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.



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.


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!!

Casler Engine mount build.

                           Finally Back on the Robin

I finally have cleared the decks to the point where I can get back on the Robin. It took 9 months to order and receive my new Casler 1/2 VW. Then I was named the lead engineer on the Embraer 190/175 Rudder/ Elevator program. That took a lot of my spare time and required quite a bit of travel to Brazil. That program is winding down and I started to have more time in the evening. In addition to the Robin, I have been redesigning the nose landing gear of a Tiafun Motor Glider and the fabrication of a new 1-26 sailplane Nose Cowl. All of the heavy work is finished so I set to work on the Robin. The first task was to locate the engine in its proper location relative to the center line of the fuselage and in the correct vertical position. My main criteria for the location was to have the old prop spinner contour blend into the existing fuselage contour. So I set up a three strap hanger from the overhead of my shop. I used three turnbuckles on each strap so I could fine tune the positions.

Casler hanging from ceiling
the thrust line was set 1 degree downward. I used a piece of long welding wire as a spline. once the curves blended from the cowl to the fuel tank fairing, I knew I had the correct vertical height. On this new cowl, the cheek cowls will be functional like the original Limbach installation in the Fourier RF4D. If you look carefully at this side view you can see the welding rod spline.

Welding rod Spline
Once I had the engine in position I added temporary wood trusses to keep the engine from moving. I used Hot Glue to temporarily attach them.

Temp braces
In this picture you can see round aluminum pucks that were fabricated to dummy up the rubber engine mount. Since I intended to build in place, I want to be able to tack weld the engine mount together. I am using a thru bolt style Lord Mount that was available thru WD Granger industrial Supply. This mount is inherently safe because there is a thru bolt that will keep everything in place should a rubber bushing fail. The  most common style of engine mount used on light engines have a section of rubber bonded between two plate with a threaded stud. These mounts require a safety strap should the rubber fail.

Lord Mount

 I mentioned this in my previous post from the original engine mount for the 2 stroke, but this is worth repeating . The basic principal of designing a proper engine mount is to ensure that on the airframe side of the rubber isolator, there exists a stiffness path for all 6 degrees of freedom. in order for an isolator to absorb engine vibration, it must convert the vibratory energy into heat. It can only do this when the vibration input node is being reacted by a stiff load path. Consequently, there will be a series of tubes that will supply the needed stiffness. The first being a square pattern that ties all 4 mount pads together.

corner mount pad

 I fabricated four corner mount pads. The two tangs extending outward are the weld pads for the trus. The corners are made of 3 pieces of 1/8" 4130 steel. I used a hole saw to cut two circular pads to be welded to the main base. The mounts require 3/8" of spacer per the mount spec. I have just finished fabricating the square backing and I will cover that in the next blog update. But before I sign off tonight I want to share a trick that an old machinist taught me 35 years ago when I was a safety crash test technician at Ford Motor Company in Dearborn Mich. There is a trick to drilling a large hole in thin material. I have what is known as a Deming Drills. Large diameter drills with a reduced shank. These drills will tend to chatter and elongate the hole when they are used. Except if you use the "Trick"

"The Trick"

use a small piece of cloth soaked in cutting oil. I like Tap Magic. Center the drill on the work piece and then place the folded rag between the drill and the work piece. Apply pressure with the drill press and then turn it on. The rag will stop all chattering and create a perfect started hole.

Until next update


IT'S HERE!!!!!

My Casler 1/2 VW just arrived. I just started unpacking it. Here are some pictures. I need to finish the Tiafun first however. 

my new Casler

 Thank You Scott!!!

Tiafun Cowl repair cont.


I got back from Brazil Thursday morning. After a day of nonstop sleep I got back to work on the cowl repair. I finished cleaning up the internal blister layup and blend sanded the new laminate into the existing laminate. 
inside ply's trimmed and blended
most structural glass repairs only need one inch per ply overlap to fully develop the tensile strength of the ply. In the case of the blister, this was a 6 ply repair with 3 structural plys on the inside and outside, each layer overlapping the existing laminate by 1 inch. This is truly overkill, but meets the requirement of restoring original capability, plus one ply. I will spray the inside of the cowl with a grey trunk paint and this damage will be completely hidden. The outside micro balloon and epoxy was completely cured when I got back from my trip. I rough sanded the micro with 100 grit sandpaper with my orbital DA sander. This reduced the high points and leveled most of the surface. There will always be slight pin holes left that will need to be filled with Pinhole filler or spot putty.

micro balloon blend sanded
This picture shows the micro balloon after it was rough blended with the DA. The surface is wave free but there are still small pin holes and very shallow applicator marks. I followed up with the DA by using a flexible rubber sanding block with 180 grit paper. This served to further level the surface. This cowl is very easy to blend out because there is severe contour. The hardest surface to make smooth, level and wave free is a flat surface. For that type of contour I will usually stop at this point and spay the surface with a sand able primer. A long sanding stick quickly shows high and low spots. I usually try to level the surface with a combination of sanding and spot putty fill. Spot putty is designed to adhere to primer paint.
catalyzed spot putty
This is my preferred spot putty. This is far superior to the Auto-Zone lacquer based products.  This material is catalyzed like auto body filler. Its ready for sanding in 10 minutes. The cycle time of your products makes all the difference in speeding up the repair. I could have used light weight auto body fill in place of the epoxy micro in the earlier step, but since I knew I would be in Brazil for 5 days, I knew the time didn't matter. Epoxy Micro is a very strong filler and is superior to Automotive bondo.

blended spot putty
 This is what the spot putty looked like after an initial rough sanding with 100 grit. I finished the feathering with 180 grit and there was hardly any left.

There was one other issue with this cowl and that was a corner Dzus fastener had pulled thru the cowl laminate. Normally I would look for an oversize repair fastener and enlarge the existing hole with a step drill or tapered reamer. I would then install a repair fastener. But that would require a crimp tool and a special set of repair washers. This fastener fit just fine but had no retention on the back side of the laminate. So I cleaned the metal with  rotary wire wheel and sand blasted the metal to a dull finish. I then mixed some T-88 structural epoxy adhesive with a small amount of chopped cotton fiber. I then bonded the Dzus fastener back in place and formed a fillet around the inside surface. This will stop the fastener from backing out.The main difference between structural epoxy adhesive and epoxy laminating resin is the addition of molecules of an elastomeric. This dramatically increases the peel strength of the adhesive and stops all tendency for  delamination

This lower cowl had been repaired previously after the original nose gear collapse. I have noticed that the door underwent extensive reconstruction and is over 1/2" thick in some places. The leading edge of the door sits below the existing contour and quite frankly looks terrible. I am thinking about blending the the existing contour to the door contour after I rework the door hinges to slightly lower it. The idea is to get a smooth surface when the climb door is closed for soaring. That will be the subject of the next entry in the blog. My friends at the airport have repositioned the Tiafun fuselage into a larger hanger where I will have full access to the engine. Since the plane skidded on its fwd engine mount, I am unsure of the actual condition of the metal engine mount. There is flaking paint on the mount and I suspect its just a bad paint job, but I will pull the engine and remove the mount so I can sand blast it and inspect the for cracks. Dan has already bought a new Saur fwd engine mount that fits like the original. I will prime the mount with zinc chromate and finish with a polyurethane top coat. The last thing you want to do is powder coat any aircraft parts.Powder coating is a plastic film that is fused to the metal in an oven. It doesn't not have any ability to act as a rust conversion coat and in a short period of time it will develop cracks and water will migrate.

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.