Swappable Pitts Special S1S - an aerobatic classic

Swappable Pitts Special S1S

In the summer of 1973, when the Rolling Stones and Tony, Orlando & Dawn were topping the charts, my mother and her flying partner Bert scratch-built a Pitts Special S-1S based on plans from Aviat Aircraft. Bert was an excellent mechanic and designer, and even my aunt was enlisted as  a pro-seamstress. The project took 2500 man- and woman-hours of work, and some of my earliest memories are linked to the build - sewing and doping the canvass, the wood and steel skeleton, and the smell of oil and engine parts. 

Their Pitts was serial number 427 and was tagged CF-BRT. They flew it in local airshows and trained in basic maneuvers. The S-1S is a one seater, so no rides for the kids unfortunately, but we spent a lot of time around the plane on the ground and watching it fly. In 1978 the plane was involved in a major crash - a bungee in the landing gear broke and the gear failed. My mother flew it around for a while to use up fuel, and belly landed on the grass rather than bailing out and ditching it, but unfortunately BRT pitched end-over-end. She walked away with minor injuries, but BRT was an almost total wreck. They re-built it through 1978 and got it back in the air, and it is still flying today. 

The Pitts S-1S is a classic, and I am partial to it over some of the newer performance aerobatics planes, probably because I spent so much of my childhood at aerobatics competitions (we even got to the Worlds in 1980 and saw Leo Loudenslager win the Aresti cup with his Laser 200 that is now in the Smithsonian) where the Pitts featured so prominently.

So I came at this project from two points of view. One, I wanted to have a good, tough, aerobatics plane and have been unhappy with a couple that I have bought commercially, since they seem to require too much speed on account of their weight. I wanted a light plane that could fly at more scale speed but still have the power to do the moves. On the other hand, I thought it would be fun to take on the Pitts, a challenging plane from a design perspective and see if we could make one for the swappable series with excellent flying characteristics with foamboard. So I set about making a swappable CF-BRT, as close to the original in form as possible, with no real idea how it would perform. 

The results were WAY past my wildest expectations. This is one of the most fun planes I have ever flown. It’s as gentle as a kitten if you want, but when you open it up it’s amazingly agile in the air. It takes off in a shot, even from grass (which surprised me because the wheels are scale), and lands on grass well too. Initially I had the throws dialed way down, but I quickly went to high rates and now that’s all I use - in a word, if flies great. 

Below, I provide build instructions, and plans at the end. Above and below are some videos of it in the air. Flying videos of tiny planes too far away are not the most interesting, so I focused on low passes etc. just so you can see it flies smoothly. I also have some wing-cam clips of basic moves, rolls, loops, hammerheads, again to show the plane works. My friend Ed did the other video (below), complete with music, as usual - Thanks Ed! 

The movies are mostly from the maiden flight day and the next day. As I said, the flight characteristics were beyond my expectations - the plane is light and virtually floats and is easily able with a modest power set to do any of the basic moves a real Pitts would do. You might notice some differences in the plane in different clips - I made some cosmetic modifications to the cockpit and rear deck after construction to make it more realistic (noted below), and put the stripes on it after the maiden (Ed's movie below is all from the first day). 

Build instructions:

This plane is a bit tricky to build because the two wings are different, the top is flat and swept back, the bottom has dihedral but is not swept. This gives most of the characteristic looks of the Pitts, especially from head on. I used tape for the colour as I went, but that adds a lot of work. If you don’t want to have a sun-burst on the wing, painting would simplify it. I don’t recommend this for a first time build. Compared to my other designs like the Messerschmitt and Mustang, this is harder, even harder than the B25 bomber (though WAY easier to fly). But if you have built a couple planes, make this one since it flies great. 

Step 1 - top wing. The overall construction is much like the wings on other builds I have done with a few exceptions. There is no dihedral, but it’s swept back, so the spar is different as is the joint between the two halves. The bottom panel ends in a line parallel to the fuselage and correspond to where the struts will connect. Glue the short spar into place so it ends where the bottom panel ends, and I always glue skewers into the folds of the upper panel for added strength (see picture). Fold and glue wings as usual. To join the two halves and get the right sweep-back angle, short joining-spars are first inserted and glued in to each wing at an angle so they meet face to face when the wing halves are joined. In addition, two popsicle sticks are inserted tight to the front and back of the inside space and glued into one wing. Now join the wings with lots of glue on both spars, sticks, and the joint itself, and tape once the glue hardens. A semi-circle is cut away from the trailing edge joint to suit your taste (I re-cut because the first was not big enough). Cut ailerons. 

Top Wing Halves

Join halves with short joiner spars and popsicle sticks (for sunburst instructions see below).

 Cut out wing above cockpit.

Step 2 - bottom wing. Again the overall form is similar to the top wing, but with  a few degrees of dihedral (guided by spar in plans), and here the bottom panel ends where the servos are installed. Join as usual and cut ailerons. 

 Bottom wing set up and joining.

Step 3 - make the fuselage. This is a standard box fuselage, short and stubby.

Fuselage and two wings. 

Step 4 - assemble the tail. The horizontal and vertical stabilizers are straightforward, and give the Pitts a lot of its look too, as well as huge control surfaces. The elevator is forward of the rudder, so I cut a pie slice out of the horizontal stablizer above, and the fuselage below to allow a popsicle stick to be added to join the two halves of the elevator, for extra strength (see pictures). Glue the two stabilizers together at 90 degrees, then join them at the tabs on the top of the fuselage, and again make sure they are true as the glue hardens. Once assembled, trim the bottom of the fuselage or rudder (whichever extends beyond the other so the line of the rudder and fuselage are the same (see picture). Servos are installed under the horizontal stabilizer and connected to the controls, nothing unusual here. 

Notch fuselage for elevator movement.

Join stabilizers and check mobility.

 Once tail joined to fuselage, trim the bottom of the fuselage to match the sweeping line of the rudder. 

Step 4 - Install lower wing, and layout the wing struts. Insert the lower wing into the slots in the fuselage and glue into place once the wing is centred and 90 degrees to the fuselage. The wings are joined with two “I”-struts. I reinforced these with 0.5mm CF (just cut out with scissors using the same pattern, then hot glued to the foam). They would probably function without it, but I suspect in a crash the reinforcement will be useful. Now dry-fit your I-struts to the top wing as shown, so they close up the openings at the end of the bottom panel, and hold them in place with some tape. place the top wing on the bottom wing so that the bottom surface of the strut comes just to the leading edge of the wing and check the alignment. The bottom curve of the I-struts should be close to the curve of the wing - you can trim a bit if it is way off, but make sure the wings stay parallel - measure the inter-wing distance on both sides at the struts, and make sure it is about the same. The wing tips should be at about the same position on either side, and the centre of the top wing should be a few cm forward of the bottom wing. If that all looks good, then outline where the struts contact the bottom wing, when you have everything centred and lined up perfectly. At the same time, measure the distance from the top of the fuselage box to the bottom of the top wing and bend yourself some piano wire struts as shown, and glue this down to the fuselage box, as shown. These will eventually be taped in place to the wing for extra strength. 

Lower wing install.

Bend wires and install the fuselage struts. Position at front most short former.

Step 5 - Install fuselage decks. Now its time to cover up the top of the fuselage. First, glue all the formers into place. These are fairly standard, except that there are several in the front, to support the change in shape at the cowling, and these extend over the sides of the fuselage to create the whole front shape - this is an important part of the way a Pitts looks - the front cowling flairs. Leave the front-most former unglued until later (see below). 

Test fit formers (note that the back formers of your's will look a little different because I moved the cockpit forward and raised the back deck height of mine later - a big improvement)

When the formers are done, dry fit the three paperboard deck pieces and make sure they fit. In the centre piece, you will need to trim out a little notch on either side for the wire struts to pass though. Leave some extra room here, you will hardly see it, but you don’t want it too tight when you are rushing to get it in place before the glue sets. When you are happy, start with the middle deck, which forms the cockpit. I glue first, tape later, so all formers that join this deck are glued, and the paperboard centred and then held down into place until the glue hardens, then tape down the two sides. Then do the rear deck in the same way. Before you do the front-most former and cowling, make sure you fit your powerpod. I also insert a velcro strap through the top of the fuselage that will wrap around the battery, for extra support (see picture).

 Cover the centre section first. Note the notches for the wires at the front. 

Then do the back section (and your's will be higher and further forward). Then fit your powerpod. Note the velcro strap inserted at this stage - do it now before you cover the cowling section. 

Lastly, install the front-most former, which is special because it has a piece of paperboard glued to it that makes the front of the cowling. Cut out the former and the paperboard, glue them together and then trim the paperboard to match the former around the top. This leaves enough space to get your powerpod in and out (it has to be a short powerpod, so it can drop down and clear the front former, but it does work - see picture), but covers over much of the front to give it a nice look. Once this is set, glue and tape the front cowling so that it extends forward to be even with the front-most former. It should extend out the rear to past the last former so from behind it looks like there is a space under the cowling. 

Shown with powerpod in place. Not the first former has a paper/tape face. Glue it into place and check the powerpod still goes in and out okay. 

With the cowling installed the motor should be clear of obstructions. 

 Cowling from behind - gives the oval look to the nose, which is a distinctive Pitts Special feature. 

A note here: I originally had the back deck about 1cm lower and the cockpit about 2cm further back. It did not look good, and made the fuselage look too long and thin. Some of the pictures of the build are from this version. After a day, I ripped the back deck off and added some height to the back former, then installed a new former further forward to make the back of the cockpit and it totally changed the look of the plane. In the plans, this is all simplified for you - I just wanted to explain why some of the pictures might look a little different. 

Step 6 - Install top wing. Now its time to join the two wings. First, remove the temporary tape on the I-struts, and glue them into place on the top wing with hot glue, making sure they stay at 90 degrees to the bottom of the wing. Once both are hardened, re-position the struts to your outlines on the bottom wing, and check everything is still lined up well. If so, glue the bottom of both struts, place them on these marks, and hold into place until the glue hardens. Re-glue the joints on both sides as squeegee away the excess glue. Your wire struts to the fuselage should also be in the right place now, and you can just tape them onto the top wing with packing tape. Your wings should not be REALLY strong, you should feel no wobble between the two wings. 

Tape struts on to re-test fit - should sit on the wires.

Check (and re-check) that wings are parallel and struts conform to shape of both wings fairly well.

When you are happy with the fit, glue struts to top wing.

Re-test, and if all's well glue struts to positions marked on bottom wing, hold them into place and re-glue joints.

 Lastly, tape wire struts to top wing. 

Step 6.5 - connecting the ailerons. I wanted dual ailerons, so I had the bottom ones on a servo and connected them to the top wing ailerons. Based on the way it flies, I strongly encourage this since its does not have crazy roll rates, just scale. So, THIS turned out to be WAY harder than I imagined, knowing nothing. Originally I thought I could just put two horns on the top and bottom behind the strut, and hook ‘em up, but its not that easy. First try the top ailerons would go up, but not down, then they would go down and not up. Only then I decided to read up on this and learned you can only connect them where they are directly above one another, so I redid it at the very edge (see pictures). I had a couple pin hinges around, and I used these and taped them together with a skewer. This works brilliantly, although does not look great. A wire would be better and smaller pins, but it's what I happened to have on hand. 

Join the ailerons near the tips - as close as you can to straight up and down. Test to make sure top aileron has full motion in both directions before gluing anything. 

Step 7  - Landing gear. A rear wheel is installed on a little length of thick piano wire that is bent in any way you see fit to attach it to the rudder. Tape that on. The front landing gear are made of two pieces of 2mm piano wire as shown. One is much like the ones that Bixler always makes, The second one provides more support out the back, it is a simple squared off U-shape with loops on the ends. Loop these over the wheel axels before you put the wheels on. Now put three skewers in the fuselage as shown below, and use short stiff elastics to attach as shown. I use old bike inner tubes for this - just cut sections like slicing salami and you have great elastic bands that never wear out. This set up looks realistic, but also gives you great stability and some shock absorption. This plane eats up grass take off and landings, even with fairly small wheels. I cut out a piece of paper in a triangle and taped it between the two to finish the look. Over time the tape wears out, just re-tape it.  

Landing gear held in place by bike inner-tube elastics.

Paper triangle taped to two gear wires.

Step 8 - extra decorations. I made mine up to look like a classic Pitts S-1S to match CF-BRT, with the sunburst top wing (made by covering the wing with red tape, then cutting the lines with a very sharp razor blade and peeling the tape triangles off. Do this BEFORE you install the wing - see pictures), and classic double pinstripe with the diamond (this would also be easier to do before you put the wings on, but I did it later and it was no big deal). Stripes were all cut out of white tape, surprisingly simple (you know those things that you could spend twice as long finding it on the internet as it took to just make it up from junk you have?). The windscreen was cut from garbage clamshell packaging, and is just as easy as it looks. I put a couple pieces of back tape on the nose to look like the air intakes. On the tail I put an EAA sticker (classic logo, of course) and the registration tag letters CF-BRT were just printed on my computer on paper, cut out and taped on with packing tape - presto. Done. 

The sunburst is made by lightly cutting the tap with a very sharp blade and a flexible straight edge of some sort (above), then peeling it back (below)

A windscreen, of course (cut from clamshell packaging - see plans). Pin-stripes are just white paper tape. One the cowl I just bent it around a bit to make the distinctive stripes (below)

Taped some paper letters for the CF-BRT on the tail to finish it off. 

Then I thought I needed a pilot, so I put my “Bixler Doll” in the seat. 

The pilot functioned well at basic moves. 

Step 9 - Power setup. This plane takes a standard powerpod, except because it is so short and stubby, you have to cut the end off (at least a couple cm, but why not just cut it in 1/2 to reduce weight). To be honest, I have begun making all my powerpods 1/2 length since they never are the fuselage anymore like in the first FT planes. 

 Anyway, I set mine up with a NTM 2826 1300 kV motor, a 40 amp ESC, and a 9X4 prop. It runs on Nanotech 3S batteries, and either 1300mah, or 1500mah fit into the battery slot fine. I make little foamboard boxes for my batteries, and in this case it helps complete the look too. With this set up the plane can take off in a few feet, can go vertical, and do anything you want, but it is not stupidly overpowered so it looks non-scale, or flies badly because of torque. I balanced it on the spar of the top wing about 5cm from centre (remember because it is swept back, the position of the spar is not constant), and this seemed ideal. 

 The first flight was a dream - it just took off and flew, no trim was even needed. I started with low rates on my ailerons and elevator (like 50%), but quickly went to full 100% because I loved the way it rolled and climbed, and I did not find this made it hard to handle.  In my first flight, the brand new  25amp ESC burned out and I learned it does great landings on grass, and soon learned it takes off well too, so I am really pleased with this plane. Seems like a great trainer for aerobatics - its powerful and agile, but can be slowed right down to soar gently, so it does everything. 

 Of the planes I have designed, or even built from scratch, this is easily the most fun one to fly. I wish I knew what it was like to fly a real Pitts, but I will have to make do with this little one, and imagine they are similar - agile but not twitchy, fast enough to do what you want but not so fast it takes a lot of space, a great glider with soft landings. I hope some of you can work up the energy to make this, because its flight characteristics are excellent. 

Here are some pictures of the final product, and the build plans are provided below. Please comment if you build one - I would like to know how it goes and how it flies. 

Plans: