STALL AND SPIN
One of the first questions a pilot might ask, when converting to a new
aircraft type, is "What's the stall speed?" The reason for the enquiry
is that usually, but not always, the approach speed chosen for landing
is 1.3 times the stall speed. Stall is an undesirable phenomenon
in which the aircraft wings produce an increased air resistance and
decreased lift, which may cause an aircraft to crash.
The stall of the wing occurs when the airflow no longer can go around
the airfoil's nose (leading edge) and separates from the upper wing
surface. It happens when a plane is under too great an Angle of
Attack (AoA). For light aircraft, without high-lift devices,
the critical angle is usually around 16°. The picture below shows
a stalled airfoil :
Geometric Angle of Attack is the angle between the airfoil chord line
and the direction of flight. The angle of attack measured relative
to zero coefficient of lift is called the Absolute Angle of Attack
(Absolute AoA). There's also the Pitch Angle, which measured with
respect to the horizon.
For symmetric airfoils the Absolute AoA is equal to the Geometric AoA,
whereas for cambered airfoils these two angles are different as shown
For airfoils of one family the symmetric airfoil stalls at a higher
Geometric AoA compared with the cambered airfoil, however the cambered
airfoil has higher lift coefficient and stalls at a higher Absolute AoA.
As mentioned in the chapter Forces in Flight, the lift force is proportional to the density of
the air r,
the square of the airspeed V, the type of airfoil and to the
wing’s area according to the formula :
Lift force = 0.5 * r
* V2 * wing's lift coefficient * wing area
Since lift coefficient is proportional to the angle of attack, the lower
airspeed is the higher the angle of attack has to be in order to produce
the same lift.
Thus, stall may occur during take-off or landing, just when the speed is
low. At low speed the aerodynamic forces are smaller and, if a
non-experienced pilot tries to lift the aircraft at a too low speed, it
may exceed the critical angle of attack and stall occurs. If
you're flying near the stall speed and make a steep turn, the aircraft
will stall. That's because, if the aircraft stalls for instance at
20 knots in straight level flight, it will stall at 28.2 knots in a 60
degree banked turn. The rapid reduction in speed after passing the
critical angle of attack means the wing is now unable to provide
sufficient lift to totally balance weight and, in a normal stall, the
aircraft starts to sink, but if one wing stalls before the other, that
wing will drop, the plane falls out of the air. The ground waits below.
Stalls may also occur at high airspeeds. If at max airspeed and full
throttle the pilot suddenly applies excessive up elevator, the aircraft
will rotate upwards, however, due to aircraft's inertia, it may continue
flying in the same direction but with the wings at an angle of attack
that may exceed the stall angle.
Stalling at high-speed gives a more dramatic effect than at low speed.
This because the strong propeller wash causes one of the wings to stall
first that combined with the high speed produces a snaproll followed by
a spiral dive. This happens very fast causing the aircraft to dive
at full throttle and unless there's enough height for recovery, the
crash will be inevitable.
An aircraft with relatively low wing loading has a lower stall
speed. (wing loading is the aircraft's weight divided by the wing
area) Since the airfoil also affects the stall speed and the max
angle of attack, many aircraft are equipped with flaps (on the wing
trailing edge), and some designs use slats (on the wing leading edge).
Flaps increase the wing's lift coefficient, but the simple ones may
reduce the stall angle. Slats, on the other hand, increase the stall
Aircraft that are designed for Short Take-Off and Landing (STOL) use
slots on the wing's leading edge together with flaps on the trailing
edge, which gives high lift coefficient and remarkable slow flying
capabilities by allowing greater angle of attack without stalling.
The leading edge slots may prevent
the stall up to approximately 30 degrees angle
of attack by picking up a lot of air from below, accelerating the air in
shaped slot (venturi effect) and forcing the air around the leading edge
upper wing surface.
The disadvantage of the slots and flaps is that they produce higher
Since the high lift is only needed when flying slowly (take-off, initial
final approach and landing) some designers use retractable devices,
closes at higher speeds to reduce drag.
Such devices are seldom used in model aircraft (especially the smaller
ones), mainly due to its complexity and also the increasing of wing loading,
which may counter-act the increased lift obtained.
The wing's aspect ratio (AR) also affects the overall lift coefficient
of the wing. For a given Re, the wing with higher AR (with long wingspan
and small chord) reaches higher lift coefficient, but stalls at a lower angle of
attack than the wing with low AR as shown below :
However, for a given wing area, increasing the aspect ratio may result
in a too small wing chord with a too low Reynolds number, which may
significantly reduce the lift coefficient. This is likely to occur with small
Another method to improve an aircraft's stall characteristics is by
using wing washout, which refers to wings designed so that the outboard
sections have a lower angle of attack than the inboard sections in all flight
The outboard sections (toward the wing tips) will
reach the stalling angle after the inboard sections, thus allowing effective
aileron control as the stall progresses. This is usually achieved by building a
twist into the wing structure or by using a different airfoil in the outboard
section. A similar effect is achieved by the use of flaps.
aileron drag is a further factor that may cause an aircraft to stall.
When the pilot applies aileron to roll upright during low speed, the downward
movement of the aileron on the lower wing might take an angle on that part of
the wing past the critical stall angle. Thus that section of wing, rather than
increasing lift and making the wing rise, will stall, lose lift and the aircraft
instead of straightening up, will roll into a steeper bank and descend quickly.
Also the wing with the down aileron often produces a larger drag, which may
create a yaw motion in the opposite direction of the roll. This yaw motion
partially counteracts the desired roll motion and is called the adverse yaw.
Following configurations are often used to reduce aileron drag :
- Differential ailerons where the
down-going aileron moves through a smaller angle than the up-going.
- Frise ailerons, where the leading edge of the up-going aileron
protrudes below the wing's under surface, increasing the drag on the
- And the wing washout.
Stall due to aileron drag is more likely to occur
with flat bottom wings. Since differential ailerons will have the opposite
effect when flying inverted, some aircraft with symmetrical airfoils designed
for aerobatics don't use this system. The picture below illustrates an
example of a Frise aileron combined with differential up/down movement.
Another factor that affects the aircraft's stall characteristics is the
location of its centre of gravity CG. A tail-heavy aircraft is
likely to stall at higher airspeed than one with the CG at the right location.
Recovering from a stall :
In order to recover from a stall, the pilot has to reduce the angle of
attack back to a low value. Despite the aircraft is already falling toward the
ground, the pilot has to push the stick forward to get the nose even further
down. This reduces the angle of attack and the drag, which increases the
After the aircraft gained speed and the airflow incidence on the wing
becomes favorable, the pilot may pull back on his stick to increase the angle of
attack again (within allowable range) restoring the lift. Since recovering
from a stall involves some loss of height, the stall is most dangerous at low
Engine power can help reduce the loss of height, by increasing the
velocity more quickly and also by helping to reattach the flow over the wing.
How difficult it is to recover from a stall depends on the plane. Some full-size
aircraft that are difficult to recover have stick shakers: the shaking stick
alerts the pilot that a stall is imminent.
A worse version of a stall is called spin, in which the plane spirals
down. A stall can develop into a spin through the exertion of a sidewise
moment. Depending on the plane, (and where its CG is located) it may be
more difficult or impossible to recover from a spin. Recovery requires
good efficiency from the tail surfaces of the plane; typically recovery involves
the use of the rudder to stop the spinning motion, in addition to the elevator
to break the stall. However the wings might block the airflow to the tail.
If the centre of gravity of the plane is too far back, it tends to make recovery
much more difficult.
Another circumstance that may cause loss of control is when a hinged
control surface starts to flutter. Such flutter is harmless if it
just vibrates slightly at certain airspeed (possibly giving a kind of buzzing
sound), but ceases as soon as the airspeed drops. In some cases however,
the flutter increases rapidly so that the model is no longer controllable.
The pilot may not be aware of the cause and suspect radio interference instead.
To reduce the flutter, the control linkages should not be loosely fitted and the
push rods should be stiff. Long unbraced push rods can create flutter as
vibration whips them around. In some difficult cases the control surface
has to be balanced, so that its centre of mass (gravity) is ahead of the hinge
line. It should be located at about 60-65% of the length of the control surface
from its inner end.