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.









Wing construction

main wing assembly jig. This used
every clamp I own
The wing construction started in April of 2009. The spar mold was built up using sections of MDF board. The first project was to lay up a test wing spar panel to verify the design concept. The spar itself was a "C" section shear web with .125" carbon pultrusion "Graphlite" rods laid up as a secondary bond in the corners of the "C". The rods themselves were completely encased or "bedded" with a mixture of cotton flox and epoxy mixed to a paste consistency. This is an area that is a source of additional unneeded weight in the wing design. All future spars will eliminate the use of the round rods. The voids between the rods add considerable weight when multiplied by the 44 feet of total spar length (there is a 2 foot overlap in the root). The first test panel was also designed to static design criteria. Later it was discovered that the test wing exhibited excessive deflection during test and prototype wing was revised to limit deflection by the addition of more Graphite pultrusions.
One of the design problems associated with the use of carbon pultrusions is load fitting termination. I debated a number of design concepts for the wing root attachments, but finally decided on overlapping the root spars. Because of this, the Robin Prototype wing is actually asymmetrical in configuration. The left wing sits 1.5 inches ahead of the right wing. The two spars overlap and sit between a box frame section in the fuselage. Two large shear pins pass through both walls of the box frame and both spars. The advantage here is that the wing bending moment is internally reacted by the overlapping spars and not in excessive structure built into the carry through fuselage box. The disadvantage that I later discovered during construction is having to drill the two holes!! This problem is so great that the overlapping of the wing spars has been eliminated on the production design and an alternative spar attach scheme is being used. The other great disadvantage in the original design is ease of assemble and disassembly in the field. The final design is far easier to field assemble.

aft ribs being assembled
 In the years I have been designing ultralight airplanes I am amazed at how many designers are actually ignorant of the design load conditions of the wings. There is a belief by many amateur designers that the worst case design condition regarding "Drag" is a rear acting load that is trying to tear the wings off the plane in a terminal dive!! Consequently these designs have all featured some kind of "drag" wire that attaches from the fwd fuselage to the wing spars. In reality the aft acting shear load or drag on an aircraft is quite low. In terms of actual numbers consider the condition of equilibrium of an ultralight pointing straight down in the zero lift condition. The total drag acting on the aircraft is exactly equal to the gross weight. In the case of the Robin this would be 550 lbs.



 Now divide that number between all of the components and you will find that the actual load on the wings themselves is in the area of 80 lbs per panel. The actual design criteria that determines the horizontal shear load is the abrupt pitch up maneuver at maximum maneuvering speed. When the aircraft is flying straight and level at the max maneuvering speed, the forces of lift and weight are balanced, as are the thrust and drag. But when the nose attitude is abruptly increased the lift vector shifts forward and increases the load acting in the fwd direction on the wing. 
Leading edge skin being trimmed


This induced vector in the case of the Robin is around 375 lbs acting mid span in the fwd direction. So you can see that the actual horizontal loading is around 8 times greater in the fwd direction as it is in the aft direction. This is why the early aircraft used to fail by the wings folding fwd. To react this load in a weight efficient manner, I took advantage of the deep leading edge "D" cell box. There is a leading edge spruce spar and leading edge fitting designed to react this load. The load is reacted by the fuselage in a couple between the nose spar and the main wing attach pins. Horizontal shear is reacted by bearing the wings spars into the main attach box.

Both Wings D cells laid out

I chose the the configuration of the wing to try and achive laminar flow over the first 33% of the wing. The airfoil is a Wortmann FX 170, low speed, high lift laminar airfoil. This airfoil is designed to be laminar over the first 32% of the upper surface, because of this I placed the wing spars at the 33% chord point. The nose "D" cell also acts as a closed torsion member to react wing twist due to aileron deflection and airfoil pitching moments. The aft section of the wing is covered in light weight Dacron fabric. I intend to bond the fabric with a heavy "pinked" edge right at the 32% chord point. This is designed to emulate sailplane turbulator tape that is usually placed at this point on modern sailplanes. The idea here is to force rapid transition of the laminar flow into turbulent flow that will be contained in a small but attached boundary layer. This is one of the riskiest concepts in the prototypes design. I am banking on a benign stall characteristic because of the low wing loading. Its not called "Experimental Aviation" for nothing!! As I said in my opening mission statement, I want to push the design beyond the low drag of my old Wren.


main aileron bay rib. All bonds between
 foam and wood are with 5 minute epoxy
The remainder of the wing is constructed very similar to my old Wren and that of The Sky Pup, I am using Styrofoam core for the rib shear panels and spruce wood for the rib caps. The big advantage here, besides ease of construction is a wide rib cap. This greatly increases the fabric bond area and eliminates the need for rib stitching. If I were to redesign the Robin into a LSA or a  higher speed homebuilt, these ribs would become wooden trusses and require rib stitching. You can get away with this in an ultralight when your maximum level speed is only 63 mph.

Lower Aileron Cove after forming with ammonia

The current prototype wings weigh 48 lbs per panel; This is higher than my original estimate. Because of this, the wing spar is being redesigned to eliminate the round carbon rods and the fiberglass shear web. I have identified a 9 lb per panel weigh savings in this area alone.



Root of wing showing pre cured fiberglass stiffeners




In the next post I will show pictures of the wing test and the test configuration

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