Robin Ultralight: January 2013

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.

Brake pedal installation

I'm sorry I'm so late in updating the blog. I have been very busy!! 6 months ago I agreed to take on a side job for a Gulfstream G3 Engine swap. I am designing a new engine installation for the Right side pylon. This plane will be a flying test bed for the manufacturer of the new engine. I have a hard deadline for complete release of the engineering drawing of Feb 9. Consequently I am slammed for time and the Robin has to take a back seat. That's the bad news, the good news is I have earned enough money to purchase a 4 x 8 x 19" 5 axis N/C router!!!! I am expecting delivery to Houston Texas in May. I can't tell you how excited I am to get this tool. Not only will it allow me to make an exquisite wood kit for the Robin, but it will also make possible my next project. I am going to design and build an all wood laminar flow single place high speed airplane powered by an industrial Suburu 40 hp engine. Speaking of industrial engines, I recieved my replacement engine for the MZ 34 I currently have on the Robin.

23 hp Vanguard Engine

When I get back on the project I will swap this engine for the MZ 34. Stripped down and using a forged billet flywheel this engine will weigh 64 lbs. That will put the prototype over the part 103 254 weight limit. However I anticipate that changes to the wing and landing gear will bring her back under 254 lbs. More on those changes later. I plan on mounting the prop on the flywheel directly. Research has shown that a direct drive should yeild 154 lbs of static thrust. By Mounting the prop on the flywheel side the airflow thru the cowl will be in the same direction as the engines design. I will also be able to mount the engine using the PTO mounting fasteners.

I have finished the new replacement landing gear installation. I fabricated a cover plate that can be removed to service the gear.

new gear cover plate

landing gear center cover.

The part 103 Robin will go back toi the original Fournier mono wheel. If a direct drive installation works, the prop clearance issues are such that I could use a non retractable gear. Saving even mode weight.

The existing rudder pedals were removed and modified to add toe brakes. This installation is very clean. Here are some pictures of the installation:

toe brakes

toe brake pedals were added to the existing rudder pedals, The toes brake position is adjustable. I will set the position after I get a chance to sit in the cockpit. The brakes are cble actuated and are modified Go Cart drum brakes.

break cable routing

there is enough relief loop in the brake cables to allow full rudder pedal adjustment.

rudder return pulley

I decided to update the Blog this evening because I received a question from a reader about the rudder return cable design. The main change I would make to this design is to raise the cable attach points to the end of the rudder pedals. This still works pretty good. I like this a lot better than the spring return I used to have. You will notice that the pulley bracket can swivel horizontally. The fwd pivot hole is oversize and allows the pulley bracket to also rotate up and down. The threaded rod is also the cable tensioner.

This is a better picture of the return mechanism.

Finally in December it was my privilege to meet a blog reader from Sweden Dr Jörgen Åstrand. He is Fournier RF4 and Piper Cub Owner and possible a future neighbor here in Texas. It was sure nice meeting up here in Texas. Ironically he was also a reader of my Friend Ed Piper Cub Blog. Only at the last minute die we all realize that he was coming see us both during his vacation.

Dr Jörgen Åstrand and myself.

KJHKHKJHiu