Robin Ultralight

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.

New Landing Gear

I have been patiently waiting for my new Aluminum Landing gear from Grove Aircraft in San Diego. My friends Mike and Neal Lafrance actually own the hanger next to Grove.. Even with that kind of pull, I still have to wait 8 weeks for the gear. As Robbie explained it to me, its out of his hands because the gear has to go to a heat treat shop 3 times during its construction. I have 3 more weeks to wait!!! I decided that the simplest way to add a landing gear to the Prototype was to attach it to the outside of the fuselage in the same location of the old one. I needed to add some additional bearing blocks and increase the area of the two diagonal stiffeners . Truth be told, I should have done that for the original gear.My goal is to have all of the modifications done and the Fuselage recovered by the time I get the gear. I want to be able to just bolt it in and then flip the plane over.

reinforced gear frame

in this picture the extra doubler on the two diagonal members are shown. all of the vertical loading of the gear is reacted by these members. They also need to react the aft braking load by a heel and toe reaction between the gear fitting bolt and the back wall of the fitting. underneath the plywood is 1.5" of birch plywood bearing blocks. .

outer landing gear fitting

This picture shows the outboard fitting for the new landing gear attach fitting. There will be a narrower fitting that is bolted to the inside of the gear frame. Both plates will be welded to a .125 plate that will act as a bearing plate. I will have two 3/8" bolts permanently brazed to the 1/8" plate. I will counter bore into the birch doubler to clear the bolt head. Sitting on top of the plates will be a radius block. The gear will be sandwiched between two of these radius plates. The isead here is to limit the gear bending moment to the gear itsself. The radius blocks will act like two fulcrums.

Unfortunatly this new gear will add 8 lbs to the Robin and may put the prototype out of the part 103 category. I'm sure with the redesigned wing and the single cylinder 2 stroke engine, Part 103 will still be possible. After my talk on Labor Day, the universal opinion of everyone I talked to was to scrap the 2 stroke and Part 103. My Friend Murry Rozansky who is the ESA Treasurer, suggested that I look at the 23hp Briggs and Stratton Vanguard conversions. After some research, I have decided that this is the perfect engine to adopt. The Prototype is actually perfect for it from a weight and balance point of view. My CG will move forward to 22% at full fuel. 26% empty. The static margin increases by 10%. This engine will be far more reliable than the 2 stroke. I will however fly this plane on the MZ 34.

2 comments:

Anonymous said...: Hi Mark,
Just a weird random thought..

..could a landing gear spring be made from a 1985-mid1995 GMC Safari/Astro fiberglass leafspring?; September 1, 2013 at 9:29 AM
Mark Calder said...: Theoretically I guess it could, but the configuration is all wrong; September 1, 2013 at 2:29 PM