COZY: Approaching Deep Stall


Kevin R. Walsh
 

Andrew-

I think you've gotten some good responses. The box of CG and Gross Takeoff Weight is a bit of work, and the stall behavior will change pretty significantly. The CG position makes much more difference than the takeoff weight, but because the canard loading changes from both together, forward CG, or mid CG at heavy load leads to a canard stall that is a mush. Aft CG and light loading creates a very sharp canard stall bob cycle up and down. For me the most exciting was the max gross at aft CG, as it is a pretty significant pitch buck. IIRC my full power, max gross, Aft CG test had a very high deck angle and a pretty quick nose drop that got my attention. But in every case the aircraft did exactly what it should have.

Also realize that you have a test to do with trimming to a fixed airspeed, then deviating by 5-10 KIAS and watching the time required to return to trimmed airspeed. If you have an EFIS that is recording all of these things for you, it is possible to download that data and plot the resultant decay of the sinusoid. As you move the CG and max gross around this is a good indicator of how far you are from the neutral point. If you do some fancy math you can actually use those decay rates to identify the neutral point, but I think that is beyond the scope of most of this testing.

The very valuable point is that if during any of this testing you find that the airspeed (and therefore pitch) oscillations are not being damped, or much worse, become divergent, then you know to knock it off, head back to terra firma, and recalculate your CG. In each configuration doing that stick free stability test should precede the stall, because it gives a good indication of how much margin you still have. Because in no case inside the box of a properly built and weighed Cozy should that be the case.

Kevin


On Wed, Aug 24, 2022 at 1:17 PM 'Andrew Anunson' via COZY Builders Mailing List <cozy_builders@...> wrote:
I have aft CG testing coming up soon.  What are signs that I'm approaching a deep stall?

Thanks,
Andrew Anunson
Cozy MKVI #1273
Pound, VA

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Kevin


Marc J. Zeitlin
 

Kevin R. Walsh wrote:

<Some extremely useful stuff>

... In each configuration doing that stick free stability test should precede the stall, because it gives a good indication of how much margin you still have. Because in no case inside the box of a properly built and weighed Cozy should that be the case.

While it is the case that pitch stability goes down as the CG moves aft, and deep stall susceptibility goes up as the CG moves aft, in a COZY you will reach the deep stall point long before you reach the stick free neutral point. So having some amount of speed stability, hands on (stick fixed) or off (stick free), is not necessarily an indication that you're safe wrt deep stall.

In Long-EZs and COZYs, the neutral point seems to be around FS-105. In Long-EZs, the deep stall susceptibility point is somewhere around the same fuselage station, however in COZYs, with the wider fuselage forward of the CG, the deep stall susceptibility point is somewhere in the FS-103 region with the shorter canard, and somewhere around FS-102 (the rear end of the original approved CG range) with the longer canard.

This is why, if folks leave the canard long to achieve a higher front seat loading capability, I STRONGLY recommend moving the CG RANGE forward by 0.5" - 1", depending upon what was tested in Phase I.

In any case, I'd use the pitch buck behavior as an indicator of how close to tickling the dragon you are, rather than assume that if the airplane is pitch stable, that's showing margin against deep stall, because it isn't.

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Marc J. Zeitlin                      marc_zeitlin@...
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