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The French Civil Aviation University
081 Principles of Flight
Aerodynamic Forces
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Content:
1) Forces acting on an airplane
2) Lift
3) Drag
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How does an
airplane fly ?
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1- Forces acting on an airplane
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In straight and level flight at constant speed with no turning
moments the aircraft is said to be in equilibrium, it means:
THRUST = DRAG
WEIGHT = LIFT
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2 -THE LIFT
The purpose of the wing is to produce enough upward force to
raise the aircraft mass.
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 Chord Line : The chord line is the straight line intersecting the leading and trailing
edges of the airfoil
• Relative airflow: The direction of the airflow with respect to an airfoil
• Angle of Attack (AoA): The angle measured between the relative wind and the chord
line of the airfoil
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The physics principles used for the creation of Lift
Newton's Third Laws
To every action, there is an equal and opposite reaction.
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The physic principles used for the creation of lift
Bernoulli’s Principle
As the velocity of a fluid ( in this case the air) increases , its pressure decreases
P+V=C
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 As air flows over the top surface of an airfoil it is accelerated, therefore pressure is
reduced.
 The pressure difference between the low pressure on the top of the wing and relatively
higher pressure on the bottom of the wing creates an aerodynamic force that we call Lift.
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Some videos:
Bernouilli`s principle:
https://www.youtube.com/watch?v=gGpdUQh_DCM
Newton’s law :
https://www.youtube.com/watch?v=mn34mnnDnKU
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3- LIFT FORMULA
L= ½ . ρ . V2 . S . CL
Where:
L = lift
Cl = lift coefficient ( AoA, camber, surface condition )
(rho) = air density
V = air speed
S = wing surface area
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2 ways to control the LIFT !
L= ½ . ρ . V2 . S . CL
Air speed or Angle of Attack (AoA)
 The faster the airspeed, the more
lift
produced
 As AoA increases lift increases,
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However there is a Limit !
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3 - THE DRAG
Drag is the aerodynamic force that opposes an aircraft's motion through
the air. Drag is generated by every part of the airplane.
The aircraft are submitted to two types of drag:
1- Induced drag: Drag caused by the lift
2- Parasite drag: a) Form drag
b) Interference drag
c) Friction drag
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Form drag is created when the airflow has actually separated
from the airfoil
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Interference drag is created where two different objects meet and disturb the
free stream airflow
Examples of this include where:
• The wing meets the fuselage
• The elevator meets the fuselage
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Skin friction drag is the force between the free stream air and any
object passing through it
KEEP THE AIRPLANE SURFACE CLEAN !!!
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As the airspeed of an airplane increases parasite drag continues to
increase exponentially
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Induced Drag
• Created by parts of the plane that create lift
• Greater the lift, greater the induced drag
• Induced drag is proportional to lift
Lift create wingtip vortices and induced drag
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Factors which affect induced drag are:
- wing shape
- aspect ratio
- lift and weight
- speed
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Total Drag
Total drag = Parasite drag + Induced drag
Note:
• At low airspeeds induced drag is high and parasite drag
in low
• At minimum drag speed both induced and parasite drag
are at a minimum
• At maximum speed induced drag is low and parasite
drag to high
• Minimum drag occurs where the induced drag = parasite
drag
Minimum Drag Speed = Best Glide Speed
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DRAG FORMULA
D= ½ . ρ . V2 . S . CD
Where:
CD - Co-efficient of drag – shape and angle of attack
ρ (Rho) – Free stream air density
V – airspeed (TAS)
S –wing surface area
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Summary
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REFERENCES :
 https://www.boldmethod.com
 https://www.nasa.gov
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THANK YOU FOR YOUR
ATTENTION
Any Question ?
Mail : [email protected]
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