A stall occurs when the angle of
attack of an aerofoil exceeds the value which creates maximum lift as a
consequence of airflow across it. This angle varies very little in response to
the cross section of the (clean) aerofoil and is typically around 15°. At the
stall, the airflow across the upper cambered surface ceases to flow smoothly
and in contact with the upper surface and becomes turbulent, thus greatly
reducing lift and increasing drag. Changing the effective configuration of a
wing by the deployment of leading edge or trailing edge devices will directly
alter the angle of attack at which an aerofoil stalls. However, all this
assumes a clean wing and for any aerofoil, contamination of the normally smooth
surface by frozen deposits will result in a change to the angle of attack at
which a stall will occur.
The Angle of Attack of an
aerofoil – the incidence of the wing to the incident airflow - is not the same
as the pitch attitude of the aircraft as displayed on the corresponding primary
flight instrument and many aircraft do not have an instrument which displays
angle of attack. However, when flying straight and level with a particular
aircraft mass and prevailing density altitude, for every wing angle of attack
there is a corresponding indicated air speed. Because of this, the indicated
airspeed at which an aircraft has been shown to stall in given circumstances is
determined during aircraft certification and included in the AFM. This
indicated speed, Vs, provides a fundamental reference for all other AFM
aeroplane performance calculations which involve indicated airspeed and helps
define the Flight Envelope for each aircraft type and variant.
The indicated speed at which a
particular aircraft operating at a particular total weight will stall when in
straight and level flight at a given air density (which broadly corresponds to
altitude) will not be very different for the climb and descent case at that air
density – but it will increase as bank angle increases since in a turn, the
lift from the wing must exceed the weight being supported. As air density
decreases with increasing altitude, more lift must be generated by an aerofoil
to sustain flight and so the true air speed at which an aerofoil will stall
will increase. However, this fact is not of direct relevance to the pilot since
the airspeed displayed is indicated air speed derived from the ambient air
density. Thus, the indicated speed at which an aircraft will stall will be the
same at any altitude.
On a propeller driven aircraft
with engines ahead of the wing, the slipstream from each engine can change the
angle of attack for the area of the wing that it affects, when compared with
the angle of attack of those parts of the wing in the free airflow. Thus the
wing will stall in parts, not all at once. Therefore, the power set, and the
resulting slipstream effects, will change the speed at which stalling becomes
apparent in any particular configuration.
Stall Awareness
A stalled wing may be accompanied
by one or more of the following:
- Buffet
- Poor pitch authority.
- Poor roll control.
- An inability to arrest descent.
On all transport aircraft, some
form of stall protection system is a certification requirement. Such a system
is likely to incorporate stall warning based on the continuous direct
measurement of angle of attack moderated by wing configuration in order to
enable recovery from an incipient or approaching stall. For aircraft with a conventional
control column, this usually includes a stick shaker which provides tactile as
well as aural alerting. At the onset of a full stall, most stall protection
systems activate a stick pusher to ensure that aircraft pitch attitude (which
although it is not directly related to angle of attack varies in the same
relative sense) is automatically reduced as an essential component of recovery
to safe flight.
It is important to note that stall protection systems cannot take account of the possible effects of contaminated aerofoil surfaces and also that any such contamination may not necessarily be symmetrical in respect of the overall aircraft. It should also be noted that on modern Fly-By-Wire aeroplanes, the stall protection system has become part of a wider Flight Envelope Protection System.
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Credit: skybrary
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