which will then transfer to the final part as a negative scribe mark. 

The surface finish of the plaster is perfect. But if it gets scratched, its not the end of the world, scratches will become small resin ridges in the lay up mold, I usually wet sand the molds with 400 grit paper, this will knock off any resin ridges. 



There is quite a gap between the master model and the table. I want to use this space to extend the surfaces. Its always a good idea to have excess tool surfaces that are set beyond the final trim line. In a production environment these would be used for the vacuum bag seal surface. 









Here is a side profile with the old spinner in place. I like the lines!!!  This should end up looking pretty good. The spinner was designed originally for the MZ-34 engine. This engine was a much more compact design and placed the spinner much closer to the firewall. Since I didn't want to remake the spinner and the backing plate lay up tools, I elected to stay with the original. That presented a real challenge when I reset the upper surface contour. It turned out ok, but it basically determined the center line of the crankshaft for this new engine mount position. The real problem was the lower lines blending into the existing fuselage lines. If I had stayed with the original fuselage line, the cowl would have had an inverted curve and looked terrible. So I re-faired the fuselage by creating two false longeron lines and increasing the depth of the cowl. The results however speak for themselves. 


foam block extension surfaces
So here is where I'm at right now. I blocked up the gap at the base with my floral foam.  I will saw and sand the foam until the surface is at or slightly below the extension surface. I will then sweep some plaster over the foam to stabilize it and then follow up with a final sweep of Bondo to finish the surface. Its pretty tough right now getting a lot of work done, in the late afternoon its been over 95 degrees and the heat index is over 106. So I probably wont get this next step finished until this week end when I can get out to the shop early before the heat builds. Texas is a wonderful place to live, 9 months out of the year!!!  From late June until early September however, it's hotter than hell!!!
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