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.









LOWER COWL

When I left off on the cowl, I had flipped the Robin over and  supported her with a false spar attached to the wing spar attach bulkhead. The fiber glass surface of the cowl  was sanded down to knockoff the high spots. A coat of automotive bondo was then swept over the entire surface. The first goal of the final sanding is to eliminate large waves in the surface. Later the goal will be to fill small pin holes and divots.
initial rough sanding
I will sweep in the junction between the cheek cowl and the cowl side. In order to do this I want both surfaces to be relatively wave free. I added additional bondo on both of these surfaces and used my air file board to level them.
new circular drag
I made a larger Circular drag for the lower surfaces. The method I use is to sweep the fillet in 3 stages.
1st sweep
After the first sweep, I let the bondo cure and knock off high spots with a tubular sander with 36 grit paper stuck to it. There is no sense in using anything finer at the initial stages.

2nd sweep


Again, I  let every thing cure and then knocked down the high points with a tubular sanding block. I used flat sanders on the flat areas and tubular sanders for inside fillets and compound contour areas. Never mix the two because you will leave streaks in the bondo.

third sweep
As you can see the fillet is fully formed. I swept a thinned coat of  Bondo over the entire surface. I thinned the Bondo with a product called resin honey. Its actually Styrene monomer. It thins out the Bondo and will fully cure once it is catalyzed. After the entire surface is swept with thinned Bondo, I drop down to 80 grit and continue leveling the surface. I am now paying attention to small depressions and pin holes.

blocked out with 80 grit

I'm getting pretty close to a wave free surface. At this point its time to remove the spinner. The actual cowl needs to close up the area around the crankshaft. The spinner was added initially to aid in the fairing of the foam blocks.
spinner disc installed
I cut a disk of aluminum the same diameter as the spinner base. I also cut a 2" hole in the center that matched the crankshaft prop hub flange. This centers the disk. I used Bondo to bond the disk to the exposed foam in the cowl. I then added Bondo around the edge of the disk to fill in gaps between the disk and the cowl surface.

finished disk
More filling and sanding and the cowl is now ready for its last fill and sand step. I will use a catalyzed spot putty.
spot putty

Spot putty is like bondo but much thinner and consists entirely of hollow micro balloon sphere filler. This makes it much easier to sand and allows me to now drop to 180 grit sanding paper.
spot putty coat
The entire surface is coated and then blocked out using 180 grit paper.


sand able primer
 After sanding the spot putty, I sprayed a coat of high solids filling primer. This is super easy sand-able paint and is used to fill sanding marks. The paint is blocked out with 220 grit paper. After the sanding is finished I used a LED flash light to inspect the complete surface looking for minor imperfections and pin holes. I circled every defect with a sharpie and re coated locally with spot putty

inspected surface
I also struck a level line in preparation for the next step I will add a split plane that will be used to form a flange on the fiber glass splash. I am not making the actual cowl mold here. Normally this step would he done with Ultra-cal Plaster, but that would add over 200 lbs to the fuselage which is already nose heavy. What I am making here is a lift of the OML (Outside Mold Line) surface of the cowl. I will still need to add inlets. That will come after I make the OML splashes.

split plane
This is the beginning of the fabrication of the splash split plane. I was aided by 15 year old Eloi Mathieu. Eloi was a very enthusiastic helper. He is the Brother of Perrine Mathieu who once worked for me at Triumph Aerostructures. Eloi had his first opportunity to run a band saw. It took a few tries but he rapidly got the hang of it and cut out boards with very good precision. Its still not 100% possible to fill the small gaps between the board and cowl surface so the gap is filled in with modelling clay.

modelling clay
This shows the precision of Eloi's cut vs mine. His board is on the left. The lower half of the cowl is now ready for 3 coats of PVA  (Poly Vinyl Acetate) release agent. I need to be able to shoot gel coat within 12 hours of spraying PVA. In high humidity, the PVA will absorb water and swell.

Thats the subject of my next blog entry




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