Pitts S1S for 100 watts scratch build - RC Groups
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Feb 14, 2007, 12:07 PM
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Build Log

Pitts S1S for 100 watts scratch build

Way back on a rainy Sunday afternoon in August of 2004, Chris Parent and I met at a Kinkos copy center to brainstorm on a design for a Pitts Special bipe. It is hard to not have warm fuzzy feelings about Curtis Pitts’ cute little creation. Chris had very good luck with FF scale Pitts models and found them to be surprisingly stable and forgiving. He also flies a highly modified Potensky EPP foam Christen Eagle (dubbed the “Pitted Special”) that is a capable, yet gentle aerobat. He reasoned that we could create a design for a high performance scale aerobatic plane that even I could fly.

We set out to arrive on a design that would have good scale aerobatic performance on the 100 watt power systems we use in other planes. Target weight would be about 18 ounces. Chris had some great documentation including accurate three-views. The Kinkos has a 36” roll-fed copy machine, so we had the 3-views blown up to various sizes to compare. We settled on what is approximately 1/7th scale, with a 28-3/4” wing span. Chris made some changes to the outline for better flight characteristics. The chord was bumped up from a scale 5” to 5-3/8” to bring the loading down a bit. He increased the fin-rudder area a bit and snuck the rudder hinge line forward to gain even more rudder area. He also added 1 degree positive incidence to the stab.

I scanned the three views and imported the bitmap into CAD and traced it. Chris and I then sketched in the structure. We diverged a bit at this point, as Chris wants to build a S1C with 2 long ailerons on the bottom wing and a semi-symmetrical airfoil. I want an S1S, which has four short ailerons and a symmetrical airfoil. This build log is about the S1S. Time to get this one done for the coming season.
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Feb 14, 2007, 12:13 PM
confused poet
HanksGB's Avatar
Very Cool, I will be following with Interest.

Feb 14, 2007, 02:11 PM
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Thanks for the interest Henry.

The Pitts tail has such nice curves that it naturally lends itself to laminated outlines ala Pat Tritle. The structure mimics the full scale’s welded tube structure. It should look authentic with the sun shining through the covering. Plan was bonded to fomecore, then cut out to make forms. Three laminations of 1/32 x 5/32 were wetted with ammonia and glued with aliphatic, then taped around the forms.
Feb 14, 2007, 04:11 PM
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The parts that needed to be cut from sheet stock were nested in the drawing in 3” or 4” wide strips. I printed the parts out on 11 x 17 sheets using my inkjet printer, then took them to Kinkos to be copied on to paper using toner. I put a reference scale on the parts sheets I printed, then brought a ruler with me to Kinkos in order to tweak the scale to a true 100%. It is surprising how inaccurate these copy machines are at printing in the proper scale. I cut the copies into 3” and 4” strips, laid them face down on the balsa sheet stock and transferred the toner to the balsa by rubbing the backs of the copies with dope thinner. The resulting printed strips were cut out the old fashioned way.
Feb 14, 2007, 06:36 PM
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Looks good Mike, I'll be watching closely.
Feb 14, 2007, 08:53 PM
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Thanks for the compliment Paul. The cockpit details on your Shoestring came out great. I hope you start a thread on the HE-51 build.

The lower wing has 11/64th of dihedral each side, so I built it in two pieces with a 1/32 ply brace in place of the shear web at center. There is 1/32 balsa shear web on front and back of spar out for two bays, then just on the front in the third bay. No shear web outboard of the interplane struts.

The top wing is flat, so knocked together easily. I included tabs on the bottom of the ribs to make it easy to build the wings straight. I didn’t have the ambition to laminate the wingtips. It would have been a nice touch. Ailerons will be cut out later.
Feb 14, 2007, 10:19 PM
Design > Build > Fly
Qrome's Avatar
What motor are you planning on using? This is the size of Pitts I would like. Looks great.
Feb 15, 2007, 04:58 AM
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Mike Smart's Avatar

I'm not sure that 100W will be enough for a Pitts of that size. It doesn't sound very big at that span, but the Pitts is a very stubby aeroplane with a lot of frontal area. I would have thought that you would want at least 125 to 150 W/lb for decent aerobatics.

Feb 15, 2007, 08:53 AM
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Qrome, Thanks for the compliment. A 10mm x 10mm stick mount is drawn in, so there are lots of possibilities. Depends on how the weight piles up and what might allow for balance without ballast. I'm leaving it open for now.

Mike, You are probably correct. When we originally planned this thing, we wanted a scale aerobat that we could use our 1200-1350mah 3 cell packs in. We were only getting a good 100 watts out of them at the time. These days we can get 15 amps out of those packs without pushing them hard. Chris is a FAC scale FF champ and can build incredible amounts of lightness into his planes, so his is bound to be much lighter than mine. He is also a scale aerobatics purist who finds great challenge in performing routines with minimum power. He regularly performs spectacular flights with planes under 100 watt/lb. I'm a hack builder and flyer. My plane is bound to be overweight, so I'm sure I'll be looking for a power system in the 150 watt neigborhood. I suspect that 125 watts/lb would get me two or three clean rolls on a vertical upline before the momentum is lost. That is all I'm looking for. Chris and I both have Stevens Park Caps. His is about 16 ounces and flies spectacularly on a Razor 300 @ about 115 watts. Mine is a sledgehammer in comparison, @ 21 oz with a Hacker B20-15L and 180 watts. Chris will be modeling the S1C, which originally flew on 65 hp! I'll probably need more power to be happy.
Feb 15, 2007, 09:18 AM
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I have a very difficult time building straight fuselages, so I’m going to have to split this one to have any chance at an accurate result. The full scale Pitts has a longeron that runs the full length of the upper fuse. This is the natural point for a split. I drew the parts so that the bottom of the fuse could be built upside down directly on the plans.

The landing gear design is a direct rip-off from Bill Stevens (I only steal ideas from the best designers). The landing gear wire will be sandwiched with balsa in between 1/32 ply plates. That assembly will slide into the pocket built into the bottom of the fuse. This allows the landing gear to be easily removed and put into a vice for straightening after the occasional mishap.

Building the fuse upside down on the top view caused me to arrange the formers with the motor stick cut outs backward, building in left thrust! Also this is where two dimensional drawing meets three dimensional reality and I realize that I didn’t allow for the motor and gearbox to extend past the nose former. At least it is easy to do the fixes now.
Feb 15, 2007, 04:29 PM
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Originally Posted by Mike Smart

I'm not sure that 100W will be enough for a Pitts of that size. It doesn't sound very big at that span, but the Pitts is a very stubby aeroplane with a lot of frontal area. I would have thought that you would want at least 125 to 150 W/lb for decent aerobatics.

Mike, you're probably correct on the power requirements. I also would be looking at more power.

As far as listening to Mike Tully say he's just a hacker flier and builder...
Don't you believe it!

I've seen a couple of his planes and I've seen him fly, I wish I could be that good.
Feb 16, 2007, 10:07 AM
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Paul, You’re flattering me, keep it up! Honestly, if you do a nice covering job over a hack build, then you can fool a lot of people. I never succeeded in FF because my planes weren’t straight enough to fly. RC cured that, as you can usually turn them away from crashing into the ground. As far as my flying goes, you’ve seen me fly my Miss June, which has magical anti-gravity properties. An unfair advantage to be sure. I'll let you give it a try to see for yourself.

The wing front has dowels into a 1/32 ply plate on the fuse. I plugged the wing front into place then glued in the rear screw plates after making sure the wing incidence was parallel to the work surface. I cut wing saddles and applied the forward 1/32 sheeting while the fuse bottom was still pinned to the plans in order to minimize twisting. The second former back from the front was drawn directly from the very good reference 3-view I started with, but was surprisingly far off from the correct shape. Needed lots of trimming to be sheetable. I re-cut the motor stick holes to get the correct thrust angle and put on the top formers. Had to drill some holes in some of the pieces to compensate for the overly heavy LHS sheet stock. I must have accidentally grabbed some 1/16th oak. The cowl top and turtle deck were then sheeted.

I applied some bits and pieces to the lower wing bottom in order to fair into the fuse bottom shape. Too many model Pitts’ have a simple slab bottom, and that always bugs me. The full scale Pitts is built like a big model, so getting the shape right is pretty easy. Three plies of ¼” sheet were glued together and carved to shape the nose cowl. It is aligned with dowels and attached with 1/8 diameter rare earth magnets.
Last edited by Mike Tully; Feb 16, 2007 at 10:14 AM.
Feb 18, 2007, 12:43 PM
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The cockpit area is a natural place to put the battery hatch. It provides access to the middle of the big, fat fuselage. Like the nose cowl, the cockpit/battery hatch is aligned with 1/8” dowels and held in place with rare earth magnets. I’m keeping my fingers crossed that the plane will balance with the battery accessible from this opening. This Pitts will be about the same size as the Aero-Lab Pitts kit. Joel Dirnberger makes plastic parts for that kit, so I ordered a canopy from him. It was only $3.00! Not quite the right shape for this application, but sure beats making a plug and trying to form my own.
Feb 20, 2007, 05:25 PM
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Most Pitts kits and ARFs cheat on the cabane struts. I wanted something scale looking, so went searching the Steens Aero website and sifted through the archives of full scale Pitts build photos to look at wing attachment shots. The Steens website is a great resource for anyone building a Pitts model.

The cabane struts were cut from basswood stick stock and sanded oval. A 3/32” OD aluminum tube was glued to the top. The tube was lashed in place with Fireline fishing line. Epoxy was drizzled over the lashed assembly which was then blasted with a heat gun to cause the epoxy to thin and wick into the lashed area, creating a strong 3D weave.
Feb 21, 2007, 04:39 PM
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Now I remember why I’ve always avoided biplanes. Aligning the wings is agony. Copying the interplane struts from the 3-view only got me a rough starting point. I cut some out from fomecore for a trial fit. Not even close. Now another design flaw appears – the canopy/battery hatch cannot lift out and clear the wing. The top wing must be lifted an extra 1/8” to allow clearance. I then cut two piece interplane struts with pivoting bottoms out of ply and began the tedious process of lining the wings up. After several evenings of tweaking and measuring, I was able to use the adjustable ply struts as a template for the permanent struts.

The wheel pant design is another blatant rip off from Bill Stevens. Why mess with perfection? They simply can’t be beat for ruggedness and ease of fabrication. They capture the wheel in place so that no wheel collars or other hardware are needed. If you haven’t built a Stevens model, then go to his website and download an instruction manual to see how things go together. He is an innovative engineer.

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