Forces of Flight
Scalar and Vector Quantities
A scalar quantity has only magnitude
A vector quantity has both magnitude and direction
Scalar Quantities
Vector Quantities
Length, area, volume,
speed, mass, density,
pressure, temperature,
energy, entropy, work,
power
Displacement, direction,
velocity, acceleration,
momentum, force, lift, drag,
thrust, weight
The Car: A Vector Example
• A car drives on a straight
road heading due east at
60 mph...
• The speed of the car is
60 mph East
60 mph. This is the first
part of the vector.
• The speed tells us the magnitude; size of
the vector.
• The direction of travel is due east. This is
the second part of the vector.
Weight or Gravity
• A negative force
• A force always directed towards the
center of the earth
• Dependent upon the mass of all the
airplane parts, plus the amount of fuel,
plus any payload on board (people,
baggage, freight, etc.)
Weight or Gravity
• During a flight, an airplane's weight
constantly changes as the aircraft
consumes fuel.
• The distribution of the weight then
changes, so the pilot must constantly
adjust the controls to keep the airplane
balanced, or trimmed.
Lift
Lift: The force that directly opposes the weight
of an airplane and holds the airplane in the
air.
Simplified:
Lift: The upward force that
works against the
downward pull of gravity
Lift
• A positive force
• Needed to overcome the
weight force
• Lift is directed perpendicular
to the flight direction.
Lift
• Amount of lift depends on
several factors including
– Shape of the aircraft
– Size of the aircraft
– Velocity or speed of the aircraft
– Shape of the wing
– Density altitude
Lift
• Most of the lift is generated by the
wings
• Air pressure below the wing is greater
than air pressure above the wing
Drag
Drag: The force that resists any object
trying to move through a fluid.
Simplified:
Drag: Resistance of the air (technically a
fluid) against the forward movement of an
airplane.
Drag
• As the airplane moves through the air,
the air resists the motion of the
aircraft.
• Directed along and opposed to the
flight direction.
Drag
• Factors that affect the amount
of drag
– the shape of the aircraft
– the "stickiness“ of the air
– the speed of the aircraft
– The type of material used on the
aircraft
Thrust
Thrust: The force which moves an aircraft
through the air.
Simplified:
Thrust: The push that moves an airplane
forward.
Thrust
• Overcomes drag
• Airplanes use a propulsion
system to generate the thrust
• The amount of thrust depends
on many factors associated
with the propulsion system
– type of engine
– the number of engines
– and the throttle setting
Thrust
• The direction of the thrust force
depends on how the engines are
attached to the aircraft.
• When engines are located under the
wings, parallel to the body, thrust acts
along the body centerline.
Forces Summary
• The motion of the airplane through the
air depends on the relative strength
and direction of the forces you’ve
been shown.
• If the forces are balanced, the aircraft
cruises at constant velocity.
• If the forces are unbalanced, the
aircraft accelerates in the direction of
the largest force.
Image Resources
• National Aeronautics and Space Administration (NASA). (n.d.).
Virtual skies: Aeronautics tutorial. Retrieved June 24, 2009, from
http://virtualskies.arc.nasa.gov/main/maeronautics.html