Robin Ultralight: Engine Cowl baffles and Rudder Pedals

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.

Happy Labor Day!! The weather has finally broken in Texas, Today is a beautiful day and the first time in 2 months that I can work in the shop with my shirt on!! You guys have no idea how uncomfortable you can get after sanding fiberglass or welding all day with no t shirt on!!! I have been using this 3 day week end to good advantage. I started on the cowl cooling baffles. This is an extremely critical portion of this design. I have learned the hard way about the importance of engine cooling. In an effort to save as much weight as possible I removed all of the engine driven fan blower and shrouding on my old Kawasaki 440 engine I was using on My Wren. I knew I needed to correctly duct the inlet air across the cylinder, but I completely neglected controlling the total air flow. Air is lazy, it seeks the path of least resistance, and usually that is anywhere but the air passage between the cooling fins, I initially thought I could fabricate a Styrofoam mock up of a duct and then cover it with one layer of graphite cloth. MEK would make short work of the core after the Graphite cured, but I found that in building the duct, the volume had to first decrease then increase to the final volume. This would choke the flow. The idea is to slow the flow before it passes thru the cooling fins, but not restrict it. i tried a few different ideas but finally settled on a scheme where the cowl was divided in half before the engine. The only path for the inlet air will then be the passages around the Cooling fins.

Baffle Supports

I started the construction by removing the carburetor. I used two aluminum angles and picked p the 4 screws that attach the reed valve body. The next step was to fabricate an aluminum baffle that attached to these angles and spanned from the upper cowl to the lower cowl and from the firewall to the front edge of both cowls. This was a tedious effort. I started by cutting a sheet of .020 aluminum oversize.

beginning of the Baffle

I kept trial fitting the lower cowl until I had a 1/2" gap between the baffle and the lower cowl and fire wall. All along the periphery of this baffle an angle will be added to attach a 2" wide silicon Cowl baffle strip.

Baffle with cut out

after the divider was built , I fabricated a split aluminum duct that shrouds the cooling fins. Once sealed up the air has no place to go except across the fins. I had to trim back the upper Left corner to clear the upper cowl.

Trimmed duct

After the duct was trimmed I fabricated a 1/2" wide curved flange that will attach the Silicon Baffle.

Baffle flange attached

I am now waiting for my next order from Aircraft spruce to arrive with the Baffle and a pre-drilled retainer strip.
I believe that i am dealing with the two most efficient companies in the world when I order from Aircraft spruce and its shipped by UPS. They are a very well oils close meshed team. I can order something on a Monday and I will be installing it the following Thursday.

Once I got as far as I could with the baffles I started installing the adjustable rudder Pedals. During the construction I realized that I didn't need to use 3/8 4130 tubing for the cable guides, 1/4" Copper tubing worked fine. i was able to force 3/16 Nylon hydraulic tubing through the center after I bent them. These will act as strain reliefs and cable guides.

Rudder Pedals being trial fitted

3 comments:

Jeff Page said...: Mark,
Your Robin is a truly beautiful creation. Thanks so much for sharing your work thus far. Do you foresee using the Robin in a powered sailplane mission? Would it be practical to run a pull start chord to the cockpit?

I’m in the beginning stages of building a Legal Eagle XL and am getting a lot of enjoyment and learning a bunch with my build.

Keep up your great work,
-jeff page
http://windsor-legal-eagle-xl.blogspot.com; September 16, 2011 at 1:56 PM
Mark Calder said...: Jeff, thanks for the kind comments. The robin IS an ultralight motor glider. Its roughly patterned after the Fournier RF4D motor glider.; September 16, 2011 at 3:32 PM
Mark Calder said...: I have removed the pull starter from the prototype, I am not taking any chances on going over 254 lbs. i have identified around 20 lbs of weight savings for the final design. Because of this "niceties" like a starter and brakes will be possible.; September 16, 2011 at 3:35 PM