Robin Ultralight: December 2012

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.

New Landing gear brakes.

Hello to all the readers of Kitplanes magazine!! I was fortunate to meet Dean Sigler at the 2012 Experimental Soaring Associations Western Conference in Tehachapi California. It was sure great meeting and seeing everyone over the Labor Day week end. From the emails I have received I see that the January edition has already made it to the subscribers base.

The first thing I realized when we went to the airport for the initial Taxi test, is that I need brakes!!!! For the initial flight my buddy Ed was going to keep me from rolling into other planes and hangers, but in reality that's not really practical.
With this new Grove spring gear, it quickly became apparent that I was going to have a axle bending problem. The axle is a 4130 steel tube 3/4" x .125" wall. I do not want to heat treat the axle, so I am using the 4130 in the normalized condition. That limits the stress to about 95 ksi in tension and 74 ksi in compression. The old landing gear design tucked up inside the hub and limited the bending moment because the point of load application was only 1.75" The new spring gear increases the cantilever distance to 3.57", That distance by the way is the point the axle is cantilevered to the point of load application which is the center of the hub. When brakes are added the distance increases by another inch. The gear will almost bend now just sitting on the ground!!! To solve this problem I decided to make a sleeve collar that is 1.24" in diameter at the point of greatest moment. this will limit the bending in the axle back to the original design stress. I bought some 3.5" diameter 7075-T6 round stock and proceeded to make a ton of chips.

Sleeve collar being turned

Here is the beginning of the Sleeve hub being turned on my Lathe.
This is my faithful 36" Grizzly Bench lathe. I've had this lathe for 20 years, and believe it or not I have moved this lathe all over the country chasing the latest and greatest aircraft design contract!! I'm getting too old for that stuff these days however!!

This gear design is not only way too heavy, its also too damn expensive. As you can see in the next picture, 8 hours of my life was spent standing in front of my lathe!!

Chips and turnings

I filled that trash can 5 times!!
I have decided that the final design will be offered in two options, thanks in large part to all of the letters I have received. I will offer the original retractable mono wheel as well as an option Taylorcraft style internal bungee landing gear.

sleeve collar and axle

This shows the outside of the sleeve collar, the part not shown is a flange that mates to the gear leg. In this picture its very apparent how far the axle would have had to extend. The inside face of the wheel bearing rides on the outboard face of the sleeve. The flange of the sleeve is bolted to the gear with 4 .25" dia bolts. The brakes that I finally adapted are from Azuza manufacturing. I purchased them online from Go Kart Galaxy
http://www.gokartgalaxy.com/

ask for Clint. These brakes are mechanically actuated so I ordered all of the cable , cable ends and fittings. I have plenty of excess cable that I think I can route the cable to the actual rudder pedals. That will be the subject of the next blog entry.because I haven't designed them yet!!!

inside view of gear leg

The 3/4"l axle is welded to a 1/8" steel plate that is bolted to the inside face of the gear. I had to grind out a small recess at the very bottom of the gear to clear the actuation lever. I also had to flatten the lever and remove the 1/4" offset. The flange of the sleeve is also shown in this view.

Cable anchor bracket.

I added a bracket to attach the adjustable cable anchor. All in all this turned out to be a really clean installation. The cable will route along the trailing edge of the gear and will be attached to it with heavy duty nylon tie wraps

Drilling the axle hole

If you were wondering how I was able to accurately drill the 3/4" axle hole, this is how I did it.