Angle of Attack

[mikehallam]

FWIW My 1960's Jodel had the perfect aural device for the usually most dangerous deliberate slow phase of flight, i.e. approach & landing.
A simple leading edge vane & buzzer. You didn't have to look away to hear it begin to sound on the sweet spot for finals. After all it's wing airflow break away we're discussing, so it doesn't really matter at what angle.
mike hallam.

Originally Posted by Capn Bug Smasher

AOA indication in the million-dollar cockpit chez Smasher is my butt which also does sideslip. Bargain!

If my bum tells me my ship is languishing and the controls are labouring at the air (ok, my hands are in on this, too) then AOA is high. If my behind says the ship feels tight and buoyant, AOA is low and it's all good. I sense the aeroplane's motion, weight and response through my magic posterior which is absolutely art rather than science.

The further away you get from flying by the seat of your pants the closer you come to flying by numbers. By the time you reach jet fighters, airliners and tankers this justifies investing in a proper AOA gauge, one that gives you useful, calculated metrics like best range and endurance, rather than vague notions like "low" (aircraft feels firm beneath me), "high" (mushing around) and "dead" (clenched.)

[Shaggy Sheep Driver]

I see we're starting to get the 'fly by your bum' and 'de skilling' arguments this subject usually generates. Soon some bright spark will mention 'stick position'.

Pull too hard in a loop exit in a Yak 52 and be even a tiny bit out of balance and BANG, you flick. No time for any info from your bum, and there isn't any anyway. Not all aeroplanes have gentlemanly pre-stall buffet like a Chippy does!

De-skilling? I think not! If you can pull to nearly the stall angle in steep turns and aeros, you are losing no skill but gaining information, maybe life saving information if you're low down, that you didn't have before.

That Russian device Beags mentions sounds the dog's whatsits to me. Sounds typically Russian; elegantly practical. Pity they didn't fit one to the Yak!

[Pace]

The other point worth considering is the rough and ready 1.3 Xs the stall in a given configuration.
That is drummed into pilots as the approach and landing VREF which is not actually accurate. 1.1 1,2 1.3 1.4 1.5 they are all just numbers as is the principal idea that an aircraft has to land at or near the stall. It can be landed way above the stall if your not worried on published stopping distances

The AOA will give you a much more realistic picture

Pace

[Capot]

I do understand the thought behind "rather have AoA than ASI". But in my dotage I tend to drift off a bit......

.............It's a gusty day, maybe 10 Kt to 20 Kt, 30 degree X-wind, and young Lucy, who has been taught to use an AoA indicator from Day 1, and is now flying solo circuits on an aircraft with one, 6 hours after getting her PPL.

Her approach is watched by people who notice that she seems to be over-controlling with the elevator, and that she is having difficulty staying on the centre-line.

At 300 ft or so, the nose drops in a stall from which she does not recover. RIP Lucy.

Now, I wonder what caused that? Would she have been better off watching her airspeed like a hawk, and keeping it a few knots high because of the gusts? Did the AoA indicator kill her? Perhaps she should not have been flying on that day, but she had a PPL and a mind of her own. That's not the point, which is that the AoA indicator is obviously useful, but could be a very false friend if you don't really know how to fly.

Perhaps I'm just over-cynical about magic bullets.

[Capn Bug Smasher]

Pull too hard in a loop exit in a Yak 52 and be even a tiny bit out of balance and BANG, you flick.

I see your point, true, the butt isn't a precision AOA instrument or even a stall indicator. Listen to your butt tell you your AOA is high, or low, but don't ask it for exact science.

Wing loading decides how much precision you can achieve flying with your bum. A Cub bobbing along like a cork is much better than the big bad Yak slicing through the air.

No time for any info from your bum, and there isn't any anyway.

Yeah there is! You're in a loop, you're under G. Your squashed butt says your AOA is high, right there. About unexpected flicks: see butt sensitivity and aircraft design, above.

[wanabee777]

Leighton Collins on angle of attack

Here's an excellent treatise on the subject of AOA:

FROM THE ARCHIVES: LEIGHTON COLLINS ON ANGLE OF ATTACK, 1965 | Article - Tue 28 Apr 2015 07:02:24 PM UTC | airsoc.com.

[Shaggy Sheep Driver]

Your squashed butt says your AOA is high, right there. About unexpected flicks: see butt sensitivity and aircraft design, above.

I don't understand this. Are you saying every time 'G' squashes your bum you should ease off because the AoA is high? I don't see how you'd ever complete a loop if you did that!

The trick is to pull as hard as you can but not so hard you exceed that stall angle. To do that, you need to know when you're approaching the stall angle. To do that you need an aeroplane that buffets before it departs (the Chippy) or an AoA indicator.

[Capot]

Thanks wannabee for an interesting and illuminating article. I think his final thought is on the button;

While we do not feel that an angle of attack indicator is a cure-all, and certainly here, as always, the proponents of a new instrument seem to reach too far and demand too much of it, we’d still like to have one.

Why? Here's the last para of the add-on re flying a twin on one engine (BTW the pic is an Aztec, I think, which I did my twin rating on; I recall that achieving level, and straight, flight on one was a minor triumph at any altitude.)

Although an angle of attack indicator may not cure all of everybody’s problems, it can certainly take care of some of everybody’s problems, and tough ones at that. Besides it is comforting to know what every bird knows, i.e. the exact angle at which the wing is set to the relative wind, no matter where it’s coming from. That’s what counts.

I would prefer to say "...angle at which the wing meets the air..." because the wing goes through the air (perhaps motionless up to that point) and because I have a tiny mind. I agree 100% with what he says there.

PS; discussions like this always remind me of Alan Bramson's famous description of aircraft that could be flown with outrageous AoA; "Thrust-supported Contraptions". (NB I do mean AoA, as opposed to climb rate and angle.)

[Pace]

Now, I wonder what caused that? Would she have been better off watching her airspeed like a hawk, and keeping it a few knots high because of the gusts?

Cabot that is the very point I am making regarding 1.3X the stall in a given configuration there are times when 1.3X stall is dangerous on approach and you may have to look at a much higher figure and sometimes a much much higher figure


Pace

[9 lives]

I wouldn't recommend one of the current GA 'AoA' devices until someone has found a way of directly detecting actual difference between chord line and relative airflow and displaying it on a simple indicator.

BEagle, is this based upon your personal experience with a modern GA AoA system such as the Alpha Systems brand?

Having had experience with "real" (vane type) AoA systems, and earlier pneumatic AoA systems, I was startled to see how well the modern system worked.

That said, after hours of testing, and confirming good function, I was not sold that I needed it - but it worked!

Pull too hard in a loop exit in a Yak 52 and be even a tiny bit out of balance and BANG, you flick. No time for any info from your bum, and there isn't any anyway. Not all aeroplanes have gentlemanly pre-stall buffet like a Chippy does!

And not all aeroplanes are type certified (for stall warning characteristics) as a Chipmunk is. It seems there is a reason for that! A type certified GA aircraft will give the pilot tactile stall warning in all situations.

[Camargue]

thing is shaggy, if you are doing aeros, last place you are looking is inside at the cockpit at gauges!!

I do think a properly trained pilot can do with out an aoa indicator and anyone doing aerobatics knows you can stall at any speed - 9g in a extra equates to about 170kts (give or take)

the extra buffets before stalling - just the more G you apply the shorter the period between buffet and departure. To practice finesse I sometimes split s and hold the plane on the buffet at a shade under 5g

but having said that anything that reliably aids safety cant be a bad thing

[wanabee777]

As I recall, the McDonnell F-4 Phantom had a variable pitch aural AOA indicator (through the headset) which allowed the pilot to give his full attention to his outside combat environment while keeping the plane's performance as close to the "burble" as possible without stalling.

I don't know the G limits, within which, their typical air combat maneuvers were conducted.

PPruner's???

[Capn Bug Smasher]

I don't understand this. Are you saying every time 'G' squashes your bum you should ease off because the AoA is high? I don't see how you'd ever complete a loop if you did that!

No - sorry - I didn't mean to imply you should ease off - or do anything at all! Except, maybe, to proceed with caution. All I meant was when you've got G, you've got high AOA. That's all! Nothing more and nothing less. High AOA isn't a bad guy by default, you can fly at high AOA all day long and that's OK so long as you know you're doing it. As you say, how else would you do a loop?

The trick is to pull as hard as you can but not so hard you exceed that stall angle. To do that, you need to know when you're approaching the stall angle. To do that you need an aeroplane that buffets before it departs (the Chippy) or an AoA indicator.

Yes, exactly, glad we're on the same page. My bum only tells me roughly what my AOA is. If I want precision I need a gauge or an aeroplane design that tells me.

[Pace]

I could nevertheless see further development in AOA technology from Hud display to autopilot coupling and auto stall avoidance

Pace

[BEagle]

AoA triggered stick shaker and stick pushers have been around for years!

The VC10 had such a system (due to ARB issues concerning 'super stall'), which was quite positive in action. During air tests of the VC10K, we took it to the 'stick shaker' stage, but the VC10C could be taken to the stick pusher if an additional AoA display was fitted. Intentional stalling (i.e. at the stick pusher) was prohibited in the VC10K variant though.

There was also a 'lift rate' modifier which ensured that the stick shaker would be triggered at the correct AoA for configuration and would also operate sooner if it detected a rapid change in AoA...

It was quite common to get the odd rattle of stick shaker during flapless or slatless approaches. One reason being that, in one VC10K2, the lift rate modifier wiring was found never to have been connected and was tucked behind the soundproofing....

But on another occasion, I was flying a nice steady flapless approach at exactly the correct speed when suddenly the klazons went off and the stick pusher operated ! It was certainly possible to override the system, so having done so we went around, dumped the stall protection system (it was common to do so when refuelling off another tanker, due to disturbed airflow over the AoA probes) and landed off a normal approach.

The reason for the spurious operation was that the aircraft had been sitting in the grubby, dusty environment of the operational theatre and hadn't had a wash for ages. So the AoA probes had probably been binding until a gust on my approach dislodged one, which then went from the cruise value to the actual value, fooling the stall protection system into premature operation by spurious detection of a rapid AoA change....

So no thank you - please do not consider an autopilot with an 'auto stall avoidance' device. Airbus includes excellent envelope protection features in the autoflight system, but if the pilots don't understand them or take the totally wrong action for unreliable airspeed indication, all the clever robots in the world won't guarantee protection.

[Croqueteer]

All a/c have an angle of attack indicator. It's called the stick/column and when its aft of a point which you can paint on the fus, the wing will be stalled. Don't bother quoting different flap positions etc because it always works out more or less to the same spot. For example if you are tightening a final turn, be very aware of stick position!

[9 lives]

Don't bother quoting different flap positions etc because it always works out more or less to the same spot.

Not in single Cessnas, the flap setting will affect the neutral position of the pitch control considerably.

[Shaggy Sheep Driver]

All a/c have an angle of attack indicator. It's called the stick/column and when its aft of a point which you can paint on the fus, the wing will be stalled. Don't bother quoting different flap positions etc because it always works out more or less to the same spot. For example if you are tightening a final turn, be very aware of stick position!

Ha! Didn't take long! As I said in post #22:

Soon some bright spark will mention 'stick position'.

Camarque: No, of course one isn't fixated on any instrument in aeros, but one would take occasional glances as one does with the other instruments to confirm that at this speed and this 'G' the AoA isn't too near the top of the yellow arc. In a sustained pull (max performance 360 for instance) one can pull to the limit!

[Pittsextra]

to confirm that at this speed and this 'G' the AoA isn't too near the top of the yellow arc. In a sustained pull (max performance 360 for instance) one can pull to the limit!

Errr do you want to word this better?? "pull to the limit" of what limit of the movement range of the stick?? or something else?

[Shaggy Sheep Driver]

Errr do you want to word this better?? "pull to the limit" of what limit of the movement range of the stick?? or something else?

Oh... probably something else, Pittsextra.