Robin Ultralight: May 2015

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 Casler Cowl Cont:

I am writing this update from my hotel room in San Jose Dos Compos Brazil. This is my last day here. This was a quick trip, 3 days. I came down here for a design review on the EMB 175 elevator. I am the design lead for the Embraer 190/175 Rudder and Elevator. This was our last day and when we got ready to go into Embraer, we got a phone call telling us the place is on strike. This is the 5th time I have been shut out of Embraer.

Anyway, I made significant progress on the cowl since the last update. I am almost finished with the shaping of the upper half of the cowl. I will start where I left off.

lower template installed

I completed the side view template on the lower portion. I had to disconnect and re-position the lower fabric stringer on the fuselage. Unfortunately, I will also have to build new fabric stringer stand offs at the side of the fuselage. The fuselage lines will not blend with the new engine position.

The next step is to set the plan view templates.

plan view template r/h side

plan view template l/h side

Before I could set these these templates I had to add a temporary fabric surface on the fuselage. this is the surface that the cowl will sit on so I needed to mock it up. The shape is determined by the clearance envelope around the engine. I am holding everything in place with hot glue.

The shape was set a little long so I could trim the cowl to size the outlet area.

The next step is to block up the upper cowl with foam blocks. there are two separate shapes that need to be formed, the first is the transition surface between the spinner and the fuselage, the second is the shape of the cheek cowl. The two surfaces will eventually transition to each other with a 2" radius.

upper blocking

the blocks are fitted as close as possible and attached with as little hot glue as needed. In all cases, I try to attach at the lower surface well away of the final contour. Sometimes that's not possible and the glue has to be dug out before foam shaping.

once the blocks are in place I used a hand saw and butcher knife to rough carve the shape.

rough carved

you can start to see the first shape appear. I will use a hand sander and sand along the vertical template horizontally to establish the side profile. Once this profile is established, and the foam is rough carved. Then the true magic happens with this method. The foam is used to sand the foam. Both materials will abrade each other at the same rate creating a spherical interface.

this is the result.

foam sanded

You can now see the smooth transition between the spinner and the fuselage contour. The next step is to block up the cheek cowls. I ran out of the grey foam so I bought a different brand for the cheek cowls. As long as I don't mix and I use the same foam to sand itself, its not an issue. I will jump to the image of the final foam sanded cheek cowls.

Add caption

now the shape of the upper cowl is apparent. The next step is to lay one ply of light fiberglass cloth on top of the foam. This will allow for the final surface finishing with Automotive Bondo. I have decided that I need to make a full mold for this cowl, Anything less will be heavy and ugly. I will work on the upper half of the surface until its ready to be splashed (initial mold) then I need to flip the whole plane over so I can work on the under side. that will be a real job!!!

with a layer of fiberglass

I put a layer of aluminum tape on the spinner and the fuselage so I could lay the fiberglass on it. I put some mold release wax on that tape so it could be removed later. The fiberglass is draped over the transition nicely, but it still has too small of a transition radius. I used polyester resin for this step so I could move on to the Bondo step later in the afternoon. Before I added the Bondo, I block sanded all high spots and generally roughed up the fiberglass so the Bondo would adhere.

bondo surface and circular drag

This shows the first coat of Bondo and the circular drag I made out of Aluminum to sweep in the transition radius. Now its a matter of sand, sand and more sanding until the surface is perfect. Then I will flip it over.

I will be back to Texas tomorrow Morning and I will have all week end to work on the project.

see you at the next update

Engine mount Installation cont:

I completed the in-situ design of the engine mount. At this point in the project, I am designing as I go and not laying out anything on the computer first. I do check loads and stress however! The addition of another motor other than the original MZ 34 is less than optimum. I never designed for these engines so their installation is not extremely clean and light. I know I could do a lot better and make a cleaner installation from scratch. I increased the area of the 4 longerons that terminated at the firewall. I attached 4) 4130 steel angles to the ends of these longerons. The main issue with this method of attachment is the lack of a load path for vertical shear. I polled my designers at work and Dave Magerstadt, an excellent designer and former ESA treasurer, came up with a solution!

shear intercostal

I bonded a 3/8" plywood intercostal to the firewall. Before I did I added 4 fiberglass doublers to the old firewall. I fabricated 4 fittings that picked up the 1/4" engine mount bolt and shear tied into the wood intercostal.this will react the vertical shear that the motor weight x 6.2 g will induce.

This is another view of the firewall. I have already added the ceramic firewall that will protect the fiberglass firewall bulkhead. Later the ceramic and the wood intercostal will be coated with Contengo. an ablative latex coating designed to char rather than burn.

iso view of firewall

I fabricated 4 fittings from Aluminum that tie the ends of the intercostal to the single 1/4 engine mount bolt. The loads are low, about 287 lbs per fitting. The next two pictures show the Contengo coating and a view of the engine mount installed with the shear fittings.

the white coating is Contengo,

shear fitting

this shows clearly the load path I am developing. If I did not add these intercostals, the vertical shear would be reacted by differential bending of the upper and lower fitting. This is a recipe for disaster, an inaccurate load path invites unpredicted failures in the basic joint. Thanks to Dave's Idea, I have a good load path reaction.

Contengo

Contengo!!

engine mount

After I welded the Mount and primed it with Zinc Phosphate I asked my buddy Ed Gardner of MFR Inc to top coat it.

http://www.wepaintguns.com/

Ed is the Picasso of Pistols, the Rembrandt of Remingtons the Wyeth of Winchesters.... You get the idea. Visit Ed site and you will see some beautiful work!! Some of it is mine too!! I asked Ed to topcoat the mount with Urethane paint. While he was at it, he top coated my friend Michelle Craig s 1946 Talyorcraft wing fittings and the cowl of the 1984 Tiafum Motorglider I am repairing for Dan Stofman.

Unfortunately the engine mount was also a compromise. I had to add two .032 gussets to react the non optimal truss. I would have liked to have terminated the truss member into the the other tubes cluster, but I had to revise the tube angle to clear the intake manifold. This creates a local overturning moment that needs to be reacted by a gusset plate. Not efficient, but again, all the load paths are accounted for.

Engine mounted

Well the good news is the plane didn't fall over on its nose when I released the overhead straps. The plane was tail heavy with the light weight MZ 34. That will be remedied with this engine!! Its 50 lbs heavier and sits a full 12" fwd of the old MZ 34's position. This means that I have to build another new cowl. I have decided not to make a full mold for this cowl since this is a prototype. I do not want to invest the time and money making a mold for a one off cowl. My plan is to sculpt the floral foam and then cover the foam with squares of aluminum foil attached with spray glue. I will then wax the surface and lay up on top of the foil. I will fill, fair and sand the cowl until the finish is acceptable and then remove it from the foam form. I will split it while it is on the mold. To start the process, after work today I stopped at the local Michaels hobby store. I'm usually the only guy in the store and all of the woman think I'm a gay florist when I buy out the floral foam.

all of the foam from the store

This is all the foam from the Mansfield TX. Michaels store. Thats the trunk of my new Scion FRS by the way!!!

I usually confirm the clerks doubts and tell them I have a "Big Wedding"

So the first step in developing a proper cowl shape is to lay out a Masonite template of the side view and plan view or in nautical terms, "Max Half Breadth" My desire is to use the existing spinner and backing plate. I have molds for both of those parts. So I need to develop lines that blend from the spinner to the fuselage lines. This was pretty easy to do on the upper loft.

upper cowl spline template

this is good clean line. Much longer than the original Robin Cowl with the MZ 34, but sleeker. not a bad start. I will semi permanently attach this template on Butt line Zero. The lower template proved to be a bigger problem. The drain sump and the lower case caused a reverse inflection of the lower line. I use a piece of 1/8" welding rod as a spline strip. I had to cut loose the lower fabric stand off on the fuselage to achieve a smoother spline. The following picture shows the final result.

lower spline template

I still need to develop the plan view templates, but once finished. I will cover the engine in Aluminum foil to protect it and then block in the spaces with floral foam. That is when the real fun begins and I will get to express my hidden artistic talents!! I will have more as I complete this mold. Stay tuned!!