Robin Ultralight: Fuel tank

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.

The design of the Robin Fuel tank was based on a similar concept I used on my Wren. In keeping with the spirit of die hard home building, I have designed the tank so it can be built as a one off design, but for my project I chose to build a full master model and female tool because I plan on offering this part as a semi kit. As part of the goal of eliminating as much drag as possible, I decided to recess the fuel filler neck into the FWD fairing. There is a fuel door that will be located just fwd of the canopy hinge. The tank capacity is 4.9 gallons US, this is in keeping with the US part 103 requirement of no more than 5 gallons or less fuel.

Fuel tank mold

A master model was constructed from birch plywood and MDH board. This was for the production fuel tank. If a one off tank is being built, the tank is mocked up out of Styrofoam and covered with aluminum duct tape. This temporary "Master" is then set in place using dabs of automotive "Bondo" or body filler onto a flat plate of Masonite backed by MDH board. A 1/2" radius fillet is wiped around the lower edge. The master and the exposed Masonite plate is waxed with at least 5 coats of carnuba wax.

A word about waxing and waxes is in order here. One of the most expensive mistakes I made on the Wren production was to use Silicon based mold release wax with a high temp tool. everything was scrapped after the resin cured. Silicon anything has no place around a composite shop. Always keep this material far away. When waxing for mold release the technique here is to create numerous release planes. A good commercial mold release wax is used, but as an option you can also use Johnson's floor paste wax, this is also a Carnuba palm product. The first coat of wax is usually a very heavy one. Open pores in the mold will draw in the liquid state wax. The wax is allowed to dry and then buffed with a wheel or a rag. After each buffing the surface is water hardened or "spit" shined. This creates a hardened layer or release plane. Repeat this 4 more times at a minimum, the last coat however is not water hardened, just wiped and buffed, the reason for this is that the wax will retain some "tooth" or surface roughness that will not cause the gel coat of resin to bead up.

overall view of the tank assembly

Once the outer shell is molded, it was released from the tool and all of the internal fittings were installed. The final step is to lay up another 3 plys of 8 oz cloth on a flat waxed Masonite surface. The precured tank mating flange is scuff sanded to remove all gloss. A mixture of cotton Flox and epoxy is applied to the flange surface and the tank is then set onto the uncured lower layer. I built a wooden picture frame that slid over the the tank and rested on the flange. I then applied a small amount of weight to the frame to ensure no gaps and to have positive squeeze out of the Flox.

detail view showing internal fitting reinforcement

I used precured- plates of 1/4" thick NEMA grade C fiberglass sheet to reinforce the fitting attach points. This worked quite well. The fittings are Brass barbed commercial fittings with a NPT pipe thread. I drilled and tapped the plates for this these. I scuff sanded the bonding surface on the plates and the inside of the tank. I then bonded the plates in place using T-88 epoxy. The fittings them selves were screwed into the backing plates and provided all the needed clamp pressure. This is an area where it does not pay to go light on the adhesive. All excess squeeze out is used to fillet seal the edges of the reinforcement plates.
The filler neck was purchased from Aircraft Spruce and Speciality, it is a standard weldable aluminum filler neck. An exact diameter hole is bored into the top of the tank and a double flox fillet is used to secure it in place, The aluminum must be scuff sanded and wiped down with MEK.

Bottom view showing pick up tube

A copper tube is brazed to the fuel outlet fitting. This ensures that fuel will draw from the tank at high angles of attack. This image shows the lower surface after bonding. This surface is supported by the load shelf in the fuselage. The final step in building the tank is to slosh it with a half cup of resin. This is primarily for a fillet bond on the inside of the tank between the side wall and the lower surface. This is designed to react peel forces that would occur due to over pressurization.

view looking FWD

The rear of the tank has two additional barbed fittings. These will attach to a fuel line that attaches to a sight gauge on the instrument panel

2 comments:

Unknown said...: Mark forgot to tell people to make certain all the gel coat is removed from the tank, prior to filling with fuel, as he did not one time on the Wren, but did a most excellent dead stick landing in south Wichita on the first test flight. dale a nulik; January 16, 2014 at 8:57 PM
Unknown said...: Mark forgot to tell everybody to remove gel coat and anything remaining in fuel tank prior to filling with fuel, and flying as with one wren, he forgot to, but did a most excellent dead stick landing in south Wichita one afternoon. dale a nulik; January 16, 2014 at 9:00 PM