# Aerodynamics by ewghwehws

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Aerodynamics
This presentation is not designed as a
flight instruction, knowledge of current
circulars.
Much information was obtained from the
Handbook of Aeronautical Knowledge)
Aerodynamics
“A branch of dynamics that deals with the motion of air and other gaseous fluids
and with the forces acting on bodies in motion relative to such fluids.”

Source: Merriam-Webster Online
Forces of Flight
Forces Of Flight
Lift
Weight
Thrust
Drag

   Forces can be broken into vectors
Forces In Flight
During straight and level un-accelerated
flight
   The forces are in equilibrium
Definitions: Forces Of Flight
Lift
Lift

In addition to allowing the aircraft to fly,
changing the force of lift on various parts
of the aircraft allows the aircraft to be
maneuvered in flight

   We will review how lift is created first
Physics Review
Newton’s laws summed up.
   1st - A body at rest tends to stay at rest and a
body in motion will stay in motion unless
acted on by a force.

   2nd – Force = Mass * Acceleration

   3rd – For every action there is an equal and
opposite reaction
Bernoulli’s Theory of Lift

Bernoulli’s principal summed up:

   as the velocity of a fluid increase, its internal
pressure decreases.
Bernoulli's Theory
Bernoulli’s Theory of Lift
Bernoulli’s principal

The airfoil acts as part
of the venturi
accelerating the
airflow over the top of
the wing and lowering
its pressure
Pressure Distribution Causes Lift
Pressure moves from high to low
   A boiler explosion is an example of this force
Theory of Lift
The relatively high pressure below the
wing wants to move to the low pressure
above the wing
The pressure differences create the force
of lift
Lift
Lift
Changing the basic shape of the airfoil
allows for different flow patterns about the
wing changing the amount of lift created

Changing the amount of lift created by
various airfoils on the aircraft allows the
pilot to maneuver the aircraft in-flight

   The various flight controls allow us to do this
   We will review terms before discussing
maneuvering flight
Terms
Terms
Chord Line
Chord Line : An imaginary line leading
from the leading edge to the trailing edge
of an airfoil
Relative Wind
Relative wind: the airflow that is parallel
and opposite the flight path

• It is important to note
that the relative wind
relates to the direction of
flight and not the pitch
attitude of the aircraft

• All of the airfoils at the
left have the same pitch
angle but they do not
have the relative wind
Angle Of Attack
Angle of attack: Referred to as “AOA”, is
the angle made up between the chord line
and the relative wind
Angle Of Attack
Forces On An Airfoil
Forces On An Airfoil
If lift is produced we will have drag
Changing The
Amount of Lift Produced
In order to change the amount of lift, we must change
the pressure distribution about the airfoil

Increased or decreased velocity or mass airflow over the
top of the airfoil means a change in lift

We can change the velocity of the airflow over the wing
in two ways

   1. We can increase the velocity by increasing the airspeed at
which we are flying

   2. Change the AOA by using the flight controls
Review
AOA changes velocity and mass of
airflow over the top of the wing
   Increasing AOA allows us to increase lift
   Decreasing AOA reduces lift

Note:
Angle of
attack of
the
bottom
airfoil
produces
lift.
Review
Increasing the speed at which you are
flying also creates increased velocity of
airflow over the top of the wing creating
more lift.

This being the case, when we accelerate
lift would be increased. In level flight we
don’t want this. Remember Lift must =
Weight. So what do we do?
Maintaining Level Flight
Accelerating or decelerating presents a
slight problem if we want to maintain level
flight.

During level flight we are producing lift
nearly equal to weight (L=W)

If we change speed, we must change AOA
so that L=W.
Maintaining Level flight
Stalls
Airspeed and Stalls
In the previous slide you noted that we
had to increase AOA as we slow to
maintain lift
We can only increase AOA so much
before we induce an aerodynamic stall
Though airspeed is often associated with a
stall, a stall can happen at any airspeed
and in any attitude if the critical AOA is
exceeded
Airspeed/Stall Connection
Stalls
For a given airfoil, we can only increase
AOA so much before we experience an
aerodynamic stall

Airfoil in normal flight            Airfoil in stalled condition
Stalls
A stall occurs when the AOA exceeds the
critical AOA
At this point airflow can no longer stay
attached to the top of the airfoil
High pressure is allowed to move on top of
the airfoil reducing the pressure differential
and thus reducing lift
Drag also increases
Stalls

Stall: beyond the critical AOA,
High pressure air from below
the wing is allowed to move
on top of the wing. This
reduces lift and creates drag.

To recover from a stall, you must
reduce the AOA and regain smooth
airflow over the wings
Stalls
Stalls
Stalls
Generally aircraft are designed to have the
root stall first
   This gives the pilot an indication that a full
stall is imminent if nothing is done to correct
the situation (Airframe buffeting)

   This also allows for some aileron control
during imminent stalls since the aileron
portion on the wing is not yet stalled
Stalls
Stalls
Airfoil Design
Airfoil design effects the flight
characteristics
The airfoil used on a specific aircraft
depends on what the aircraft is designed
to do

   Fighter
   High speed transport
   Training aircraft
   …
Spins
Spins

Spin = Aggravated stall that results in
autorotation
Prerequisites for a spin
   Stall
   Uncoordinated
Spins
Spins
Spins
General exit strategy - PARE
   Power- Idle
   Ailerons – Neutral
   Rudder – opposite the spin
   Elevator – foreword

Refer to your POH for specific procedures
Maneuvering Flight
Maneuvering Flight
The flight controls allow pilots to maneuver
the aircraft by changing the pressure
distribution (and the amount of lift created)
on various parts of the aircraft.

The primary flight controls do this by
changing the AOA to change the lift
created thereby allowing the aircraft to
pitch bank and yaw
Primary Flight Controls
Primary Flight Controls
The Rudder Ailerons and Elevator all
change the AOA to change lift and
maneuver the aircraft.

This could be the
rudder ailerons or
elevator. They all
change AOA and
thus the lift produced
Axis Of Rotation
Maneuvering Flight
The aircraft rotates about three axis that all pass through the aircrafts center of gravity

Pitch control
The elevator has negative AOA and
provides “tail down force” during normal
flight
Pitch Control
Pulling back on the yoke increases the
elevator AOA, increasing tail down force
allowing the nose to rise
Pushing forward on the yoke reduces AOA
as well as tail down force allowing the
nose to drop
Pitch Control
Pulling back on the yoke allows increased
tail down force resulting in an increase in
pitch. The result is a climb.
Changing The Amount of Lift
Ailerons allow the aircraft to roll
   Ailerons – change AOA on the wing tips
Ailerons
Ailerons
A section of the left wing has a higher AOA while a section of the right wing has
a lower AOA. The left wing is producing more lift and the right is producing less
lift. The result is the aircraft entering a bank. Banking allows a turn.
Rudder
Rudder
The rudder allows the aircraft to yaw about
the vertical axis. The rudder helps in turns
and during crosswind landings.
Stability
Stability
Stability refers to the aircrafts tendency to
Stability
Static stability – The initial tendency after
the aircraft is displaced

Dynamic stability – The tendency over
flight after displaced
Static Stability
Dynamic Stability
Stability
Aircraft manufacturers design stability into
the aircraft to reduce pilot workload

•Lateral Stability
•Longitudinal Stability
•Directional Stability
Longitudinal Stability
Definition of CG
   Location affects stability
Stability
Longitudinal stability about the lateral axis
(Pitch)
   Forward CG is more stable
Stability
Longitudinal stability about the lateral axis
(Pitch)
   Nose down = increased speed = more tail
down force which acts to raise the nose
Longitudinal Stability
Forward CG decreases performance
   It’s a trade off between performance and
stability

http://www.aero.und.edu/multimedia/weight2.html
Stability
Lateral stability about the longitudinal axis
(Bank)
   Dihedral
Stability
Lateral stability about the longitudinal axis
(Roll)
   Sweepback – more or less span wise flow
Stability
Directional Stability about the vertical axis
(Yaw)
   Keel effect
Maneuvering Flight
Maneuvering Flight
power to climb at the same airspeed
   Component of weight acts as drag during
climbing flight
Maneuvering Flight
Turning flight – The horizontal component
of lift causes an aircraft to turn
Propeller
The propeller is a rotating airfoil (Thrust)
   AOA at hub is higher due to slower speed
Propeller
   Left turning tendencies
Propeller
   Left turning tendencies
Maneuvering Flight
   Adverse Yaw – Higher AOA = more induced
drag
Maneuvering Flight
Load Factor – To maintain level flight in a
turn, total lift must be increased
   We do this by increasing AOA
   The wings must now support more weight
than the aircraft weighs
   With an increase in lift we see an increase in
drag as well
   The higher AOA means we can stall at a
higher airspeed
Maneuvering Flight
   Higher AOA means higher stall speed
Drag
Drag
Drag is broken into two types

   Parasite

   Induced
Drag
Parasite drag:
   Further broken into

Form drag
Skin friction drag
Interference drag
Drag
Parasite / form drag
Drag
Parasite / Skin friction drag
   Airflow is stagnate near the surface of the
wing
Drag
Parasite / Interference drag
   Airflow is disrupted by how various objects
are arranged on the aircraft
Drag
Parasite drag increases with velocity
Drag
Induced drag
   A vector of lift is canted aft. This is induced
drag
Drag
Induced drag
   Any time a wing is producing lift their is
induced drag
   The higher the AOA the more induced drag
   Slow aircraft = high AOA = high induced drag
Drag
Combined drag curves gives us total drag
   1 airspeed gives us least total drag
Wake Turbulence
Wingtip Vortices
Aircraft producing lift produce a wake
(wingtip vortices)
Wingtip Vortices
Airflow moves from
high to low – at the
wing tips it is allowed
to spill over the top
creating a vortex
Wingtip Vortices
Also called wake turbulence
   This can be dangerous to all aircraft
An aircraft that is heavy / Clean and Slow
creates the strongest wake turbulence
Avoid flying into the region of possible
wake turbulence
Wake Turbulence
Avoid flight where the wake turbulence
could be.
Taking Off
Landing
Secondary Flight
Controls
Secondary Flight Controls
On most training aircraft secondary flight
controls are limited to:

   Flaps
   Trim
Secondary Flight Controls
Flaps
   Allow decreased minimum flight speed
   Allow increased drag / Steeper descents
Often used during landing for this purpose
Secondary Flight Controls
Flaps
Trim
Accelerate you get more tail down force
   Not desirable if you want to maintain level
flight
Trim