axes of an aircraft will be covered more
in depth in the flight controls chapter
however it is important to know the
three axes that an airplane can move on
the first is pitch the pitch or lateral
axis is the point that a plane will
pitch its nose up or down on the roll or
longitudinal axis of an aircraft is the
point where the airplane will roll to
the left or right and the yaw or
vertical axis is the axis that the plane
will yah on or move its nose left and
right it is also important to note that
each of these lines runs through the
airplanes center of gravity each
aircraft responds to control inputs
differently and therefore handles
differently for example a smaller
aircraft might respond rapidly to
control inputs while a large transport
aircraft may feel sluggish and slow to
respond to inputs these features can be
designed into an aircraft to facilitate
the particular purpose of the aircraft
by considering certain stability and
maneuvering requirements there are two
types of stability static and dynamic
static stability refers to the initial
tendency or direction of movement back
to equilibrium
in aviation it refers to the aircraft's
initial response when disturbed from a
given angle of attack slip or Bank
positive static stability is the initial
tendency of the aircraft to return to
the original state of equilibrium after
being disturbed neutral static stability
is the initial tendency of the aircraft
to remain in a new condition after its
equilibrium has been disturbed negative
static stability is the initial tendency
of the aircraft to continue away from
the original state of equilibrium after
being disturbed dynamic stability refers
to the aircraft response over time when
disturbed from a given angle of attack
slip or Bank this type of stability also
has three subtypes positive dynamic
Stability neutral dynamic Stability and
negative dynamic Stability these act in
the same way as the static subtypes but
these subtypes explain the same relation
over time as shown on the above graph
stability in an aircraft effects to
areas significantly movement and
controllability in designing an aircraft
a great deal of effort is spent in
developing the desired degree of
stability around all three axes but
longitudinal stability about the lateral
axis is considered to be the most
affected by certain variables in various
flight conditions longitudinal stability
is the quality that makes an aircraft
stable about its lateral axis an
aircraft with an unstable longitudinal
axis will have a tendency to dive or
climb and can become unsafe to fly
static longitudinal stability or
instability in an aircraft is dependent
upon three factors location of the wing
with respect to the center of gravity
location of the horizontal tail surfaces
with respect to the center of gravity
and the area or size of the tail
surfaces the above picture shows these
forces balanced in flight
the center of lift in most asymmetrical
airfoils has a tendency to change its
fore and aft positions with a change in
the angle of attack the change in lift
tends to move forward with an increase
in angle of attack and to move aft with
a decrease in angle of attack this means
that when the angle of attack of an
airfoil is increased the center of lift
by moving forward tends to lift the
leading edge of the wing still more this
tendency gives the wing and inherent
quality of instability most aircraft are
designed with a nose heavy or a center
of gravity that is in front of the
center of lift even when the horizontal
stabilizer is level during flight
downwash from the wings pushes the
horizontal stabilizer down generally the
manufacturer of the aircraft will give
the horizontal stabilizer an angle that
will provide the most stability during
flight at a given speed and power
setting as shown above for a nose heavy
plane when the air speed is slow and
there is less downwash on the tail the
nose will pitch over more aggressively
thrust can also affect the longitudinal
stability of an aircraft reduced power
causes less downwash and causes the nose
of a nose heavy airplane to naturally
pitch down which is a desirable
characteristic in an aircraft in order
to make stall recovery easier
stability about the aircraft's
longitudinal access which extends from
the nose of the aircraft to its tail is
called lateral stability there are four
main design factors that make an
aircraft laterally stable dihedral sweep
back heel effect and weight distribution
a dihedral design is when the wings are
connected to the fuselage a few degrees
higher than from being perpendicular
therefore when the aircraft is viewed
straight on the wings points slightly
upward like a V the angle of attack
stays high if one wing is lowered and
will attempt to lift back to its stable
position if the other wing is lowered in
the process it will then be lower and
raise up until the oscillations subside
in addition to a dihedral design
aircraft may also have wings that are
swept back this design allows the lower
wings leading edge to hit perpendicular
to the relative wind this causes greater
lifting effect and the wing restores to
its original position
an aircraft always has the tendency to
turn the longitudinal axis of the
aircraft into the relative wind this
weathervane tendency is similar to the
keel of a ship and exerts a steadying
influence on the aircraft laterally
about the longitudinal axis when the
wing dips the weight of the lower half
of the plane acts as a pendulum to
steady the aircraft this effect is
called the keel effect and its
effectiveness can be changed by weight
distribution to make more of the
aircraft under the center of gravity
stability about the aircraft's vertical
access the sideways moment is called
yoing or directional stability yawing or
directional stability is the most easily
achieved stability in aircraft design
because the vertical fin and the
fuselage after the center of gravity are
the main contributing parts that caused
the aircraft to weathervane or point its
nose into the wind if the area of the
fuselage aft of the center of gravity is
larger than in front of the center of
gravity the aircraft will be vertically
stable also to add to the stability a
vertical fin is added much like the
feather on a narrow the fin being placed
on the tail helps keep the wind from
Ewing the aircraft to the left or right
free directional oscillations or Dutch
roll is the tendency of the aircraft to
yaw and roll in a figure eight formation
however most aircraft are built with
spiral instability instead this
instability causes the plane to yaw in
one direction and if not corrected the
aircraft will enter a deep spiral dive
however this tendency is usually easily
controlled by the pilot
an aircraft is not turned like a car or
a boat to turn the aircraft must be
banked when the aircraft is banked the
lift acting does not act straight up and
down in a turn the lift component is
broken up into a vertical and horizontal
component the horizontal component is
the force that is involved with turning
the plane to either side if the lift and
weight continued to act straight up and
down in a bank the plane would remain
pointed in the same direction the rudder
is used to correct any deviation between
the straight track of the nose and tail
of the aircraft a good turn is one in
which the nose and tail of the aircraft
track along the same path if no rudder
is used in a turn the nose of the
aircraft jaws to the outside of the turn
the rudder is used to bring the nose
back in line with the relative wind the
important thing to consider is that the
horizontal lift component not the rudder
is used to turn the plane during a turn
the aircraft loses some of its vertical
lift which requires back pressure on the
elevator to maintain a consistent angle
of attack also during a turn the
increased drag causes the airplane to
lose airspeed if a constant speed
constant rate turn is required then
additional power may be needed
an aircraft is balanced whether it is in
straight and level flight or in a climb
however to transition from straight and
level to a climb there must be back
elevator pressure applied when this
happens the wings have an increased
angle of attack and momentarily generate
more lift if no power was added for the
climb the airspeed would gradually
diminish because the thrust required for
a climb is greater than the thrust
required for level flight an aircraft is
able to climb at a certain airspeed only
because of excess thrust when this
thrust is gone the aircraft will no
longer be able to climb and will start
to lose airspeed this same principle is
applicable to descents during a descent
elevator pressure is applied forward
which causes the wings to momentarily
have less lift this causes the nose to
drop and the airplane will start to
descend this time the power must be
reduced to maintain a constant air speed
when an aircraft rapidly loses lift on
its wings a stall occurs because the
critical angle of attack for the airfoil
has been reached stalls are one of the
most misunderstood areas of aerodynamics
because pilots often believe an airfoil
stops producing lift when it stalls in a
stall the wing does not totally stop
producing lift rather it cannot generate
adequate lift to sustain level flight
most aircraft are designed to stall from
the wing root out to the wingtip this
provides the ailerons to have some
authority during a stall the stalling
speed of a particular aircraft is not a
fixed value for all flight situations
but a given aircraft always stalls at
the same angle of attack regardless of
airspeed weight load factor or density
altitude a stall can happen at low or
high speeds and during turns not just
when the aircraft is flying slowly in
the picture above the pilot is traveling
at 100 knots but pulls back on the
elevator sharply the result is that
because gravity and centrifugal force
prevent the aircraft from immediately
changing directions so the angle of
attack is increased from low to very
high and the aircraft could stall when
centrifugal forces are added such as in
a turn the stall speed increases most
training aircraft are built with the
center of gravity in front of the center
of lift this means that during a stall
the aircraft will rotate about its
center of gravity and nose down this
condition is better than having the
center of gravity behind the center of
lift because if the nose cannot pitch
down during a stall the aircraft will
remain in a stalled state until it
contacts the ground if ice is allowed to
accumulate on the aircraft during flight
the weight of the aircraft is increased
while the ability to generate lift is
decreased as little as point 8
millimeters of ice on the upper wing
surface increases drag and reduces
aircraft lift by
95% pilots can encounter icing in any
season anywhere in the country at
altitudes of up to 18,000 feet and
sometimes higher
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 air foils and produce forces that
create the thrust to pull or push the
aircraft through the air a cross-section
of the blade shows that the top surface
has a camber much like the top of the
wing and the bottom is flat much like
the bottom of a wing the chord line in a
propeller is the line drawn from the
leading edge to the trailing edge on a
propeller the thicker end is the leading
edge
as shown above each section of a
propeller blade moves downward and
forward the angle that the propeller
strikes the relative wind is its angle
of attack which produces thrust also
this blades camber acts just like a wing
to create forward lift the pitch or
blade angle says how much of a bite the
propeller has or how much air it handles
in one rotation on most smaller aircraft
the pitch or blade angle is set and
cannot be changed but in larger aircraft
the pilot may change this to adjust the
load on the propeller and to control
engine rpm these propeller systems are
called constant speed propellers
propeller efficiency can range anywhere
from fifty to eighty seven percent
depending on how much it slips
propeller slip is the difference between
the geometric pitch of the propeller and
it's effective pitch the difference is
geometric pitch refers to the
theoretical distance a blade should
travel through the air and the effective
pitch is the amount the blade actually
travels if the effective pitch is in a
shorter amount of time than the
geometric pitch the propeller travels
through the air too quickly and that is
referred to as a slip
the reason a propeller is twisted is to
ensure that when the blade is spinning
the angle of attack of the blade to the
air stays the same if the blade were not
twisted for example the part of the
blade near the hub would be traveling
slower than the part of the blade near
the tip this causes the part of the
blade near the hub to potentially have a
negative angle of attack and the part
near the tip to have a high angle of
attack and be stalled during cruise
therefore to maximize the efficiency of
the blade the blade is twisted
to the pilot torque or 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 airplanes three axes
these four elements are torque reaction
from engine and propeller corkscrewing
effect of the slipstream gyroscopic
action of the propeller and asymmetrical
loading of the propeller or p factor
torque reaction causes the aircraft to
rule because the engine components are
moving in one direction so according to
Newton's third law of physics on the
airplane above there must be an equal
and opposite reaction most newer
aircraft are built with their engines
offset to counteract this tendency when
the airplane is on the ground this role
tendency is translated to the vertical
axis and causes yaw during takeoff this
yah tendency is canceled by the pilots
use of rudder
because of the high speed rotation of
the blade the slipstream ends up being a
corkscrew which yaws the aircraft about
its vertical axis the more compact the
spiral the more predominant the force
however as speed increases the corkscrew
along gates and becomes less effective
when airborne this effect also causes
the aircraft to roll on its longitudinal
axis the rolling moment caused by the
corkscrew flow of the slipstream is to
the right while the rolling moment
caused by the torque reaction is to the
left these forces would seem to cancel
each other out but they vary greatly and
it is the pilots responsibility to
control each tendency regardless of
which is more prominent
to understand gyroscopic action you must
first understand how gyroscopes respond
to forces the picture on the top shows a
force being applied to a gyroscope from
the force acting upwards on the
gyroscope it should be noted that the
response is yaw instead of the expected
pitching motion this is because when a
gyroscope is spinning the resultant
force always takes place 90 degrees in
the direction the gyroscope is spinning
in an aircraft we can simulate that same
force by pitching the nose up and down
the applied force is from the tail of
the aircraft and the resulting force
takes effect 90 degrees in the direction
the propeller is spinning in this case
counterclockwise from the pilot's view
and therefore the plane would yaw to the
right the pilot must control this motion
with proper use of the elevator and the
rudder
when an aircraft is flying at a high
angle of attack the blade of the
propeller moving downward has a bigger
bite than the blade moving upward this
causes the center of thrust to be moved
to the right side of the prop disk area
which causes a Ewing movement toward the
left the reason is that the blade moving
downward has a greater speed through the
air because it's moving into the wind
while the blade moving up travels with
the wind because the propeller is an
airfoil the faster the blade moves
through the air the more lift it
produces therefore the faster moving
blade causes an unbalanced forward
thrust called P factor or the
asymmetrical loading of the propeller
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