Note: In the last article we discussed drag and the use of
streamlining to reduce drag. We will come back to drag in
future parts in order to introduce another form of drag ...
drag which is caused by lift. Before we come back to drag
we will discuss lift.
the following article in 1984 as a manual which came with
a computer flight simulation.
Lift is created
by the wing passing through air. A cross-section of the wing
(airfoil) is shown in Figure 2, where some important terms
2. Airfoil Terminology
The mean chord
line is an imaginary line that extends from the leading edge
to the trailing edge of the airfoil. It is further extended
in Figure 2. Relative wind is the airflow caused by passing
the aircraft through an airmass. Relative wind is approximately
opposite to the flight path. Angle of attack is the angle
between the relative wind and the mean chord line. Figure
3 illustrates these terms in level, climbing, and descending
3. Angle of Attack: Shown in level, climbing, and descending
flight. (Angle of attack is the same in all three examples
Note that angle
of attack does not have the same meaning as aircraft pitch
attitude. The angle of attack in Figure 3 is purposely the
same in all three examples (level, climbing, and descending
flight) so as to emphasize this difference. In actual flight
your angle of attack will often be different during different
phases of the flight.
In order to understand
how lift is produced, we must explore the theories of Bernoulli
the conservation of energy in fluid flow. Assuming a constant
density (no compression) of the fluid, energy is held constant
by decreasing pressure with increasing velocity or, conversely,
by increasing pressure with decreasing velocity. This incompressibility
assumption holds nearly true for airflow as well, at least
at the low speeds flown by light aircraft. The classic graphic
illustration of this theory usually depicts a tube of varying
diameter (Figure 4).
4. Bernoulli's Favorite Tube
Figure 4 shows
an enclosed tube with airflow. Mass flow within the enclosed
tube is the same at all points since the air can’t escape.
An incompressible fluid (or low-speed airflow) must increase
velocity at the narrow point to maintain mass flow. If energy
is to be conserved, then an increase in velocity (kinetic
energy) must be balanced by a decrease in pressure (potential
Figure 5 illustrates
airflow past an airfoil at a positive angle of attack. The
airflow over the top of the wing has a higher velocity than
the airflow under the wing and, consequently, a lower pressure.
A basic rule in physics states that when an imbalance exists,
a force will result tending to relieve that imbalance. In
the case of our airfoil this force is directed upwards, from
the higher pressure to the lower pressure. This force is known
5. Bernoulli Explanation of Lift
As late as the
mid 1960’s many flight instructors were emphasizing
Bernoulli’s law as the major contributor to lift theory.
This concept does go a long way in explaining lift when looking
just at airflow immediately adjacent to the wing. Bernoulli’s
law, however, doesn’t explain the forces of airflow
deflected by the wing. Indeed, most modern instructors give
credit to Newton for explaining the majority of lift production.
third law states that for every action there is an equal and
opposite reaction. Figure 6 is a repeat of Figure 5 with labels
changed to emphasize action-reaction theory.
6. Newton Explanation of Lift
Down wash is caused
by the airfoil altering the direction of airflow downwards.
This will occur as long as there is a positive angle of attack.
Downwash is easy to understand no matter what shape the airfoil
takes. In this age when fighter jets use thin, symmetrical
airfoils, you can see why deflected air is considered to be
the major contributor to lift.
As a pilot, you
must learn how to control lift during takeoff, climbs, level
flight, turns, descents, and landing. You can generally increase
lift in two ways; increase airspeed or increase your angle
Given a constant
angle of attack, an increase in airspeed increases pressure
differential and downwash, and therefore increases lift. Given
a constant airspeed, an increased angle of attack increases
pressure differential and downwash, thereby increasing lift.
As a pilot, you must manage both airspeed and angle of attack
in order to gain the desired flight goals. A good example
would be an airspeed transition from fast cruise flight to
slower flight when entering a crowded airport traffic pattern.
You reduce power and the aircraft decelerates. Since the weight
of your aircraft is unchanged, you must produce constant lift
during the deceleration. In order to produce constant lift,
you must increase the angle of attack slowly until the aircraft
is stable at its new slower speed.
There is a limit
to the angle of attack that you can use to generate lift.
You can alter the relative wind airflow only so far before
the wind refuses to change anymore.
Figure 7 shows
an airfoil at three different angles of attack. The top illustration
shows an airfoil at the same angle of attack used in the previous
discussion of lift generation. The middle airfoil shows an
increased angle of attack. Notice that the airflow is separating
from the surface near the upper trailing edge of the wing.
The bottom airfoil is at stall angle of attack. The point
of airflow separation is so far forward that we don’t
even see a downwash vector; Newton’s downwash is gone.
Velocity in the area aft of the separation point is very low;
Bernoulli’s suction is gone. And with neither law still
in effect, there is no way to maintain lift.
7. Airflow Separation With Increasing Angle of Attack
What is a stall?
A sudden loss of lift due to airflow separation from the wing.
How do you stall an airfoil? Stall is a function of angle
of attack. You can stall an aircraft at any airspeed and pitch
attitude if you exceed the stall angle of attack.
How do you recover
from a stall? Simply reduce the angle of attack.
Stall angle of
attack depends on the airfoil shape, and is usually somewhere
between 10 and 20 degrees. Generally speaking, thin airfoils
will stall at a lower angles of attack while thick airfoils
will stall at a higher angles of attack. Also, symmetrical
airfoils will stall at lower angles of attack than airfoils
with more bulge on the upper surface (higher camber). Figure
8 shows the lift characteristics of a typical airfoil used
on training aircraft. Lift increases steadily until stall
angle is reached. After this point, lift drops off suddenly.
8. Lift versus Angle of Attack
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