AoA vs ASI

[QDMQDMQDM]

I'm bored.

"Angle of Attack may be simply and cheaply measured in all light aircraft. The lack of AoA indicators in modern aircraft is a sad accident of history and a significant detractor from flight safety. An AoA indicator should be used in addition to the ASI. Discuss."

[englishal]

Quite agree.....:)

[Miserlou]

Sorry but I fully agree, too.
Well not quite fully actually.
The AoA gives an exact reading of the condition of airflow over the wing in any and all conditions.
Once you have an accurate AoA indicator there is no need for an ASI.
So the AoA should be installed 'instead of' the ASI.

[eharding]

I'm bored.
"Angle of Attack may be simply and cheaply measured in all light aircraft. The lack of AoA indicators in modern aircraft is a sad accident of history and a significant detractor from flight safety. An AoA indicator should be used in addition to the ASI. Discuss."

Indeed - you could have a cunning system whereby a flap of metal on the leading edge completes an electric circuit at a specific angle of attack, thereby releasing the door to a small cage in the cockpit containing a rabid ferret trained to bite the handling pilot in the ankle, and hence advise the pilot of an incipient stall.

We could make millions with this. I'm fairly sure about the ferret aspects, not so sure about the rest of the mechanism. We might have to negotiate on the Rabies feature, and maybe we could have the ferret pull the ballistic recovery handle on more modern types?

[IanSeager]

Angle of Attack may be simply and cheaply measured in all light aircraft

The word cheaply is the main problem.

Ian

[Mark 1]

The cheaper sensors use two pitot heads mounted at different angles; AoA is calibrated from the difference in total pressure of the two heads. It seems to work quite well.

There was a lot of good debate on tthe subject on this thread recently:
http://www.pprune.org/forums/showthread.php?t=240618

[QDMQDMQDM]

The word cheaply is the main problem.

There's one flying on a Super Cub which uses the following material, not pricey:

82" length of cold water type 3/4" PVC pipe and two Adell type clamps for attaching to aircraft

-gas welding rod to mount yarn on

-yarn

-length of 1/4" aluminum tube for color range markings

[IanSeager]

There's one flying on a Super Cub which uses the following material, not pricey:

82" length of cold water type 3/4" PVC pipe and two Adell type clamps for attaching to aircraft

-gas welding rod to mount yarn on

-yarn

-length of 1/4" aluminum tube for color range markings

By the time someone has designed and built one, tested it, got STCs for all sorts of GA aeroplanes and marketed it, I'm still betting that it would be expensive.

Ian

[n5296s]

Err... what for exactly? Inexperienced pilots are likely overwhelmed by everything they have to watch (I certainly was, especially during my IR training or when I started to train in the Pitts). And experienced pilots know how to fly the plane without one. Precisely the people who could in theory benefit from one (e.g. the guy who took his Bo into an apartment building in LA a couple of years back) almost certainly wouldn't be watching it or know how to interpret it anyway. If the stall warning (which is after all an AoA indicator, albeit somewhat binary) doesn't get your attention when you're in the conditions for an accelerated stall, you're almost certainly beyond hope.

Really the best thing to teach people is never to get into the situation where an AoA indicator would help them in the first place, i.e. slow flight. If I define "slow" as Vs1*1.4 (~75 KIAS in my 182), then even a 60 degree bank won't hurt you. You've no business to be flying that slow anyway except on final or on traffic duty. Anything slower is on the back of the power curve which is definitely a bad place to be.

Of course if people were taught spin recovery then it wouldn't matter anyway. But maybe that's a separate thread? You should be able to recover from a one-turn spin (i.e. enter, spin, recover) in 500' in any small plane. And, yes, I know, one-turn is really only an incipient spin taken a bit far, *real* spins start after three turns, but if you get that far then you're either very shortly to be dead, or you're doing it on purpose.

n5296s

[sir.pratt]

so what does it mean when my aoa indicator (wool tail) is facing the wrong way?

[Final 3 Greens]

So the AoA should be installed 'instead of' the ASI

How do you comply with Vne, Vfe or Va with only an AoA indicator?

[n5296s]

so what does it mean when my aoa indicator (wool tail) is facing the wrong way?

You're going backwards? Very handy for knowing when to initiate a stall turn...

[Miserlou]

The AoA meter will always be accurate as it measures the only parameter which the wing cares about, the angle of attack. The speed is irrelevant an the ASI only 100% accurate at one set of loading and atmospheric conditions.
If presented in analogue form you would still mark the AoA with Vs, Vx, Vy, Vfe, Vno and read it in exactly the same way. You could also add a scale which would give an approximate speed deviation.
Of course there would be circumstances where the system would not be relevant, for example, aerobatic types where high speed, zero wing loading would confuse the instrument.

One should bear in mind that best climb and best angle speeds vary not only with weight but with altitude.

[sir.pratt]

You're going backwards? Very handy for knowing when to initiate a stall turn...

bit late for a stall turn i think......

[englishal]

I have AOA indicators......They are called Leading Edge Slats. When the AOA increases they pop out ;)

[Final 3 Greens]

If presented in analogue form you would still mark the AoA with Vs, Vx, Vy, Vfe, Vno and read it in exactly the same way.

Congratlations, you have just created an ASI. I rest my case.

[Miserlou]

Yes, an ASI which is accurate regardless of aircraft weight, density altitude, load factor or position error.

As stated in the opening post, it is 'an accident of history' which saw airspeed become the measurement of a wings performance.

I wonder how many lives have been lost due to this misrepresentation.

[QDMQDMQDM]

If presented in analogue form you would still mark the AoA with Vs, Vx, Vy, Vfe, Vno and read it in exactly the same way.

Congratlations, you have just created an ASI. I rest my case.

F3G,

This is so missing the precise point of the whole difference between an AoA indciator and an ASI that I am surprised!

David

[OpenCirrus619]

If presented in analogue form you would still mark the AoA with Vs, Vx, Vy, Vfe, Vno and read it in exactly the same way.

Congratlations, you have just created an ASI. I rest my case.

You would, of course, need various scales calibrated for different levels of G.
Guess you'd need a G-meter as well - so you know which scale to use.
Possibly a magnifying glass would come in handy - approaching 0G the scale is going to get VERY small.

Just my thoughts (which are probably not worth the proverbial penny).

OC619

[Final 3 Greens]

QDM

To be honest, I wasn't being entirely serious.

[apruneuk]

I think that is commonly accepted that most stall/spin/crash events happen when the aircraft is near to the ground, slow and turning and when the pilot has been distracted. The commonly practiced scenario both in PPL, CPL and FI exams is the turn onto Final with 2 stages of flap on a PA28 or Cessna.
How would an AofA indicator allow for the fact that stalling AofA is less with flap than clean?

[QDMQDMQDM]

How would an AofA indicator allow for the fact that stalling AofA is less with flap than clean?

It's higher with flap down. That's the point of flaps. Your AoA indicator will tell you the wing's AoA. You can then decide its significance, as you do for airspeed.

If AoA was commonly used on the approach instead of airspeed, every pilot would know that stalling AoA is higher with flap than without, a fairly critical piece of flight information.

I rest my case.

[apruneuk]

QDM

Flaps come in a variety of shapes and sizes but consider the Fowler flaps on a Cessna. They increase wing area and camber, thus increasing coefficient of lift. From the lift formula it can be seen that with an increase in S (wing area) and Cl, the same amount of lift can be generated at a lower V (very useful for approach to land). It also means that more lift will be generated for any given angle of attack than for a clean wing, hence a better view of the runway.
Another side effect of flap deployment is that the wing will stall at a lower AofA than clean due to the increase in induced drag as speed reduces. This is partially countered on transport aircraft with the use of leading edge devices to re-energise the boundary layer over the upper surface of the wing but these are not found on most typical training aircraft.
If you still don't believe me, next time you go flying try some stalls (at a safe height, of course!) with and without flap and note the result.

[QDMQDMQDM]

Hmm,

apruneuk you sound far more knowledgeable than me so I'd better let someone else confirm I am wrong, lest I dig mysefl a (Bigger?) hole.

QDM

[apruneuk]

QDM

No, I'm wrong! The AofA is measured as that angle between the relative airflow and the chord line of the wing. The chord is a straight line joining the leading and trailing edges of the wing and as such will clearly give a higher AofA for a given NOSE ATTITUDE with flap deployed. Although the wing shape is now different it should stall at roughly the same AofA but at a lower nose attitude.
Who's digging a hole now!

[QDMQDMQDM]

Phew! I was scared you were blinding me with science for a moment.

[norilsk]

All I know is when I get to 21 degrees Alpha I push!

[Miserlou]

AoA remains AoA even under different g conditions which is one of the benefits thus Vx and Vy remain the same.

Re. flap settings. An inner scale, or colour codes for the flap fixed settings.

"I can never remember is the cyan scale 20 or 45 degrees?"