Robin Ultralight: Engine exhaust and cowl internal baffles.

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.

The next step in getting this engine ready was to fabricate new exhaust pipes. The twin pipes that came with my Casler engine were 90 degree pipes designed to stick out the bottom of the engine. That's pretty crude in my mind and a 1.5' diameter tube creates a lot of drag sitting 6 inches behind the prop in the prop wash. I needed these pipes to snake around inside the cowl and exit on the outlets of the side cheek cowls. I started the project by ordering some pre bent tubes from Columbia Mandrel bending. They did a really good job. I ordered some 180, 90 and 45 degree sections along with a straight section of tubing.

raw tubing from Columbia Mandrel bending

this took some time and a lot of cutting and fitting. the pipes were a 3 dimensional bend. I had to route the pipes under the push rod covers and along side the block. This required multiple bends.

Here are pictures of the L/H and R/H pipes

Left hand Pipe

Right Hand Pipe

the cowl was buttoned up and the clearances checked once again.

nice tight fit. I have good clearance to the fiberglass cowl. I will wrap these pipes in heat resistant insulating tape.

After the pipes were fabricated, I started n the internal cowl baffles. The inlet air has to be forced thru the cooling fins. I started fabrication with a cardboard mock up.

This was time consuming. the shapes are just approximate at this stage of the mock up. have a needle contour gage that is very handy in creating offset curves. The baffle plates will be made from .020" 7075-T6 aluminum sheet. the seal edges will be lined with silicon rubber baffle seal. That will be pop riveted to the aluminum sheet.

here is the lower baffle with the cut out mocked up for the exhaust pipes. you can see the small bent flanges that seal the bottom of the cylinder fins.

I made the first one and them promptly bent the flanges in the wrong direction. Oh well practice makes perfect. I should have the baffles all wrapped up by this coming week end. After that, I can do the final trim and finish on the cowl, I will also install the final nut plates and then prime the cowl once more. The next step will be to finish the engine installation and then get a prop and try and start it!!!

1 comment:

Gary Gower said...: Great cowling... The airplane is coming out very nice.
Just one advise: Remember that VW as any other aircooled engines depend of the cooling of the oil, The heads and more specific, the exhaust valves.

Try that the air gets as much to the head and valve cover area as possible, also the tin tubes over the push rods, act as oil return and help cool the oil.
Some air flow pressurized under "carter" fins will help.

Keep the good work... Thanks for the updates.

Gary; October 26, 2016 at 4:04 PM