Last update: July 7th, 2019
Introduction
I’ve always been enamored by this plane. I can’t explain why. Perhaps it’s the ratio of wings to fuselage length, or the round curves of the tail. It just seems like the perfect plane to bore holes in the sky leaving a trail of smoke. The decision to build the plane was a given, but which specific one to build turned out to be a much harder decision. Truth be told, this isn’t my first Pitts. In the early 2000’s, I inherited a 48″ Pitts built in the early 80’s, decked out with an “Orange Crush™” livery – from my brother. It came with a seized .60 2 stroke engine, a fiberglass fuse and a box of splinters that held the remains of the wings. This was my first restoration project. I spent many months reconstructing and covering the wings, replacing fuse formers, switching the power plant to an inverted OS 90FS with on-board glow driver and that classic Pitts red and white look.
It looks pretty, small enough size to fit in the car fully assembled and flew. The problem is, it doesn’t fly well. It could be that the wings are not perfectly aligned at the correct incidences, or the weak landing gear, or the CG isn’t perfect (which I had to guesstimate, since the original plans were not available). Perhaps that’s why my brother crashed it so many times and why his RC flying pals affectionately referred to it as “Orange Crash”. Whatever it was, it made me decide to retire the plane to become hangar-art and I vowed to find another Pitts to build… some day.
R.I.P. traplet.com
Wind the calendar forward a decade or three, add a new career, relocation across the continent, other RC builds, ARFs (and crashes)… and you land on the end of the 2016 flying season and my commitment to finally finding a Pitts to build. I knew I wanted a scale Pitts S2 that I could transport assembled in the car. I also wanted to install a gas engine and a smoke system. I was happy to build a kit, or from scratch, as long as I could find a design that met my requirements. There are a LOT of options out there and online auction sites even offered some vintage kits that looked promising. None of them met my needs for one reason or another. After much research, I ended up selecting the 55″ Duncan Hutson design.
Despite the plethora of Pitts build threads on the internet and a well-known, award-winning scale designer, it seems NO ONE fully reviewed this specific design. Aside from a partial start, there’s no-easily found evidence anyone has ever successfully built or flown this plane. Being an optimist (or just dumb) and adventurous person, I decided to go for it! I ordered the short kit with a cowl, wheel pants and canopy from Traplet Plans. The online order went through their US branch quickly, however after not hearing back for weeks I sent several emails and began to worry. Things did not look good and got even scarier when the US office closed. I was now dealing with the UK headquarters. After several more weeks of unexplicable difficulties, some long distance phone calls and numerous emails, they finally shipped my order. Days after my shipment arrived, it turned out the company was bankruped and shut down. After many months of being offline, it looks like the plans are now available from Sarik Hobbies, if (after reading this) you are interested.
Unboxing and initial inspection
Upon arrival, I was excited to look over the contents of the package, study the plans and start planning my build. To be fair and state the positives, the cowl, canopy, wheel pants (spats), quality of wood and laser cutting met my expectations. Unfortunately, there were several issues I discovered right away – and eventually during my build that you should know about if you decide to build this plane. For what its worth, there’s a great Scale Construction book authored by Duncan Hutson that provides excellent insight in to some of the design decisions he made and building techniques intended for his designs. Unless you are a very skilled and experienced scratch builder, I suggest you read the book before building this plane. It also makes a great addition to your library in general.
Upon careful inspection of the plans and the parts I received, I found several discrepancies or problems:
– A right side WB1 rib on the plans incorrectly labeled as WB1A
– The laser cut F9 part is actually F2 in the plans
– The laser cut F6 part is actually F4 in the plans
– The WB1 and WB1A laser cut parts’ labels were swapped
– They cut 6 x WB1’s instead of 8
– They cut 10 x WB1A’s instead of 8
The items listed above are not catastrophic issues and can all be addressed by a seasoned builder. There were a few other things that surprised me, (but were not mistakes per the plans). These are things that would require some thinking, research, decisions and potential redesign:
– The wing spars are all balsa
– The fuse formers are not all ply
– Lack of precision and symmetry on some surfaces
– Light support/structure between F1, F2, LG and the lower wing saddle
– No diagonal bracing throughout the fuselage
There were also very few details about general fuselage assembly, tank mounting, radio gear positioning, stabilizer strut hardware, landing gear installation – which all had to get resolved.
I spent several evenings studying the plans and comparing them to other scale Pitts plans that offered great input to many of the issues above. I even sourced a full scale maintanance manual, which provided excellent insight in to the design and parts breakdown of the plane. While I knew I would deal with isses as they came up during the build, I wanted to get an idea of what materials I would need to order before I got started. I sketched some ideas and discussed them with my rc bulding pals for advice. After a few more rounds of deliberations, I was ready to order sticks, sheets and other materials.
Stabilizer and fin construction
I decided to sart with the “tail feathers” as a good warm up exercise. They are pretty quick to progress through and provide a sense of accomplishment early, which helps get through some of those harder to complete-in-a-reasonable-time parts.
One of the first problems I noticed on the plan was lack of symmetry in the stabilizer and elevator. The right and left sides didn’t perfectly match. I decided to pick one side and mirror it to make the other side a perfect match. With my modifications in place, I covered the plans and began the build! All joints are glued with aliphatic resin and allowed to dry overnight.
The plans call for a balsa stab TE, but given it’s only a 1/4″ stick (and I lost my Extra 300 to stab flutter), I decided to switch the TE to basswood stock. The design also calls for the stab TE and elevator LE to be butt-joined to the 1/16″ balsa cores. In my opinion this isn’t strong enough so I added 1/8″ X 3/32″ reinforcements along the TE and LEs between the “ribs” and hinge reinforcements. The elevator wire in the plans is angled to match the shape of the elevators’ inner contour. That would make final assembly difficult when the time comes to perminently glue the elevator wire and hinges in place. I decided to switch to a conventional 90 degrees design and reinforce the elevators to distribute the wire’s load on a wider surface area.
I also decided to widen the top and bottom angled stringers that are used to glue the stab to the fuse. This will provide a surface to anchor the covering after the stab is glued in place.
Perhaps a more graceful alternative to the 1/8″ x 3/32″ butt-joint reinforcements might be to add an extra 1/8″ to the stab-core’s TE and elevator-cores’ LE, which would slot-in to 1/8″ grooves cut along the center line of the 1/4″ sq’ TE and LEs for a snug fit. This is what I ended up doing for the fin and rudder. Otherwise, the fin (with basswood TE) and rudder are built in exactly the same way as the stabilizer.
Landing gear and cabane struts
The landing gear wires are fabricated out of 5/32″ and 3/16″ piano wires. The plans are pretty vague on specifics. There are multiple ways to interpret the design so this was my take on it. I decided to create a left and right half to be joined at the back which rests on F3. I added some braces to F3 to carry the upward forces when the plane lands. The front ends of each half would be bent forward and braced on the LG block. I also created wheel pants brackets out of collars and brass plates. This can be purchased as pre-made hardware, but I had the raw materials so I made them myself. The rear end of the LG was slightly simplified as I couldn’t understand what that extra jog is for and it complicated the bending even more. The LG was then test fitted on F3 to ensure it matches the correct angles. The cabane struts are well described in the plans and are made in a very similar fashion.
Fuselage construction
It took a while for me to decide how to build the fuselage. There are literally no notes or clues on how to do this in the plans. It obviously has to be straight and square to avoid unwanted twists or warps. There are a few methods I found and I decided to start the build with a center-line “crutch” that would have reinforcements where the formers would be glued. I decided to add a diagonal brace between F4 and F5 and determine the loaction for additional braces later.
Since there was no drawing for this in the plans, I had to draft my own by drawing a main symmetry line and accurately measuring the widths and locations of the formers along the center-line. I also took this opportunity to figure out how the tail post would be constructed (absent from the plans), and what modifications I would need to make to the engine mount box for my power-plant of choice (DLE 20RA). I decided to also add a 2 degree right thrust which required some fun calculations to ensure the engine’s hub would end up on the symmetry and center-line. This drawing helped me precisely trim the longerons’ aft ends to form a base for a 1/4″ sq’ tail-post, placed in-front of the fin post.
The F4 – F6 formers are 1/8″ balsa. This should be fine because of the other structural elements. However, while building the “fuse crutch”, I realized how much pressure the formers will be under. While the crutch supports prevent the formers from buckling at the center-line, the other fuse bracing and stringers could crush the formers or just deform them. To resolve this, I decided to reinforce the balsa formers in “strategic places” with 1/16″ ply and some sticks. A simple alternative to this would be to create the formers from ply instead of balsa. Lite (1/8″) ply formers could have additional lightening holes to reduce some of the added weight. This seems to be the ubiquitous approach many other designs use.
I decided to make several modifications to the fuselage design to increase structural integrity, be better suited to the larger engine and my not-so-soft landings from time to time. I also wanted to incorporate a hatch opening to hide the switches, gas/smoke fill lines and offer other access. The next step is to increase the inside and bottom thickness of the longerons with 1/8″ ply doublers between F1 to F4 to create a 3/8″ sq’ section. Then create a doubler for F9 between F2 and F4 with an extension that overlaps with F8. After gluing both F1s together, widen the notches for the longerons in F1 and F2 and increase the width of the holes in F2 to accommodate the thicker F9 braces. Cut notches in F2 and F4 for additional stringers between F1 and F5 (see hatch construction for more details). Support the crutch longerons on a set of straight beams. Set F1, F2 and F3 aside. Squarely align, dry fit, and glue F4, F5, F6 and the tail fin post, leave overnight to dry.
Rotate the assembly vertically and clamp F2 to the support beams. Ensure everything is level and pin a plum line to the tail post to hang through the formers. Adjust the assembly so the string rests precisely next to the center line on the formers. Place rubber bands around the longerons in the F1 position to ensure the crutch shape is correct before gluing F2. Mix up a batch of 30 minute epoxy to glue F2 and gussets carefully in place, ensuring everything remains square and true. Let dry overnight. Repeat the process for F1.
After the epoxy for F1 has dried, rotate the fuse back horizontally and clamp it upside down on the the support beams. Dry fit F8 and F9 through F1, F2, F4 and F5. Insert (without gluing) F3 in its location to ensure the shape of F9 is formed correctly when glued. When everything fits well and all the gussets and blocks between the F8-F9 overlap are ready, mix up another batch of 30 minute epoxy and carefully glue everything in place ensuring true joints throughout. Leave F3 unglued in place, for easy access during the landing gear installation steps.
With the main longerons clamped on the support crutches, fit and glue the next set of 1/8″ x 1/2″ stringers. I opted to glue them from the tail post through to F1 (even though they start at F2 in the plans), for additional support. I chose dense balsa for the stringers so bending them is a challenge but they offer more strength. A pair of stringers (left and right), are placed in the fuse former slots and glued in one pass using rubber bands at the front and clamps at the back. The left and right side apply equal forces to avoid torquing the fuse laterally. Repeat the process for the second pair, which start at F2. The longerons will be trimmed later before sheeting the front of the fuse. Even though this process shouldn’t introduce any warping, I always double check just in case. You can never be too careful.
The plans show no diagonal braces but many other designs do, including the full scale and other RC scale Pitts. I decided to install basswood 1/4″ sq braces between F2 and the longeron to where F4 and F9 meet. This is a major area of stress on the plane and I wanted to stiffen it. Then I installed balsa 1/8″ x 1/2″ braces between F4, F5 and F6. These diagonals create a strong, rigid structure (just like the full scale design) with very little added weight. As with previous steps, care was taken to ensure the fuse remains straight and true.
The landing gear block needs to be a hardwood. I couldn’t easily source any maple, which is what I usually use for the job, so I found a nice strip of walnut at my local hardware store. I trimmed it to size per plans and marked the installation location on the F8 formers.
The stresses between F1 and F2 are significant, handling the loads from the engine and wings. The potential loads between the LG block and F8 are up to how soft landings are. Let’s just say in my case, it’s important to beef this area up! I used 3/8″ square basswood sticks to diagonally brace between F1 and F2, as well as strengthen the landing gear block. To ensure strong bonds between all these joints, they were epoxied at the same time and left to dry. This image also shows the extended F9 doublers and support blocks glued between them and F8. With the LG block in place, the LG wires can be test fitted and after any required adjustments, F3 can be glued in place.
Tanks, hatch, servo tray and cockpit
At this point, while there’s still access, I focused my attention on the internals of the fuse. Since one goal was to keep the plane assembled there needs to be a way to the tanks if needed. At the same time, I wanted to hide the power switches, charging plugs and fuel/smoke-oil filling tubes. To achieve all this, I designed a hatch for the front cockpit between F2 and F4. It is held down by pegs on one side and magnets on the other. To mount the tanks close to the CG, I created a vertical tray and rails between F1 and F3.
There was still plenty of room to mount a power switch and charge plug on the front of F4. The smoke pump fit neatly at the bottom of F4, which needed more reinforcement for the occasion. I mounted an instrument panel in the scale location made with a high resolution print and faux lenses made out of various sized googly-eyes. A servo tray was installed under the instrument panel with the throttle and rudder servo, as well as the receiver.
With the internals taken care of, it’s time to focus on the bottom of the fuse, but before we can install the stringers, we have to bend them. The long, graceful curves of the bottom of the Pitts fuse may have been inspired by a boat hull. They span the entire length of the fuse. However, there are several vague details in the plans which have to be deciphered before hand. How the bottom wing fairing (“belly pan”) is installed, how the stringers join at the rear post and how the tail wheel is supported are all left to the modelers imagination. What is clear however, is that the stringers need to be arched. To achieve this I soaked the stringers for several days in water. Once wet they are fairly flexible and able to be bent along a jig I created for the task. The stringers are left to dry for a few days and keep the arch nicely after they are removed from the jig.
The bottom stringers can be installed to span the entire length of the fuse, (if all the slots in the formers were lined up, but they are not…), however I ended up segmenting them between F2 to F3, F3 to F4 and F4 to the tail post. I did this to accommodate modifications to the design for the bottom wing fairing and a bottom front hatch to access the fuel/smoke tubing when needed. However, since the segments started as one continuous arch, they all line up well and don’t visually disrupt the bottom curve.
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