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.

Vertical Fin Construction

One of the reasons I like to build with wood is the ease of construction. The construction of the vertical fin required only 3 days to complete. The main reason it took this long is because of the cure cycle time of the T-88 epoxy. 

A word about  adhesives. One of the great leaps in technology that occurred during WWII was the development of Epoxy Adhesives. T-88 adhesive was developed by Howard Hughes for the HK-1 or Spruce Goose. Prior to the use of synthetic adhesives all wood bonding agents were organically based, either they were derived from curdled milk (Casein Glue) or from the fat of rendered animals. (Hide Glue) these products can all be attacked by bacteria and will eventually break down. All of these early adhesives required good fit up between the wood joints and high clamping pressure. 

But along came Epoxy. A good wood joint occurs when the wood fails under test away from the joint. The adhesives need only be slightly stronger than the materials they are splicing. Epoxy adhesives are different from epoxy laminating resins. The main difference is viscosity and the addition of polymers  (rubber) into the resin matrix. These polymers are what make adhesives different from resins. In a bond joint, the stresses tend to concentrate at the ends of the joint. The addition of the rubber molecule allows the peaking stresses to be slightly relived.  This dramatically increases the most important property of an adhesive, the peel strength. In the construction of the Robin there are joints between wood and wood, wood and foam and wood and fiberglass. The foam I am using for the ribs in the case of the tail feathers and wing is extruded 3 lb density Styrofoam sheet. Testing has shown that this foam will fail at approximately 90 psi in shear. The design allowable by the way is only 15 psi. Therefore there is no need to use a high strength adhesive like T-88 in any joint between Styrofoam.  All Styrofoam joints in the Robin use 5 minute epoxy. This dramatically speeds up the build process. The remainder of the joints will all use T-88. light clamp pressure is all that is required, the only requirement for the epoxy joint is contact pressure. The shear strength of the T-88 is around 5000 psi, where as the shear strength of the Spruce is around  900 to 1200 psi depending on the grain direction. 5 minute epoxy is in the region of 2700 psi.

Top sheet of the Vertical fin drawings

This is the top drawing of the beginning of the Vertical fin section of the plans. The fin airfoil is a symmetrical section . There are two spars, a leading edge and a trailing edge. The leading edge is skinned with 1/32nd plywood and is supported by a series of perpendicular nose ribs. 

Completed front and rear spars
All spars in the empennage are built off of a 10 foot x 8” flat jig board.  The main spars are assembled on the jig board first, and then the jig board is used to locate the rear spar.  Construction begins with the assembly of the two spars.
Birch plywood is used for the shear webs of both spars. There are intermediate blocks of wood separating the spar caps called intercostals. They serve two purposes, they react the compression component of the shear load and serve as a panel breaker reducing the size of the shear bays. The maximum allowable shear stress before buckling is a function of the shear bay size. 

Another view of the completed spars and skins
 This picture shows the pre-trimmed leading edge skin and rear spar gusset plate. this turned out to be a mistake, the final contour came our slightly larger than the design and these parts ended up being too short. The way to build these parts are to cut them slightly oversize, form them fit them and then trim them.

By following the print exactly these foam ribs are cut out. The edge bevels are carefully laid out and also cut out. The loft of the vertical fin is a constant section between the front and rear spar. This simplifies the construction.

The ribs are used to jig the spars together
 This is why its important to cut the ribs accurately and with the correct bevel angles.The ribs are being used to locate and jig the two spars together. Again, these bonds are all with 5 minute epoxy allowing the builder instant gratification!!!!!

Rib caps being added

this is an idea that belongs to Steve Wood first used on his Sky Pup. By bonding spruce wood rib caps to the foam core. this ensures that all axial bending loads will be transmitted through the wood and all shear loads will be transmitted through the foam. Because of the thick low density section, the foam is an efficient shear web. analysis has shown that a built up wood truss rib would be slightly lighter, i decided the simplicity of the construction outweighed the ounces I could save. where this design differs from the sky Pup,is in the use of the rear gusset plate and fwd skin. These pieces overlap the rib caps and create a nice clean aerodynamic joint. the bond between the foam and wood is again 5 minute epoxy. the later skin bond will be T-88 adhesive.

Nose ribs being added

 This view shows the nose ribs being bonded with 5 minute epoxy. The technique for cutting out these ribs is to first make a metal template from either Aluminum or Sheet tin. I use a 24 pitch metal cutting band-saw blade. Rather than use the front of the blade, i guide of the back of the blade where there is no kerf or teeth. The templates all have at least 3 #30 holes to accept some round toothpicks that are used to hold the foam to the template.

Leading edge being soaked in Ammonia

This is where more magic happens, this forming technique was developed during WWII by the US forest wood laboratories in Madison Wisconsin. Again, this work supported the non strategic materials aircraft program. the Ammonia soaks through the total thickness of the plywood. This usually takes about 30 minutes to be effective. The wood is not damaged and neither is the adhesive. The wood fibers are softened and will undergo dislocation when being formed. After the ammonia drys, the wood is good as new and the strength  is  unaffected. The wood will not have any spring back either. This is the beginning of the wrapping process for the leading edge. This is the oversize sheet that I previously mentioned that had to be remade.

This next step is best done outdoors!!!

a series of Velcro belts and clamps are used to wrap the skin around the leading edge foam ribs. try to start from the center and gently fold the skin downward. There are some space blocks under the belts that allow them to stand off from the from spar web. Let the set up dry over night. he next day the bond is made between the wood and the foam. Use T-88 adhesive here for all joints, the working time need to be the same at all of the joints, so 5 minute cannot be mixed into the joints. Before the skin is bonded to the ribs, a coating of epoxy resin is rolled onto the inside surface as a moisture barrier.

Final Assembly
3 days and 3.1 lbs!!!!!!




Wing tip master model process.

The Robin has a number of compound contour fiberglass parts. All of these parts are available for purchase, and will save a great deal of time when they are, however, in keeping with the spirit of die hard homebuilding, the Robin plans include all of the contour templates to build the parts. I have used this master model technique for many years. This process involves the use of 1lb/cu ft green urethane floral foam. This foam is readily available at Hobby Lobby and Michael’s craft stores. I always watch for sales at these places and load up when ever the price is right. I have actually built a full size fuselage master model using hundreds of the foam blocks. I will never forget the night I bought out the complete inventory of the Michael’s store in Wichita Kansas. I showed up at the checkout stand with 6 shopping carts full of foam. The lady at the register eyed me suspiciously; I knew what she was thinking, so I confirmed her doubt. I bent my wrist, struck a pose and in my best lisp I told her, “I have a HUGE!!! Wedding” The all knowing nod of her head said it all!!!

The basic principal of this process is exactly the same as the process amateur telescope builders use to make their optical glass lenses; they use the glass to abrade the glass. Since both materials abrade each other as the same rate, a spherical interface is formed. In my molding process, the foam is precut to the approximate shape and sections of the blocks are hot glued together. Care must be taken when doing this to ensure that the glue line will be beneath the eventual final contour. Its also important not to mix blocks of a different manufacturer for a given mold. All foams are not created equal and when the final shaping occurs the foam sanding block will not abrade the mold foam equally.

The following is a series of photos that illustrate the process used to make the wing tip master model. At the end of this process, I complete the master model by filling and fairing using automotive finishing processes. For the one time builder, the final step would be to seal the foam surface. I use spray paint of any type, sand able primer is acceptable. This surface is waxed with up to 5 coats of carnauba wax (NEVER, NEVER, NEVER, use a silicon based wax!!!!)  The outer surface of the foam is then laid up with a layer of fiber glass. The resin is allowed to cure and the surface is filled with either lightweight automotive Bondo or a slurry mixture of epoxy and micro balloon . The skin is then released. Building a part in this manner is slightly heavier than molding a part, but structurally they are the same. 

The process starts by building full size templates of the contour sections and the tip rib. There are two ways to do this, full size laser cut cardboard templates can be ordered, or the plans can be scaled up on a copier. The section stations drawing, have grid lines that are used to check for the exact scale factor when copied. Glue all of the paper templates to a piece of sheet aluminum or thin steel sheet. A word about adhesives here is important, do not use a water based adhesive, the preferred adhesive is 3M 777 spray. If you use a water based adhesive, the paper will swell and the contour lines will be distorted.  The reason I use metal templates is to facilitate cutting the foam on a band saw. I use a 24 tooth metal blade with minimal kerf. I guide the back side of the blade on the template when I cut the foam. All of the templates will have at least 3 #30 holes drilled into them so a round toothpick can he used to hold the foam to the template. Experiment with the correct drill size prior to drilling the templates to get a tight fit to the particular brand of your tooth picks.

using the tip rib as a sanding guide.
 All of the foam blocks are precut on the band saw to exactly 3 inches of depth. In some cases there will not be enough height of the block for a particular template. In these instances, the blocks must be spliced. Use a minimum of hot glue to do this and try to visualize where the bond line will be in the final contour. It’s very important that the foam splice glue not be near the final contour.

The base rib in the case of the wing tip master is a Styrofoam (extruded foam only) rib. Since this is the point where the contour begins the transition to a constant section, there is no need to shape this surface. It is used as a guide to sand the much softer floral foam.

using the base rib as a sanding guide
 once the plan view contour is sanded into the foam, the corners of the foam blocks are cut off with a sharp butchers knife of a hand held hack saw blade. Cut the blocks back to within a 1/2 inch of the final contour. Do not over cut. Repairs are difficult.

at this pont in the process, the magic happens!!! blocks of scrap foam are used to sand the foam. 
a block of scrap foam is used to sand the foam
Since the contour varies on the part, choose different blocks for the different areas of contour. Just a few swipes is all that is needed to bring the contour into shape.

this shows how the sanding block is starting to take on
 the contour of the mold.
Larger blocks are used for the less radical contours; smaller blocks are used near the trailing edge where the contour becomes more severe

final shape of the master
Dont worry about the gaps between the blocks, they will be bridged in the next step, which is to drape the foam with a layer of fiberglass cloth. In the case of the wing tip, the final lay up is only 1 ply of 8 oz BID cloth. There is no need for a heavier laminate because the tip cap bonds to a full rib and the contour of the tip cap gives it stiffness. remember, this plane is only flying at 63 MPH!!!

This is  the final finish for the master because I was making a full mold for this part. As an alternate to making a tool, the one time builder would be finished at this point.

This is what the final tip looks like after it is molded

3.8 OZ!!!!!

WingPanel Test

Test set up with members of the "Brain Trust"
Early on in the project I decided to test to destruction a full size wing panel. This was a tough decision because of the cost and time required to do this. But as it turns out,  this was indeed a fortunate decision. The test set up consisted of a strong back constructed from steel square stock and I-beams bedded into two 3 foot holes drilled into the limestone that makes up my part of Texas.  I was fortunate enough to be able to draw on the local “Brain Trust” from Vought Aircraft, Bell Helicopter, Lockheed Martin and American Eurocopter. All fellow designers and stress engineers and most importantly, friends in the business. I learned a long time ago on my Wren project that it pays to have a second set of eyes available.
Test set up showing nose angle

The wing panel itself was mounted inverted with the nose angled downward at 14 degrees. This is to simulate the abrupt pull up condition at the max maneuvering speed. This will induce the leading edge compression load that will verify the leading edge spar and the fitting.

The actual load was 90 lb bags of ready mix concrete. Fortunately a neighbor was just starting a fence project and was gracious enough to let me “borrow” them for a day. The load schedule was designed to emulate the constant loading of a rectangular “Hershey bar” wing with the parabolic drop off that occurs near the tip. The bags were to be placed in 1”g” increments. The design criteria I established was to have no skin wrinkling less than 2.5 “g” because I am trying to maintain laminar flow. After the first “g” of load was placed, it was realized that this wing would have too much deflection. It was decided to continue the testing to see where the weak link was in the design.

At appox. 3.5 “g” the deflection was 21 inches at the tip . When we attempted to add the next “g” load, it was noticed that there was severe web buckling between the main shear pins. This area of the wing used a box spar with a 1 inch foam core. The bond had failed obviously between the main web and the foam. A complete shear failure occurred shortly after the buckling was noticed and the wing broke. 
Shear web failure caused by web instability
Investigation of the foam bond joint revealed that there was never a complete bond to begin with. I built this portion of the spar on a very hot day and due to the insulating properties of the foam, the epoxy and micro balloon mixture had an exothermic reaction and never bonded to the web. I realized that this could easily happed to any builder so I immediately revised my design to eliminate the foam box design and use bonded vertical stiffeners instead. I found a product used in the electric motor business called NEMA grade C fiberglass insulation. This is a fiberglass sheet product made from 120 style glass and polyimide epoxy resin.. Its produced in an autoclave and is rated to 450 degrees working temp. Besides being a high quality laminate and product, its also cheap and available in numerous thicknesses. I was using ¼” thick plates as the main intercostals at the shear pins. I used .063” thick plates for the vertical stiffeners.
I decided to retest the wing to verify the new stiffeners and to verify the shear capability of the root web. The wing spar shear web lay up schedule is constant out to wing station 36, because of this I cut off the damaged inboard section of the spar and rebuilt the outboard 18 inches of the spar by adding a new shear pin intercostal. The outboard section of the wing was damaged when the spar struck the ground so that portion was cut off and discarded. I bonded a wooden plate to the outboard section of the undamaged spar so that I could gain enough leverage to emulate the design bending moment at the new “root” It was decided to test the spar in the up right position. Load was applied with a floor jack through a load cell I designed. The cell consisted of a hydraulic piston with exactly one square inch of area. Automatic transmission oil was pressurized and the pressure read by a hydraulic pressure gauge. The readout was one for one. The cell was calibrated at a local truck weigh station.
Testing verified that the shear web was fully stable at the design ultimate load of 1325 lbs. The final design of the prototype wing incorporated bonded vertical stiffeners and roughly double the amount of graphite used in the test spar.

Wing construction

main wing assembly jig. This used
every clamp I own
The wing construction started in April of 2009. The spar mold was built up using sections of MDF board. The first project was to lay up a test wing spar panel to verify the design concept. The spar itself was a "C" section shear web with .125" carbon pultrusion "Graphlite" rods laid up as a secondary bond in the corners of the "C". The rods themselves were completely encased or "bedded" with a mixture of cotton flox and epoxy mixed to a paste consistency. This is an area that is a source of additional unneeded weight in the wing design. All future spars will eliminate the use of the round rods. The voids between the rods add considerable weight when multiplied by the 44 feet of total spar length (there is a 2 foot overlap in the root). The first test panel was also designed to static design criteria. Later it was discovered that the test wing exhibited excessive deflection during test and prototype wing was revised to limit deflection by the addition of more Graphite pultrusions.
One of the design problems associated with the use of carbon pultrusions is load fitting termination. I debated a number of design concepts for the wing root attachments, but finally decided on overlapping the root spars. Because of this, the Robin Prototype wing is actually asymmetrical in configuration. The left wing sits 1.5 inches ahead of the right wing. The two spars overlap and sit between a box frame section in the fuselage. Two large shear pins pass through both walls of the box frame and both spars. The advantage here is that the wing bending moment is internally reacted by the overlapping spars and not in excessive structure built into the carry through fuselage box. The disadvantage that I later discovered during construction is having to drill the two holes!! This problem is so great that the overlapping of the wing spars has been eliminated on the production design and an alternative spar attach scheme is being used. The other great disadvantage in the original design is ease of assemble and disassembly in the field. The final design is far easier to field assemble.

aft ribs being assembled
 In the years I have been designing ultralight airplanes I am amazed at how many designers are actually ignorant of the design load conditions of the wings. There is a belief by many amateur designers that the worst case design condition regarding "Drag" is a rear acting load that is trying to tear the wings off the plane in a terminal dive!! Consequently these designs have all featured some kind of "drag" wire that attaches from the fwd fuselage to the wing spars. In reality the aft acting shear load or drag on an aircraft is quite low. In terms of actual numbers consider the condition of equilibrium of an ultralight pointing straight down in the zero lift condition. The total drag acting on the aircraft is exactly equal to the gross weight. In the case of the Robin this would be 550 lbs.

 Now divide that number between all of the components and you will find that the actual load on the wings themselves is in the area of 80 lbs per panel. The actual design criteria that determines the horizontal shear load is the abrupt pitch up maneuver at maximum maneuvering speed. When the aircraft is flying straight and level at the max maneuvering speed, the forces of lift and weight are balanced, as are the thrust and drag. But when the nose attitude is abruptly increased the lift vector shifts forward and increases the load acting in the fwd direction on the wing. 
Leading edge skin being trimmed

This induced vector in the case of the Robin is around 375 lbs acting mid span in the fwd direction. So you can see that the actual horizontal loading is around 8 times greater in the fwd direction as it is in the aft direction. This is why the early aircraft used to fail by the wings folding fwd. To react this load in a weight efficient manner, I took advantage of the deep leading edge "D" cell box. There is a leading edge spruce spar and leading edge fitting designed to react this load. The load is reacted by the fuselage in a couple between the nose spar and the main wing attach pins. Horizontal shear is reacted by bearing the wings spars into the main attach box.

Both Wings D cells laid out

I chose the the configuration of the wing to try and achive laminar flow over the first 33% of the wing. The airfoil is a Wortmann FX 170, low speed, high lift laminar airfoil. This airfoil is designed to be laminar over the first 32% of the upper surface, because of this I placed the wing spars at the 33% chord point. The nose "D" cell also acts as a closed torsion member to react wing twist due to aileron deflection and airfoil pitching moments. The aft section of the wing is covered in light weight Dacron fabric. I intend to bond the fabric with a heavy "pinked" edge right at the 32% chord point. This is designed to emulate sailplane turbulator tape that is usually placed at this point on modern sailplanes. The idea here is to force rapid transition of the laminar flow into turbulent flow that will be contained in a small but attached boundary layer. This is one of the riskiest concepts in the prototypes design. I am banking on a benign stall characteristic because of the low wing loading. Its not called "Experimental Aviation" for nothing!! As I said in my opening mission statement, I want to push the design beyond the low drag of my old Wren.

main aileron bay rib. All bonds between
 foam and wood are with 5 minute epoxy
The remainder of the wing is constructed very similar to my old Wren and that of The Sky Pup, I am using Styrofoam core for the rib shear panels and spruce wood for the rib caps. The big advantage here, besides ease of construction is a wide rib cap. This greatly increases the fabric bond area and eliminates the need for rib stitching. If I were to redesign the Robin into a LSA or a  higher speed homebuilt, these ribs would become wooden trusses and require rib stitching. You can get away with this in an ultralight when your maximum level speed is only 63 mph.

Lower Aileron Cove after forming with ammonia

The current prototype wings weigh 48 lbs per panel; This is higher than my original estimate. Because of this, the wing spar is being redesigned to eliminate the round carbon rods and the fiberglass shear web. I have identified a 9 lb per panel weigh savings in this area alone.

Root of wing showing pre cured fiberglass stiffeners

In the next post I will show pictures of the wing test and the test configuration