Robin Ultralight: August 2011

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.

I have built 4 sets of rudder pedals so far and I hated every one of them for various reasons. The biggest is the lack of easy adjustment. A few weeks ago I won a raffle at our local glider club TSA (Texas Soaring Association). My prize was a one hour aerobatics lesson. I had a blast but also got the chance to see a properly designed adjustable rudder pedal . I'm not ashamed, I will always copy a good idea and give credit where credit is due. I guess good design is making sure you always copy the best!!! Any way, I owe the seat loft lines to an add in Soaring Magazine for a Jantar Sailplane and now I owe the rudder Pedals to an ASK 21 sailplane. I have to say that I have been very impressed the deeper I look into the design of modern German sailplanes Anyway here is what I ended up with. The material will be here later this week and I will start fabricating it. The cowling is completely finished but there still are tons of small but minor details to finish.

3D model of the adjustable rudder pedals

The pedal assembly slides fore and aft on a set of bronze Oilite bushings. the red cam on the tube is a pawl with a tapered bolt . it is spring loaded to the latched position. The pawl is connected by a lanyard to a tennis ball that sits in a holder between the pilots legs. The rudder pedal carriage is also spring loaded with a large light compression spring such that when the pawl is released the pedal assembly is forced rearward. The pilot adjusts the pedals by pulling the latch and pushing the pedals until he is comfortable. the pawl engages the center tube every 1 inch. the total travel distance is 14 inches. the two rudder pedals ride on a welded fixed shaft that is attached to the sliding outer tube. The pedals also ride on bronze Oilite Bushings. Not shown are two small Teflon shoes that ride on the ends of the pedals and on the floor board. This is to address the one criticism I have of the two systems I looked at, they tended to rock in the inboard/outboard direction. The key to this design however are the two "S" shaped tubes.

inboard side profile of rudder pedals

The rudder cable passes through a nylon liner inside of the "S" tubes. The cables are anchored to the engine mount where the turn buckles are located for the rigging of the rudder. The cable only moves at the end of the rudder pedal, the cable exits at the bottom of the tube right at the pivot axis center line. As the pedals are adjusted fore and aft, the cable slides through the nylon liner in the 'S" tube.

Well its been 35 days over 100 degrees and the old shop is a bit unbearable to work. I started sleeping after work and getting up at 9pm so I can work in the cool of the night. I'm making progress.I just finished fitting the spinner to the backing plate and trimming the spinner for the prop. I have the upper cowl trimmed and fitted.along with the prop and spinner.

Spinner, ground adjustable prop and upper cowl

Upper cowl trim was previously determined by a fiberglass mock up cowl. I am using 7 oz graphite for the cowl because it is 10 times stiffer than fiberglass and can be laid up with only one ply. The edge bands got a extra doubler. The upper cowl, spinner and backing plate weighs 1lb 1oz.

view looking aft

I'm having a tough time finding metric 8mm x 1.25 grade 8 prop bolts with drilled heads. If anyone has a source please e-mail me at Planebuilder@yahoo.com

R/H side

So while its too hot outside to work, I have been working on a redesigned rudder pedal. I have a fixed seat, since the pilot can weigh as much as the plane, he must be positioned right in the center of the CG range. Consequently the rudder pedals must be adjustable. I had the opportunity 2 weeks ago to take an hour of aerobatic instruction in a ASK 21 sailplane, The rudder pedal adjustment mechanism is similar to an ASW 21 and is pretty typical of German sailplanes. Any way, if its good enough for them, its good enough for me.

Robin Ultralight

Welcome to my Robin Blog.

Adjustable rudder pedals

Carbon cowl