Longitudinal Stability of the Sonex Line

[Andy Walker]

Thanks Chris and Kip, sounds like I was remembering correctly. I agree Chris, we should be able to pick one each from the dynamic and static stability columns. :)

I have flown three LSAs, a Zodiac 601XL and two different Tecnam P-92 Echo Supers.

The Zodiac is neutrally stable, but overly sensitive in pitch. Since roll takes much more force, the plane doesn't really feel well balanced at all. The word "twitchy" comes to mind; I spent way to much effort trying to maintain straight an level in that airplane, it always wanted to be diving or climbing.

The Tecnam is slightly positively stable dynamically and statically. If you disturb it at all and then let go of the stick, it will try to slowly return to straight and level at the trimmed airspeed. this makes that airplane easy and very comfortable to fly, and super stable for cross-country flying.

The very small amount of stick time I've had in Sonex/Waiex tells me it's right in the middle. Sensitive without being twitchy, balanced in control forces, and "stable enough" to make it not too much work to fly cross country. I think for the Sonex mission of a fun airplane with some traveling capability, it's right on the money.

[radfordc]

I'm curious who voted "negative dynamic stability." If I'm remembering my aerodynamics correctly, that means that if you put the aircraft in an oscillation state (say, but 'flicking' the stick back and then releasing it), it will continue the oscillation (in this example, pitching first up, then down), in ever increasing amplitude, until actively corrected or until the airframe can no longer handle the forces acting on it.

This is the worst possible handling trait for an aircraft, and is very dangerous. I certainly have never seen this kind of behavior from a Sonex/Waiex/Xenos. I have always heard of the Sonex described as having neutral stability on all axes, and in fact recall John Monnett stating that is how the airplane is designed to behave.

Am I wrong or have my definitions confused?

I described my plane as "negative dynamic stability" (and so did the guy who reviewed the Sonex for Kitplanes magazine). As I stated in my earlier post, my plane will depart from a trimmed condition and won't recover back to level flight by itself. I don't think that any plane that has negative stability will do the "oscillation" you describe. Negative stability means when the nose pitches up or down it just keeps on pitching. A plane that has positive stability will oscillate up and down in decreasing amplitude until it reaches its trim speed.

My plane has negative stability but "just barely". Nothing happens very quickly...you can take your hand off the stick and the plane continues on for a few seconds. Eventually, it will start to pitch. Once the pitch starts it slowly increases if you don't bring it back to level. I don't think my plane is indicative of most Sonex's.

[radfordc]

Here is the Sonex response to the Kitplanes review.
http://www.sonexaircraft.com/press/lett ... ditor.html