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.









Canopy frame and Windshield


Completed Canopy Frame
 The canopy Frame is designed to pivot at the extreme FWD upper edge. The canopy is designed to pivot 90 Degrees Vertical to allow plenty of room for entry. At the 90 degree position the canopy is removable due to a slot in the canopy hinge fittings. The Fwd portion of the frame is built up similar to the FWD fairing. The process starts by bonding a horizontal foam shear web to the two canopy side rails. The side rails are made from laminated 1/4" birch Plywood.  Multiple pieces are laminated together and then a final Bevel cut is made on the table saw.  After the foam is Bonded to the shear web, a light layer of fiberglass and epoxy is laid. The main purpose of this ply is to protect the foam in the event of a fuel leak. 
Canopy shear web and rails
The foam formers are added after the glass cures.
As mentioned earlier, all foam to wood Joints are made with 5 minute Epoxy. This greatly speeds up assembly time.








Canopy frame with FWD stringers and formers.

The formers and stringers are added similar to the FWD fairing. 1/32nd Birch Plywood fabric transition gussets are also added. The alignment of the rear most former is critical as it must be angled to clear the control stick and provide a FWD flange for the actual canopy to sit on.




Canopy partially open
Unfortunately the Shop ceiling is not high enough for a full extension of the canopy. Once opened there are cable guy braces and a small strut to keep the canopy in place when opened. The rear frame and Yarmulke were added to the rails during the early build. The next step is to make the windshield template.



Windshield in place

The Windshield is a flat wrap design, there are no compound contours that would require heat forming. The purpose of the Yarmulke was to remove compound contour and act as a sun shield. Because of the number of variables with a plans built frame, the actual windshield profile must be developed from a template. I used taped together poster board sheets to develop the template. Once this is derived, it is transferred to the Lexan. The Actual Windshield is .032" thick therefor it is very easy to trim with the use of metal tin snips. The final attachment to the frame will be with polyurethane automotive windshield adhesive and countersunk 1/8" pop rivets. These are really nothing more than clamps used to hold the canopy down for cure.
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Canopy Frame and Yarmulke

I originally called the compound contour section of my canopy the "beanie" but after my first trip to Israel I made the acquaintance of Bob Goldrich, a fellow Aircraft engineer and small plane enthusiast. I have decided to rename the "beanie" the Yarmulke in his honor. The Hebrew word for the Yarmulke is kippah which literally means "Dome" If you could see Bob's Yarmulke, you would immediately notice the similarity. In Judaism the purpose of the Yarmulke is to remind the wearer that there is a higher power above his head, in the Robin, the purpose was twofold. It provides a sun shield and offers a way to transition the compound contour to the single curvature flat wrap of the windshield. Again in the spirit of die hard home building, I have laid out the loft lines and construction procedures using my floral foam master model technique that can be used to fabricate this part. Again, since I intend to offer this part as a molded component I continued with the floral foam process and made a full female lay up mold. I will outline the steps I took to build this part. This is very similar to the earlier part of my blog about the wing tip master model process.

Templates

As in the case of the Wing tip, a full set of loft sections will also be offered full size or as laser cut paper patterns. It is possible however to enlarge the sections from the drawing. There is a reference square grid pattern that can be used to adjust the enlarge percentage. It is only necessary to enlarge only one half of the loft. Once the scale is correct, a full size (100%) copy can be made and used for the opposite side. After the templates are generated, they are glued to a sheet of 1/8" Masonite board. Use a solvent based spray adhesive like 3M 777 to adhere the paper to the Masonite. Do no use a water based adhesive, it will cause swelling of the paper and the accuracy will be lost.

Yarmulke plans
 This is the top drawing of the Yarmulke loft. If careful attention is paid to the assembly and shaping of the foam, a very accurate copy of the surface can be duplicated. Or you can just buy the pre molded part!!









Foam blocks being cut out
Just a reminder about this process, its very important that foam from the same manufacturer and same batch be used for this process. Do not mix. When the blocks are pieced together, make sure that the adhesive (either hot glue of 5 minute epoxy) is always used well below the final contour of the part. Glue in the seam will disrupt final shaping.




Blocks being assembled on a Pink Styrofoam base

The base of the yarmulke is cut out from 2 lb density pink or Blue Styrofoam. This material will not sand readily compared to the green floral foam. All of the pre cut blocks are bonded to the base with 5 minute epoxy. Hot glue will melt the Styrofoam. After the glue cures, the foam is rough shaped with a butcher knife. It is only necessary to trim within a 1/2" of final contour.



After final shaping
A foam block is used to sand the foam. The principle here is the same as grinding optical glass. The foam abrades itself as it sands the master model. The result is a spherical interface. the results are actually quite amazing using this method. Do not worry about the gaps between the blocks. Study this image when you get ready to bond the blocks together. This will give you a good idea when the bond lines should be. 
view looking aft

After the final shaping 3 layers of 8 oz BID cloth is laid up over the foam. its a good idea to seal the foam first. I have tried a number of methods. The best results I have obtained used sand able spray primer. At least 6 coats are sprayed on and let dry. A couple of coats of carnuba mold wax is then laid on top of that. followed by the 3 ply of glass. The idea is to seal the open cell foam so the epoxy resin doesn't leech into the the foam. If it does this, it will draw in air. I usually coat the foam first with raw resin and let it sit. If there are any open pores, they will fill with the resin.


Fiber glass being trial fitted
 The fiber glass is trial fitted before wetting out.
















 After Cure and before release the final surface is finished out. You can use epoxy mixed with micro balloon to the consistence of peanut butter as a surfacer/filler. or you can use automotive Body filler or "Bondo" for the home builder, that is the final step before fitting and trimming. The next Blog entry will show the construction of the actual canopy frame.
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