Stall AOA

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danw
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Stall AOA

Post by danw »

Hi Guys,

I'm having difficulty reproducing stall characteristics based upon Angle of attack (not based on airspeed).

I learnt in aerobatic training, that the angle of attack is always based upon the stick position. So with the stick all the way back, this will always put it past the critical angle of attack, regardless of airspeed.

However flying quite a few planes with Accu-feel, and the Stall AoA quite low (10.5), I cannot produce a stall by pulling the stick all the way back. My FSX sensitivity is at max as well with no null zones, so it should give the highest AoA possible in FSX by pulling all the way.

The only way I can produce a stall, is by letting the airspeed drop really low. In reality the only effect of low airspeed is to cause the pilots reaction to pull the stick back, and it's the pulling of the stick that should cause the stall not the airspeed itself.

If I'm wrong and it's just some aircraft that aren't working, can you please point me to which non-accusim aircraft I can try that have correct stall with accu-feel?

Is this something that's being improved for the upcoming v2 of Accu-feel? I also feel the stall behavior is off, we should be able to sit at the critical angle of attack and stall/unstall repeatedly by moving the stick a tiny amount forward or back. But instead the stalls are always quite violent losing far too much altitude than reality.
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ft
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Re: Stall AOA

Post by ft »

Have you checked the actual control surface deflections?

The best way to simulate high-performance aircraft with non-powered controls with spring-centered controllers IMNSHO is to have the joystick deflection represent the control force applied by the pilot. It's the closest we can get to an actual q-feel system, which are required for very good reason out in the real world.

When we fly in real life, we fly based on forces applied. We often think we fly on control deflections, but the stick/yoke neutral point can vary a lot and we won't be aware. Change the control forces, and we notice immediately.

This is reflected in the certification requirements/standards, as they're all about control forces.

If this is what A2A have done, full joystick deflection simply means a preset maximum amount of force applied to the simulated stick. At high speed, the resulting elevator deflection isn't enough to reach critical (stall) Aoa.

I don't think you'd have the arm power to stall a Mustang or Spitfire at high speed. I've read accounts of pilots really straining to pull out of high-speed dives.

As for stall characteristics, while some aircraft will fly fully stalled (rather fun, innit?), the Mustang wasn't one of them. From my reading (while waiting for Dudley to chime in with actual knowledge) you'll get a brief buffet and then a wing drop reminiscent of a snap roll. A benign aircraft which will really bite if mishandled. Very appropriately named, I think!

Nice to have your aero knowledge on board, Dan! Looking forward to more threads with titles like this one. :)

Cheers,
Fred
Be warned: Aero engineer, real life pilot, sim programmer. Nothing good can come out of that.

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DHenriques_
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Re: Stall AOA

Post by DHenriques_ »

danw wrote:Hi Guys,

I'm having difficulty reproducing stall characteristics based upon Angle of attack (not based on airspeed).

I learnt in aerobatic training, that the angle of attack is always based upon the stick position. So with the stick all the way back, this will always put it past the critical angle of attack, regardless of airspeed.

However flying quite a few planes with Accu-feel, and the Stall AoA quite low (10.5), I cannot produce a stall by pulling the stick all the way back. My FSX sensitivity is at max as well with no null zones, so it should give the highest AoA possible in FSX by pulling all the way.

The only way I can produce a stall, is by letting the airspeed drop really low. In reality the only effect of low airspeed is to cause the pilots reaction to pull the stick back, and it's the pulling of the stick that should cause the stall not the airspeed itself.

If I'm wrong and it's just some aircraft that aren't working, can you please point me to which non-accusim aircraft I can try that have correct stall with accu-feel?

Is this something that's being improved for the upcoming v2 of Accu-feel? I also feel the stall behavior is off, we should be able to sit at the critical angle of attack and stall/unstall repeatedly by moving the stick a tiny amount forward or back. But instead the stalls are always quite violent losing far too much altitude than reality.

Interesting topic! :-))

There are a lot of things involved here, both real world and with the simulator. All are relevant.

First the real world;

I taught aerobatics all through my career as an instructor. AOA and it's direct relationship to stall as opposed to airspeed has been something that was a long time coming in aviation. Early on, instructors incorrectly taught many many students to associate stall with airspeed due to published stall speeds. This of course was the easy approach since it referenced airspeed indicators and pilots handbooks.
The problem with all this was that we were teaching pilots all about 1g stall at a specific gross weight and very little about stall above 1g at any airspeed and any flight attitude. Trust me, it was a mess. It took years to correct this thinking and I was right in the middle of it screaming my head off to the powers that be. In the United States, our GA manufactures were pushing the "safety" of flight and how "easy" and "pleasant" it was to
fly THEIR airplanes. This thinking reflected itself in how the FAA published flight standards testing formats, so many CFI's got in the totally incorrect (and in my opinion, unsafe) habit of teaching students to "watch that airspeed".
Anyway, today we have a much better understanding of stall and its direct relationship to angle of attack. Today, the good CFI treats 1g stall for what it always should have been, an INTRODUCTION to stall. From that entry point the GOOD instructor takes the student into the realm of stall where it occurs most often and most dangerously.......that area being stall above 1g or accelerated stall.

I have to admit I've never been a huge fan of associating stall with stick deflection although it is associated. I much rather prefer to teach cue association, physical, visual, and audible. Stall linked this way in my opinion makes a pilot much more stall oriented through the 3 dimensional maneuvering arena associated with aerobatics...........and in fact with normal flying as well. I want pilots to "feel" the aircraft at all times. Doing this and flying this way not only makes the pilot aware of exactly where the stall angle of attack is, but as well how CLOSE to that stall AOA that pilot is at any given instant right side up, inverted, and at any airspeed.

For the simulator;

Getting things right with a flight model has to be one daunting process. Fortunately at A2A we have some REAL good people working these issues. I don't have the programming expertise to comment deeply on what's involved with the 1's and 0's, but I can say that all the factors mentioned are relevant. Stick deflection, elevator, stabilator, etc. deflection in degrees vs the aoa curve are all relevant. To get it all right, GW and CLmax have to enter into the stall equation as well as g. Quite frankly I have no idea HOW they work their magic and get it right. They send me the airplane and I fly it and send back how it looks and feels to me on a realism basis. They do a tremendous job.

Dudley Henriques
:-)

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danw
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Re: Stall AOA

Post by danw »

ft wrote:Have you checked the actual control surface deflections?

The best way to simulate high-performance aircraft with non-powered controls with spring-centered controllers IMNSHO is to have the joystick deflection represent the control force applied by the pilot. It's the closest we can get to an actual q-feel system, which are required for very good reason out in the real world.
That's a good point, I don't know how FSX does it but it could be mapping maximum stick deflection not to the maximum elevator deflection but to a normal movement range at that airspeed.
ft wrote:
If this is what A2A have done, full joystick deflection simply means a preset maximum amount of force applied to the simulated stick. At high speed, the resulting elevator deflection isn't enough to reach critical (stall) Aoa.
So have A2A done that for only Accu-sim planes, or for Accu-feel stall effect as well?
ft wrote: I don't think you'd have the arm power to stall a Mustang or Spitfire at high speed. I've read accounts of pilots really straining to pull out of high-speed dives.
Not a Mustang, because high speed is something like 400mph :)
But maybe an aerobatic trainer like a Robin, it's possible to pull the stick all the way back and possible to overstress the aeroplane. Trying to fly the RV-7 and various other planes with Accufeel I'm trying to get a realistic thing happening in FSX but it's not satisfying me. A2A Mustang though - no problem!

This is not a topic about the Mustang at all though, but about Accufeel and other aircraft. I like Accufeel for the sound effects, but the flight model enhancement part of it I'm not getting it to work right yet.
ft wrote: Nice to have your aero knowledge on board, Dan! Looking forward to more threads with titles like this one. :)
I don't have enough knowledge unfortunately, being a very early student of aerobatics it's all basics at the moment. But that's why I'm here to see if I can improve my knowledge by using FSX as a training tool.

Do you guys use Accufeel?
Which aircraft do you use it with?

I get the feeling from your posts that you're referring to the work A2A does on Accu-sim aircraft produced by themselves, understandable confusion given the similar titles.
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danw
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Re: Stall AOA

Post by danw »

Dudley Henriques wrote:
AOA and it's direct relationship to stall as opposed to airspeed has been something that was a long time coming in aviation. Early on, instructors incorrectly taught many many students to associate stall with airspeed due to published stall speeds. This of course was the easy approach since it referenced airspeed indicators and pilots handbooks.
It still happens :) Some instructors go over the AoA in the brief, and the wing etc. But in the air they are only watching the airspeed and exclaiming "watch your airspeed or you'll stall!" Not aerobatic instructors but regular guys that is..

Dudley Henriques wrote: I have to admit I've never been a huge fan of associating stall with stick deflection although it is associated. I much rather prefer to teach cue association, physical, visual, and audible. Stall linked this way in my opinion makes a pilot much more stall oriented through the 3 dimensional maneuvering arena associated with aerobatics...........and in fact with normal flying as well. I want pilots to "feel" the aircraft at all times. Doing this and flying this way not only makes the pilot aware of exactly where the stall angle of attack is, but as well how CLOSE to that stall AOA that pilot is at any given instant right side up, inverted, and at any airspeed.
This is good to learn and good skills to have, to fly by feel and by practice. Although in level flight stick deflection should still matter, and I'm not getting it to work in Accu-feel how I'd like. Maybe I just have the wrong planes and conditions.. but I'd like to see others experience in Accu-feel to learn from.
Dudley Henriques wrote: For the simulator;
They send me the airplane and I fly it and send back how it looks and feels to me on a realism basis. They do a tremendous job.
For their A2A airplanes I have no problem, the Piper Cub, and the Mustang are like a dream come to FSX after all the toys planes we've been flying before that. Maybe I'm greedy, I want Accu-sim on other planes too hence my tweaking of Accufeel to try and get it right
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danw
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Re: Stall AOA

Post by danw »

A quick update, I've had some success!

I think it's more a problem with the individual aircraft flight models than Accu-feel. Plus I was misunderstanding the power of Accu-feel - it doesn't change the flight or stall modelling it just gives some effects.
Even if we set accu-feel to be 10.5 degrees stall AoA, it doesn't stall at that angle if the actual aircraft flight model has a higher stall AoA.

So I tested on the Lotus L-39, which has a lower required angle of attack for stall - and everything works fine. Aircraft with a higher angle of attack stall though, like the MS default aircraft, are not able to hit stall angle of attack with a joystick in normal flight, only at other attitudes or slow speed induced stalls.

So am I right in thinking the Stall AOA setting is not to alter the stall or flight model - but just to provide the buffet effect?
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CAPFlyer
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Re: Stall AOA

Post by CAPFlyer »

Well remember that with a Cessna, you can fly the plane at full aft yoke because its critical AoA is quite high (14* if I remember correctly off hand) and unless you are at or below stall speed or over Vmc, full control deflections will not always produce a stall. I was run through several exercises when doing my Mountain Flight Training in Colorado where my instructor had me fly the plane and make maneuvers including climbs descents, and turns while sitting on the edge of the buffet with full aft stick. I changed altitude on power alone. It gave me great insight on how to handle a plane down on power or in a tight situation where I need as little horizontal speed as possible to give space for maneuvering.

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danw
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Stall AOA

Post by danw »

I think even in that case the stall is still based on angle of attack. It's just that level flight the relative airflow is from almost level in front.

But at stall speed with reduced lift, the relative airflow starts to shift coming from below the plane as weight wins over lift.

This allows the higher angle of attack to occur at the same yoke position, which adds to what Dudley was saying its not stick position alone..

So I think I agree, really it depends on the airplane doesn't it, and the stick position teaching is an oversimplification.

Although if I hold the stick at the critical angle and power off, it doesn't cause a long stall because the aircraft will sort itself out at that angle once it re orients to the new relative airflow.

Im starting to ramble now.. time to go for another FSX flight ;)
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ft
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Re: Stall AOA

Post by ft »

Dudley,
regarding relating stall to stick deflection, I not only agree that it's a bad idea. I'll go one step further and say it is both impossible and useless. As I said, we fly based on control forces but generally have no idea about where that stick force puts the actual controls, let alone control surfaces. Cues is really the only way.

Dan,
you're right that I was taking the A2A P-51 perspective rather than Accu-Feel. I failed to notice the sub-forum... and the slight cue that you wrote "accu-feel". Ooops! I hope I contributed anyway. I would indeed expect to be physically able to use full elevator deflection in most GA aircraft throughout most of the speed range, definitely well above Va... above which it isn't really interesting anyway. :)

Stall is always based on AoA. There are a few factors which does change the critical AoA, but for our purposes it can probably safely be assumed to be a fixed alpha. If the lift reduces and the airplane descends, longitudinal stability dictates that the nose will pitch down rather than the AoA increase.

CAP,
the elevator position is, by and large, an AoA selector. Elevator position and CoM determines the overall lift coefficient. Unless you have a severely forward CoM (or an interestingly designed aircraft, or both), full up elevator should always take you above the critical AoA and into the stall. 1G stall speed really has nothing to do with it, nor does Va (which is what I assume you intended to type rather than Vmc, and although Va is rather interesting in this scenario from another perspective).
Be warned: Aero engineer, real life pilot, sim programmer. Nothing good can come out of that.

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