Rans S-9 Chaos

The S-9 Chaos has been in production since May of 1987, and has remained virtually unchanged since production began. Minor refinements have been implemented to the fuel, control, and cooling systems. A new cowl and landing gear changed the looks a tad a few years back, but for the most part the plane has remained close to original form. It is simply hard to solve the equation significantly better than the way the S-9 does. Maybe with improvements in engines, some weight reduction, and control system upgrades we could see a crisper S-9, even so the stock Chaos is pretty well dialed in for the mission at hand. That mission is low cost, exciting aerobatics. Lots of ultra light type planes claim Akro status but none can do the figures with the agility and precision of the S-9. The reason is the mid wing design allows amazing strength to weight. The original S-9 weighs in at 280 lbs. with the Rotax 503. It seems incredible to roll and loop a plane that light at speeds near 100 MPH. 

Our latest demo plane weighed in at 430 pounds with 220 pounds of fuel and pilot that comes out to a power to weight of 10 per horsepower with the 582. 10 to 1 is a decent ratio, bettering that makes it even more exciting. The goal of anyone building an aerobatic plane should be to keep the weight down, and making a lighter S-9, although a challenge could be possible. Most of the ideas I have center around making the plane more mission specific, for example, our new S-9 has two tanks for the occasional cross country. Dropping one tank saves 8 pounds. It may be a little spendy, but titanium landing gear might shave 4 to 5 pounds, and lighter fabric and paint overall could kick out another 2 to 4. Close study and diligent attention to detail could eventually decrease a typical S-9 with a 582 down to fewer than 400 pounds empty. That would be almost a ˝ pound per horsepower less! 

The first flight found me with a plane nearly in perfect rig. After a 20-minute flight confirming all was in the green and buttoned down, it was time to test the aerobatic side. I could tell from the climb performance (well above a 1000 FP) and the light handling that this was going to be fun.

I did a couple of clearing turns, checked altitude and let it rip. From level flight I rolled into a tight right aileron roll. Level again for a brief moment, I repeated the right roll connecting it to three more, holding a level line. The roll rate is impressive between 110 to 120 degrees per second. 

Next I did a series of left roll, stop level and right roll, left or right the roll seemed symmetrical. I did four, eight and sixteen point rolls followed by 12 to 14 second slow rolls. All was purring along fine, the S-9 was in its element, the engine never skipped a beat inverted or not. Throttle response was smooth and power was at about 75%.

From level flight I pulled a loop, goosing the throttle to full power on the up line, making a tight round loop that I let continue into several more. I studied the altimeter noting the loop was less than 300 feet in diameter. Several more loops later I had found great fun in giving it a shot of power on the up line. It was a neat way to make a tight loop. It reminded me of riding a powerful Sea-Doo, pumping the throttle when ramping up a wave to get max airtime, only this was much drier and total three-dimensional fun!

Ok the basic roll and loop come off without a hitch, it was time to rock and roll, and that is what the S-9 does best. I pulled into a long vertical line topping out in a hammerhead to the right, with a half roll on the down line I entered a hammer to the left, just a little more rudder on these would be nice. 

The vertical line was at least 300 to 400 feet, and allowed a complete vertical roll with barley enough to hammer out at the top. Takes a lot to go straight up, but I was quite pleased with what 65 HP could do. 

On the down line the S-9 has plenty of drag, so you are not easily busting redline, so you have time to adjust the line or roll either way. Keeping it pegged straight down requires a little stick and rudder, but small moves here.

I performed some lomcevacks by taking it on a 45 up line then hard over to the right with forward stick and punching left rudder. From the ground these look slow motion, and may even be hard to recognize as a lomcevacks, but they are a gas, since the Gs are low. In fact that is one of the pleasures of doing Akro in a light energy plane the g-load are never more than +4mostly under +3.5. 

The optional Hooker harness goes a long way to feeling secure in the plane. I noted no adverse body movement, making it very relaxing to execute a figure knowing not matter the motion I was firmly behind the controls. 

Stalls came off with the traditional buffet with a straight-ahead drop. A slight relaxing of the stick and you are flying again. Noting the indicated airspeed at stall was 45MPH; I was looking forward to gentle landings. At 800 AGL, I slowed to 70MPH for the pattern then on final kept is nailed on 60MPH. Kissing the pavement at just above 50MPH, the landing was something any experienced tailwheeler could do. This is a short quick plane and paying attention on landing is required (as with any tailwheel plane!). However to learn the S-9's traits takes only a few minutes, and it is forgiving enough to allow some less than professional moves. A bonus is the awesome crosswind capacity, 90 degrees at 20 kts is easily demonstrated. Again it is simply getting to know the breed. 

I have heard about pilots not being so enthused about the S-9's runway manners. In most cases pilot skill not being a factor, the planes that would not behave, the builders had made changes to the main gear or tailwheel. On one occasion a builder simply did not have the tailwheel steer springs tight enough.

In other cases it was not following the recommended procedure. We stress to take off and land with the plane in the three-point position. This is the trick to easy tailwheel operation in both the S-9 and S-10. It is hard for experienced tailwheel pilots to follow this simple advice, but to understand the design of the S-9 is to know why this advice works. The static angle of the S-9 is 9 degrees; that was done on purpose. It placed the gear far enough forward to prevent nose over, and it gave it short, light, and strong gear. There is also very little prop clearance. At such a shallow angle there is no need to get the tail up to rotate, the S-9 lifts off when it has flying speed, and does so in the same distance with tail up or down. In fact the tail down take off comes out a little shorter. That is the science behind it, now if we can just get the point across to the veteran tailwheel pilot!

After a few touch and goes, it was time to call it a day. With just a few minutes of sunlight left, I closed the hanger door thinking about the shiny new plane left inside. How a humble little plane has the ability to take a normal day in a pilot's life and turn it upside down literally, I guess that is why it is called the Chaos!