The Gull
Part I: A modern take on a classic.
Background and Inspiration
The Gull project began on the morning of March 30, 2021. I was talking to some friends in an RC soaring chat room when I learned there would be a scale soaring aerotow event at a field near me in one month’s time. I was still furloughed from work but I had just received my COVID-19 vaccination and I was excited to finally go to a flying event. The only problem was I didn’t have anything to fly. I had a handful of two or three meter thermal gliders, but nothing appropriate for a scale aerotow. I’m an avid scratch builder and it didn’t take long for my friends to challenge me to build something for the event (or maybe I challenged myself and chose to blame them). We decided that something in the four meter range would be the most practical. I was ready to get started, but what exactly should I build? The time constraint made a true scale project out of the question, and I prefer designing my own planes anyway. I decided to design something that captured the essence of a vintage sailplane, but with a more modern feel.
I had recently become fascinated by vintage sailplanes and had spent some time studying photos of them. I fell in love with gull wings and sunlight shining through cloth covering. The gull shaped wing had to be a feature of my new plane. I studied photos of gliders like the Slingsby Petrel, Slingsby Gull, and Göppingen Minimoa. Their subtly gull shaped wings are elegant, but I didn’t want to go through all the trouble of building a gull wing to be subtle. I wanted the wing design to be a prominent feature. I contemplated how much dihedral I could give the inboard wing sections before they became absurd. I took a few minutes to look at golden age free flight models, many of which feature exaggerated gull wings like what I envisioned. I wanted the fuselage to bridge the aesthetic gap between wooden gliders and their sleek fiberglass counterparts. The outline of my new plane was beginning to materialize in my mind. This is my favorite part of starting a new project. All the daydreaming and imagining slowly morphing into a solid idea, ready to be put down on paper and ultimately brought to life with countless bits of wood.
By this time I felt sufficiently inspired to sit down at the drafting board, or CAD program as it were. I was equipped with a well-stocked workshop and an abundance of free time. The race was on to build a new glider in 30 days.
155 days later it finally took flight.
Design
Now that I had established some broad design criteria it was time to dive into the details. I like to start with the wing and design everything else around it. Most of the design work took place in CompuFoil 3D. Step one was choosing an airfoil. I turned to Mark Drela’s proven AG series. The inboard wing panel uses the AG35 airfoil and the outboard panel transitions from AG35 at its root to AG38 at the tip.
Next, I spent some time experimenting with different planform options. The temptation to build an elliptical gull wing was huge, but sanity prevailed and I settled on a simple tapered design. The inboard panel’s root chord is 14’’ and tapers to 12.5’’ over its 24’’ length. To establish the distinctive gull wing look, the inboard panels have a dihedral angle of 15 degrees. The outboard panel tapers from 12.5’’ to 6’’ over its 48’’ length. The outboard panels have no dihedral. Small wingtips would be added later to round out the tips. With the wings mounted on the fuselage the wingspan would be 12’ 8’’ or about 3.85m. The goal was to maximize wing area without sacrificing the aspect ratio too much. The wing area is 10.6 square feet and the aspect ratio is 15. I decided the wings would have ailerons and spoilers but no flaps. Once I finished defining the wing parameters I exported a planform view to AutoCAD where I could sketch out the fuselage shape and proportions.
The fuselage was the most challenging part of the design. I wanted to meld a retro looking wood fuselage with the shape of more sleek and modern composite designs. After a bit of trial and error I finally had something that looked proportional to the wing. Armed with the critical dimensions, I switched from AutoCAD back to CompuFoil to refine the profile shape and create the formers. This was a bit more of a challenge than I anticipated! After five iterations I was finally happy with the results. There are 15 formers spaced at 4’’ intervals for a total length of 60’’ not including the vertical stabilizer. The formers have jig holes so they can slide onto a metal pipe to keep everything aligned during construction.
Lightness was a concern throughout the design process. Lightening holes were incorporated wherever possible. I planned to use balsa for everything I could and only use plywood when balsa was impractical or structural loads dictated it. I did not have a specific target weight in mind for the project; I was just shooting for less than ten pounds.
The tail surfaces were much easier to design than the fuselage. With the wing span, wing area, and fuselage length already defined, I had all the information I needed to calculate the vertical and horizontal tail volumes necessary for controlled flight. Once I knew the target surface area, I experimented with different spans, planform shapes, and chord lengths until I felt like they fit the aesthetic of the plane and met the aerodynamic requirements. The horizontal stabilizer features a modified elliptical planform and uses the HT14 airfoil at the root and transitions to HT12 at the tips. The span is 30’’ and the projected area is just over one square foot. The vertical stab was only loosely defined at this point. I knew how big it needed to be, but the design didn’t crystallize until after construction began.
It’s important to remember that I was working as quickly as possible to meet a deadline. By this point it was late at night on March 30. I went from inspiration, to design, to CAD drawings in one day. Of course, not everything had been fully thought out yet. Drawings for the vertical stab, wing mount, and landing gear mount wouldn’t be done until the build was underway. It was time to get some sleep. The next day would be a build marathon.
Construction
I walked into my shop the morning of March 31 filled with enthusiasm. Construction began with the wing spars. I had never built a gull wing before, but I knew that strong accurately shaped spars would be the key to success. The joint where the inboard and outboard panels met at a 15 degree angle would be under a lot of stress, so the spars needed to be made in one piece to effectively transfer flight loads. The spar design was pretty standard. The top and bottom caps are 1/8’’ thick by 1/4’’ wide made out of hard balsa reinforced with carbon fiber and fiberglass. 3/32’’ thick vertical grain balsa shear webs complete the beam structure.
To accurately produce the 15 degree bend in the spar stock I made a jig out of MDF and covered it with tape and mold release wax so the parts wouldn’t stick. I cut 3’’ wide balsa sheets to the right lengths; then prepared 3’’ wide strips of unidirectional carbon fiber and fiberglass. The composite reinforcement is thicker at the wing root and tapers to only two layers at the tip. The layup is one full length piece of unidirectional fiberglass plus four layers of unidirectional carbon fiber that start full length and get progressively shorter. The idea is to maximize strength at the wing root and minimize weight at the tip to reduce yaw inertia. The layup was vacuum bagged on the jig and left to cure overnight.
April 1 was a productive day. I laser cut the parts for the horizontal stabilizer and assembled it. Next, I laser cut the main wing ribs out of 3/32’’ balsa. I opened up the vacuum bag and popped the spar blank off the form and carefully cut it into quarter inch wide strips on the table saw. Wing assembly could finally begin! I used the spar form as a jig to build the wing on. Starting with the left wing, I clamped the bottom spar to the jig and glued on the ribs. Then I glued in the top spar cap, the leading edge strip, and the small 1/8’’ stringer spars that support the leading edge sheeting. By this point the wing was stiff enough to remove it from the jig and glue in the remaining stringer spars and the trailing edge pieces while being careful not to build a twist into the wing. After repeating the process with the right wing I switched my attention to the fuselage.
I laser cut the fuselage formers out of 1/8’’ poplar lite ply, then slid them onto the metal tube that would act as a jig to keep everything aligned. I wanted the stringers to be full length but the Sitka spruce I had was only 48” long. I looked through my wood stash to find something long enough and found some old growth spruce from a 1912 pipe organ that I had salvaged after a church disassembled it. The wood had tight, straight grain and its old age made it a perfect fit for my vintage inspired sailplane. I cut the spruce into 1/8” square sticks and began assembling the fuselage. Most of the stringers had to bend pretty far to conform to the fuselage’s curves. There was no good way to clamp or pin them to the formers while wood glue or epoxy cured, so I used CA. CA glue and spruce don’t get along well, but baking soda can be used as a catalyst to help them bond. Balsa sheeting and trusses would be added later and would essentially gusset every joint to reinforce the brittle CA joints. Life got in the way a bit, and work continued sporadically from April 2 to April 9.
On April 9 I took the partially completed wings, fuselage, and horizontal stab outside and set them up so I could see the plane take form.
Inspired by my progress, I continued to work diligently for the next ten days. The wing mounts were designed and built during this time. The front half of the fuselage was sheeted with 3/32” thick balsa. The top and bottom of the aft portion were also sheeted, but the sides were left open and would later receive diagonal trusses. This was partly to reduce weight in the tail and partly because I love the look of open framed construction. The nose was shaped out of a block of balsa.
The plan was to use the fuselage as a form to create the canopy. My two attempts at making a clear plastic canopy failed, so I decided to move on to the vertical stabilizer and come back to the canopy later.
The fin features a straight leading edge with a rounded trailing edge. Laser cut leading and trailing edge pieces made assembly of this complex part fast and accurate.
To finish up the wings I glued on the rest of the leading edge sheeting, taking the utmost care to prevent twisting or warping the wings as I went. I also glued in the diagonal trusses on the ailerons and cut 1/8’’ thick balsa to make the spoilers.
By April 19 the entire plane was framed up. I took it outside and mounted the wings. For the first time, I could see all the parts come together to form a complete aircraft. I spent about an hour walking around the plane admiring it and planning the next steps to complete the project.
That same evening I found out that the aerotow event was canceled. This came as a bit of a relief to me, as working around the clock to finish the project was exhausting. The task of completing and covering this large glider had become daunting. No longer faced with a deadline, I chose to slow down and savor the rest of the build.
©2021 Thomas Martino
This is the first part of a two part series. Next month, Thomas wraps up the build and talks test flying. All images are by the author. Read the next article in this issue, return to the previous article in this issue or go to the table of contents. A PDF version of this article, or the entire issue, is available upon request.
