Robin Ultralight: September 2016

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.

I prepped the lower cowl for layup by applying 7 coats of Carnauba mold release wax. Followed by a buildup of Rexall Partall 10 PVA release film. Since the lower cowl needs a surface extension into the upper cowl, that mold was also recoated.

upper and lower molds joined

After both molds are joined together I shot more primer onto the surfaces. You want to be ready to do the layup immediately after the primer flashes off. This will give you the best mechanical and chemical bond to the resin.
The seam between the molds is stuffed with modeling clay.

modeling clay

The clay is pushed into the seam and then gently scraped off with a sharpened tongue depressor. nothing will stick to the clay because its an oil base clay. This will keep the laminate near this joint from becoming porous and it makes a smooth transition between the mold halves.

final layup

As I mentioned earlier in this blog, I need to make an inner flange that will be secondarily bonded to the lower cowl and form a flange that the upper cowl will sit on. In order to do this, I need to lay up the lower cowl at least 2 inches beyond its normal trim. Remember the actual parting line between both cowls is etched into the upper cowl surface. This is the reason I needed to join the upper cowl. Here is an image of the initial lay up. the surface is covered with polyester fabric, the same fabric I cover the plane with. In this case its functioning as Peel ply. I like to lay a final layer of Peel ply on my wet lay-ups especially at any ply splices. The peel ply beautifully transitions these ply's together. The other reason is in the area of the overlap flange, I want a smooth surface with no rough ply edges.

Its HOT today in Texas, humid too. after I finished the lay up, my tee shirt was soaked clean thru. I actually wrung a cup of sweat out of this thing!!! I can hardly wait for fall. The high temp keeps dropping one degree per day. By mid October, it will be wonderful again in Texas!!!

multi tool trimmer

I released the upper cowl and here is what is left. Even in this high heat, I needed to let the laminate cure for 48 hours. Its important that the resin become fully cured almost to the point of being brittle. The tool I like using for trimming is a vibrating multi tool. I use a grout cleaner as a cutter. There is carbide grit bonded to a steel disk. This will wear thru beautifully and with very good control. The actual final trim of the cowl is one inch below the trim that you see.

inner flange being released

Here you can see the secondary flange lay up I did on top of the original cowl lay up. I laid 7 layers of wax on the inside of the cowl before I laid up the inner flange. Here I am parting the inner flange from the cowl. once I have released the inner flange, I will re trim the cowl an inch lower at the scribed trim line.

more inner flange

here is another view of the inner flange. You can see that it will lock in and "key" perfectly to the inside surface (IML) of the cowl.

bonding the inner flange

Here is the inner flange being bonded to the IML of the cowl. you want to make sure you use an actual structural adhesive here and not thickened laminating resin. . Structural adhesives all have an elastomeric polymer added to the matrix. This dramatically increases the peel strength of the bond. This will keep the flange from peeling away from the cowl. I used T-88 structural adhesive.

Trial Fit

Time for a trial fit up. I joined both halves and temporarily attached them with Clecos. Eventually I will replace each Clecos with a bonded nut plate. Before I can actually set the position of the cowl, I need to mount the spinner and backing plate. The cowl must sit with a 3/16" gap between the outer surfaces and the spinner. the surfaces must all be true and faired. So before I can finish mounting the cowl, I had to lay up a new spinner and prop backing plate. Fortunately I already have the molds.

Another view

The cowl I about 6 inches too long, this is why I made the extension surfaces on the master model. I wanted to be sure I had more than enough excess.

Backing plate mold

here is the backing plate mold. This will get 3 ply's of .014" thick Graphite. The Lathe will be used to true it up and trim it.

Spinner mold laid up

here is the spinner laid up in its mold. the white fabric is peel ply. This is a very hard mold to release because you cant start a edge peel. But I coated the mold with Wax and PVA. Once cured, I will submerge this mold overnight in water and hopefully it will float out after the PVA dissolves.

Backing plate being trimmed

I made two wooden backing plates and clamped the backing plate between them. The hole in the backing plate is much larger than the center bolt. This is so I can use a dial run out gage and adjust the position of the backing plate until it runs true.

More in a few days or so!!!!

Robin Ultralight

Welcome to my Robin Blog.

COWL AND SPINNER PROGRESS

LOWER COWL AND NEW SPINNER