AIRCRAFT STALL
CHARACTERISTICS
At a constant mass and air density, the coefficient of lift (CL), and consequently the total amount of lift, is dependent on the
angle of attack. Graphically, lift increases almost linearly from an angle of attack of 0◦ to 4◦, and then at an increasing rate
to 15◦. Lift cannot be obtained without incurring the penalty of drag. At a specific angle of attack, depending on
circumstances, the lift generated is insufficient to sustain the aircraft in level flight. The angle of attack at which this occurs
is the stalling angle or critical angle and is the point at which CL MAX is attained. An aeroplane will not stall unless the
critical angle is exceeded. The magnitude of the critical angle is independent of the mass of the aeroplane, it is therefore the
same for all aeroplane masses; mass only affects the value of the airspeed at which the stall will occur.
The critical angle can be anywhere between 8◦and 20◦ to the relative airflow; its exact value is dependent on the type of stall
entry, the wing profile, the planform and the aspect ratio. Except at high Reynolds numbers, an aeroplane, in the same
configuration, in subsonic flight will always stall at the same critical angle of attack unless shock-induced separation occurs.
The commencement and spread of the stall is dependent on the wing design.
At angles of attack greater than the critical angle, the total drag and CD continue to increase but the total lift decreases
dramatically, as shown in Figure below:
A further consequence of increasing the angle of attack beyond the critical angle is that the stagnation point
will move aft along the lower surface of the wing. The wing will continue to produce some lift, albeit very
little, up to an angle of attack of 90◦.
The low-speed stall occurs at a particular angle of attack of the wing to the relative airflow- not at a specific
airspeed. As the angle of attack and CL increase, the IAS must be reduced to maintain the lift formula in
balance. The speed attained at the stalling angle is the stalling speed, which is directly proportional to the
aeroplane mass.
Factors Affecting the Low-Speed Stalling Angle
1)
S l a t / Flap Setting: The use of leading-edge slats, be they automatic or manual, cause negligible drag but do delay
the stall to a higher angle of attack than would be possible without their use.They re-energise the boundary layer,
enlarge the low-pressure area over the upper surface of the wing, thus producing a larger CL MAX, and cause the
separation point to move further back from the leading edge of the wing. Leading-edge flaps have a similar effect and
can increase the maximum lift generated by 50%. Trailing-edge flaps when deployed will decrease the critical angle of
attack to between 8.5◦ and 14.0◦ and increase CL MAXby between 20% and 80%, depending on the flap type. A large
extension of trailing-edge flaps usually causes a considerable increase of drag and produces a low stalling angle but
contrary to this, extension of the triple slotted trailing-edgeflap, such as the Fowlerflap, increases the critical angle to
22.0◦ and significantly increases the CL MAX by approximately 110%. The greater the number of slots the higher is
the value of CL MAX .
2) Ice Accretion : The formation of ice on the surface of an aeroplane has a dramatically adverse effect on its
performance; frost has a similar effect. The most dangerous type of ice accretion is that which occurs when flying
through cumulonimbus clouds or through supercooled water droplets ahead of a warm front because of its rapid
accumulation. The effect that ice accretion has on an aeroplane’s performance includes:
a. increased mass; b. decreased total lift; c. decreased CL MAX - that is the most serious effect; d. increased total
drag; e. up to 25% increased stalling speed; f. reduced stalling angle; g. changed aerofoil shape; h. abnormal stalling
characteristics
Most aircraft are designed to have a nose- down
pitching moment at the stalling angle. This occurs
because the CP moves rapidly aft at the stall,
increasing the wing restoring moment and reducing
the angle of attack. This induces the wing to produce
lift once more and is therefore a self-correcting
characteristic. The shape of the wing determines the
point of stall commencement and also the spread of
the stall across the wing surface.
The Effect of Wing Design on the Low-Speed
StallAs the angle of attack increases, because of the
movement of the CP the pitching moment of
the wing changes and affects the angle of the
downwash impinging on the tailplane thereby
changing its pitching moment. Because the
greatest amount of lift is generated at the wing
roots, on entering the stall the CP moves aft on
a straight-winged aeroplane and, because the
wingtips stall first, forward on a swept-wing
aircraft.
Swept Wings: Of all wing
shapes, swept wings have the
greatest tendency to tip stall first
because of the spanwise flow of
the boundary layer. The three
elements that combine to form the
wing-tip vortex on swept-wing
aeroplanes are leading-edge
separation, flow around the wing
tips and spanwise boundary layer
flow. The greater the sweepback
the greater is the boundary-layer
spanwise flow and the magnitude
of the vortex drag. Increased
angle of attack also increases the
spanwise flow of the boundary
layer.
Swept wings produce a nose-up pitch tendency due to the separation of the thickened boundary layer from the upper surface of the wing tip
causing it to stall first, which induces the stall to spread across the wing from the tip to the root on the upper surface; consequently the CP moves
forward and in towards the wing root. This results in a decreased wing-restoring moment and the maximum downwash being concentrated
inboard, which increases its effect on the tailplane.
Although the effectiveness of the elevators is diminished by the turbulent airflow over the tailplane, the increased downwash on the tailplane
causes the tail moment to exceed the wing moment and causes the nose to pitch-up. Swept wings produce a nose-up pitch tendency due to the
separation of the thickened boundary layer from the upper surface of the wing tip causing it to stall first, which induces the stall to spread across
the wing from the tip to the root on the upper surface; consequently the CP moves forward and in towards the wing root. This results in a
decreased wing-restoring moment and the maximum downwash being concentrated inboard, which increases its effect on the tailplane. Although
the effectiveness of the elevators is diminished by the turbulent airflow over the tailplane, the increased downwash on the tailplane causes the tail
moment to exceed the wing moment and causes the nose to pitch-up.The CL for a swept wing is diminished in direct proportion to the cosine of
the sweep angle. This is because the effective fineness ratio of the wing to the airflow is increased, which decreases the acceleration of the airflow
over the upper surface of the wing.
Elliptical Wings
The commencement of the stall of
an elliptical wing is evenly spread
along the trailing edge and
progresses forward uniformly
towards the leading edge. The
reason for this is that the wing has
a constant lift coefficient from
root to tip and all sections of the
wing reach the stalling angle at
the same time. It is the most
efficient wing design but is
difficult to manufacture.
Rectangular WingsThe stall of an unswept rectangular wing
does not occur over the whole wing
simultaneously, it commences at one
particular point usually at the wing root
near the trailing edge and spreads outward
and forward to the rest of the wing. This
pattern of progression is caused by the lift
coefficient being much greater at the wing
root than at the wing tip.
An untapered wing has less downwash
moving inboard from tip to root and
therefore stalls at the root first, which is a
desirable feature. At or near the stall the
wing-tip vortices are large enough to affect
the airflow over the tailplane decreasing its
effective angle of attack.This causes a
tendency for the aircraft to pitch nose-up.
The separation of the boundary layer from the upper surface of the
wing for a rectangular, low aspect ratio, wing usually commences at
the wing root at a point close to the trailing edge of the wing when
the angle of attack is approximately 8◦. It then gradually spreads
outward and moves forward with increasing angle of attack until at
approximately 15◦ angle of attack it is located at between 15% and
20% of the length of the mean aerodynamic chord from the leading
edge of the wing.