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.

Still waiting for the Casler Engine

I have another 3 months to go before I get the engine. I wrote Scott Casler to make sure he didn't forget me.
I have asked a Woman who works with me at Triumph/Vought to help with the redesign of the wing and landing gear. Once Michelle has some preliminary drawings of the new wing I will post some. The new wing will be similar to the Woodstock wing and will look more like the original Fournier wing. The wing will be all wood and be a tapered design. There is a very significant weight savings due to a tapered wing. The big disadvantage of a tapered wing id poor stall characteristics. I am going to use two different low speed laminar airfoils chosen to have a constant coefficient of lift at the local Reynolds number. This will emulate a square Hershey bar wing with a tendency to stall in the root first. I do not want to mechanically twist or wash out the wing because it is really inefficient.

The landing gear will go back to a single wheel with out riggers. Again this is similar to the original RF4D and is very weight efficient.

Thats really all I have to report right now. Hopefully I will have some preliminary drawings to show in the near future. 

Casler 1/2 VW

Just a quick update, I have ordered a Scott Casler 1/2 VW engine. I'm tired of being in the engine manufacturing business. Unfortunatly Scott is so busy, I have to wait another 6 months for the engine. In the mean time, i am working on a complete redesign that will make the Robin look even more like her namesake the Fournier RF4D. I purchased a set of Woodstock plans and I intend to incorporate a similar wing. I will go back to the single mono wheel and will fix it for the ultralight version. I bought a N/C router a while back and my redesign will take advantage of this. Eventually I would like to sell an experimental and ultralight version powered by full and 1/2 VW engines. I will fly my current plane and incorporate all lessons learned.

Latest Update



I am sorry its been so long since Ihave done an update to the Blog. Events have conspired that demanded all of my time. I had to take an emergency leave from work to help my mother who unfortunatly was diagnosed with terminal cancer. I could write for weeks on the ins and outs of Hospice care!! My Mother unfortunatly passed away after 4 weeks. At least she wasn' in any pain thanks to the Angels in Nurses uniforms who work for Hospice at Henry Ford Hospital!! 

my poor old ma, God rest her soul!
This is a hoot!! This is a picture of my Ma 3 years ago at Halloween dressed as Jimmy Leyland the current Detroit Tigers manager, my ma was a huge Tigers fan and during the Funeral, the organist actually played "Take me out to the Ball Park" when we were walking her casket out of the church. She wouldn't have had it any other way!!! 

When I got back to Texas, I recieved notification of shipment of my new Router. That took a good two weeks of my time dealing with the shipping agents, customs and secondary shippers. But it was definitly worth it!!  I took a bit of a risk by ordering this router from China. I have had some quality problems, but thankfully all of the important stuff like Bearings, gears and servos came from Tiawan and Japan. My biggest issue turned out to be poor Chinese quality control on the wiring. Servo motors are very sensitive to noise and because of this they require good grounds and solid connections. The latter was the root of my early problems. I ended up soldering every crimped connection on the machine.
The proud owner and his new Router

The router is 4' x 8' x 19" it has a variable speed 7.35 hp spindle. I can machine wood, aluminum, plastic and steel with this. It took a while to get this up and running, but thanks to my Buddy Ed who is an electronics genius, we worked out all of the initial bugs. I plan on using the router for fabrication of a all wood airplane kits. On the side we are also using it to cut some beautiful laminated wood gun stocks. Here is a picture of a couple of test projects we cut using standard clip art from our 3d software files.

Cat and the Eagle
These projects were roughed out with a 1/4" ball nose cutter. after the rough cut the finish cut was done with a 1/16" ball nose cutter. The router took a pass every .007" Each piece took 1 hours to finish cut.

Back to the Robin!!!

I decided to swap out engines. The MZ 34 proved itsself to be an unreliable choice. The engine just would not cool. The vibration is terrible and because of the earlier problem of it dieseling and not shutting off, I decided it was unsafe. When I went to the SSA Western homebuilding convention in Tehachapi Ca, I was talked into swapping the engine for a Briggs and Stratton Vanguard V twin. I am in the process of adding that engine. Unfortunatly I am forced to leave the cylinders unfaired and in the breeze just like other aircraft that use the same engine. This in my opinion detracts greatly from the original lines of the Robin which was designed to emulate the Fournier RF4D. Consequently, I am drawing all of the final plans using a 1/2 VW or a modified Generac industrial engine. Both of these engine are opposed Boxer configurations.
 The first thing I needed to do to use the Vanguard was to strengthen the fwd fuselage to accept the higher shear loads. I decided that the simplest method for the modification would be to sheet the fwd section with plywood. I had to double up the fwd longerons so I could bolt new fittings for the new engine.

side truss with doublers
  This is a view of the fwd fuselage side truss . I designed the original truss so close to the existing loads, that I had to double up all of the members between the landing gear and the firewall. Also you can see the large plate doublers added to the fwd longerons to accept the mount fittings. Unfortunatly this meant that I had to uncover the whole fuselage!! That is not a bad thing actually, because I had a number of areas that were not well thought out regarding the final shape after covering. This will give me a chance to clean all of that up.

side sheeting

After the doublers were addded, I then sheeted the sides with 1/16" Finnish Birch plywood. The circular cutout is for access to the brake pedal adjustment screws.

I am stripping the engine of a great deal of its weight, just the steel shrouding alone adds 5 lbs. The flywheel is 15 lbs! I have debated inverting the engine and making a new ignition , But since this is going to be a one off design I have decided to just turn the flywheel down and use the stock ignition. I am driving the prop from the flywheel side and mounting the engine on the PTO side.

PTO of Vanguard
On this engine I am again designing a dynafocal mount system. To refresh everyones memory, a dynafocal mount is designed to theoretically support the engine at the exact center of gravity. The theory is that all vibration nodes are zero crossing in there amplitude when they pass through the center of gravity. Since there is zero displacement at this point if it were possible to attach here, there would be no induced vibration into the mount. Obviously this is impossible so the next best thing to do is focus the input axis of the rubber shock mounts through the center of gravity. To find this angle a process called tri-filing is used.

Trifiling process

I made up some small lugs with a small hole in them. I attached a length of welding rod through that small hole and then hung the engine from that rod. I then took an angle measurement and that determined the input angle to the center of gravity. I then designed the mount to these angles. The picture above shows 4 angles lugs that the engine mounts attach to.

This brings everyone up to date with my progress.

Now forf the bad news, as I write this I'm sitting in the Admirals club at DFW airport waiting for a flight to Brazil!!! My company Triumph Vought just won a contract to design the aft section of the new EMB 195 airliner. Yours truly is in charge of the Empannage!! I will be at Embraer for 4 weeks starting tomorrow. To all of the Brazillian blog followers, if you are around Sao Jose Dos Compos, please write me at : Planebuilder@yahoo.com.


Brake pedal installation

I'm sorry I'm so late in updating the blog. I have been very busy!! 6 months ago I agreed to take on a side job for a Gulfstream G3 Engine swap. I am designing a new engine installation for the Right side pylon. This plane will be a flying test bed for the manufacturer of the new engine. I have a hard deadline for complete release of the engineering drawing of Feb 9. Consequently I am slammed for time and the Robin has to take a back seat. That's the bad news, the good news is I have earned enough money to purchase a 4 x 8 x 19" 5 axis N/C router!!!! I am expecting delivery to Houston Texas in May. I can't tell you how excited I am to get this tool. Not only will it allow me to make an exquisite wood kit for the Robin, but it will also make possible my next project. I am going to design and build an all wood laminar flow single place high speed airplane powered by an industrial Suburu 40 hp engine. Speaking of industrial engines, I recieved my replacement engine for the MZ 34 I currently have on the Robin.

23 hp Vanguard Engine

When I get back on the project I will swap this engine for the MZ 34. Stripped down and using a forged billet flywheel this engine will weigh 64 lbs. That will put the prototype over the part 103 254 weight limit. However I anticipate that changes to the wing and landing gear will bring her back under 254 lbs. More on those changes later. I plan on mounting the prop on the flywheel directly. Research has shown that a direct drive should yeild 154 lbs of static thrust. By Mounting the prop on the flywheel side the airflow thru the cowl will be in the same direction as the engines design. I will also be able to mount the engine using the PTO mounting fasteners.

I have finished the new replacement landing gear installation. I fabricated a cover plate that can be removed to service the gear. 
new gear cover plate
 landing gear center cover.

The part 103 Robin will go back toi the original Fournier mono wheel. If a direct drive installation works, the prop clearance issues are such that I could use a non retractable gear. Saving even mode weight.

The existing rudder pedals were removed and modified to add toe brakes. This installation is very clean. Here are some pictures of the installation:
toe brakes

toe brake pedals were added to the existing rudder pedals, The toes brake position is adjustable. I will set the position after I get a chance to sit in the cockpit. The brakes are cble actuated and are modified Go Cart drum brakes.

break cable routing
there is enough relief loop in the brake cables to allow full rudder pedal adjustment.

rudder return pulley

I decided to update the Blog this evening because I received a question from a reader about the rudder return cable design. The main change I would make to this design is to raise the cable attach points to the end of the rudder pedals. This still works pretty good. I like this a lot better than the spring return I used to have. You will notice that the pulley bracket can swivel horizontally. The  fwd pivot hole is oversize and allows the pulley bracket to also rotate up and down. The threaded rod is also the cable tensioner.

 This is a better picture of the return mechanism.

Finally in December it was my privilege  to meet a blog reader from Sweden Dr Jörgen Åstrand. He is Fournier RF4 and Piper Cub Owner and possible a future neighbor here in Texas. It was sure nice meeting up here in Texas. Ironically he was also a reader of my Friend Ed Piper Cub Blog. Only at the last minute die we all realize that he was coming see us both during his vacation.

 Dr Jörgen Åstrand and myself.


New Landing gear brakes.

 Hello to all the readers of Kitplanes magazine!!   I was fortunate to meet Dean Sigler at the 2012 Experimental Soaring Associations  Western Conference in Tehachapi California. It was sure great meeting and seeing everyone over the Labor Day week end. From the emails I have received I see that the January edition has already made it to the subscribers base.

The first thing I realized when we went to the airport for the initial Taxi test, is that  I need brakes!!!! For the initial flight my buddy Ed was going to keep me from rolling into other planes and hangers, but in reality that's not really practical.
 With this new Grove spring gear, it quickly became apparent that I was going to have a axle bending problem. The axle is a 4130 steel tube 3/4" x .125" wall. I do not want to heat treat the axle, so I am using the 4130 in the normalized condition. That limits the stress to about 95 ksi in tension and 74 ksi in compression. The old landing gear design tucked up inside the hub and limited the bending moment because the point of load application was only 1.75" The new spring gear increases the cantilever distance to 3.57", That distance by the way is the point the axle is cantilevered to the point of load application which is the center of the hub. When brakes are added the distance increases by another inch.  The gear will almost bend now just sitting on the ground!!! To solve this problem I decided to make a sleeve collar that is 1.24" in diameter at the point of greatest moment. this will limit the bending in the axle back to the original design stress.  I bought some 3.5" diameter 7075-T6 round stock and proceeded to make a ton of chips.

Sleeve collar being turned
Here is the beginning of the Sleeve hub being turned on my Lathe.
This is my faithful 36" Grizzly Bench lathe. I've had this lathe for 20 years, and believe it or not I have moved this lathe all over the country chasing the latest and greatest aircraft design contract!! I'm getting too old for that stuff these days however!!

 This gear design is not only way too heavy, its also too damn expensive. As you can see in the next picture, 8 hours of my life was spent standing in front of my lathe!!

Chips and turnings
I filled that trash can 5 times!!
I have decided that the final design will be offered in two options, thanks in large part to all of the letters I have received.  I will offer the original retractable mono wheel as well as an  option  Taylorcraft style internal bungee landing gear.

sleeve collar and axle
This shows the outside of the sleeve collar, the part not shown is a flange that mates to the gear leg. In this picture its very apparent how far the axle would have had to extend. The inside face of the wheel bearing rides on the outboard face of the sleeve. The flange of the sleeve is bolted to the gear with 4 .25" dia bolts. The brakes that I finally adapted are from Azuza manufacturing. I purchased them online from Go Kart Galaxy

ask for Clint. These brakes are mechanically actuated so I ordered all of the cable , cable ends and fittings. I have plenty of excess cable that I think I can route the cable  to the actual rudder pedals. That will be the subject of the next blog entry.because I haven't designed them yet!!!

inside view of gear leg
The 3/4"l axle is welded to a 1/8" steel plate that is bolted to the inside face of the gear. I had to grind out a small recess at the very bottom of the gear to clear the actuation lever. I also had to flatten the lever and remove the 1/4" offset. The flange of the sleeve is also shown in this view.


Cable anchor bracket.
I added a bracket to attach the adjustable cable anchor. All in all this turned out to be a really clean installation. The cable will route along the trailing edge of the gear and will be attached to it with heavy duty nylon tie wraps

Drilling the axle hole
If you were wondering how I was able to accurately drill the 3/4" axle hole, this is how I did it.

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.


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