AOA and Canard Stall testing

[lelievre12]

A while back we discussed the use of newer 'performance' envelope limiting autopilots now available. Amongst other things one can limit the allowable AOA before the autopilot will nudge the stick forward. For example to keep you away from a deep stall if you are at aft CG.

However I was just reading the CG testing of Chris Esselstyn's Cozy 540 RG where he reports;

CG of 100.24 – Power off stall, 53 kts, and 16 degrees fuselage attitude

CG of 100.57 – Power off stall, 50 kts, and 13 degrees fuselage attitude

The above data seems counter intuitive to me where an aft CG sees the Cozy stall at a lower fuselage angle/AOA than a forward CG would.  Can someone help me get my head around what is happening here? I would have thought the aft CG would stall at a higher AOA as the canard had more authority.

 

Edited July 2, 2021 by lelievre12

[Kent Ashton]

It does not make any sense to me either but  it could just be a difference in how he flew the airplane, how he measured the deck angle, whether there was a difference in fuel weight between the tests, etc.   .33" of CG change does not sound like enough to really tell much difference and seems susceptible to measurement errors.    OTOH, loading a 180# person in the passenger seat is a big change.

My own spreadsheet shows that moving 25# of ballast from just ahead of the rudder pedals to the pax seat would move the CG aft .84".  Removing that 25# of ballast and putting a 180# pax in the seat would bring the CG forward 3.22".  (Standard Cozy IV - no guarantee on the nums)

When I did my tests, I started with the CG ballasted in the "first flight" box and removed about 5# at a time--shifting the CG aft--until I did not like the canard stall indications.  I do not recall how I adjusted for the fuel weight though.
 

[lelievre12]

I guess there may be a variable between fuselage attitude (relative to horizon) and AOA unless the plane is carefully flown straight and level? 

eg. if at stall the plane starts losing altitude then the descent glide angle will need to be added to the fuselage attitude to arrive at the AOA. Perhaps the second Cozy 540 tests were actually not straight and level. 

If that is the case then I guess the Garmin AOA detection will still  work for stall alerting as it detects actual air AOA and ignores attitude.

 

BTW, Chris reports 50KIAS stall speed with rear CG. Will a Cozy really fly that slow? I guess if it is light enough, yes.

 

Edited July 2, 2021 by lelievre12

[Kent Ashton]

1 hour ago, lelievre12 said:

If that is the case then I guess the Garmin AOA detection will still  work for stall alerting as it detects actual air AOA and ignores attitude.

BTW, Chris reports 50KIAS stall speed with rear CG. Will a Cozy really fly that slow? I guess if it is light enough, yes.

My airplane, which may or may not be typical, can hold level flight at about 55KIAS (uncalibrated ASI) at the ballast I usually fly with.  I bet I could get down to 50 if I removed the ballast.  It gives me a calculated GG of 103.55" (Nat shows 102" as an aft limit) but that's dicey.  I have flown it without ballast but I am careful not to get down to those speeds.

I don't put much stock in AOA indicators.  Better to just do some tests and decide on a ballast/CG that appears comfortable.  All bets are off if you stall these in a vertical or very nose-high attitude.  As I understand it, that can bypass all the normal canard stall indications and fall-off into wing-stall.

[Marc Zeitlin]

6 hours ago, lelievre12 said:

BTW, Chris reports 50KIAS stall speed with rear CG. Will a Cozy really fly that slow? I guess if it is light enough, yes.

If people quote IAS, rather than CAS, the number means absolutely nothing in relation to another aircraft, for what should be fairly obvious reasons. It's a useful # for THAT plane, since it tells you where IT stalls given the airspeed indicator pointing to <something>, but as Kent says, his uncalibrated 55 KIAS might be exactly the same as my 60 KIAS / 65 KCAS (and probably is, at the same GW and CG). If you don't know the airspeed error, IAS is meaningless.

With Chris's empty weight of over 1300 lb., MAYBE at very light weights he might be stalling in the low 60 KCAS region, but 50 and 53 KCAS aren't going to happen. There are very few, if any Long-EZ's that stall that slowly, much less COZY's.

And as we've probably discussed here a few times, and I discuss in my "Canard Aircraft Aerodynamics" presentation available here, from Columbia, 2019:

http://cozybuilders.org/Oshkosh_Presentations/index.htm

an AOA indicator on a Rutan derivative canard airplane is kind of useless. There are a couple of folks that have installed them, one on a Velocity, in particular, but the owner could not explain to me what it was indicating to him or how he had calibrated it. He liked it a lot, though.

[lelievre12]

Thanks Marc. As ever, super informative.

I missed the Columbia talk but I think the answer to my question is here:

"Can measure MAIN WING AOA to prevent deep stall, but would need to calibrate AOA indicator BY STALLING MAIN WING (that’s not going to happen in any reasonable world and deep stalls can easily be prevented just by keeping CG in the approved range)"

However I don't think you need to stall the main wing to calibrate the AOA.  The calibration if the AOA could be done in the Phase 1 testing like Chris Esselstyn where he started with FWD CG then slowly worked his way back until the aircraft until started to lose stability and stall recovery was less and less prompt. That setpoint would be what is needed.  ie. set the AOA to the comfortable flight limit, no need for the actual full stall. 

Why would you do this? Well I don't entirely agree with your statement "stalls can easily be prevented just by keeping CG in the approved range". My SQ2000 has a questionable "approved range" which cannot be relied upon. Further,  Chris's testing suggested that power on stalls were more likely to provoke a deep stall at an aft CG.  Or put another way, accelerated stalls are harder and more dangerous to explore in Phase 1 testing than simple power off stalls.  Therefore the AOA indicator can help provide additional input in these more difficult  tests pilot so the power off 'comfort' baseline is not crossed. 

I fly with an AOA in my spam can and have learned much in my circuit work on how close or far I am from the stall in accelerated turns and various weights.  The real merit of AOA is knowing what is happening in accelerated dynamic flight, not static straight and level.  For example, on a gusty day, its quite alarming what your wing is doing unless you add some speed on short final. And of course the critical base--->final turn is very interesting to watch on AOA if you are cranking her over tight because you turned too late.  In a canard at very flight condition you will expect to get a nose drop every time, however knowing the main wing AOA in each of these conditions won't hurt in such a beast as an SQ. 

Edited July 3, 2021 by lelievre12

[Marc Zeitlin]

2 hours ago, lelievre12 said:

However I don't think you need to stall the main wing to calibrate the AOA.  The calibration if the AOA could be done in the Phase 1 testing like Chris Esselstyn where he started with FWD CG then slowly worked his way back until the aircraft until started to lose stability and stall recovery was less and less prompt. That setpoint would be what is needed.  ie. set the AOA to the comfortable flight limit, no need for the actual full stall. 

And if you know what your CG is when you reach that point in any stall modality, and you're not comfortable going any further aft in CG, then the AOA isn't calibrated to stall, but just to some arbitrary main wing AOA. Which is all fine and good, but since the main wing AOA at low speeds is going to be very different depending upon where your CG is (fwd or aft) and how heavily you've loaded the canard/elevator, it's not in any way giving you the same information that it does in a conventional aircraft, where it's measuring the AOA of the FORWARD wing, which is the critical one for stall behavior.

2 hours ago, lelievre12 said:

Well I don't entirely agree with your statement "stalls can easily be prevented just by keeping CG in the approved range". My SQ2000 has a questionable "approved range" which cannot be relied upon. Further,  Chris's testing suggested that power on stalls were more likely to provoke a deep stall at an aft CG. Or put another way, accelerated stalls are harder and more dangerous to explore in Phase 1 testing than simple power off stalls.  Therefore the AOA indicator can help provide additional input in these more difficult  tests pilot so the power off 'comfort' baseline is not crossed. 

First, with respect to the meaningless information provided by the SQ2K factory with respect to CG range, you address this directly in your first paragraph, where you stated that you slowly work your way back in CG during stall testing until you either reach the aft limit as defined in the POH or else determine that stall behavior is changing in an uncomfortable manner, and you use THAT as your rear CG limit. So you can/will ignore the POH and determine your own rear CG limit, which then become the aft end of "the approved range" for THAT aircraft.

With respect to Chris's testing, with which I'm not totally familiar, the nose bob is definitely more pronounced under power (particularly full power) and the deck angle is VERY steep, so it's an uncomfortable (at least for me) condition. The higher nose bob certainly creates situations where main wing AOA can increase dynamically to too high a level.

As a correction to terminology, though, power on stalls are NOT "accelerated" stalls. An accelerated stall means a stall that occurs at more than 1G, NOT a stall that occurs under power. Accelerated stalls (which I tested in my plane thoroughly) can be done with power on or off, and just require a constant bank angle - I stall tested at 15 degrees, 30 degrees, 45 degrees and 60 degrees of bank.

The same comments as above apply to accelerated stalls - the main wing AOA will change substantially with CG location, so what is barely OK at rear CG will be more than adequate at fwd CG, and the AOA will display something very different. So my comment about "staying in the approved CG range" still applies - if you're forward of YOUR plane's aft CG limit, there's no deep stall danger (all other blah, blah, blah about vortilons, fuel baffles, ballast, etc. still apply).

With respect to power on stalls with larger nose bobs, even if you had an AOA gauge calibrated to some arbitrary AOA of the main wing, a very large bob could put you past that dynamically if you're not being careful, but again, that will have been tested in Phase I in order to set the rear CG limit appropriately (something that the AOA gauge cannot help with, as it's not yet calibrated, and once the testing is done, you no longer need the AOA gauge).

3 hours ago, lelievre12 said:

I fly with an AOA in my spam can and have learned much in my circuit work on how close or far I am from the stall in accelerated turns and various weights.  The real merit of AOA is knowing what is happening in accelerated dynamic flight, not static straight and level.  For example, on a gusty day, its quite alarming what your wing is doing unless you add some speed on short final. And of course the critical base--->final turn is very interesting to watch on AOA if you are cranking her over tight because you turned too late.  In a canard at very flight condition you will expect to get a nose drop every time, however knowing the main wing AOA in each of these conditions won't hurt in such a beast as an SQ. 

I am quite familiar with the usage of AOA gauges in conventional aircraft, and am in full agreement that when it's measuring the AOA of the FORWARD wing, it's an incredibly useful instrument that should be required by 14 CFR part 91.205. And while you are right that the AOA gauge won't HURT in a canard aircraft, it's not giving you any useful information that you don't either already have or do not need, IMO.