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.









Horizontal construction





Top drawing of the Horizontal plans
    The construction of the Horizontal is similar to the Vertical fin and the rest of the empennage. The process starts with building the main spars. All of these spars are built up on a level jig board. The construction of this jig board is critical. The actual construction surface is MDF or particle board back up by ladder frame of 2 x 4's. All construction grade wood will have some type of warp in it. By sighting down the board itself, you can determine the warp direction. The 2 x 4's are placed with the narrow edge against the back of the construction surface. Choose both boards such that the warps in each board are in the same direction. Build up a ladder frame 112" long and 7.5 inches wide.

Place 8 cross blocks evenly spaced between the two main members. Cut and fit the construction surface 112" long by 11.5 wide. Rip a full sheet of MDF or Particle board 11.5” wide and stack up the two boards together. Stagger the joints such that they do not line up with each other; Glue both boards together using Titebond Glue. After they glue sets, lay the construction surface on top of the back up ladder frame.  Stretch a chalk line from one edge to the opposite side. Space the chalk line off of the construction surface with a block of ¾” thick wood. Use a spacer block at both ends. The idea here is to shim the construction surface such that the gap is exactly ¾” all along the surface. After the shims are in place along one edge, secure the construction board to the ladder frame by driving some sheet rock screws through the shims and into the ladder frame. Once one edge is level. Use some more door shims to level out the aligned edge. Place the shims under the ladder truss. Once the aligned edge is level, start leveling the construction board in the perpendicular direction, using door shims between the construction board and the ladder frame. Once the construction surface is perfectly level, it can be used for the build up of all of the spars.
The spars of the Horizontal, Elevator, rudder and Vertical fin are all built up on this assembly board. All of the spars have some degree of taper to them (relative to a 90 degree angle) All of the spars are first constructed from square stock spruce. After the assembly is bonded together, the bevel angles are cut on a table saw. Use a hollow ground planer blade for all of these cuts.


One spar of the Horizontal being assembled

 I use 3/8 x 3/8 corner molding for the locator blocks. I will stretch a chalk line to locate one surface and then make a template that locates the opposite side. Usually my template consists of a parallel board that spans at least 2 of the opposite locating blocks. Glue the blocks in place with Titebond glue

After the spars are built up, the blocks can be chiseled and sanded off and the jig board reused.



Since the horizontal is a constant section, all of the ribs are identical. Because of this, it pays to make a more durable cutting template. I am using galvanized tin sheet here. A tracing of the rib shape is first made by either copying from the plans using a copy machine calibrated to the correct scale, or the full size loft or laser cut paper templates are used. Drill some #30 holes in the template, this is an approximate size, experiment with the round toothpicks such that they will wedge into the holes after sticking out about 3/8". Use these toothpicks to locate and hold the foam blanks. Use a band saw to cutout the foam. As mentioned earlier, I always use a metal cutting blade with minimal kerf and guide off the back of the blade. This ensures that the ribs are easily cut and repeatable.


ribs located to the spar


After the ribs are cut and the spars built, assembly begins on the Horizontal. Again, 5 minute epoxy is used to bond the ribs to the spar, this greatly simplifies and speeds up construction.Use a 90 degree wooden triangle as a guide to set the ribs perpendicular to the spar. The ribs here are shown being assembled to the front spar. If a rim sits on a plywood doubled , trim the rib equal to the thickness of the doubler.


Front spar being added

 The Front spar is located to the standing ribs. 5 minute epoxy is used to bond these ribs again. The cans were added for weight. Only contact pressure is required between the rib and the spar. If a rib sits on a doubler, trim away the thickness of the doubler before bonding.
 

Nose rib template

 The nose ribs are constructed similar to the main ribs.


Nose ribs being added to front spar
 The Nose ribs are bonded to the front spar per the drawings. The small tab is removed from every rib where it sits on an intercostal. All bonds are made with 5 minute epoxy


Learing edge splice sheet

 The leading edge is spliced at 3 points along its length. At each location the outside contour of each rib is reduced by the thickness of the splice sheet.. This is done by carefully sanding. At each splice location the nose ribs are also doubled up. The splice plate is pinched together as shown with a clamp. It is then sprayed with unscented Ammonia and allowed to soak through and dry.


splice plate being bonded to nose splice ribs

 After the Ammonia drys, bond the splice plate to the ribs with 5 minute epoxy as shown.


start of construction of leading edge form tool

 The leading edge skin nose radius must be formed into the plywood sheet. Its important that before you actually form this skin that it was cut out with the face grain running the length of the part. The assembly jig table is used to construct the forming tool. Standard hardware rafter or truss brackets were used to mount two parallel 2 x 4 walls. This photo shows one wall installed. Use a chalk line to set one edge and then make a tool that locates the opposite side, similar to the spar jig block procedure


completed forming tool

 a series of "Broken arm" clamping fixtures were made from 2 x4's and piano hinges. These will be used to apply pressure on the leading edge tube used to form the inside surface. The precut leading edges are all wetted with spray ammonia and then folded in the center and shoved between the two walls.


leading edge being formed

 The whole idea in forming the skin is to impress the leading edge radius. The remainder of the skin will wrap over the nose ribs and form to contour. I could have made a smaller former and formed one skin at a time, but that would have added 3 additional days to the process.



formed leading edge

This is what the skin looks like after 24 hours. There was a little spring back, so I will use a smaller radius forming mandrel (tube) the next time I do this. This part was OK and it wrapped and bonded to the nose ribs with the correct contour.

Complete empennage assy

This the final empennage assembly, all of the other components are built similar to the horizontal and Vert Fin. After the leading edge was formed and bonded to the nose ribs (I used epoxy and Micro balloon mixed to a consistency of peanut butter for the foam to wood bond) The reason 5 minute wasn't used is because of the working life. Before the skin is attached the 1/8" thick cap strips were bonded to the foam ribs.

Everything in this empennage assembly weighs 9.8 lbs and took 2 weeks to build.
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