Aircraft Control / Hershey Bar Wing, Etc.

[jedi]

Two of our highly qualified, well known, members had a brief discussion and slight disagreement on the “Hanger Flying” pages. The following post mentions “Thread Drift” and I agree having started a thread with that title.

I believe this subject belongs under the “Aerodynamics “ title and I want to be a fly on the wall” and listen to where this conversation goes.

I will copy relevant posts from “What out-of-production aircraft would you buy the rights, if you wanted to start a 'kit-plane' company?“ for reference, not in chronological order.

I guess I don't get what you are saying. A roll command near the stall that induces a spin is undesired yaw that is divergent. It just keeps on spinning until the pilot neutralizes the ailerons, reduce AOA to un-stall the wing and then stomps on the rudder to stop the divergent yaw that the unstable wing leveraged onto the airframe, unless it hits the ground first.

Anyway, this is thread drift, so I'll stop with that comment.

The Hershey Bar wing is yaw unstable. That's good if you want to tumble the plane for aerobatics, but bad if you stall turning crosswind after takeoff.

Most pilots are taught that the constant-chord wing has good stall characteristics because the root tends to stall first and then it progresses outward with the tips last to go. That's true so long as you maintain neutral ailerons. Deflect the ailerons and the drooped aileron will stall early, and to compound the problem it is a high drag wing tip (hence the yaw instability). The result is an unsymmetric stall with a strong yaw that throws the plane into a spin at low altitude. This will most likely be a fatal crash.

Build a constant-chord wing with enough washout and the problem gets noticeably better. A few planes do this, but most do not, especially in the EAB markets. The washout generally needs a jig to build it right, and most builders just want to build their wing on a flat table.

My thought is to build the simple Hershey bar wing, but then eliminate the ailerons. Replace them with a swing-tip that tends toward yaw stability instead of yaw instability, and that improves the span loading in the rising wing instead of reducing it. Ailerons are good for aerobatic flight, but if you are interested in coordinated flight near stall, I think ailerons are a bad solution.

="REVAN, post: 778299, member: 46058"]
Not true. It has deep meaning on the implementation of control laws needed to fly the aircraft. Those control laws (i.e. - pilot training, perception and skills) are the primary issues when it comes to stall-spin avoidance. The swing-tip was meant to improve that situation by changing the system dynamics to improve system stability and control authority in the vacinity of a stall.

University of Washington, Aeronautical and Astronautical Engineering, Class of 1995. Now you have.

Yes, a tapered and untwisted wing will spin harder than an untapered and untwisted wing. That's why people think the Hershey bar wing is a safe wing. Truth is, that without flaps it is still pretty spin prone. Deflected flaps add washout. Build that washout into the wing and it will be more spin resistant when the flaps are up too.

I'm thinking this shows up in the fatality statistics for turning crosswind being more likely for fatal spin than turning base or final. When turning crosswind, the flaps are usually up. The wing is in a more unstable configuration and the plane is more likely to spin. Turning final, the flaps are usually down, the wing less unstable and the plane less likely to spin, even if the wing is partially stalled.

If folks want to keep discussing this, maybe it should be taken to a new thread.

Stability of a wing alone is meaningless in the context of an aircraft. This is true in pitch, as always, but it's even more true in yaw, where stability is absolutely dominated by vertical surfaces. There are very few aircraft that are unstable in yaw, and all the ones I'm aware of look like it -- no vertical tail or other vertical surfaces.

You’re conflating roll/yaw coupling and yaw stability. Even near stall, introduction of a yaw command will not cause yaw divergence; however, introduction of a roll command can cause undesired (but not divergent) yaw. And of course in the stall itself you can have control reversal of roll, but again, generally not divergent (or at least, not with a low time constant).

Others are free to comment.

 

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