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.









Fuselage Fabric and Cable Fairlead


This has been a very productive week end, and as I write this, its still not over. I'm taking a break to let the last base coat dry on the fwd fairing and the canopy. When I finish writing this entry I will paint the first color coat on the fuselage. On Saturday morning I started the process of building a master model of a small streamlines blister that will hide the rudder cable exit fair lead. The whole process took exactly 1 hour, from the time I started until I was able to lay up the first blister. I started the weekend by finishing the turtle deck and the Vert. fin covering. Both of the projects were very simple and straightforward. Before I painted the base coat on the Turtle Deck my Buddy Ed was at again with my I phone camera, He took this picture from the rear near the horizontal attach.

Second Chantz Ballistic Chute
 This will be the last time this is visible.
















Tail lowered so I could access the top of the turtledeck
While I am finishing out this plane, my Buddy Ed is painting a newly recovered wing of a Citabria. He was telling me that the complete weight of a traditional fabric covering for a plane like a Cub can be as high as 60 lbs. My plane is equivalent in area to a J3 Cub, and so far I estimate the complete fabric and finish weight is about 8 lbs.




Me and Boo

Other than Ed, Boo is my only friend who comes and visits when I'm working. I'm starting to get excited, I actually see the light at the end of the tunnel. The number of final jobs can now be counted on one hand. I installed my new tachometer this afternoon and revised all of the wiring. That was one of the last nagging projects I needed to address. From now on every time I install a rudder, aileron or elevator, it will be with final hardware.




Robin and Boo man!



The Lines of the Robin are really evident in this view. The fabric is literally the planes skin, and that can leave nothing to the imagination. I have received quite a few compliments on the lines of the Robin, but its really Rene Fournier who should receive credit. I tried to stay as true to his original design lines as possible.







Rudder cable fair lead lug

This is a picture of the rudder cable fair lead lug. A 3/4" nylon split Piper style cable fair lead installs here. I couldn't spend all of the effort to streamline the Robin just to leave 2 square inches of exposed frontal area. So on Saturday I started the process of making a master model and mold for a lightweight fiberglass blister that will fair this and be bonded to the fabric.






Blister loft
The project started by laying out a simple 3x body of revolution. Since the lug is 1" tall and 1.9" wide, a 3x after body was laid out with simple conic curves. Two copies of the plot were made.



















Loft glued to a floral foam block
Since the blister is half as tall as it is wide I cut one pattern in half and then glued them to a block of Urethane floral foam block. The leading edges were aligned with each other.












After cutting on the band saw

It was off to the band saw with the foam. first the side profile was cut and then the center was left in the excess block which was used as a cradle while the plan view profile was cut. In terms of lofting this block represents the max half breadth and the inboard profile. All that is left is to use a small bit of the foam as a sanding block and round off the corners. You work both sides evenly because as the sanding block werars it transfers the contour to the block. By switching left to right, you get an equal shape


foam block positioned on my tooling plate
I used a small dab of bondo to adhere the foam to the tooling plate. The plate is well seasoned with hundreds of coats of wax already applied over the years.













Foam with one layer of fiberglass applied
I used Polyester resin and a single layer of 1.5 oz fiberglass. The surface needs to be sealed and a final finish needs to be applied.














Bondo added

After thee resin cured, a sweep coat of Bondo was added. I usually thin the Bondo with MEK prior to mixing it. Any way, a layer was added and the the Bondo was sanded. Don't screw around with fine paper at this point. The first coat was worked down with 80 grit. After a nice and level surface is achieved, I swept in a 1/2" fillet radius of Bondo between the table and the block. 180 grit paper was then used to further level the surface. A final sweep coat of Bondo was then added and 320 sand paper was used to smooth it out. 5 coats of Carnuba mold wax was then added
Gel Coat applied

I didn't want to get my Gel Coat gun dirty, so I just poured catalyzed Gel coat on over the form. Kind of looks like a rotten egg doesn't it?

The Gel Coat is allowed to tack off and when its about the consistency of firm rubber, more resin is poured over and layers of roving mat are added to reinforce the Gel Coat.







Mat being added

Now it looks lime some primordial animal!!! The thickness of the Gel Coat and fiberglass all but assured this would cure with heat due to the exothermic reaction of the resin. The heat actually melted the mold wax and the blob just popped off the tooling plate.









Mold
This is the mold when it was released. I trimmed the edges up and then sanded the Gel Coat with first 320 dry and then 400 wet sand paper. The smoother you make the mold the easier it will be to release the final part. So far up to this point I had spent about one hour. I was now ready to lay up the first blister.









Two types of Fiberglass

Now this is a lesson learned. I am now laying up a fine surface cloth before I lay a heavier structural ply. With contact wet lay up, the laminate is very porous and the final finish is prone to pin holing. So I now use a light weight fiber (1.5 oz) surface ply and then I follow with one ply of 8 OZ Rutan style BID fabric. Epoxy is always used for fly away parts. Polyester resin is not very dimensionally stable and will eventually warp.





Blister Temporally installed

This is what it will look like. There will be an 1/8 " slot cut in the after body and I will slip the cable thimble and shackle thru it. The blister will be cocked to align with the cable. I will bond it on with fabric cement.
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