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.









Final Jobs Before covering

Merry Christmas!!   For the last 3 years, this vacation has been my chance to make real progress on my project. I had hoped to be able to take off most of the month, but we had a huge number off RFQ's ( Request for Quote) in the last month. I normally work on an advanced composite design project, unless we have quotes to do. Had my project just been my Advanced R&D project, I could have taken off. Oh well. Anyway, I got smart and bought 3) 55 gallon Barrels and set the Robin on top of them. This also gave me a chance to weigh her with the wings on. I am not going to make ultralight weight without a Ballistic Parachute. I am close, 246 lbs, but that without cover. On this plane however is a ground adjustable CGS prop that weighs 9.5 lbs. I am using that Prop to dial in a fixed pitch Spruce prop that will weigh 1.5 lbs. Subtracting that weight from the total and that gives me a weigh allowance of 16 lbs for covering. I have estimated 18 lbs. Anyway, with a Second Chantz http://www.secondchantz.com/ ballistic chute, I will be a legal Ultralight. as I mentioned earlier in my blog, by wing is 19 lbs over weight. 5 lbs of that weight is excessive resin packed into the void between the round carbon pultruded rods. The other excessive weight is the fiberglass in the spar and the leading edge. I have begun working on a redesigned wing, I do not like the current attach method either. This and the desire to reduce weight has driven me to a redesign. My new wing design will be constructed of plywood and graphite pultrusions. There will be a fixed center section, this one redesign will save 5 lbs, due to elimination of the overlapping spars. The attachment method will be more akin to the beautiful German sailplane designs I have been studying. There will be no physical attachment of the ailerons or the wing other than two main shear bolts. All fore and aft and vertical shear will be reacted by sockets and pins. The breakdown of the wing will be greatly simplified by this redesign, I would anticipate that installing the wings or breaking them down will take about 10 minutes.
up in the air
If my span was one foot longer, I couldn't build her in my shop. man I miss my old shop in Wichita!! I picked these barrels up at a local Feed store, believe it or not, but they were marked as having concentrated lime juice from Iran!!!  I thought we had an economic embargo in place for 25 years??? The scales are also shown in this view. I needed to elevate the plane so I could walk around it without ruining my back. I needed access to bottom so I could terminate the aileron cables. I designed the wire routing so that they could pass from root rib to root rib without passing through any redirection pulleys. In the root of each wing rib, I added 2 split Piper Style cable guides. The cables are being redirected less than 1/2 of a degree. Very acceptable!!! The ailerons actuate with very little friction.
Aileron Cables

Here is a view looking fwd from the tail . This area is accessible after the seat is removed. the Main wing shear pins are also shown. These ended up being 3/4"  grade 8 pins. They are modified automotive grade 8 bolts. Strength wise, automotive Grade 8 bolts are stronger than typical aircraft bolts. AN aircraft bolts are actually equivalent to Automotive grade 5 bolts, the big difference is the pitch of the thread.  The cables break at 3 AN-115 shackles with 3) 3/16" clevis pins.
cable cutout

This was a little "oops". At the limit of the bottom travel of the aileron, the turnbuckle interfered with the upper aileron fairing. 3 minutes of work with the rotary rasp and a circular sanding wheel, and all was fixed. The aileron cable pulleys are free floating and seek their own angle thru the range of aileron travel. All 4 pulley brackets are identical. The design has two cotter pins that retain the cable when the lines are slack. For inspection purposes, there will be an inspection cover in the lower covering. The Ailerons are Frise type ailerons.


The big issue with a long sailplane wing is adverse yaw. Plain hinged ailerons will generate more drag when deflected downward than upward. This is due to the dynamic pressure being greater on the lower surface than the upper. Since drag is a function of dynamic pressure, the down traveling aileron will normally have more drag that the up traveling aileron. This causes the nose to yaw in the direction opposite of the turn. The pilot must anticipate this and lead with rudder prior to staring the bank. This makes  for a poor flying airplane. Adverse yaw is usually eliminated by the use of differential bell cranks. But this would have required 1 bell crank bearing and two Heim or Rod ends per aileron. That equates to about $200 per airplane. The alternative is a Frise style aileron. I chose the Frise because of three reasons, it was lighter, cheaper and the dipping of the nose causes an increase in drag and an aerodynamic balancing force that reduces control stick forces. The German Gliders that I have been flying, have friction and aerodynamic forces so low, there is very little force required.
Elevator Tip Cap

One of the last jobs I did today was trim and bond the elevator tip cap. Tomorrow, I will add a small foam bulkhead that closes out the Horizontal and the elevator tip caps.


I have three more jobs to finish before i can start covering, i need to install the Primer tubing and install the Throttle. once they are finished, I have only to finish installing the Rudder and Vert. Fin tip cap. I held off until this week, and it turned out to be a good thing, My  Buddy Ed Gardner http://mmwauto.com/ gave me a very cool Christmas gift, one that everyone on this blog will eventually appreciate. Its a 170 degree digital Video recorder. Its so small I can mount it in the Vert. fin Tip cap. I will record my first flight and taxi tests with this.
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