Changing a Perfectly Good Airplane

Designing and building a modified RANS S-9, part 1.


Over the years, Michael Ryer has flown a bunch of airplanes in the 5000 hours his logbook reflects. Subjectively, he liked some and hated others. Positive subjectivity, for Michael at least, is the good feeling he gets when he moves the stick aft and the G loads come on smoothly and in a linear fashion as opposed to stick movements where nothing happens and then, all of a sudden, the airplane reacts. Objective data is hard measurements in pounds, inches, gallons per hour, and so on, provided when an airplane is flown and inputs/results are compared to a performance standard. This article is about how subjective requirements were built into an existing, objective airplane.

Change is hard for most people, unless they are in charge. When we compare similar homebuilts, homebuilders seem more than willing to leave a “thumbprint” on designs. Some changes, like a paint scheme or interior leather color, address personal taste and have little or no effect on performance or safety. Other changes alter performance characteristics or structural integrity, and these must be made very carefully.

The completed right wing. The leading edge was reshaped by adding balsa wood and fiberglass to the leading edge tubular spar.

Nearly five decades ago, when Michael was in his 20s, he built a Pitts S-1S Special to meet his need to fly upside down and stop wearing out his old Citabria. He built the Pitts and flew it for over 30 years, accumulating nearly 3000 hours in the little red biplane. Flying contests and airshows, he got really good at tumbling through the sky, making noise, smoke, and snap rolls in front of spectators.

He built other airplanes and drifted away from hard-core aerobatics. In 2003, he sold the Pitts and became an old-guy weekend flier, flying a Rans S-6S (his 10th homebuilt). He moved to Kansas and kept his airplane at Stearman Field near Witchita, a place infected with an active aerobatic club. Watching airplanes like Extras, Pitts, and clipped Cubs rekindled the urge to loop and roll. Like any serial builder, Michael needed to build another airplane—another aerobatic airplane.

Michael retired from the aerospace industry as a manufacturing engineer. He knew his way around flying structures from building launch vehicles and airplane components for various companies. His new toy needed to safely perform old-guy aerobatics, help him relive his glory days, and have a great ramp presence, like a Staudacher or One Design, whenever he returned to the surly bonds of earth. Michael knew the Rans S-9 Chaos and its designer, Randy Schlitter, through building the S-6S. The S-9 is a strong airplane (+6, -4 G) and performs well on a 65-horsepower Rotax 582.

In the early 1990s, Michael had seen Schlitter fly the S-9 in an airshow. The airplane had remarkable performance and did positive-G maneuvers nicely. Having seen the airplane in action, Michael still felt it was a good starting point for all the changes he wanted to make to the little beastie. He set the following goals for the proposed changes: better inverted performance, more roll rate, great aesthetic appeal, use the 65-horsepower Rotax 582, and have minimal airframe weight gain. He settled on the S-9 and started work in January 2012.

With the change criteria set, it was time to get started on the project. But first Michael needed to establish relationships with technical experts to assist him with the changes. He knew he could build the airplane from a manufacturing standpoint, but he was not a design engineer. He needed someone who could engineer the contemplated changes in a way that would meet the criteria, yet still be within his manufacturing capabilities. The last thing he wanted to do was turn a safe airplane into an unsafe one.

Remember Stearman Field, mentioned earlier? Well, one of the airport bums Michael ran into was John Wells, a glider guider and Fly-Baby pilot. John and Michael got to know one another through flying around the area. Michael discovered John was a retired design engineer with at least a passing knowledge of how to engineer the changes being contemplated. Over many gallons of coffee at the Stearman Field Bar and Grill, John agreed to be Michael’s principal design consultant and mentor. Michael also got to know the folks at California Power Systems. Michael needed their help for changes to the engine cooling and exhaust systems. Now, with the “concurrent engineering team” in place, Michael was off to the races figuring out how to design and build the modifications.

Scanning the Barnstormers website, Michael tracked down an uncompleted S-9 project in Pennsylvania. Using a borrowed pickup truck, he and his amazingly tolerant wife spent four wintery days retrieving the fuselage, landing gear, and tail surfaces. It was a good starting point for all the changes he wanted to make.

The completed wing skeletons, ready for covering.

New Airfoil Structure and Wing

Michael wanted to address the existing design’s inverted challenges by looking for a different airfoil for negative-G work. The original S-9 airfoil is flat bottomed, like a Piper Cub. The new airfoil needed a more symmetrical shape to allow the airplane to more easily perform inverted maneuvers. Next, Michael wanted to improve the roll rate. For simplicity, the original wing had ailerons attached with four simple centerline hinges, like gate hinges, between the aft tubular wing spar and the forward tubular spar of the aileron. The near-one-inch gap between the wing and aileron creates a considerable discontinuity for the airflow to negotiate as it moves across the gap between the wing and the aileron. The original aileron design provided a roll rate of 100 degrees per second. Michael’s goal was to reach a roll rate of 240 degrees per second.

The original S-9 airfoil was designed by RANS and is similar to a Clark Y.

The new airfoil is the RAF 34, which is similar to the M6 airfoil used on the Pitts.

To maintain structural integrity of the wings, no internal changes were to be made. The airfoil shape, totally new ailerons, and redesigned wingtips were the only changes made to the wings. The changes to the airfoil shape came from a subjective analysis of the existing airfoil against other airfoils. Michael’s experiences flying aircraft with the airfoils formed the basis of this comparison, along with the mission profile of flying old-guy aerobatics.

The bottom of the covered wings with the ailerons attached.

The original S-9 airfoil was designed by RANS. It is closest in appearance to a Clark Y. The S-9 airfoil has a flat bottom. When inverted, a Clipped Cub (which has a similar USA-35B airfoil) requires nearly full-forward stick and full power to maintain level flight. So, the Clark Y had to go. It simply demanded too much from the aircraft when inverted. A clipped-wing T-Craft, with the 23012 airfoil shape, performs well inverted. The last airfoil in this subjective evaluation was the Munk M6. Michael decided to go with the M6 because it was a good inverted performer and had been used on the Pitts.

With the airfoil selected, Michael proceeded to determine how to adapt the M6 shape to the S-9 wing rib structure. The ribs are built of half-inch aluminum tubing, cold formed to shape. The existing top surface shape was very similar to the shape of the M6 airfoil. The bottom, however, presented a problem. The M6 bottom shape has a distinct “knee” at about the 60% chord point of the airfoil. This posed a problem for manufacturing. The round tubing could not make such a tight bend without being mandrel formed.

Now it was back to the drawing board. John introduced Michael to a valuable resource at the UIUC website. Michael was able to pull up the M6 airfoil and all its technical performance data (lift and drag curves, station notations, and so on) along with a listing of other, recommended airfoils. Within the list was the RAF 34 airfoil. John took his apprentice aside and helped him understand the interesting subtleties of the airfoils being considered. The performance curves of the RAF 34 were nearly identical to the M6. With the RAF 34, Michael got a nice smooth underside curve from leading edge to trailing edge for ease of manufacturing (no sharp bends). John pointed out the last 20% of the upper surface was “reflexed,” resulting in gentle delayed stall characteristics. This was a win/win for the design and manufacturing team. The only challenge would be accommodating the sharper leading edge radius of this airfoil.

Typical S-9 ailerons are attached to the wings with simple hinges, resulting in a gap between the two structures.

The basic wing structure was changed by adding a new shape to the leading edge and a new aileron attachment method to the trailing edge. The new leading edge was made by shaping the leading edge tubular spar with balsa wood and adding a fiberglass overlay for impact resistance. The original internal structure of the wing (drag/anti-drag bracing, fuel tank, wingtip structure, lift strut attach points, and so on) was unchanged. The wing ribs required rebending the lower tube to the proper shape, and remaining internal structure was built using the assembly steps provided by the RANS builder’s manual. To eliminate the round tube shape of the S-9 rear wing spar, and to serve as the attach point for the ailerons, Michael developed a box structure. It resulted in a design very similar to every 100-series Cessna ever built. John reviewed the new structure. He added some additional reinforcement at the aileron hinge points and a few more rivets to resist buckling of the box structure when it was G loaded. Now that the airfoil shape was resolved, the next change was to the ailerons.

The covered right wing with the aileron installed.

New Ailerons and Wingtips

The existing S-9 ailerons use the trailing edge wing spar as the attach point. The aft wing spar is a 2-inch tube, and the leading edge spar of the aileron is a 1-inch tube. The two structures are joined with simple hinges, resulting in a gap of nearly an inch between the two structures. Airflow between the bottom of the wing and the top of the aileron is stopped by a gap seal. Also, the trailing edge of the aileron is a half-inch tube. The aileron is essentially rectangular in cross section. The roll rate of the S-6S, which has the same aileron design as the S-9, is slower than a Cessna 150. The two airplanes have similar wing and aileron areas.

Piano hinge was used to connect the new ailerons to the trailing edge spar box.

With the wing ready for a new aileron, the aileron shape was redesigned as an aluminum-skinned triangular structure. The structure had a leading-edge spar, a torque box at the control horn station, and full ribs at each end. The skins were stiffened in the same manner as the RV series of aircraft with L-section stiffeners running between the spar and the trailing edge. These stiffeners are riveted to the skin. The new aileron is the same shape as the original S-9 and one inch deeper in its chord. Piano hinge was used to connect the aileron to the trailing edge spar box. Michael, following John’s recommendation, also mass balanced the ailerons to reduce flutter probability.

The last wing item to address was the wingtip and its shape. The original wingtip was a very rounded, tubular structure with the end of the aileron set back from the wingtip some 6 inches. Michael originally drew up a Mooney-style wingtip with the aileron mass balance as part of the wingtip. John helped Michael understand the need to hold the end of the aileron back from the wingtip by a few inches, about the chord span of the aileron. If Michael really wanted to get efficient ailerons, he needed to add end plates to the ailerons so air could not flow through the gap created when the aileron is deflected up or down. He did this on his symmetrical-winged Pitts and it worked. He has not done this yet on the S-9, and it is on the winter downtime project list.

The aileron shape was redesigned as an aluminum-skinned triangular structure. Note the balance weight to reduce the probability of flutter.

John and Michael, having completed the wing work, were ready to move on to the next part of the project—but first, it was time to enjoy a good Cabernet. We’ll talk about the fuselage, tail group and flight testing the S-9 next time in part 2.


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