Robin Ultralight: February 2012

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.

Latex House Paint Covering method.

Big milestone today. I started finishing the fabric. I decided a while ago after some research that the Latex house paint system was the lightest covering method I could use. The major covering methods are mostly dissolved thermoplastic. All of these processes are heavy. The latex system I believe is also similar to the Stewart covering system, http://www.stewartsystems.aero/default.aspx in fact I am using their adhesive called Eco Bond. There has been a ton of research done on using Latex House paint. A friend of mine in Wichita, Duane Life covered his J3 Kitten and I have always been impressed with the finish. A lot of good work and research has been done on this process, the most impressive is Drew Fidoe's blog http://www.bowersflybaby.com/tech/latex.html, other good sites are the reprint of Charles Threewit's article http://www.lazair.com/housepaint.htm and this article in Ultralight News http://www.ultralightnews.ca/articles/housepaint.htm

Fresh Bloom

The process starts with a good base latex Acrylic latex enamel. I am using Sherwin Williams D-100 gloss exterior enamel The base color of my Robin based on my Friend Bills design is Gun Metal Grey. This may be too dark, so after the base UV coat is applied I may pick another lighter color if Bill decides I need it. The formula for using latex involves adding a viscosity modifying agent called Floetrol. This smooths out all brush and roller marks. Each coat is additionally thinned with standard windshield wiper fluid. The links give the ratios.

The day started nicely with a fresh bloom in the flower box. We had a few 85 degree days last week. Like I said earlier, Texas is wonderful in the spring, winter and fall!!!

I started with the ailerons. I figured they were the easiest to recover if I screwed up.

Single coat on Ailerons

The procedure for finishing goes like this. After the paint is well thinned and mixed with Floetrol. it is rolled on with a foam roller. Right behind the roller you follow up with a foam brush and push the first coat into the open weave. Let that dry and and complete the same process on the opposite side of the part you are working on. The second coat is put on with only the roller and is rolled 45 degrees in the opposite direction of the first coat.

Ed doing the Brushing duties

Its amazing how well this covers. By the end of the day, all of the control surfaces and Horizontal were finished and I had used less than a quart of paint. The beautiful thing about that is its mostly water and it evaporates away.

Ailerons with second coat

Depending if Bill likes this color, the next step will be a spray over coat at Eds house in his spray booth. At this point the fabric on the surfaces sounds like a snare drum when you tap it. Its pretty exciting seeing it at this stage.

Coating the elevator

This is me doing the elevator. I tried to measure the weight gain at this point, but I found that my scales were not sensitive enough to measure the difference. I will know exactly when I reassemble everything an re weigh, but from the looks of it my weight gain will be measured in ounces. My estimate of the paint weight was 12 lbs. Based on that I was going to be 10 lbs over weight.

Finished Rudder

The Rudder was recovered after the trailing edge was replaced. Again, I could not measure any weight gain.

Wings

This is another view that shows the wings. They came out beautiful!! Over weight , but very clean and smooth.

Horizontal with one coat

The Horizontal came out beautiful. Its very smooth and very clean. I can hardly wait to fly this thing.

Parachute trip, revised rudder and covering.

So its been a very busy 3 weeks. I have been traveling on my job and last week end my friend Ed and I drove to Phoenix Arizona to buy a Mini-Max project, I was looking for a Second Chanz 550 or 600 ballistic parachute, This particular model weighs 13 and 17 lbs respectively. Because of the 30 lb weight allowance the FAA allows, this particular model will give me the 10 lb margin that the prototype appears to be overweight. I found a project for sale and one of the items was a model 600 ballistic chute. I called my Buddy Ed Gardner and asked him if he wanted to split the whole deal, he gets the complete project, I get the parachute. The final price was a really good deal, circumstances combined to force Mike Gurtler to part with his project.

Mike and his old Project

I got to hand it to my Friend Ed, he had a really good idea about how to pack the project, he bought 2 foot wide packing wrap, similar to Saran Wrap. We covered each section three times. That turned out to be the perfect number. We hit nasty weather coming back thru the mountains near El Paso. On the following day (Sunday) we were in ice, rain and snow all the way to the Dallas Fort Worth area

We spent about 2 hours packing the project in Phoenix.

Mike and me just before shove off

Ed and I drove my old 96 Chevy Z71 Pick Up. (219,000 miles) I have had this truck since it was new and I have religiously maintained her,but 2 years ago I had to put in a new transmission. Other than that I have not touched her. I attribute her long life to the use of Mobil 1 oil . Anyway, she made the trip

ready to shove off

We both got one hell of a deal, I got a $3200 parachute and Ed got an airplane complete with all covering materials , engine and a brand new composite ground adjustable prop. I wont mention the price, but our gas was just about equal!!!
I accused Ed of being really anal when it came to packing up the project, well that turned out for the better, he decided to do one last walk around through Mikes garage and guess what!! There was my parachute sitting on top of a trash can!! That was the whole reason I bought half of this project. It would have been near impossible to ship the chute to me once I had left. The pyrotechnic would have to have been removed.

I finished my Wing Walk and started covering the L/H wing. I then went back to the rudder. Turns out I designed the trailing edge too light. After I covered the rudder I noticed the trailing edge was scalloping and the fabric was losing its tension.

wing walk skin installed

This picture shows the wing walk after installation. The outer glass surface has been scuff sanded.

using the Multi-tool to cut off the training edge

So I used my muiti-tool to cut off the trailing edge. as you can see in this photo, its only .125" thick and .75" wide. This was actually unstable in torsion. After the fabric was shrunk, the trailing edge rolled over. I am modifying the trailing edge to be similar ti the rest of the airplane. I will detail the Trailing edge attachment in the next blog.

I prepped the L/H wing for cover, but will not actually be able to cover it until Tomorrow.
I tried to mount my new Ballistic parachute and discovered that due to the wing being overweight, I cannot mount the chute behind the pilot (at least not now) The weight and balance right now was the rear limit at 30% with no fuel and 22% fwd CG with full fuel. Still working on the chute installation. I am thinking that I will mount the wings after cover and then check the final balance. This will determine where the parachute goes

Wing Walk and some test photos.

I started modifying the L/H wing to reinforce the leading edge so I could add a wing walk. The redesign was pretty straightforward. I removed the existing wing skin with my multi-tool. That thing zipped right through the fiberglass. After the skin was removed, I reinforced the foam ribs with a ply of 8 OZ fiberglass on each face of the exposed ribs.

lateral stiffeners

I then slotted the ribs every 2 inches and inserted a wooden stringer. the Stringer was sized to react 250 lbs between the farthest span (6") so basically each stringer can support the weight of one step. In reality the load will be distributed between at least 3 members. The stringers were potted in with micro balloon and epoxy. Using the leading edge lay up mold, I laid up a 2 ply skin of .012 thick carbon fiber.

leading edge walkway skin

2 ply's of graphite and a single lightweight ply of glass comprise the walkway skin. The leading edge was taper sanded to allow blending into the existing skin.

Right now the skin is curing after having been bonded 8 hours ago. Tomorrow I will be able to flip the wing over and finish sanding the skin in preparation for covering. I should be able to finish the covering tomorrow.

rib stitching

My airfoil is a Worttman 170 low speed laminar flow airfoil. It has a reflexed trailing edge on the upper and lower surface. Because of this, the fabric tension after shrinking will try to pop off the reverse curve surface. So I elected to rib stitch the trailing edge where the reflex is. Everywhere else on the wing, the fabric is bonded.

wing re test

The original wing test spar has been sitting out in the weather for over 2 years. The ultra violet radiation has attached the foam and the fiber glass. Periodically I retest the wing. The original wing was rebuilt after initial destruction testing and re mounted to the back side of my test stand. A 2 x 10 wood extension was added to increase the applied moment such that it equals the actual moment on the wing. This is a little conservative because the wood does not deflect similar to the wing. Because of this the stress is concentrated more toward the root, there is no relief that would normally occur it the complete wing was allowed to deflect.

anyway, this is a picture of the wing at just a little over limit load. There is 1200 lbs of force being applied. In the previous picture you can see my load cell that I designed. I made a piston with exactly 1 inch of area. I then bored a cylinder out of some scrap aluminum.I drilled and tapped into the cylinder and installed a 3000 psi pressure gauge. I filled the void with transmission fluid. The actual load now is a one to one read out. I calibrated the load cell at work on a universal testing machine one day at lunch. There is a little hysteresis error, but that was under 100 lbs. Close enough!!!