Robin Ultralight: August 2012

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.

Just a quick update.

I took the Robin to the Airport for first taxi and I discovered a landing gear design error. I built the gear with a horizontal core of Spruce wood. Under the gear clamps, the wood crushed slightly and the gear released or started to slide out of the gear clamp. This is a basic design flaw and because of it, I have fired the designer!!!!

After I hired me back, I went back to the drawing board and started on an all aluminum spring gear. I will either build it myself or Have Robbie Grove fabricate it up for me.

In the mean time, if you are in the Southern California area, specifically Tehachapi on Labor Day Weekend, you are invited to attend the ESA western conference at Tehachapi airport. My original plan was to haul the Robin out for her first Public showing, but the gear problem took care of that plan. I will be speaking on the Robin design @10:30 am on Sunday.

If you can make I would love to meet any of the Blog followers.

Mark Calder

Final Prep

I am preparing for the first taxi and flight test. Its incredible how many little "final" jobs are left. I'm really up against a deadline, I am a guest speaker at the SSA western conference on Sunday Sept 2 at 10:30 am. I really would like to have a first flight before that speech!!! If you are in Southern California on Labor Day weekends, you are invited to the conference. I'm looking forward to the opportunity.
I started finishing out the design in anticipation of the first flight, I mean I really started getting serious!! The first problem is the canopy, the fwd hing idea is really bad. There were many reasons to scrap the idea, but the biggest was I couldn't run the engine with the instrument panel installed on the canopy. I finally got the engine running reliably. The gasket for the needle seat finally arrived and once installed, the engine rang smooth through the entire RPM range. I was able to put another hour running time on the engine. She should be fully broke in by now.

New Canopy hinge

I decided to hinge the canopy on the R/H side. I installed a 3 foot long piano hinge and a holding lanyard . The immediate result was to greatly simplify the wire routing. I removed all hardware related to the fwd hinge, and I realized the I needed a second latch for the fwd edge of the L/H side. The existing design has a latch on the aft end of the canopy, but not one farther forward. I have calculated that there is about 100 lbs of lift on the canopy. To be safe and to keep the canopy from racking I decided to add a second fwd latch.

New fwd latch

I used the existing latches and modified them for the Left side only. I added a compression spring to both latches so they will normally be fully latched. This eliminates the latch plate I had mounted on the inside of the longeron. I was worried that if I added a second latch plate it would make entry and exit into the cock pit difficult.

Rear latch

Another problem also reared its head and that's the need for a small sliding window to access the latches so the canopy can be locked down. I am adding that window as I write this. I should have it done tomorrow. I am adding the window right above the rear latch.

One of the cool things about being in the small world of aviation, is the small circle of friends you realize you actually have. My Friend Ed was in contact with a Pilot of Erickson Helicopter of Medford Oregon through his blog http://nc35447.blogspot.com/

Erickson was in the Midlothian Texas area setting a huge air conditioning unit on a local Walgreen's Warehouse. Ed is actually from the Medford region and he had a number of mutual friends with Fran, but when the guys showed up here in Midlothian, the circle finally closed. Erickson has an operation in San Diego at Gillespie field. A few months ago I was there visiting my friends Mike and Neal Lafrance. This is where the circle closes, Fran is very good friends with Mike and Neal. Mike gave Fran a ride in his Luscombe a few years ago.

Fran Tebbe

I invited Fran to sit in the Robin.

If you guys are aver privileged to meet the guys from Erickson Helicopter, you are in for a treat. These are some of the nicest guys I have ever met!! I have to apologize because I am bad with names but another Erickson guy took a "ride" in the Robin

Mr Warren

There is Ed in the background

After the Erickson guys left, I decided to convert the Robin to a two place side by side

Gretta and Gracie

Eds girls Gretta and Grace took a quick ride in the Robin. Believe it or not, we were once this small.

Canopy Lanyard

So the final count down continues, I need to add the side window, finish the fuel door and route the Pitot static tube. Once that is finished, I will do one final assembly to catalog all of the assembly hardware and then break her down and take it to the airport

Seat Cushion and Citabria Cowl Repair.

I have been waiting for a copper gasket for the Tillotson HR carb needle seat. I have been waiting a month, and believe it or not, it actually arrived while I was writing this blog. In the mean time while I was waiting, I fabricated a new seat head rest and added a leather covered head rest and seat cushion. It really came out nice! Every one who has sat in this seat said it is extremely comfortable.

head rest and seat cushion

I fabricated a light weight head rest support from .020 aluminum sheet.

Shoulder, lap and crotch harness

Good view of the seat belts and adjustable rudder pedals. The read "Tee" handle is the Ballistic chute deployment handle.

Another view

Citabria Cowl Repair

This is the plane I have been flying at Big Q Aviation in Midlothian Texas http://www.bigqaviation.com/
The Cowl was getting pretty tired and while the wing was being rebuilt, it was decided to repair all of the elongated cowl fastener holes. Once a fastener gets loose, a "sawing" action occurs on the fiberglass holes and the holes start to elongate. Some of the holes were so badly elongated that there was just a thread left on the original cowl. That was the case for 16 of the fastener holes. The rest were just slightly elongated and could be repaired by filling with chopped cotton fiber and epoxy. The original Cowl was hand laid up out of fiberglass cloth and polyester resin. The basic principle of repair says you need to restore the existing structural capability. In the case of a composite repair, that usually means all of the original fibers plus 1 extra. The strength of the original material is the determining factor on what taper ratio will be used to scarf the ply's . High strength uni fiber repairs like on a wing spar will require a taper ratio of at least 100 to 1. The lower modulus materials used on the this cowl could be repaired with as little as 15 to 1. the minimum however per FAA guidelines is 20 to 1.

repair schematic

The basic idea here is to clear out the elongated hole, in this case a slot is opened up along the cowl edge.The edges of the cowl are then taper sanded at a 20 to 1 ratio. A sheet of waxed .020 Aluminum is then bonded to the outside of the cowl with 5 minute epoxy. 7 plies of equal size semi circle doublers are then laid up into the tapered space. After they cure , the edges of the splice are taper sanded . The process is really simple, only the edges are sanded until a space in the middle equal to the original slot remains. After the splice is sanded, a final overlay ply is added (original ply plus 1 extra)

corner edge repair

After the inside is repaired, a very slight taper is sanded into the outside surface, and a single ply of fiberglass is laid up. This way the repair is actually a double lap shear/Tapered scarf. This is a very good structural repair because all tendency for the tapered splice to Peel away is reacted by the over lay ply's.

Double hole repair

This is a good example of what the repair looks like after taper sanding. Its possible to actually count the ply's as they drop off. Notice how the center of the repair laminate has not been sanded.

single hole repair.

This picture clearly shows the internal taper sanding. One final overlay ply will be added over this repair.

overlay ply with peel ply

a very good technique is to finish the repair lay up with an overlay of Dacron Peel ply. This adds a professional touch and serves a very good structural purpose, the hard edge of the repair laminate is tapered out with a resin rich edge. This serves to smooth the hard edge and eliminates the tendency for the repair to peel away. After cure, the peel ply is pulled off the laminate. This material is the same Dacron I used to cover the Robin.