Robin Ultralight: Cowl continued

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.

After the divider plane was constructed, the master was waxed with 7 coats of carnuba wax. Never use silicon based anything around fiberglass. Years ago I destroyed a $10,000 high temp fuselage mold by using silicon wax for a release agent. When you use carnuba, water harden or "spit shine" between each coat. sprinkle a little water and buff out the wax. the idea is to build up multiple release layers. In addition to Carnuba mold release wax, I shot the last wax coat with PVA (Poly vinyl acetate) film. this is a water soluble release film that 100% guarantees a final release. I have months of sanding in this master and the last thing I need is to destroy it with a stuck splash.

PVA release applied

The hot weather here allowed the PVA to fully dry in less than 10 minutes. Thats a record for me. Its really tough to dry this stuff in high humidity and cold temps. This is a fail safe release, if it becomes stuck, all you have to do is crack open the edge and squirt some water in the gap. Capillary action will suck the water to the rest of the mold eventually. I have
never had a tool stick using PVA.

After the PVA cured I sprayed a coat of gel coat. I spent some money years ago and bought a commercial gel coat gun. Its the only way to go. It uses a disposable paper cup that speeds up clean up. I allowed the gel coat to almost fully cure and then I wiped a bondo fillet radius in the sharp corner at the divider. I need a radius here because the fiberglass mat will not conform to a zero inside radius.

gel coat and bondo fillet

The most important thing to remember when making a mold like this is to ensure the thin gel coat face is supported by the backing fiberglass mat. This is not the final tool, so I can live with small areas where the gel coat may break open. But if this were a final tool, I would immediately lay up a layer of finer glass cloth once the gel coats tacks off. I would use a open weave like a 285 Crowfoot that complies to compound contours. I would take my time and using a paint brush stipple out the first layer to ensure no air bubbles. But in this case, a single layer of mat is all I need.
The next step is to lay up the fiberglass mat. I'm using Polyester resin, because its cheap and cures fast. Its suitable for this intermediate splash and my final tool but it must be laid up in at least 3 layers for the final tool. Polyester will continue to creep and move around after cure. Its best to move swiftly through this intermediate steps before it deforms too much. To keep the resin from kicking off to quickly, transfer it to an open pan after mixing. this reduces the exothermic reaction. I like to pour at least 1/2 the resin immediately on the tool. I will mix the resin in batches. In this case it took two batches of 28oz . do not use the same mixing stick when you stir succeeding batches, also use new brushes. the older resin can force the new resin into a premature cure.

lower cowl fiberglass layer

When you use mat you need to use a serrated roller to work out the air bubbles. They work remarkably well too. I let this cure and then the next day using ropes, a come-along and hand winches I flipped the Robin over. I'm sure she was getting dizzy sitting on her head for these last 6 months.

Robin right side up

Its been a while since I have seen her like this. I like the way the spinner will blend into the new cowl. I used the old spinner I built for the old motor. The side profile was designed to blend into that shape so why ruin a good thing?
The same process is again repeated on the upper surface. I will fly thru the next few picture without much explanation

high solids primer

I needed to do some minor sanding on the upper surface. especially in the area of the spinner. when I cut off the old spinner I damaged some of the original surface. The key to success is sand, sand and more sanding. when you get tired of sanding, sand some more!!

Wax and PVA added

Same as the bottom. Wax and PVA were added.

See the light area in the blend radius. When I looked at the left hand vs the right hand surface I noticed the tangent line of the blend radius wasn't even. This is usually referred to as the highlight line in studio surfacing. So I reduced the blend radius with my flexible sanding rod. I applied wax and PVA then shot gel coat.

Susan and Zach cleaning the released splash

About 2 hours later the splash was sufficiently cured to release both the upper and the lower cowl. In this picture two young enthusiastic friends of mine Susan Lassan and Zach Strout are cleaning the released cowls. Because we used PVA everything eventually comes off with water.

Before I released both splashes. I trimmed the edges with a Milwaukee saw. I then drilled a bunch of 1/4" holes thru both flanges so they could be reassembled with fasteners.

Zach holding both splashes

here is Zack holding up both splashes. At this point I can remove the old foam master and clean up the engine area.

outside surface

Here is the finished splash.

Casler 1/2 VW

Finally the engine is exposed to the air!!!

Now this is the beginning of the next step to make the air inlets. I am starting this out by taking a plaster splash of the inside of the fiberglass splash. I am using US gypsum Ultracal 30 molding plaster. this id the best stuff to use for a plaster mold. it has a long work life and if you run into a problem, it can be rejuvinated by adding additional water and stirring it up. When you mix plaster, always add plaster to the water and let all air bubble subside. this will ensure you do not have dry spots inside the plaster.

plaster being mixed.

here is the plaster being mixed. I bought 30 90 lb bags of this plaster at the surplus store at my company Triumph Vought a few years ago. I got each bag for 50 cents. they sell for $23 to $30 normally.

I reassembled the splash leveled the spinner plane and poured plaster into the nose. I will let this cure, split the fiberglass splash and pull out the cast. I will lay out two inlet shapes and cut them out on my band saw. I will set the table with 7 degrees of bevel so the shapes come out tapered. They will be used to develop the inlet for the next steps.

plaster splash

this is about 1.5 inches thick. this is the thickness of the air inlet

more in the next blog update!!!

Welcome to my Robin Blog.

Cowl continued

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