Robin Ultralight: Canopy frame and Windshield

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.

Completed Canopy Frame

The canopy Frame is designed to pivot at the extreme FWD upper edge. The canopy is designed to pivot 90 Degrees Vertical to allow plenty of room for entry. At the 90 degree position the canopy is removable due to a slot in the canopy hinge fittings. The Fwd portion of the frame is built up similar to the FWD fairing. The process starts by bonding a horizontal foam shear web to the two canopy side rails. The side rails are made from laminated 1/4" birch Plywood. Multiple pieces are laminated together and then a final Bevel cut is made on the table saw. After the foam is Bonded to the shear web, a light layer of fiberglass and epoxy is laid. The main purpose of this ply is to protect the foam in the event of a fuel leak.

Canopy shear web and rails

The foam formers are added after the glass cures.
As mentioned earlier, all foam to wood Joints are made with 5 minute Epoxy. This greatly speeds up assembly time.

Canopy frame with FWD stringers and formers.

The formers and stringers are added similar to the FWD fairing. 1/32nd Birch Plywood fabric transition gussets are also added. The alignment of the rear most former is critical as it must be angled to clear the control stick and provide a FWD flange for the actual canopy to sit on.

Canopy partially open

Unfortunately the Shop ceiling is not high enough for a full extension of the canopy. Once opened there are cable guy braces and a small strut to keep the canopy in place when opened. The rear frame and Yarmulke were added to the rails during the early build. The next step is to make the windshield template.

Windshield in place

The Windshield is a flat wrap design, there are no compound contours that would require heat forming. The purpose of the Yarmulke was to remove compound contour and act as a sun shield. Because of the number of variables with a plans built frame, the actual windshield profile must be developed from a template. I used taped together poster board sheets to develop the template. Once this is derived, it is transferred to the Lexan. The Actual Windshield is .032" thick therefor it is very easy to trim with the use of metal tin snips. The final attachment to the frame will be with polyurethane automotive windshield adhesive and countersunk 1/8" pop rivets. These are really nothing more than clamps used to hold the canopy down for cure.

2 comments:

Scotty said...: Looking good, I was wondering how you were going to enter and exit. I like it.; March 24, 2011 at 10:48 AM
Mark Calder said...: Well, I still didnt have it all quite figured out, you will have to be a contorsionist to get in and out of the Prototype, I did not add a wing walk, There will be a step on the landing gear however. The redesigned wing will not only be 29 lbs lighter, but it will have a wing walk and grealy simplify assembly.; March 25, 2011 at 4:23 AM