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.









Engine cowl part 3 (Cheek cowls).

The next step in the construction of the master model was fabrication of the cheek cowl cores. The Robin is a loose copy of the Fournier RF4D Motor glider. This plane was originally powered by a  flat 4 cyl. VW Limbach conversion. as such it had two side cheek cowls that streamlined the cylinders. On the Robin, I am using an upright single cylinder 2 stroke. I decided to retain the cheek cowls because they serve two purposes, they will streamline the carburetor and the air filter and streamline the exhaust. At this point in the construction of the engine cowl, I have completed the main body of the cowl and fared the firewall to the spinner. The main cowl is shaped, a layer of fiberglass was added and a light milk coat of body filler was added. I began construction of the Cheek cowl by laying out the surface loft on my 3D cad system. I plotted the plan view and section cuts space exactly 3" apart. I transferred the loft to some Masonite and cut out all of the templates.      
R/H Cheek cowl Patterns.
        
1/8" Masonite was used for the templates. The original pattern was plotted on printer paper and 3M 777 spray adhesive was used to affix them. do not use water based adhesive for this step, the paper will swell and distort. at some point the contour reverses and the same template is used for the fore and aft position.


Cowl template in position




I trial fitted the template at this point to ensure I had enough excessive flange area. As shown in this view, more tape had to be added above and below the cheek cowl. The actual part will only be a 1 inch flange beyond the cheek cowl, but excessive edge is needed for the building of the lay up tool and plaster splashes.

foam being sanded flush on the belt sander


All of the templates were lightly tacked to the foam with 5 minute epoxy of bondo. just a small dab is all that is needed, because the template is removed later. The foam is rough cut on the band saw and then flush sanded on the belt sander .





templates being bonded

the foam had to be pieced together for the larger templates. a small bond was all that was needed and it was placed such that it was buried inside the final contour.








foam blocks in place on master template


The foam was bonded with a very small dab of bondo to the master template. I want to remove the foam later after it has been shaped. in this view, the plan view off the foam was sanded flush to the template.

Top surface was sanded tangent to the foam blocks

In this view the sanding stick was used with 36 grit paper to sand the top surface tangent to the foam. The surface is sanded until the edge becomes a line.


corners sanded tangent
the same sanding stick was used to sand the corners flush. At this point the foam can be final shaped using a block of scrap foam as the sanding block

.

as you can see the results are excellent!!!




Foam shown during final shaping.
By sanding the foam with the foam, both pieces abrade each other. the result is a beautiful spherical interface. This surface is wave free and readily finishes after a layer of fiberglass is applied.







Another view of the final surface


After the foam was shaped I was able to peel the main template off without damaging the foam.









36 grit sandpaper tacked to the cowl



After the cowl was formed, it was necessary yo contour the base so it would fit flush to the engine cowl. I hit on the idea of sanding the contour into the foam cheek cowl cores. It worked like a charm!!!


cheek cowl bonded into position

After the cheek cowl was fit to the main cowl, it was bonded in place. The next step was to lay one layer of 8 oz fiber glass on the foam so I could complete the finishing. this is urethane foam, so any suitable laminating resin can be used. Because of cost and time considerations I used polyester resin. The cure was complete in 45 minutes.


L/H cheek cowl/inlet





The L/H cheek cowl was fabricated similar to the R/H cheek. The main difference is the addition of a cooling air inlet.

the cowl was also sanded into position and later attached.

L/H cowl in position


As I write this update i have completely coated and block sanded the cowls. I have added blend fillets between the cheeks and the main cowls.




The final finish and removal of the cowls is the subject of the next Blog.
Posted by

No comments:

Post a Comment

Subscribe to: Post Comments (Atom)