Stall Warner

[IO540]

In other words if you suddelnly meet a strong upward airflow, this brings the relative wind flow over the wing to greater than the max AoA and hence the wing stalls?

I think more likely the other way round.

Let's say you are flying along at 100kt, and you meet a 100kt (+10,000fpm approx) vertical airflow. (Let's ignore for now the fact it will rip your wings off). Your effective airspeed has now risen from 100kt to the vector sum of the two i.e. 141kt. I don't think the wing will stall! If it did, then the wing loading (due to this airflow) would be very small and one could fly through CBs, at any speed, without concern.

A stalled wing has a low wing loading, which is where Va (basically) comes from - at Va, the wing will stall (and unload) at the design limit of 3.8g, thus (supposedly) protecting itself from failure.

I think the thing which might cause the stall warner to go off is downward airflow. But I think this is unlikely in reality; air doesn't usually flow vertically that fast. The momentary stall warner activations we all get come from plain simple gusts, or from wind shear i.e. variations in horizontal wind speed.

[apruneuk]

englishal

No, if you yank the stick back on a 172 that is in unaccelerated straight and level flight at 100kt, it will instantly climb! (due to inertia and the increased angle of attack of the wing chord to the relative airflow). Depending upon the angle of climb induced, it may subsequently stall or continue to climb at a lower speed. Sorry to be picky...

[englishal]

No, if you yank the stick back on a 172 that is in unaccelerated straight and level flight at 100kt, it will instantly climb! (due to inertia and the increased angle of attack of the wing chord to the relative airflow). Depending upon the angle of climb induced, it may subsequently stall or continue to climb at a lower speed. Sorry to be picky...

Actually it will stall. I did it for my FAA CPL training, where we explored the envelope and did all sorts of stalls. The FAA requires all sorts of fun stuff to be demonstrated.....

If you are say at 100kts, or Va for your weight for safety sake, and suddenly yank back on the yoke, the wings stall and the stall warner will go off. This is because you increase AoA to beyond critical, no matter what your airspeed, and the wing stalls. A common misconception is that the aeroplane will only stall at slow speed, however one very valuable piece of info I was taught and had demonstrated is:

An aeroplane can stall at any airspeed, any altitude and any attitude

Back to IO's 100kt vertical airflow Wouldn't the vector sum of the two airflows put the relative airflow at 45° (+ or -) to the wing (tan-1 100/100)? Which if so, and the wings remained attached, would put it above the critical AoA of the wing (and hence stall it?)

cheers

[apruneuk]

It seems strange that nobody has mentioned the possibility of the pitot cover being left on or a bockage of the pitot head and drain hole by ice/insect eggs.
That being the case, the ASI would act in the form of an altimeter and over-read in the climb.
Obviously, in visual conditions the increased nose attitiude would ring alarm bells but this could be extremely dangerous in IMC without the usual visual clues. I would be very wary of assuming that a stall warner is incorrectly set just because the ASI reads what is expected; it is also worth remembering that the stall warner is set to sound well before the stall, not at the point of stall.

[apruneuk]

englishal

We are splitting hairs here. Of course what you say can be true - it depends how hard you yank and what you mean by "instantly".

[IO540]

If you are say at 100kts, or Va for your weight for safety sake, and suddenly yank back on the yoke

I hope you are doing this in something rented If I have to demo this for my CPL in the TB I will just refuse. Chandelles and lazy eights are as far as I am going. This is despite knowing that a TB20 is much stronger than a PA28; no known in-flight structural failure ever.

You are right about the 45 deg airflow. I can't get my head around all this right now; you may well be right. I think there is a difference between transient and longer term conditions though. The stick, for example, has no effect on the wing AoA - it's only through the action of the elevator, and relatively much later on through a pitch change, that the wing AoA changes.

[englishal]

I hope you are doing this in something rented

Yea, a 172. It worred me slightly as the instructor said "I hope the structural integrity of this aeroplane is good" before yanking on the yoke and stalling us. It was luckily

it depends how hard you yank and what you mean by "instantly".

Yank ..beep... about that sort of time

[mm_flynn]

Originally Posted by englishal

Back to IO's 100kt vertical airflow Wouldn't the vector sum of the two airflows put the relative airflow at 45° (+ or -) to the wing (tan-1 100/100)? Which if so, and the wings remained attached, would put it above the critical AoA of the wing (and hence stall it?)
cheers

Because the airflow has increased by 41%, the lift at constant AoA will double, but the AOA will move from say 3 to 46, way beyond stall. But interestingly developing say .4 lift Coef vs. the 0.2 at cruise (so lift goes up -Coef Lift is about 20% at Aoa = 3, peaks at say AoA =15 then drops to about 30% at AoA=20 and then goes back up at AoA=40 to about 40% - I had to look that one up and it surprised me). But you are now on an elevator adding power (Wind Vertical Velocity * G * M) so to reduce lift sufficiently to remain level the power output of the wing has to be decreased by a factor of 2.8 (in IO'scase) - which means pitching to an AoA of about 1.

IO's situation in English, the wing would be stalled, stall warner would be going off, but due to the increased airspeed and the fact that the coeficient of lift is high at such an extreme AoA, you are generating bucket loads of lift going up (until the wings fall off)

[Julian]

An aeroplane can stall at any airspeed, any altitude and any attitude

Indeed! Something I had rammed home during my CPL training as well but had also had experience of during my Aeros training.

The following is from this weeks AOPA newsletter and happens to be about ciritical AoA and stalls.

---

CRITICAL ANGLE OF ATTACK

Pilots learn that a stall can occur "at any airspeed, in any attitude, with any power setting," as explained in Chapter 4 of the FAA's Airplane Flying Handbook. How well do you understand this concept?

The typical method of practicing and demonstrating straight-and-level stalls and recoveries is to slow the aircraft down in the takeoff or departure configuration until it reaches the minimum controllable airspeed (MCA) depicted on the airspeed indicator, inducing the stall, and recovering. (See the February 10, 2006, Training Tips article "Pre-solo Stalls.") This is a safe way to demonstrate the effects of exceeding the critical angle of attack. But letting the discussion end there carries the risk of fixing in a student's mind the inaccurate notion that exceeding that angle of attack can be prevented simply by flying above minimum controllable airspeed (MCA). That's not so.

Suppose you are on your final approach glide with flaps down, throttle at idle, and maintaining an indicated airspeed of 65 knots, well above MCA for your airplane. Another aircraft suddenly appears below and in front of you. Your first reaction is to haul back sharply on the yoke to avoid collision. Without decelerating to MCA, the aircraft wing exceeds its critical angle of attack and stalls—unexpectedly and at low altitude. Because this is a so-called accelerated stall, the added lift induces a load on the airframe. To understand maneuvering speed's importance to stall avoidance, see Rod Machado's "A New Look at Maneuvering Speed" in the March 1999 AOPA Flight Training.

A glance at your pilot's operating handbook reminds you that stall speeds increase with bank angle. Practicing a level-flight steep turn, or increasing bank to complete a turn in the pattern, you are surprised to hear stall-warning activation at what seems to be a healthy airspeed. Remember that you are flying a maneuver with a high-load factor, meaning that the aircraft wing is producing horizontally inclined lift to turn the aircraft plus the necessary vertical component of lift to regulate altitude. It is flying closer to its critical angle of attack than in unaccelerated level flight.

Managing angle of attack at any airspeed, attitude, or power setting is the key, as the opening words above remind us.

---

J.

[flyingphil1]

True or False?

We have lost the theme of the original text .. we were talking about the (incorrectly adjusted) stall WARNER. For those who fancy a question on stalling ....is the following statement true or false ? This is from the Jepperson manual for FAA I.R. preparation "The angle of attack at which a wing stalls varies depending on total aircraft weight, bank angle, load factor and airspeed"... answer next week .. now thats clear isn't it? This also answers a previous question by default .. are licence holders arrogant.. I should say so!

[mm_flynn]

Answer - False

[apruneuk]

flyingphil1

The stalling angle of attack is the stalling angle of attack. The speed at which it is reached depends upon the weight/load factor; I would suggest that you have either mis-quoted the manual or that it is wrong/misleading.
I would also be wary of assuming that just because a stall warner has sounded at a higher IAS than you would expect that it is incorrectly set. Pitot-static errors are covered in most training manuals and may go some way to explaining your example.

[flyingphil1]

How have I got the text answer wrong ? .. I havent told you what the answer is yet!!!.. read the text, dear person ....me arrogant ... mais non !!!