Hyperstalling?

[mharms]

Over in the slope forum, they talk about something they call "hyperstalling" that can affect plank designs.

They describe it as a fairly rapid pitch oscillation or series of rapid mini-stalls that occur at low speeds. The consensus there, is that too large of elevon throw for a given CG position can induce hyperstalling. The remedy is often to reduce elevon throw and, possibly, to move the CG rearward (so less throw is required). Apparently, planks lose a lot of speed when hyperstalling and, with proper trimming, it can be induced as a kind of air braking. This is not the same as a slow phugoid oscillation.

Could someone explain what is going on, aerodynamically, when hyperstalling occurs?

Thanks,

Mark

[Sparky Paul]

Those mini-stalls are amusing. The plane pitches up a bit, recovers, pitches up a bit, recovers.. until you do something with the controls.
I've induced it easily on sharpish leading edged planks.

[Andrew McGregor]

It's the same thing as phugoid oscillation, except that the amplitude is enough to stall the airframe. Result is a much faster and sharper oscillation (the period gets much shorter because of the nonlinearities in the stall).

This is what happens when the airframe is *too* stable, so any of the usual ways of making an aircraft less stable apply to fix it.

Wind the trim all the way back in some fullsize aircraft and you can induce the same thing, albeit much less dramatically.