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A source of reliable angle-of-attack information is important for airplanes, as flight at an excessively-high angle of attack can cause the airplane to stall, which is generally considered to be an undesirable occurrence. To this end, airplanes generally have several redundant angle-of-attack sensors (usually consisting of vanes that stick out into, and align themselves with, the airflow, but sometimes using complicated pressure-sensing mechanisms instead) to provide warning of a dangerously-high angle of attack.

These sensors, however, are generally mounted on the forward fuselage, below the cockpit, which is not, for most airplanes, where high angles of attack actually cause problems. Where AoA matters is on the wings, which produce the lion’s share of the airplane’s lift and which need to be kept from stalling, and where the direction of the local airflow is not necessarily the same as that experienced by the AoA sensors on the forward fuselage. Exactly how not-same it is depends (as do quite a lot of an airplane’s aerodynamic properties) on the airplane’s airspeed, which is why readings from nose-mounted attack-angle sensors need to be adjusted to correct for the effects of airspeed, which is why (for instance) high-attack-angle protection on fly-by-wire Airbusses is inoperative in the absence of valid airspeed data.

Mounting the angle-of-attack sensors at the airplane’s wing roots, rather than beneath the cockpit, would allow the AoA to be measured directly right where it counts, eliminating the need to correct their readings for airspeed and allowing accurate attack-angle measurement even without any airspeed information at all, so why do most airplanes still have their attack-angle sensors on the forward fuselage instead of the wing roots?

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    $\begingroup$ Are you aware of any situation where the AOA measured at the wing root is different from the AOA measured at the cockpit, or a situation where that difference matters in a beneficial way? $\endgroup$ – GalacticCowboy Oct 29 at 17:57
  • $\begingroup$ @GalacticCowboy Depends how bendy the wings/fuselage are, and therefore whether the AoA measured at the wings could be significantly different from the measurement at the nose. Then again the AoA at the wing root wouldn't change as the wings bend up/down, it would change most out at the tips. $\endgroup$ – Criggie Oct 30 at 9:57
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Would installing the AOA vane (or, in general, sensor) at the wing root be more accurate than installing it near the nose? No, it wouldn't. In fact, it may be slightly worse due to the larger upwash at the wing leading edge and the vane protrusion may even negatively interfere with the wing aerodynamics.

Traditionally, AOA measurements are used for stall warning and, if required, stick pusher. During flight tests, the thresholds for stall warning activation (and stick pusher activation) were calibrated to the local AOA measurements from the vanes to satisfy the certification requirements, while minimizing $V_S$/$V_{SR}$ to maximize performance.

In FBW aircraft, if the control laws have AOA dependency, the control law would typically be designed with aircraft AOA (a.k.a free-stream AOA) as specified in wind tunnel and/or CFD. Therefore, the local AOA measurements would need to be corrected to the aircraft AOA before being consumed by the control laws. Answer to this question outlines the methods for such correction during flight test.

Addendum:

There are a number of other AOA measurement devices besides vane and Smart Probe (used on A220 and E170, E190). The most common ones are leading edge orifices or tabs used on GA aircraft (thanks @JanHudec for pointing out). These operate by sensing the movement of the stagnation point as AOA increases. The downside, of course, is that the stagnation point associated with the required margin to stall is different at varying Mach numbers and across high lift configurations.

This NACA report contains a number of wing-mounted stall warning devices tested in the last century.

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  • $\begingroup$ In large aircraft, the AoA vane is installed that way. But all small aircraft sense the position of the stagnation point directly on the leading edge with either small vane (triggers stall warning if blown up) or a pair of ports (in some cases even purely mechanically—there is a whistle in the connecting pipe that sounds if air flows up). That does not give AoA, just stall warning, but is simpler. $\endgroup$ – Jan Hudec Oct 29 at 20:50
  • $\begingroup$ @JanHudec Very good information. Are these devices nice-to-have, or are some of them needed to meet 23.207? $\endgroup$ – JZYL Oct 29 at 21:20
  • $\begingroup$ When the aircraft does not have AoA vane (most small GA aircraft don't) and need stall warning for 23.207 (sometimes the stall buffer is enough), it is needed for compliance. But it is on straight untwisted wings; on twisted swept wings it might be harder (or too hard; on swept wing it will be severely affected by side-slip) to calibrate. $\endgroup$ – Jan Hudec Oct 29 at 21:37
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Mounting the AoA sensor at the wing root indeed makes sense if spurious effects occur, such as rapid dynamic effects or non-linearities. The AoA measurement is pretty straightforward in the linear area of the flight envelope, below stall AoA, and in this area the best place for mounting the AoA sensor is in a relatively undisturbed place which makes it simpler to calibrate during flight tests. As has been done for decades.

However, times have changed, there are new requirements mandating recovery from a fully developed stall, not a linear region. New requirements on measuring AoA and stall separation may follow...

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    $\begingroup$ What you will see soon is a development of something based on the "Smart Boom" which is used on the latest test programs and dispenses with physical vanes and just uses ports at various locations at the tip of the boom to measure pressure and software calculates AOA. You'll see something like a small blister on each side of the nose with holes all over it, replacing the vane. $\endgroup$ – John K Oct 29 at 2:59
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    $\begingroup$ @JohnK: By that logic, wouldn't you want those pressure sensors on the wings themselves? After all, air pressure on the wings is directly what keeps an airplane in the air. $\endgroup$ – MSalters Oct 29 at 12:04
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    $\begingroup$ @Jimmy I think MSalters' point was: pressure sensors should directly determine if there's a stall ongoing, in situ as it were, without even needing any proxies (airspeed and/or AOA). $\endgroup$ – leftaroundabout Oct 29 at 12:35
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    $\begingroup$ @Jimmy yes – but pressure sensors are small, lightweight and cheap (unlike AOA vanes), so it would seem plausible enough to just spread 100 of them or so all over the wing. $\endgroup$ – leftaroundabout Oct 29 at 12:55
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    $\begingroup$ @Jimmy: True, but 7 fail out of a hundred you just shrug it off. Redundancy is a well-understood form of reliability. As for the "partial stalls over different parts of the wing", your AoA sensor can't deal with local icing at all. $\endgroup$ – MSalters Oct 29 at 15:30

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