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.









Cowling continued

I have finished extending the cowl surfaces to the table. This was a ton of mostly useless work. The plaster master was built slightly beyond the trim line, but the cheek cowls may or may not have been made long enough. I figured that I would just do a tangent extension of all the surfaces and then there wouldn't be any question if  the mold was big enough.


Extended surfaces
The surfaces were extended with Bondo. Like I said, a ton of useless work.




The next step was to seal the plaster. A couple of coats of Shellac thinned with Alcohol are brushed onto the surface. The dry plaster absorbs this like a sponge. Its important to seal the plaster so the release agents do not soak into the surface. My friend Duy came out Saturday to help. After the shellac dries a number of coats of Carnauba mold release wax is applied. Between each coat water is sprayed on and power buffed out. This is known as "spit shining" in the military. In reality its called water hardening. It creates a tougher surface finish for the wax and a release plane. I put 4 coats on the mold and hardened each of them. After the wax was applied, a fillet of Bondo was laid at the table base.
Duy adding Shellac



Bondo fillet
This is added so there will not be  any voids in the tight corner. The fiberglass mat will not lay up in a sharp 90 degree corner. The gel coat needs to have a solid backing or the surface will crack out. A void at the surface will cause gel coat cracking. After the fillet was laid, I scribed the master at the cowling trim line. Normally a mold is made such that both halves part at the same plane, but in this case, I want the upper and the lower mold to overlap each other by 1 1/2 " Both molds will have the same transferred scribe line for the final trim. I need to make an overlap flange for the lower cowl. In order to do this I want to lay up the actual part in the lower cowl mold. I will let that cure then I will put down a layer of silicon based peel ply and then lay up the overlap flange on the inside of the actual cowl. Since the cowl mold overlap the trim by 1 1/2" the overlay flange will be in the correct position. I will then trim the lower cowl and then bond on the overlap flange. This saves me 4 more intermediate tooling steps.
The next step is to construct the parting plane. I will cover this step with a little more detail because the clay trick is really neat and I want to show the detail of how to do it.




This is the beginning of the mold plane. I am using 1/8" tempered Masonite. I bought a sheet on Saturday and was planning to leave it in the back of my truck until I needed it on Sunday. On Sunday afternoon I went to retrieve it and it was gone!! I thought I was losing my mind too!!! Then I realized it probably vibrated out of the back of the camper shell on my truck. So I went to buy another one and sure enough, I found my original sheet busted up alongside the road out in the boonies. Because its not nice to "Mess with Texas" I cleaned it up and added it to my burn pile. Tempered Masonite has a nice smooth finish. The surface is still porous, so it too needs to be wax sealed. I rough cut the Masonite and you can see a few gaps between the Master and the template These are easily filled with modeling clay. the Template is attached to the master with small daps of Bondo. This is why the master was waxed with Carnauba wax.


Modeling clay






You can get this clay at any crafts store of fiberglass supply shop. being very generous, roll up some pieces and stuff it into the seal all along the edges of the template boards. Overfill all of the Gaps.




In order for the clay to stay in the gap, the back side of the board must have a tongue depressor hot glued to cover the gap.






Gaps covered


here you can see the tongue depressor covering a gap and the daps of Bondo holding on the board.












After the gaps are filled make a wooden scraper from a tongue depressor. sand the edge square and then add a sharp bevel.
Beveled tongue depressor


you can buy Tongue depressors by the carton from Aircraft spruce. Its cheaper that stealing them from the doctors office given the co-pays these days!!!
















After the gaps are filled its time to scrape away the excess clay. Take your time and slowly work into the clay lump. Use the template to keep the scraper flat and square to the surface, The results are very   clean and work quite well.




Before and After
This picture shows the clay before (upper) and after scraping (lower)


































filled gap


this shows how well a large gap can be filled and why its not critical that the fit up be exact between the master and the template. The next step is to wax the template with Carnauba and then spray PVA release agent on the template and the plaster. I will cover that step and the mold lay up in the next update. I have to order the PVA so it may be a few days.



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