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.










Fuel Tank Rebuild, new Trailer and Firewall rebuild


During the initial engine run up, a number of problems became apparent. The engine head was cooling properly, but the rest of the engine compartment was overheating. It also became apparent that I had too much pitch in the propeller. I reduced the pitch to the least angle available and I will use this setting for the retest. According to Leon Massa of Compact Radial Engines, the engine will really heat up at 6200 RPM. I have yet to see more than 5400 RPM. The tuned pipe I designed, was sized to start delivering a secondary boost at 6200 RPM. So hopefully the increased prop inflow will deliver more airflow for greater cooling at the higher output. The under cowl temps were getting too high after shut down, I recorded  300 degrees surface temp on the upper surface of the cowl. I have no doubts about the composite being post cured now!! As I outlined in the previous posts, I removed the fiberfrax from the firewall and then re-coated it with Fiber Frax and then a shield of .012 aluminum. I reinstalled the engine and added another heat shield between the exhaust and the fuselage fabric. Another problem with my original design was mounting the instrument panel permanently to the canopy. This turned out to be unsatisfactory because I cannot run the engine without the cowl installed. I am revising the design such that the instrument panel is on a separate hinge and will swing out of the way when the canopy is opened up. I have decided to use an original Fournier RF4D canopy in the future. I found a very good source who can make them for a reasonable price. Even though the canopy is thicker than the Lexan I am using, the elimination of the frame and housing will result in a net weight savings. Speaking of the Canopy Frame. I modified it to remove the solid bulkhead, this will interfere with the seat head rest, so I had to cut the foam core and complete a "corner Closeout"

the exposed pink foam shows the area I cut away.
the pink area was where the bulkhead was. I cut it out and then knife cut the pink foam away. I made a cut parallel to the existing laminate along the surface and then cut in at 45 degrees to remove a foam wedge approx .25" x .25" The exposed inside laminate was sanded with 180 grit paper to roughen it up so it can be secondarily bonded.



cut foam and sanded inside laminate





this shows what the edge looks like before dry Micro was troweled into the cut. Dry Micro was used, but that by itself has very poor adhesion properties. The technique here is to paint the surface first with raw resin and then trowel the Micro in.











Dry Micro
Dry Micro has a consistency similar to cool whip, its about as lite too. The key is that the mixture has a dull, non shiny surface.
















laminate added
 
2 plies of 8 oz fiber glass was added over the micro and exposed foam. Its only necessary to wet out only 1/8" beyond the edge. In approx 12 hours the epoxy will be cured to a semi hard state that allows easy knife trim. after trim, the edges can be slightly rounded over because the micro will be exposed.
This is a common technique that is used to finish out a structural edge. Sometimes, the foam us routed out flush down to a 1/4" and unidirectional roving is added in place of Microballoon, this would greatly increase the bending stiffness of the canopy frame. It is not required on an Ultralight however.

The next big job was to redesign the fuel tank, again this is a temporary solution. I am going to adapt an off the shelf polypropolyene tank, this will offer the maximum resistance to Ethanol. Its unreasonable to assume that pure gasoline will only be available in the future. I need to talk to Leon about revising the carb and Engine seals.  I am really amazed what happens to epoxy when exposed to ethanol in gasoline. 20 years ago when I made fuel tanks similar to this for my old Wren, Ethanol was not used in fuel and I never had any issues. Because of this, I would NEVER consider buying a home built composite airplane that uses Mogas. Wet wings especially like the Lancair would be destroyed.
existing fuel tank mold

The rebuild started by laying up 3 ply's of 8 oz and one face ply of 120 glass in the existing mold. The mold was prepped with 5 coats of mold release wax. This mold is well seasoned, so 3 coats is sufficient





existing mold

3 plies of 8 oz and a surface coat of 120 glass were laid up.  All seams are overlapped a minimum of 1.5 inches










Bottom surface mold form
Instead of laying up the bottom surface on a flat plate, I decided to make a foam and Bondo sump. This will become the low point of the tank. This is a temporary one time tool.







lower surface mold form

I added bondo to fill the foam surface, and then sand able primer over the top of the mold form. 10 coats of mold release wax was added and then a coat of spray able PVA water soluble release. After the mold was treated, I laid up 3 plies of 8 oz fiberglass and one surface coat of 120 glass.


bleed air being added to aid the cure of the sloshing compound

After the bottom surface was cured and released, I bonded a glass bearing block into the base of the sump. This will be drilled and tapped to accept some barbed nipples for the fuel line and primer line.  I rough sanded the inside of the main tank and the bottom surface. I mixed up some epoxy and cotton Flox to a peanut butter consistency, its very important that this mixture have a shiny surface. I then bonded the upper surface to the lower surface.  After the tank bond cured, I drilled a 3/4" hole where the fuel gauge will go and started the sloshing process. I am banking on this sealant to give me enough life of this new tank for the flight test program. I will constantly check the tank for signs of ethanol degradation

And Finally!!! The reason I was late in updating this blog was a trip I made to Pearson Georgia to South Georgia Cargo. I got a hell of a deal on a 24' enclosed cargo trailer. I will convert the interior into a camper and a hauler for my plane. The trailer was sold thru Allpro Trailer out of Florida http://www.allprotrailers.com/  Ask for Brian Robison  brianr@allprotrailers.com The price was $3800 base , but I ordered 5200lb axles, the final price came to $4300. After my buddy Ed and I picked up the trailer, we immediately headed north to Wilson, North Carolina where Ed bought a very nice Piper J3F cub project. Ed started a blog similar to this, http://nc35447.blogspot.com/2012/06/data-plate-confusion.html  give him a visit sometime.


Robins new Home away from Home













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1 comment:

Scotty said...

Looking good Mark!
I cant believe the progress this winter. You are sneaking up on it quickly. Love it!

June 25, 2012 at 8:04 AM

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