Robin Ultralight: Cowl Plaster Splash cont:

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.

When I left off I had just cast poured the nose of the Fiberglass splash. The purpose of this was to make two inlet features that will be added back into the fiberglass splash so a male plaster splash can be taken. The final lay up tool will then be made off of that male plaster splash.

Nose cast

Here is the nose cast. I made a template that gave me the inlet area and shape that I was looking for and traced it on both halves.

cutting the cast

you can see the inlet outline in Red. I knew this would trash my band saw blade, but they are expendable!!!

cut core with draft

I tilted my band saw table 10 degrees so I could get some draft on this cut. After I finished cutting, I sanded the edges smooth. I repeated this for the opposite side.

Cores installed

I placed both cores back in their original position. they self located due to the contour . I used hot glue to bond them in. I then took some modelling clay and formed a 1/2" radius all around the the part. Before I installed the cores, I coated them with a 50/50 mixture of shellac and alcohol. This seals the plaster and allows for easier release. I coated the inside of the splash with a few layers of carnuba wax. I purposely did not buff out this wax, I wanted it to have a matte finish. If it were polished, the plaster would run off of it in the next step.

surface coat

the next step is to coat the interior with a 'Milk" coat of plaster. Mixing the plaster is critical at this step. there is only one way to do it. you add plaster to the water and wait for all air bubbles to subside. Then you can mix it. This way there are no dry pockets in this coat. The method you use is to pour the plaster in and then slowly roll the fiberglass splash around coating all of the surfaces. This takes a while because thickness buildup isn't possible until just before the plaster starts to kick. I did this step twice to ensure a nice thick surface skin.

Susan laying the Hemp backing

The surface skin is backed up with a layer of Hemp mat soaked in plaster. My Helper Susan Lassan here is demonstrating this messy step. Susan is an Engineering Student at The University of Texas@ Arlington. Her Father Laszlo works with me at Triumph. Susan is going to be an excellent engineer, because she is trying to understand all aspects of aircraft construction. When she isn't helping me, she is helping another friend Michelle Craig, rebuild her 1946 Taylorcraft wing in my upper shop. I bought two bales of this hemp from surplus at Triumph. Got both bales for $5. The retail cost is around $140 because hemp is a controlled substance.

Released Male Master

It is chemically very close to Marijuana. I never tried smoking the stuff, but I understand its not the "good stuff"

Here is what it looks like after its released. I will chisel out the inlet cores and then do a ton of clean up before I start the final fiberglass molds.

Welcome to my Robin Blog.

Cowl Plaster Splash cont:

No comments: