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 Cushion and Citabria Cowl Repair.


I have been waiting for a copper gasket for the Tillotson HR carb needle seat. I have been waiting a month, and believe it or not, it actually arrived while I was writing this blog. In the mean time while I was waiting, I fabricated a new seat head rest and added a leather covered head rest and seat cushion. It really came out nice! Every one who has sat in this seat said it is extremely comfortable.

head rest and seat cushion
I fabricated a light weight head rest support from .020 aluminum  sheet.














 

Shoulder, lap and crotch harness

 Good view of the seat belts and adjustable rudder pedals. The read "Tee" handle is the Ballistic chute deployment handle.













Another view

















Citabria Cowl Repair

This is the plane I have been flying at Big Q Aviation in Midlothian Texas http://www.bigqaviation.com/
The Cowl was getting pretty tired and while the wing was being rebuilt, it was decided to repair all of the elongated cowl fastener holes. Once a fastener gets loose, a "sawing" action occurs on the fiberglass holes and the holes start to elongate. Some of the holes were so badly elongated that there was just a thread left on the original cowl. That was the case for 16 of the fastener holes. The rest were just slightly elongated and could be repaired by filling with chopped cotton fiber and epoxy. The original Cowl was hand laid up out of fiberglass cloth and polyester resin. The basic principle of repair says you need to restore the existing structural capability. In the case of a composite repair, that usually means all of the original fibers plus 1 extra. The strength of the original material is the determining factor on what taper ratio will be used to scarf the ply's . High strength uni fiber repairs like on a wing spar will require a taper ratio of at least 100 to 1. The lower modulus materials used on the this cowl could be repaired with as little as 15 to 1. the minimum however per FAA guidelines is 20 to 1.

repair schematic

The basic idea here is to clear out the elongated hole, in this case a slot is opened up along the cowl edge.The edges of the cowl are then taper sanded at a 20 to 1 ratio.  A sheet of waxed .020 Aluminum is then bonded to the outside of the cowl with 5 minute epoxy. 7 plies of equal size semi circle doublers are then laid up into the tapered space. After they cure , the edges of the splice are taper sanded . The process is really simple, only the edges are sanded until a space in the middle equal to the original slot remains. After the splice is sanded, a final overlay ply is added (original ply plus 1 extra)



corner edge repair
 After the inside is repaired, a very slight taper is sanded into the outside surface, and a single ply of fiberglass is laid up. This way the repair is actually a double lap shear/Tapered scarf. This is a very good structural repair because all tendency for the tapered splice to Peel away is reacted by the over lay ply's.
Double hole repair









This is a good example of what the repair looks like after taper sanding. Its possible to actually count the ply's as they drop off. Notice how the center of the repair laminate has not been sanded.  


















single hole repair.
 This picture clearly shows the internal taper sanding. One final overlay ply will be added over this repair.





















overlay ply with peel ply
a very good technique is to finish the repair lay up with an overlay of Dacron Peel ply. This adds a professional touch and serves a very good structural purpose, the hard edge of the repair laminate is tapered out with a resin rich edge. This serves to smooth the hard edge and eliminates the tendency for the repair to peel away. After cure, the peel ply is pulled off the laminate. This material is the same Dacron I used to cover the Robin.
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