Robin Ultralight: Omissions, mistakes and other excuses!!!

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.

There has been a nagging problem since the day I installed the fuel tank, no access after it was installed. I always figured I would correct this problem when I drew up the final set of plans, but I began to realize that if I have any fuel tank issues after I cover the plane, it will take a week to repair it. So I decided to bite the bullet and just fix it. This is all the more necessary because I just bought a set of beautiful Belite Instruments from Jim Weibe in Wichita Ks. http://www.beliteaircraft.com/avionics/ One of the gauges he makes is an electronic fuel gauge. This requires the addition of a fuel probe. I would need access to the tank anyway. I have decided to make the front turtle deck that covers the fuel tank a separate piece. I will attach the Turtle deck with two hinges. Since the access is not required on a daily basis, this will work well.

I began the process by removing the Front Turtle deck. I used a tool that has become indispensable in my shop. A Chicago Pneumatic Oscillating multi- tool. The product cleanly separated the turtle deck right through the existing bond. This tool is also very Handy when it comes to trimming cured fiberglass or Graphite. If you use a carbide tile grouting blade, it zips right through cured composites like nobodies business.

Cut off Turtle deck

After I cut off the Turtle deck, I rechecked the weight. .4 lbs. I will have to rebuild this section by adding two horizontal longerons that will close out the section.
I also needed to "close out" the foam and glass panels at both edges to accepts the fasteners that will mount the hinge

Foam close out being added

This is a typical Composite detail. When fasteners are introduced into composite, the core is usually increased locally in density to react the bolt clamp up loads, or the core is removed and the inner skin is transitioned to the outer skin or "closed out" I chose this method because this joint will be primarily in shear.

Fuselage sans the turtle deck

Unfortunately I had to remove the engine again. to do this modification.
its was a pain, but well worth the effort if I ever need to service the fuel tank.

Rudder Pedal "bow Sprit"

Another omission I righted this week end was the lack of a Pedal return spring on the rudder Pedals. I had to add a small extension tube "Bow Sprit" to attach the springs to.

I am getting pretty close, once these two jobs are finished, its on to installing the Instruments , throttle and wiring. After that is finished, I will re rig the wings and route the Aileron Cables. Then the last little job will be to make the tip caps of the Vert fin and the rudder. I am not going to make molds for these parts yet because my eyeball engineering and past ultralight experience tells me I may need more rudder. Everything is in proportion to the original RF4D, but the slower speed means the rudder and vert fin may not be as effective as the original. That's what flight tests are for.

Welcome to my Robin Blog.

Omissions, mistakes and other excuses!!!

No comments: