tutorial for aerodynamics of flight this
training tutorial will discuss
aerodynamics flight in order for a pilot
to safely execute maneuvers in flight it
is important to understand the
aerodynamic forces acting on an aircraft
in flight
the four forces acting on an aircraft in
straight and level unaccelerated flight
are thrust drag lift and weight thrust
is the forward force produced by the
powerplant
propeller or rotor and is opposed by
drag
drag is the rearward retarding force
caused by air that has been disrupted by
the wings rotor fuselage and other
protruding objects and opposes thrust
weight is the combined load of the
aircraft itself the crew the fuel and
the cargo or baggage weight pulls the
aircraft downward because of the force
of gravity and is opposed by lift lift
opposes the downward force of weight
lift is produced by air flowing over the
airfoil and acts perpendicular to the
flight path through the center of lift
in Strait level and unaccelerated flight
the sum of the forces acting on the
aircraft is zero this is for the most
part true during climb and descending
flight but things are a bit more
complicated anytime the flight path of
the aircraft is not horizontal lift
weight thrust and drag vectors must each
be broken down into two components for
example during a glide part of the
weight of the aircraft points forward
and therefore acts as thrust or as the
picture shows during a climb the weight
can oppose lift and also thrust
a pilot may hold an aircraft in straight
and level flight at many different air
speeds the pilot needs to coordinate the
angle of attack with the speed of the
aircraft for example if the plane is
flying at a high rate of speed the angle
of attack can be relatively low however
if the rate of speed is low then the
pilot must have a higher angle of attack
this is caused by the amount of lift
produced by the wings of the aircraft
the higher the angle of attack the more
lift is provided from the wing
if a pilot can maintain a high angle of
attack and a coordinated thrust level
the aircraft can fly straight and level
at low speeds level flight at even
slightly negative angle of attack is
possible at very high speed
certain aircraft have the ability to
change their direction of thrust rather
than change their angle of attack such
as the Harrier Jump Jet on the left and
the osprey on the right these aircraft
can point their thrust straight up
without having to point their nose in
that direction the Harrier jet uses
vents to accomplish this and the Osprey
can rotate its propellers to point up
this allows each aircraft to hover over
the ground or fly straight and level
very slowly
drag is the force that resists movement
of an aircraft through the air there are
two basic types parasitic drag and
induced drag the first is called
parasite because it in no way functions
to aid flight while the second induced
drag is a result of an airfoil
developing lift parasite drag is any
drag that is not caused directly from
the aircraft producing lift it is broken
down into three categories form drag
interference drag and skin friction form
drag is a drag that is caused by things
like the cowling antennas and the
aerodynamic shape of other components
how quickly air flow rejoins itself is
determined by the shape of the object
it's flowing around as the picture on
the left shows the best way to reduce
form drag is to streamline as many of
the aircraft components as possible
interference drag comes from the
intersection of air streams that creates
eddy currents turbulence or restricts
smooth airflow for example the picture
on the right shows the intersection of
the wing and the fuselage at the wing
root this area has significant
interference drag air flowing around the
fuselage collides with air flowing over
the wing merging into a current of air
different from the two original currents
skin friction drag comes from the
surface of the aircraft not being
completely smooth even though a surface
may look smooth it has a rough ragged
surface when viewed under a microscope
this non smooth surface causes any
interruption in air flow and more drag
induced drag is the second basic type of
drag when an airfoil produces lift there
is always drag this drag comes from
wingtip vortexes when the aircraft is
viewed from the tail these vortices
circulate counterclockwise about the
right tip and clockwise about the left
tip bearing in mind the direction of
rotation of these vortices it can be
seen that they induce an upward flow of
air beyond the tip and a down wash flow
behind the wings trailing edge this
induced down wash has nothing in common
with the down wash that is necessary to
produce lift it is in fact the source of
induced drag this down wash over the top
of the airfoil at the tip has the same
effect as bending the lift vector
rearward therefore the lift is slightly
aft of perpendicular to the relative
wind creating a rearward lift component
this is induced drag
drag is the price paid to obtain lift
the lift to drag ratio is the amount of
lift generated by a wing or airfoil
compared to its drag this also governs
the airfoils efficiency aircraft with
higher lift drag ratios are more
efficient than those with lower lift
drag ratios the lift drag ratio is
determined by dividing the lift
component by the drag component or above
by dividing the lift equation by the
drag equation L is the lift force and
pounds CL is the lift coefficient P is
density expressed in slugs per cubic
feet V is velocity in feet per second Q
is dynamic pressure per square feet and
s is the wing area in square feet on the
graph to the left the lift curve red
reaches its maximum for this particular
wing section at 20 degrees angle of
attack and then rapidly decreases 15
degrees angle of attack is therefore the
stalling angle the drag curve yellow
increases very rapidly from 14 degrees
angle of attack and completely overcomes
the lift curve at 21 degrees angle of
attack the lift drag ratio green reaches
its maximum at 6 degrees angle of attack
meaning that at this angle the most lift
is obtained for the least amount of drag
any angle of attack the aircraft
operates at other than the lift drag max
point will cause more drag therefore for
the least amount of drag to lift the
aircraft must fly with that angle of
attack
gravity is the pulling force that tends
to draw all bodies to the center of the
earth the center of gravity may be
considered as a point at which all the
weight of the aircraft is concentrated
if the aircraft were supported at its
exact center of gravity it would balance
in any attitude this weight force acts
downward through the airplane center of
gravity in stabilized level flight where
the lift force is equal to the weight
force the aircraft is in a state of
equilibrium and neither gains nor losses
altitude
if lift becomes less than weight the
aircraft loses altitude when lift is
greater than weight the aircraft gains
altitude
the pilot is able to control lift if the
control yoke is pushed forward or pulled
back the angle of attack changes and
therefore the amount of lift changes as
well for an aircraft to continue to
produce lift the aircraft airfoil must
be continually attacking new air on
helicopters this is accomplished by the
rotating blades in a fixed-wing airplane
this is accomplished by the airflow over
the wing the lift provided from the wing
is proportional to the square of the
aircraft's velocity lift and drag also
vary directly with the density of the
air density is affected by several
factors pressure temperature and
humidity at an altitude of 18,000 feet
the density of the air has 1/2 the
density of air at sea level in order to
maintain its lift at a higher altitude
an aircraft must fly at a greater true
airspeed for any given angle of attack
warm air is less dense than cool air and
moist air is less dense than dry air
lift varies directly with the wing area
if the wings have the same proportion
and airfoil sections a wing with an area
of 200 square feet
lifts twice as much at the same angle of
attack as a wing with an area of 100
square feet for most situations the
pilot controls lift and velocity to
maneuver an aircraft
as stated previously wingtip vortices
are caused when the high pressure below
the wing attempts to rejoin the
low-pressure air around the wingtip
causing a downward and outward vortex
the above picture is a more detailed
diagram of how this process looks on a
wing the heavier and slower the aircraft
the greater the angle of attack and the
stronger the wingtip vortices wingtip
vortices lead to a potentially hazardous
condition called wake turbulence
to avoid wake turbulence avoid flying
through another aircraft's flight path
rotate prior to the point at which the
preceding aircraft rotated when taking
off behind another aircraft avoid
following another aircraft on a similar
flight path at an altitude within a
thousand feet and approach the runway
above a preceding aircraft's path when
landing behind another aircraft and
finally touchdown after the point at
which the other aircrafts wheels
contacted the runway a hovering
helicopter generates a downwash from its
main rotors similar to the vortices of
an airplane pilots of small aircraft
should avoid a hovering helicopter by at
least three rotor disk diameters to
avoid the effects of this downwash wind
is also an important consideration when
dealing with wake turbulence because it
can push around wake turbulence for
example if there's a ten knot wind it
will push the wake turbulence in the
direction of the wind at a rate of 1,000
feet per minute if this is a problem
pilots may also wait approximately three
minutes for the wake to dissipate
ground effect is a phenomenon that
allows an aircraft to fly slower than
normal a few feet from the ground when
an aircraft in flight comes within
several feet of the surface ground or
water a change occurs in the
three-dimensional flow pattern around
the aircraft because the vertical
component of the air flow around the
wing is restricted by the surface this
alters the aerodynamics of the wing
mainly the fuselage and tail surfaces
are also affected but ground effect
mainly consists of the aerodynamic
characteristics of the wing the
reduction of the wingtip vortices due to
ground effect alters the lift
distribution of the wing and reduces the
induced angle of attack and induced drag
therefore the wing will require a lower
angle of attack in ground effect to
produce the same lift the reduction of
induced drag due to ground effect means
that less power is required to flight a
certain speed
most of the time ground effect will
produce a higher pressure at the static
source and the indicated airspeed will
be lower than normally required
when an aircraft is taking off the
reverse happens from landing the
aircraft taking off will require an
increase in angle of attack to maintain
the same lift experienced an increase in
induced drag and thrust required
experience a decrease in stability and a
nose up change in moment and experience
a reduction in static source pressure
and increase in indicated airspeed this
is something to consider on takeoff
because it could mean a pilot attempting
to take off will feel like the aircraft
can lift off the ground prior to the
recommended takeoff speed as shown above
this could cause the plane to have poor
initial climb performance and also in
extreme cases prevent the aircraft from
becoming airborne entirely
axes of an aircraft will be covered more
in depth in the flight controls chapter
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