Are there wool threads for displaying angle of attack?

Question

Like there is a wool thread on a glider's canopy for showing relative wind on yaw axis. Are there or have there been a wool thread as a long term mean of showing angle of attack?

For instance placed on lateral windows of modern airliners, like this:

(airliners.net)

Even if not very accurate, in case of zero visibility and failing instruments or contradictory readings, this low-tech sensor could help differentiate, for instance a deep stall from an overspeed.

There are two reasons why I ask about this,

1 It does not add significant weight and drag to the aircraft

2 It displays both local airflow direction and operational status, i.e it is noticable if the string vibrates, is cut, is glued to the window or is frozen. Most important is that when it is not obviously out of order, it cannot lie, the only interface between the sensor and the reality being pilot's eyes.

local airflow direction can be experimentally translated to useful AoA graduation, like drawn in the image.

Answer 1

[1]

Yes. Above shows said string on a glider. The yaw string is also used by the high-flying U-2 spy plane (photo on forum.dcs.world), so altitude is not an issue.[1]

The question is can it be seen at night (I assume that's the use case where previous accidents happened)? If you illuminate it from the inside, the window reflection will not help quickly establish the dire situation, and will add to the distraction/workload.

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1: Yaw string. In Wikipedia, The Free Encyclopedia. Retrieved 13:52, March 13, 2022.

Answer 2

An AoA indication displayed on the Primary Flight Display is in the pilot's field of view; what's proposed wouldn't be. At the point that things are bad & you're in either a stall or an overspeed and you don't know which, looking over your shoulder at the yarn isn't going to happen - pilots will be focused on the instruments they spend their days looking at, working to make sense of the situation.

The yaw string is useful, more so than the "ball" in the instrument panel, because it is directly in the pilot's field of view when doing things like thermaling.

The "AoA string" idea is great creativity, but it's not going to get any traction.

Answer 3

It's not that practical because while theoretically it could do the job if calibrated properly and placed where there is laminar flow (so the string doesn't vibrate in turbulence), since it's doing the same thing as the AOA vanes, but problem is, it's not in your line of sight, and mostly it's because you need more than just pure AOA information.

Because of all the mass, stall protection systems on airliners need to take AOA data and apply G load and acceleration/deceleration factors to calculate how much margin there is between an existing angle of attack and the critical angle of attack.

You see this when you are flying at a speed low enough for the low-speed barber pole to come into view on the airspeed tape on your Primary Flight Display. Pull a little pitch and the barber pole will move up a lot more than you would expect just from the small AOA change you made by the pitch control input. This is because the Stall Protection Computer is applying the sensed vertical acceleration (from on-board accelerometers) to present a low speed limit cue to the pilot that accounts for the maneuvering acceleration, or other accelerations like G loads from gusts, as well as AOA.

Answer 4

On an airliner it may be problematic, but I recall seeing one on a glider mounted on an L-shaped object on the side of the cockpit, just below the canopy. The short end of the L was pointing forwards with the thread attached directly behind it. I didn't inquire about its usefulness in detail, but it at least appears to be in use. It wasn't a clear Mad-Max-like item, but it may have been DIY.