Robin Ultralight: Engine cowling

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 building of the cowl started by mounting the spinner master model into position on the prop hub. as previously mentioned, I designed this master model with a 1' diameter hole that aligned with the 1" prop hub flange. After the spinner master was in position I constructed 4 templates and mounted them internally. These templates represented the side inboard profile, and the max half breadth of the plan view. Masonite templates were first fitted and then a splining stick was used to fair the curves from the fuselage to the spinner. These will become sanding guides later in the process. as I mentioned earlier, I am using 1lb/ft cu floral urethane foam. Its important that you buy enough to finish the first time. My experience is you cannot mix manufactures or different production batches. I bought all of this foam from Hobby Lobby. They had a new larger sheet product that I am using for the first time. I swear every time I buy a large batch of this foam the cashier thinks I am a gay florist!! I am outlining this process to show how I make these molds without the use of a NC router. These steps will not be necessary for a plans builder because I will have all of these molded parts available. The process is interesting however!!

foam being fitted

After the templates were mounted small gusset blocks were made to lock each of them together. In this picture you will notice a blue/green sheet of 1/4" PVC foam that was bonded to the back side of the spinner master. This is a spacer to ensure a gap between the cowl and the spinner.

All of the supporting structure and the eventual sanded foam blocks are designed to slip off the front of the engine after they are shaped and a layer of glass is added. The surfacing of the cowl will be done while it is attached. This will all be later removed and plaster splashes will be taken of the plug master. the cowling will be split horizontally through the center line of the prop hub. In addition to the actual cowling, there will also be two "cheek" cowls added after the finishing operation.

Blocks of foam are fitted to the templates. Care is taken to ensure the internal foam is cut out to clear the engine when the plug is removed.

upper foam in place

All of the foam for the upper half is in place. if you have any gaps, stuff wedges of the foam to fill these voids.

initial shaping begins

After the foam is in place, the initial shaping begins. I try to cut within a 1/2" to the horizontal and vertical templates. any tool will do, I used a hack saw blade and a small hand saw. Do under cut into the foam. I doesnt hurt to stand back and look before each cut.

Vertical template sanded to the edge

The next step is to sand with a sanding stick down to the template edge. Hold the stick perfectly level when sanding the vertical template and hold the stick vertical when sanding the horizontal templates. This is so you avoid undercutting the contour.

initial shaping of the foam

After the templates are sanded to the edges, you can start to carve away the foam in between them. Use a saw or a butchers knife for this. Do not over cut, be very generous at this stage. Once the foam is trimmed, the real magic of this process happens. Scrap foam blocks are used to sand the foam. Both materials will equally abrade each other and transfer the contour back and forth. This creates a spherical interface and it is the same principal that Telescope hobbyists use to grind their lenses, they use glass to grind the glass.

L/H view

Here is the final outcome of the foam shaping. This is a beautiful wave and dimple free surface the cleanly blends between the firewall and the spinner. The "P-51" look of the nose is now evident.

Next is the bottom surface. That will be the next blog entry

Welcome to my Robin Blog.

Engine cowling

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