Thrust, drag, lift, and weight are four forces
that act upon all aircraft in ﬂight.
Understanding how these forces work and
knowing how to control them with the use of
power and ﬂight controls are essential to
The four forces acting on an aircraft in straightand-level, un-accelerated ﬂight are thrust, drag, lift,
and weight. They are deﬁned as follows:
• Thrust—the forward force produced by the
power plant/ propeller or rotor. It opposes or
overcomes the force of drag. As a general rule, it
acts parallel to the longitudinal axis. However,
this is not always the case, I will explained in
• Drag—a rearward force, retarding force
caused by disruption of airﬂow by the
wing, rotor, fuselage, and other protruding
objects. Drag opposes thrust, and acts
rearward parallel to the relative wind.
• Weight—the combined load of the aircraft
itself, the crew, the fuel, and the cargo or
baggage. Weight pulls the aircraft
downward force because of the force of
gravity. It opposes lift, and acts vertically
downward through the aircraft’s center of
downward force of weight, is produced by
the dynamic effect of the air acting on the
airfoil, and acts perpendicular to the
ﬂightpath through the center of lift.
Gravity is the pulling force that tends to draw
all bodies to the center of the earth. The CG
may be considered as a point at which all the
weight of the aircraft is concentrated. If the aircraft
were supported at its exact CG, it would balance in
any attitude. It will be noted that CG is of major
importance in an aircraft, for its position has a
great bearing upon stability.
The pilot can control the lift. Any time the
control yoke or stick is moved fore or aft, the
AOA is changed. As the AOA increases, lift
increases (all other factors being equal).
The flow of a fluid is said to be streamline (also
known as steady flow or laminar flow), if every
particle of the fluid follows exactly the path of its
preceding particle and has the same velocity as
that of its preceding particle when crossing a fixed
point of reference.
The flow of a fluid is said to be turbulent or disorderly,
if its velocity is greater than its critical velocity. Critical
velocity of a fluid is that velocity up to which the fluid
flow is streamlined and above which its flow becomes
turbulent. When the velocity of a fluid exceeds the
critical velocity, the paths and velocities of the fluid
particles begin to change continuously and
haphazardly. The flow loses all its orderliness and is
called turbulent flow.
Bernoulli Principle (Venturi effect)
The basic concept of subsonic airflow and the resulting
pressure differentials was discovered by Daniel Bernoulli,
a Swiss physicist. Bernoulli’s principle, as we refer to it
today, states that “as the velocity of a fluid increases,
the static pressure of that fluid will decrease, provided
there is no energy added or energy taken away.” A
direct application of Bernoulli’s principle is the study of air
as it flows through either a converging or a diverging
passage, and to relate the findings to some aviation
Dynamic & Static Pressure
• Static pressure is the pressure you have if the
fluid isn't moving or if you are moving with the
• Total pressure is what acts on you as you face
into the wind and the air collides with your body.
• Dynamic pressure is the pressure of a fluid that
results from its motion. It is the difference
between the total pressure and static pressure.
The Airfoil is the shape of the cross section of the
wing. The front of the airfoil is the leading edge
and is usually a rounded section. The back of the
airfoil is the trailing edge and usually tapers to
nearly a point. The distance between the two is the
wing chord. The top surface of the airfoil is usually
always curved to allow smooth airflow and produce
• Flat Bottom — A Flat Bottom Wing is when
the lower surface of the wing is primarily
flat between the leading and trailing
edges. This type of wing has high lift and
is common on trainer type aircraft.
• Symmetrical — A Symmetrical Wing airfoil is
curved on the bottom to the same degree as it is
on the top. If a line was drawn from the center of
the leading edge to the center of the trailing
edge the upper and lower halves of the airfoil
would be symmetrical. This is ideal for aerobatic
aircraft and most lift is created by the angle of
incidence of the wing to the flight path.
• Semi-symmetrical — A Semi-symmetrical Wing
airfoil has a curved bottom section but to a
lesser degree than a symmetrical section. It is a
compromise between the flat bottom and the
symmetrical wing. This is a very popular airfoil
on sport type aircraft.
• Under-camber — An Under-camber airfoil has
the lower surface of the wing curved inwardly
almost parallel to the upper surface. This type of
airfoil produces a great deal of lift but is not
common in R/C models.
Is exerted on a body by the air (or some other gas)
in which the body is immersed, and is due to the
relative motion between the body and the gas.
The force created by a propeller or a jet engine is
called thrust and it is also an aerodynamic force.
The aerodynamic force on a powered airplane is
commonly resolved into three components: thrust,
lift and drag.
Pressure Distribution and CP Movement
With changes in angle of attack there are pressure
We have already seen how the center of pressure
moves forward with low angle of attack, and aft
when we have high angle of attack.
Is the curvature which is present on top and bottom
surfaces. The camber on the top is much more
pronounced, unless the wing is a symmetrical airfoil,
which has the same camber top and bottom. The bottom of
the wing, more often than not, is relatively flat. The
increased camber on top is what causes the velocity of the
air to increase and the static pressure to decrease. The
bottom of the wing has less velocity and more static
pressure, which is why the wing generates lift.
Is the length of a wing, measured from
wingtip to wingtip. It always refers to the
entire wing, not just the wing on one side of
An airfoil is an infinitely long, straight line which
passes through its leading and trailing edges.
Is a measure of the width of an airfoil. It is
measured along the chordline and is the distance
from the leading edge to the trailing edge. Chord
will typically vary from the wingtip to the wing
Aspect ratio (AR)
Is the ratio of the wingspan to the average
Wing loading (WL)
Is the ratio of an airplane’s weight to the surface
area of its wings. There tends to be an inverse
relationship between aspect ratio and wing
loading. Gliders have high aspect ratios and low
wing loading. Fighters with low aspect ratios
maneuver at high g-loads and are desig ned with
high wing loading.
Whatever direction the airplane is flying, the
relative wind is in the opposite direction.
Is composed of form drag, skin drag and
interference drag. It is all drag that is not
associated with the production of lift.
• Form Drag, also known as pressure drag
or profile drag, is caused by airflow
separation from a surface and the low
pressure wake that is created by that
separation. It is primarily dependent upon
the shape of the object.
• Skin drag, is created in the boundary layer.
Turbulent flow creates more friction drag than
laminar flow. Skin drag is usually small per unit
area, but since the boundary layer covers the
entire surface of the airplane, skin drag can
become significant in larger airplanes.
• Interference drag, is generated by the
components. An example is the air flowing
around the fuselage mixing with air flowing
around an external fuel tank.
Is that portion of total drag associated with the
production of lift. We can add the airflow at the
leading edge and the airflow at the trailing edge of
the wing in order to determine the average relative
wind in the immediate vicinity of the wing.
The angle between the chord line and the
longitudinal axis of the airplane is known as the
angle of incidence.
The angle between the chord line and the
relative wind is the angle of attack. As the angle
of attack increases, the lift on the wing increases.
Is the angle of attack which produces maximum
lift coefficient. This is also called the "stall angle
of attack". Below the critical angle of attack, as the
angle of attack increases, the coefficient of lift (CL)
increases. At the same time, above the critical
angle of attack, as angle of attack increases, the
air begins to flow less smoothly over the upper
surface of the airfoil and begins to separate from
the upper surface.
• Laminar Flow, the air moves smoothly
along in streamlines. A laminar boundary layer
produces very little friction, but is easily
separated from the surface.
• Turbulent Flow, the streamlines break up
and the flow is disorganized and irregular.
Wing design is constantly evolving. The number of
lifting surfaces, shape, size and materials used all
contribute to an aircraft’s performance.
Monoplane - one wing plane. The wing may
be mounted at various positions relative to
– Low wing - mounted near or below the
bottom of the fuselage.
– Mid wing - mounted approximately half way
up the fuselage.
– High wing - mounted on the upper fuselage.
Planform: The shape of the wing when viewed
directly from above.
> Inefficient from a structural, weight and
> Good slow flight/stall characteristics:
1.Stalls first at the wing root
2.Displays adequate aileron effectiveness
3.Usually quite stable
>Most efficient in terms of weight and
>Expensive and more difficult to construct
>Provides Minimum induced drag for any given
>Provides poor stall characteristics:
1. Little advanced warning of a stall
2. Difficult lateral control because of poor aileron
>Can be tapered in planform or thickness or both
>Relatively efficient with reasonable weight and drag
1.Tapering causes a decrease in drag (mostly
effective at high speed)
2.Tapering causes an increase in lift
>Tapering uses less material - meaning savings in
>Reasonable construction costs as well as good slow
>Used for high speed aircraft, Jet airliners
>Poor stall characteristics
>Low aspect ratio- more dragper lift produced
>High aspect ratio- less drag per lift produced
1.Stalls from the wingtips inward – reducing
>Shape is aerodynamically cleaner
>Offer greater strength
>High wing loading and high speeds
The amount of lift generated by the wing depends
upon several factors:
Speed of the wing through the air
Angle of Attack
Planform of the wing
The density of the air
The lift-to-drag ratio, or L/D ratio, is the amount
of lift generated by a wing, divided by the drag it
creates by moving through the air.
A higher or more favorable L/D ratio is typically
one of the major goals in aircraft design; since a
particular aircraft's required lift is set by its weight,
delivering that lift with lower drag leads directly to
better fuel economy, climb performance, and glide
Best glide or glide ratio is a constant speed in still
air a glider moves forwards a certain distance for a
certain distance downwards.
Flaps are devices used to improve the lift
characteristics of a wing and are mounted on the
trailing edges of the wings of a fixed-wing
aircraft to reduce the speed at which the aircraft
can be safely flown and to increase the angle of
descent for landing.
They shorten takeoff and landing distances. Flaps
do this by lowering the stall speed and increasing
Is a simple hinged portion of the trailing edge that
is forced down into the airstream to increase the
camber of the airfoil.
Is a plate deflected from the lower surface of the
airfoil. This type of flap creates a lot of drag
because of the turbulent air between the wing and
Is similar to the plain flap, but moves away from
the wing to open a narrow slot between the flap
and wing for boundary layer control. A slotted flap
may cause a slight increase in wing area, but the
increase is insignificant.
Is used extensively on larger airplanes. When
extended, it moves down, increasing the camber,
and aft, causing a significant increase in wing area
as well as opening one or more slots for boundary
Leading edge devices such as nose flaps,
Kruger flaps, and slats reduce the pressure
peak near the nose by changing the nose
Slots and slats permit a new boundary layer
to start on the main wing portion, eliminating
the detrimental effect of the initial adverse
Are unique in that they may also be fully deployed
on both wings to act as speed brakes. The reduced
lift and increased drag can quickly reduce the speed
of the aircraft in flight.
Dedicated speed brake panels similar to flight
spoilers in construction can also be found on the
upper surface of the wings of heavy and highperformance aircraft.
They are designed specifically to increase drag and
reduce the speed of the aircraft when deployed.
The aircraft propeller consists of two or more
blades and a central hub to which the blades
are attached. Each blade of an aircraft
propeller is essentially a rotating wing.
As a result of their construction, the
propeller blades are like airfoils and produce
forces that create the thrust to pull, or push,
the aircraft through the air.
Blade angle, usually measured in degrees,
is the angle between the chord of the blade
and the plane of rotation and is measured at
a specific point along the length of the blade.
The reason a propeller is “twisted” is that the
outer parts of the propeller blades, like all
things that turn about a central point, travel
faster than the portions near the hub.
To the pilot, “torque” (the left turning tendency of
the airplane) is made up of four elements which
cause or produce a twisting or rotating motion
around at least one of the airplane’s three
axes. These four elements are:
1. Torque reaction from engine and propeller,
2. Corkscrewing effect of the slipstream,
3. Gyroscopic action of the propeller,
4. Asymmetric loading of the propeller (P-factor).
Involves Newton’s Third Law of Physics—for
every action, there is an equal and opposite
reaction. As applied to the aircraft, this means that
as the internal engine parts and propeller are
revolving in one direction, an equal force is trying
to rotate the aircraft in the opposite direction.
Corkscrew Effect or Slipstream
The high-speed rotation of an aircraft propeller gives
a corkscrew or spiraling rotation to the slipstream. At
high propeller speeds and low forward speed (as
in the takeoffs and approaches to power-on stalls),
this spiraling rotation is very compact and exerts a
strong sideward force on the aircraft’s vertical
Precession is the resultant action, or deflection, of
a spinning rotor when a deflecting force is applied
to its rim. When a force is applied, the resulting
force takes effect 90° ahead of and in the
direction of rotation.
When an aircraft is flying with a high AOA, the
“bite” of the downward moving blade is greater
than the “bite” of the upward moving blade. This
moves the center of thrust to the right of the prop
disc area, causing a yawing moment toward the
left around the vertical axis.
A controllable pitch propeller (CPP) or variable
pitch propeller is a type of propeller with blades
that can be rotated around their long axis to
change their pitch. If the pitch can be set to
negative values, the reversible propeller can also
create reverse thrust for braking or going
backwards without the need of changing the
direction of shaft revolutions.
Adverse yaw is the tendency of an airplane to yaw
away from the direction of aileron roll input.
When an airplane rolls, it has more lift on the upgoing wing than on the down-going wing. This
causes an increase in induced drag on the up-going
wing that will retard that wing’s forward motion and
cause the nose to yaw in the opposite direction of
Stability is the tendency of an object (airplane) to
return to its state of equilibrium once disturbed from
it. There are two kinds of stability: static and
• Static stability is the initial tendency of an object
to move toward or away from its original
• Dynamic stability is the position with respect to
time, or motion of an object after a disturbance.
Positive Static Stability
If an object has an initial tendency toward its
original equilibrium position after a
disturbance, it is said to possess positive
Negative Static Stability
Is the initial tendency to continue moving
Neutral Static Stability
Is the initial tendency to accept the
displacement position as a new equilibrium.
Positive Dynamic Stability
After it is released, it will roll back to the bottom
and up the other side. It will roll back and forth,
oscillating less and less about the equilibrium
position until it finally came to rest at the bottom of
the bowl. It possesses positive dynamic stability.
Neutral Dynamic Stability
If the ball oscillates about the equilibrium
position and the oscillations never dampen
out, it possesses neutral dynamic stability.
Negative Dynamic Stability
If, somehow, the ball did not slow down, but
continued to climb to a higher and higher position
with each oscillation, it would never return to its
original equilibrium position, depicts negative
The quality of an airplane that permits it to
be maneuvered easily and to withstand
the stresses imposed by maneuvers. It is
governed by the airplane’s weight, inertia,
size and location of flight controls, structural
strength, and powerplant. It too is an
airplane design characteristic.
The capability of an airplane to respond to
the pilot’s control, especially with regard
to flightpath and attitude. It is the quality of
the airplane’s response to the pilot’s control
application when maneuvering the airplane,
regardless of its stability characteristics.
Is the result of strong lateral stability and
weak directional stability. The airplane
responds to a disturbance with both roll and
yaw motions that affect each other
Most airplanes are designed so that the outer tips
of the wings are higher than the wing roots
attached to the fuselage. The upward angle thus
formed by the wings is called the dihedral, and is
usually only a few degrees.
The rolling action of an airplane caused by gusts is
constantly being corrected by the dihedral of the
wings. If one wing gets lower than the other when
the airplane is flying straight, it will have a different
attitude in relation to the oncoming air. The result
is that the lowered wing has a greater angle of
attack and thus more lift than the raised wing
and consequently will rise.
If this rising action causes the wing to go past the
level attitude, the opposite wing will then have a
greater angle of attack and more lift. A dynamically
stable airplane will oscillate less and less and
eventually will return to its original position as the
Is the angle between the lateral axis and a
line drawn 25% aft of the leading edge.
Like the feathered arrow, the most important
factor producing directional stability is the
weathervaning effect created by the fuselage and
vertical fin of the airplane.
It keeps the airplane headed into the relative
wind. If the airplane yaws, or skids, the sudden
rush of air against the surface of the fuselage and
fin quickly forces the airplane back to its original
direction of flight.
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