Theory of Flight
The Four Forces
Lesson 2.1
Sep 2012
Reference
From the Ground Up
Chapter 2.1.1:
Forces Acting on an Airplane in Flight
Pages 15 - 20
Introduction
• Theory of Flight is a study into the basic
physics of why and how aircraft are able
to fly, maneuver and keep stable in the
air.
• Aircraft fly and maneuver in the air by
controlling the many forces acting on
and created by them.
Outline
• The Four Forces
• Equilibrium
• Couples
The Four Forces
•
•
•
•
Lift
Weight
Thrust
Drag
Lift
• Upward force that keeps aircraft in flight
• Created by wing (airfoil)
• Directly opposed to weight
• Acts perpendicular to the Relative
Airflow
Airfoil
• Definition: Any surface designed to obtain a
reaction from the air through which it moves
• Camber – Curve of upper and lower surfaces
of wing
Airfoil
• Chord – Imaginary line running from leading
edge to trailing edge
Airfoil
• Boundary Layer – Thin sheet of air that sticks to
wing as it moves through air.
– When smooth, called Laminar Layer
– At Transition Point, it becomes Turbulent Layer
Laminar
Turbulent
Airfoil
• Relative Airflow
– Direction of air flowing relative to wing (AKA
relative wind)
– Created by motion of airplane through air (it’s flight
path)
Relative Airflow
Flight Path
Airfoil
• Lift acts perpendicular to the relative airflow
• Angle of Attack – Angle between relative
airflow and chord
How Lift Is Created
•
Newton’s Laws of Motion:
1. An object in motion tends to remain in motion
(i.e. inertia)
2. An external force must be applied to alter the
state of uniform motion of a body
3. For every action, there is an equal and opposite
reaction
How Lift Is Created
• When air hits wing, it’s deflected down
(downwash). The opposite reaction is an
upward force.
Downwash
How Lift Is Created
• Bernoulli’s Principle:
– The total energy in any system remains constant. If
one element increases, another must decrease to
balance it.
How lift Is Created
• Airflow over top of wing flows faster, therefore
pressure drops
• Airflow on bottom of wing flows slower, therefore
pressure increases
Centre of Pressure
• If we consider all distributed pressures to be
equivalent to a single force, this force will act through
a straight line.
• Point where this line cuts the chord is called the
Center of Pressure (C of P).
• When angle of attack increases:
– Lift and drag increase
– C of P moves forward
– After point of stall (AKA stalling angle), C of P moves back
Weight
• Downward force due to gravity
• Directly opposed to lift
• Weight of aircraft acts through Centre of Gravity (C
of G)
• C of G is point through which resultant of the weights
of all various parts of the aircraft pass
• Always acts towards centre of the Earth
Thrust
• Force exerted by engine and it’s
propeller or jet
• Air pushed backward, causing an equal
and opposite reaction, or thrust, in
forward direction
• Opposed to Drag
Drag
• Resistance to aircraft when moving forward
through air
• Opposed to Thrust
• Types:
– Induced Drag
– Parasite Drag
• Form Drag
• Skin Friction
Parasite Drag
• Created by all parts of aircraft which do not contribute
to lift (e.g. fuselage, wheels, antennas, etc).
• Form Drag - Created by form or shape of a body as it
resists motion through air
• Reduced by streamlining
• Skin Friction - Tendency of air flowing over a body to
cling to surface. Made worse by dirt, mud, ice etc.
Induced Drag
• Induced Drag - Caused by parts of aircraft which are active in
producing lift (e.g. the wing).
• Cannot be eliminated, but can be reduced.
• Greater angle of attack = Greater induced drag
• Airflow over top of wing flows inward. Airflow under wing flows
outwards. Therefore at trailing edge, small eddies are created.
These move to tips, resulting in wing tip vortices, which are main
component of induced drag.
• Ground Effect – Vortices reduced very close to the ground,
therefore induced drag is greatly decreased
Aileron Drag
• When aircraft banks to make a turn, one aileron is down and
other is up
• Down going aileron = More lift = more drag
• Up going aileron = Less lift = Less drag
• Result is yaw in opposite direction to which bank is applied
• Fixed by using Differential Ailerons, where up going aileron
goes up more than the other goes down.
Equilibrium
• When two forces are equal and opposite, object is in
equilibrium
• Therefore, when thrust and drag are equal and
opposite, aircraft will move forward at a constant
speed
• Equilibrium refers to steady motion and not state of
rest
• If either of these forces become greater than force
opposing it, equilibrium will be lost
Equilibrium
• When thrust and drag are equal and opposite,
they are in equilibrium.
– If thrust is greater than drag, airplane will
accelerate.
– If drag is greater than thrust, airplane will
decelerate.
• When lift and weight are equal and opposite,
they are in equilibrium.
– If lift is greater than weight, airplane will climb.
– If weight is greater than lift, airplane will sink.
Couples
• When two forces (such as lift and
weight) are equal and opposite, but
parallel rather than passing through the
same point, they form a couple.
• A couple will cause a turning moment
about a given axis (couples act around
the C of G).
Couples
• Weight ahead of Lift – Nose down
Couples
• Lift ahead of Weight – Nose up
Couples
• Thrust below Drag – Nose up
Couples
• Drag below Thrust – Nose down
Next Lesson
2.2 - Theory of Flight
Wing Design
From the Ground Up
Chapter 2.1.2:
Design of the Wing
Pages 20 - 23