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.









Instrument panel, canopy and tail fairing Part 2

This is an update to the previous post. I am working on the modification of the Canopy frame to accept the instrument panel and increase the knee room. I laid out the trim of the solid bulkhead and then cut it out of the existing frame. The biggest problem was how to reinforce the core under the instrument panel attach screws. The core is 4.5lb/cu ft urethane foam. It will immediately crush when the screws are tightened up. I marked the location of the screws and then used a 1/4" drill to route out all of the foam within a 1/2 inch in each direction. The void will be filled with dry Micro balloon. This is Micro mixed to the consistency of dry dough. To ensure a good bond, the cavity is coated with raw epoxy.

Bulkhead and lower shear pane trimmed out
The tab outs are around the fabric stringers. The attach screws are between each of the stringers.

The plywood gussets are designed to smoothly transition the fabric  from a faceted flat to the curve of the bulkhead.











This is a good picture of the tab outs. The urethane foam core is shown. The lower shear panel was cut back, this exposed a Styrofoam edge. I do not like raw Styrofoam in the fuselage. One time during the Lakeland Airshow when I was building the Wren, one of the only other manufactures I ever met who was not very friendly came up to me and started yelling at me??? He complained that I was pushing the spirit of part 103 and I should just go back to Kansas and stop this madness!! He was quite famous and unfortunately(or fortunately) he is no longer with us. He was criticizing me for building a fuselage out of Styrofoam. Evidently he didn't level the same criticism at Steve Wood for his Sky Pup, but in any case, he told me that a single gas spill could drop me out of the air!!  That night around 2 am I heard a noise outside my camper , when I ran up to the Wren, she was soaked in gas. The whole lower panel was dissolved away. I sat up all night and repaired the fuselage by splicing in foam with 5 minute epoxy. I completed all of the repairs 30 minutes before the ultralights flew later that day. When Mr Pseudo-famous walked by the flight line, the look on his face proved beyond all doubt he was responsible!!

Anyway, that's why I don't like Styrofoam in a primary load path.!!

Shear panel close out
This is the closeout of the exposed shear panel edge. The technique here is to cut a 45 degree wedge away from the foam up to the cured glass. Dry Micro is then forced into the corner A layer of glass is then laid up over the exposed foam. When the micro cures, it forms a wedge shaped gusset. Once its fully cured (72 hours @74 degrees) the edge will be rounded over with a sanding block to keep from scraping the pilots legs.






boo saving me from a Frisbee attack
I live on the edge of the Hill country about 40 miles south of Dallas. Just south of me about 10 miles, it starts to open up like the Texas you see in the movies. There are lots of wild animals and recently there have been a number of pet dogs and cats that have been attacked and killed by mountain lions. But that's not the nastiest threat we have in Midlothian. While I was working away out on my shop minding my own business, a wild Frisbee attacked me, luckily My good ole boy Boo was there to protect me!!  I'll tell you, I will never have any dog other than an Australian Shepard. Its really fun to have a very smart dog!!
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