Robin Ultralight: October 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 cont.

I have finished the gear fitting installation to the fuselage. The first step was to locate the fittings using the gear as a guide. I loosely bolted the gear to the fittings and then located the fittings to the fuselage. Once located I back drilled the through fitting through the fuselage bearing blocks. I made a makeshift drill jig by drilling a 1/4" hole through a 2.5" thick block of aluminum. It was very critical to drill these holes perpendicular to the outer flange so the holes would line up with the inside flange. Normally the inside flange would be left un drilled or undersize so the final hole would open up both flanges. I was betting that I could drill a hole accurately enough so the drill would align with the inside flange.

homemade drill guide

I used to work in Wichita at Boeing Commercial aircraft, they had the most wonderful surplus yard I have ever seen. I bought a ton of forged aluminum block and plate stock. This is just a small little piece. There are commercial drill jigs, most notably the "Egg Cup " design. all of these will work, but I didnt have a 1/4" drill bushing for my Egg Cup.

After the 1/4" holes were drilled, I chased then through with a 1/2" twist drill. Opening the holes up to just slightly under 1/4" (-.003) I then fabricated eight 1/2" diameter aluminum bushings. The 1/2" aluminum stock is slightly larger than 1/2" (+.002) this makes for a nice tight transition fit between the wood and the bushings. I sanded the outside of the bushings and then cleaned them with MEK.

cleaned bushing and epoxy adhesive

once cleaned, I coated the bushings with epoxy adhesive and tapped them into place. The bushings were made with the center hole undersize (.223") so that on final assembly they will be opened up again to .250" The adhesive is used to stop the bushing from rotating when it is drilled out.

installed bushings

the bolt loads are such that they needed to bear into the wood with greater area, this is the reason there are 1/2" diameter bushings.

The landing gear fitting is square and parallel to the landing gear. The fuselage has a slight amount of contour . The load path from the gear to the fuselage is through a mechanism called "Heel and Toe" The design condition is a 1G breaking load applied to the tangent edge of the tires. This creates a large moment that needs to be reacted by the two outer bolts on the gear fitting. Since the fitting is wider than the bolt pattern, the reaction will actually be between the edge of the fitting (heel) and the opposite bolt (Toe) . Therefore the gap between the fitting and the fuselage must be filled with a material capable of withstanding compressive forces. The fitting is "Bedded" with a mixture of epoxy and cotton fiber (Flox) . Flox is mixed to a consistency of dough. I spread it under the footprint of the fitting and then installed the fitting. The inside of the gear fitting was coated with mold release wax so the fitting could eventually be released if needed.

flox bedding

once the fitting is installed, the excess bedding is scraped away.

When I installed the gear for a trial fit I saw that the clamp block was deflecting and bridging over the gear. To fix this I designed 4 more bushing that were exactly .003" taller than the thickness of the gear. They would crush to equal the gear thickness after installation

clamp bushings

after the clamp up bushings were installed, the bridging stopped and a more even clamp pressure was developed.

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New Landing Gear installation

My new gear finally arrived from Grove Aircraft.I have finished all of the fuselage structural modifications. I have built all of the attach fittings and I have prefitted all of them. The gear is designed to clamp up between two radius filler blocks. The purpose of this design detail is to limit induced bending into the fuselage. If the gear were to be clamped with just flat plates, every time the gear deflected, the gear moment would be transmitted into the truss.

new gear

this is one heavy chunk of metal!! Makes me want to think about a mono wheel again.

attach fitting and radius plate

This image clearly shows the radius plate. The gear is clamped with two 3/8" dia bolts. I plan on silver soldering the bolt head to the inside of the steel fitting. This is so the bolts wont rotate when they are torqued. there will be a 3/4" counter bore in the fuselage that the heads will sit in. There is a slight amount of contour mismatch between the fitting and the fuselage truss. I will assemble these fitting with a bedding of Cotton Flox and epoxy. The primary load direction is in compression, with the exception of the fore/aft braking load .

3/4" counter bores

The area under the counter bore has been padded up such that the width of the gear bearing plate is supported.

The attach bolts that pass through these pads are bushed with a 1/2" diameter aluminum bushing.

After the gear is installed, I will need to fabricate a fabric rib that will sit on the front and rear of the gear. An aluminum cover will connect the ribs and fair in the gear

New Landing Gear

I have been patiently waiting for my new Aluminum Landing gear from Grove Aircraft in San Diego. My friends Mike and Neal Lafrance actually own the hanger next to Grove.. Even with that kind of pull, I still have to wait 8 weeks for the gear. As Robbie explained it to me, its out of his hands because the gear has to go to a heat treat shop 3 times during its construction. I have 3 more weeks to wait!!! I decided that the simplest way to add a landing gear to the Prototype was to attach it to the outside of the fuselage in the same location of the old one. I needed to add some additional bearing blocks and increase the area of the two diagonal stiffeners . Truth be told, I should have done that for the original gear.My goal is to have all of the modifications done and the Fuselage recovered by the time I get the gear. I want to be able to just bolt it in and then flip the plane over.

reinforced gear frame

in this picture the extra doubler on the two diagonal members are shown. all of the vertical loading of the gear is reacted by these members. They also need to react the aft braking load by a heel and toe reaction between the gear fitting bolt and the back wall of the fitting. underneath the plywood is 1.5" of birch plywood bearing blocks. .

outer landing gear fitting

This picture shows the outboard fitting for the new landing gear attach fitting. There will be a narrower fitting that is bolted to the inside of the gear frame. Both plates will be welded to a .125 plate that will act as a bearing plate. I will have two 3/8" bolts permanently brazed to the 1/8" plate. I will counter bore into the birch doubler to clear the bolt head. Sitting on top of the plates will be a radius block. The gear will be sandwiched between two of these radius plates. The isead here is to limit the gear bending moment to the gear itsself. The radius blocks will act like two fulcrums.

Unfortunatly this new gear will add 8 lbs to the Robin and may put the prototype out of the part 103 category. I'm sure with the redesigned wing and the single cylinder 2 stroke engine, Part 103 will still be possible. After my talk on Labor Day, the universal opinion of everyone I talked to was to scrap the 2 stroke and Part 103. My Friend Murry Rozansky who is the ESA Treasurer, suggested that I look at the 23hp Briggs and Stratton Vanguard conversions. After some research, I have decided that this is the perfect engine to adopt. The Prototype is actually perfect for it from a weight and balance point of view. My CG will move forward to 22% at full fuel. 26% empty. The static margin increases by 10%. This engine will be far more reliable than the 2 stroke. I will however fly this plane on the MZ 34.