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.









Casler update and Tiafun Motorglider cowl repair

I guess I have more blog readers than I thought. Some of you folks cornered Scott Casler at Oshkosh last week and pushed him into starting the build on my Engine. Wont be long now!!

In the mean time, I finally finished a big side modification job and I was able to clear the decks and start on the repair of Dan Stofman's Tiafun Motorglider. 3 years ago when I went out to the SSA western workshop in Tehachapi CA, I met Dan who was a friend of Murry Rozansky, treasurer of the ESA and the guy who invited me to speak. While I was there a beautiful Tiafun Motorglider taxied up to the restaurant. Dan remarked that  was looking for that very plane. Two weeks later I was browsing the Chartis /AIG insurance salvage list and I found a Tiafun in Moriarty New Mexico. I called Dan and sent him the link. He threw a really low ridiculous bid against it and guess what?? He won. So Dan flew out to Texas and we grabbed my flat bed trailer and pick up truck and drove to New Mexico. The plane had made a hard landing in Grants New Mexico and collapsed the nose gear. The prop struck and she settled down on the front cowl. The plane was sent to a repair facility in Moriarty and Chartis paid around 50k to put a brand new Sauer 90 hp VW conversion engine and a brand new MT electric prop. The shop worked on the plane for two years. The original mechanic on the project quit and when the second finished the repairs, the plane was being taxied to the run up area and the nose gear collapsed again!!! The second Mechanic did not know that the rigging was incomplete.  Chartis got fed up with the whole affair and put it up for salvage. Enter my friend Dan!!



Dan and his new bird


It took us 8 hours to securely load and rig the plane and then another 13 hours back on the road to Midlothian

we had to add an 8 foot extension to the trailer
The primary damage to the plane was the prop, lower cowl, gear door and the actuator linkage. I sent the Prop back to MT in Miami and they rebuilt the whole prop. For $500 more we could have bought a new one. We figured the main components should be good, after all, the engine prop combination only has .8 hours total time!! I had a local IA inspect the engine run out and he found no signs of a bent crank. The blades were wood covered in Fiberglass, so they absorbed all of the impact damage and protected the engine.

The main damage to the cowl was the grinding down of a chin blister that once was used to streamline the original motor mount.



Ground down chin blister
The only other damage on the cowl was severe gel-coat cracking. I took the cowl out the the Airport and trial fitted it back to the fuselage. I wanted to check if I had any clearance issues with the Sauer engine mount.


clearance check
After looking at the new mount, I realized that the old blister was not really needed with the new engine. The cowl however has blend contour from the base loft to a blister surface. Rather than rework a much larger section, I decided to restore the original blister contour.
I started the process by cleaning up the blister opening.

Dotco motor with flapper wheel
 I cleaned out the opening using a sanding flapper wheel. I then went back to my old faithful technique of using floral urethane foam to shape the actual blister.

floral foam
 The opening was larger than the stock block size so I pre-fit two blocks together and very carefully bonded them along the very lower edge in the middle of the block. The idea is to locate the glue below your final contour so the glue doesn't interfere with the shaping.

5 minute epoxy
I like to use 5 minute wherever I can get away with it. I have no patience!! Both blocks were bonded to the exposed edge of the cowl. After it cured I carved the rough shape using a hacksaw blade.


Rough cuts
The plan here is to rough in a side view and plan view. I want to get within 1/2" of the final shape. The next step is to sand in a tangent edge. This edge will be tangent to the existing cowl contour. I use a sanding stick with 36 grit. No sense in wasting your time!!


side tangent sanding


edge tangent sanding
Now the magic happens with this process, the foam is used to sand the foam. This is why I paid careful attention to keep the glue line below contour. A block of scrap is used alternately on the left, then right side to sand the foam. The contour is constantly being transferred back and forth between the two pieces of foam. The result is almost perfectly wave free and a spherical surface.

front view after final sanding

side view after final sanding

See how pretty and easy this method is!! I have used this to build full size fuselage masters, repair Corvettes, Sailplanes and power boats. You will recall in the previous blog entry's I made my cowl. The next step was to sand back about one inch the existing gel coat.


1 inch of Gel Coat removed
The next step is to layup the fiberglass repair ply's. I like to used the old Burt Rutan Hexcel 7725 BID cloth. Its about 8 oz/yd sq and lays up about .010" per ply. I didn't bother to pre-fit these ply's, I just draped them over the foam and wet out the glass from the outside.I laid 3 bid ply's and one very thin final ply that will require very little final fill in its weave.

3 ply's of BID and one ply of fine cloth

The next day the laminate was fully cured thanks to living here in the Texas summer autoclave!!
I feather sanded the edges with my Dotco and 4 inch disk. This cut away the excess cloth and tapered the glass into the 1 inch area I removed the gel coat from/

feather sanded edges
The next step was to sand out the sever gel coat cracks. I decide to fill all of these deep cracks with a mixture of Microballoon and Epoxy. This makes for a very strong, but sand able filler surface. I let that final mixture cure overnight again. This however usually takes an extra day to cure to the hardness needed to fine sand without loading up the paper.

sanded gel coat cracks
Microballoon filler

The next day I flipped the cowl over and removed the old foam core. A wire wheel on the Dotco removes the foam in about 10 seconds. You could also sand blast it, but that's  bigger mess.

Dotco and Wire cup
And just like that, its gone!!!!

Foam be gone!!!
Again I sanded away the interior paint up to an inch away from the repair. I also taper sanded the main cowl into the inside of the blister. I then added three more internal structural ply's wetting them out without trimming.

internal repair ply.
 This is now up to date at this time. I am going to let everything cure up for the next week. I am leaving for a short business trip back to Brazil on Saturday. I will be back next Thursday and I will complete the rest of this repair and update the Blog in my next entry.
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1 comment:

Unknown said...

I'm trying to get any recent updates on the Robin U/L but can't find anything in last couple of years. I've asked for Email updates and never received anything except a note that I was already enrolled when trying the second time after 4 months of nothing. Please HELP!!!

My Email: brevardden@gmail.com

February 27, 2016 at 9:28 AM

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