Robin Ultralight: September 2011

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.

Final Cowling pictures and internal Baffling

I have finished the fire wall forward. The Fiberfrax is installed, the latex primer is applied and the fuel and electrical wires have been run. The internal baffling is finished. I have some very thin countersunk Stainless steel washers that I am bonding to the surface at every fastener. This will reinforce the countersunk holes very cleanly.

L/H side

My Friend Ed ttp://mmwauto.com/index.php/services.html did the final paint on these parts. As you can see from the pictures the finish is excellent. He has a huge down draft paint booth. Its big enough to fit both of my wings and the fuselage. The paint by the way was Summit racing single stage Ureathane paint. http://www.summitracing.com/sitemap.aspx?ibanner=FD4

R/H side

Unfortunatly I had to cut away the blister for the tuned pipe. A 4 stroke version will have the full blisters, but it will be a new cowl. I'm looking at an all composite version in the LSA/Experimental class. Wood is really light, but it does take some time to work with. My new idea is to build most of the fuselage with flat sheets of 1/4" PVC foam core pre cured in a platten press with a layer of 120 fiberglass on each face. Preliminary sizing of the shear panels show this would be adequate for a fuselage. A structure built like this would not requite much final sanding and prep.

Completed baffles

This is the final design f the baffles. Not shown is another Baffle that seals off the back half of the side blister. There is only one place now for the air to flow and thats across the cylinder cooling fins.

looking aft at my messy shop

Some more pictures

Tonight I am correcting a design problem I should have addressed when I first built the fwd turtledeck over the fuel tank. I'm making it removable. I'm buying some of Jim Weibes beautiful lightweight instruments, one of them is an electronic fuel gauge. I need to remove the tank so I can add the sender

Rudder Pedals and the Zimmer skimmer

I apologize for not making an update sooner, I’m at that phase of every project where the tiny details start to eat up the most time. A friend in Wichita once told me this is the phase where you are 90% complete with 90% left to do. My squawk list is getting huge. The number of “final” jobs range from ordering new bolts of the proper grip, to adding Fiberfrax to the firewall.

In that regard I have completed the cowling and internal baffles for the engine. I am now ready to start routing electrical and throttle cables. But before I can do any of that, I needed to pull the engine and cover the firewall with a Ceramic blanket which will act as a fire wall. On top of the Fiberfrax surface, will be a coating of Latex Contego primer. This will serve to protect the Ceramic Paper (Fiberfrax) from contamination due to fuels and will also form a charred surface in the event of a fire. This will add precious minutes in the event of a engine fire.

New rudder Pedals

I started to install the new Rudder pedals and the new cable run. I am very happy with the final design here. But again, there are lessons that were learned that will be incorporated into the final plans. This is why you should never buy a set of plans that are all “paper” Building a prototype shows the mistakes and omissions a designer will make. Building a plane from plans that has never flown in my opinion is not only dangerous, but unethical on the designer’s part. One of the features I copied from the original RF4D was the canted rudder hinge line. I like the look and the fact that deflection of the rudder will induce a small rolling moment in the direction you are trying to turn in. That effect is minor, the main driver was styling. Well that angle leads to all sorts of problems when it comes to driving the rudder. The rudder horn is rotating in a canted plane that necessitates the redirection of the rudder cables. They need to be in the same plane as the rudder horn. On my design I further complicated this problem by mounting the rudder horn on the lower rudder rib, which is not perpendicular to the rudder hinge line. This means that the connection to the horn needs to articulate with two degrees of freedom. Too complicated!!!! I have a solution for the prototype, but I will rectify this in the final design.

I am using Piper split cable guides to route the rudder cables. I have always liked these guides. In fact, we have swapped out the original oak wood guides in a 1-26 sailplane I am helping my instructor rebuild. (I am getting some real tail dragger time right now in a Citabria) The beauty of these guides is that they are split and held together with a snap ring. When they are disassembled, the complete cable assembly can be removed, turnbuckles, thimbles and shackles. They are made from Nylon and as such offer a nice smooth friction free sliding surface.

Rudder Cable bushings

This is the initial installation of the new Rudder Pedals. As usual, I forgot to add a “bow sprit” to the center body to attach the return springs to. This is being fixed. In fact, I just ordered some 6061-T6 Aluminum tubing because I want to replace the welded steel parts. My friend Ed has a nice TIG welder and claims he can weld me up a set. Just to make sure, I ordered 3 times as much as I should need. (sorry Ed!!) The rudders pedals can be swapped pretty easy and I will hold off terminating the cable that run fwd of the pedal “S” tubes.

Zimmer Skimmer

This is a little off topic, but I thought you might find this interesting; this is a picture of the Zimmer Skimmer. This plane was conceived just prior to the start of WW II. It was envisioned as a carrier based fighter. It had an extremely short take off roll due to the twin almost helicopter like propellers. The Navy was actually interested in this plane and as a venture between Zimmer and my Company Vought Aircraft, two flying prototypes were built. The Skimmer was powered by twin C-85 engines driving the propellers through a 6.1 to 1 reduction gear box. Vought Aircraft actually started construction of a real fighter powered by two Allison V-12 engines. Both planes were ready for their first flight when the Navy cancelled the program and ordered both planes scrapped. This Plane is the last remaining example and is on loan from the Smithsonian Institute. The Vought Retirees club is undertaking a full restoration. This has been an ongoing project for the club for the last 6 years and it is now finally nearing completion. Notice in one of the Pictures the ducted fan attached to the C-85 sitting under the body.

Engine Cowl baffles and Rudder Pedals

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