Plane & Pilot
Tuesday, June 25, 2013

Decathlon Xtreme

Aeronca champ…You’ve come a long way, baby!

Then I started trying to hold climb speed, which turned out to be a totally new experience for me in the type.

Citabrias climb okay. Nothing spectacular. Super Ds do much better. Quite well, actually. The Xtreme, however, is well named because it really gets with the program. Greg had told me that a good, no-sweat climb speed would be 80 mph, which is about what a normal Citabria climbs at but is well above best rate-of-climb numbers. However, I pulled the nose up. Then I pulled it up some more. Still too fast. It wasn't until we were climbing at an almost uncomfortable nose angle that the airspeed settled down to 80 mph.

Then I glanced at the VSI and got a shock: 1,400 fpm! And we were 10 mph over best-rate-climb speed. Plus, between Greg and me, we put the airplane right at gross weight (it has 610 pounds useful and we were using most of it). This is definitely NOT your grandfather's Citabria/Decathlon. Later I saw 1,600 fpm, then to amaze me even more, Greg said, "If you didn't have my big butt back here you'd be seeing a solid 1,800 fpm!" Incredible!

The extra 30 hp and more efficient prop are instantly noticeable. Plus, even with the heavier engine, through a series of weight-saving stuff like composite floorboards and lightweight starter/alter­na­tor, they netted a 30-pound reduction in weight. It all adds up to better climb performance.

I played with different climb speeds as we worked our way up to cloud base at about 5,000 feet, pushed the nose over and brought the power back to 24 square, about 75% at that altitude. I was doing clearing turns and generally scoping out the area when I looked down at the IAS. It showed nearly 140 mph, and I thought I was going downhill. But the VSI said I wasn't. Ditto the altimeter. Here we were at five grand at cruise power and showing 140 IAS. That means this thing was truing something on the high side of 150 mph. Is that possible? Their spec sheet says it is. It says the airplane tops out at 165 mph and cruises 155 mph (135 knots), which, given the kind of airplane that it is (traditional high-wing, lots of struts and other draggy stuff) is incredibly fast. However, knowing that most of American Champion's goals in building the Xtreme were aimed at improving its aerobatic capabilities, I dropped the nose a little to see if their efforts had paid off.

As the speed came up to 145 mph (which proved to be unnecessary, btw), I pulled the nose up, zeroed the elevator pressure and leaned into the ailerons, matching them with some rudder, to see if their roll rate improvement mods worked. And they definitely did. I'm positive I've done thousands of aileron rolls in Citabrias and Decathlons over the years, so I have their roll rates, or lack thereof, permanently stored in my memory banks. Citabria roll rates are best described as "leisurely." Decathlons are "okay" and notably better than a Citabria. The Xtreme, however, is a sizeable amount that's faster rolling than even a Super D. This is due to the ailerons, reportedly designed for American Champion by Kevin Kimball of Kimball Enterprises.

The new ailerons borrow on what's commonly referred to as Super Stinker technology, as introduced by Curtis Pitts on his S-1-11 Super Stinker, and as used in modified form to great success on Kimball's series of Model 12 Pitts biplane variants, as well as a few other recent aircraft designs. These are new concept ailerons in that they incorporate the tried and true concept of being "symmetrical," meaning they're significantly fatter than the wing, both top and bottom, which greatly aids in keeping the airflow attached and improving the aileron's effectiveness even at small deflections and low speeds. Where the ailerons differ over most, however, is the way the nose of the aileron interacts with the cove in the back of the wing where it's attached.

Where most ailerons are fit into the aileron wells in a way that gives minimum gap top and bottom and that gap stays constant, the Xtreme ailerons have noticeable gaps while in a neutral position, and the gap changes with deflection. As the aileron is deflected, the nose of the aileron, which is slightly pointed rather than being rounded, closes the gap, which rapidly increases the effectiveness of the aileron the further it's deflected. It's a little like on-demand power steering: Near the neutral position, there's little or no difference between the Xtreme and a normal Decathlon. However, the further the ailerons are deflected, the more the roll rate increases. I did a handful of full-deflection rolls trying to guess how much the rate is increased, and my best guess is around 30%. It's enough to be immediately noticeable by anyone used to flying the older wings. However, not everything is absolutely perfect in aileron-land.

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