Transcript Aerodynamics - Delta State University

Aerodynamics
Lecture 2
Chapter 2
Review from last class
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Balloons
Cayley
Wright Brothers
Aerodynamics
Da Vinci
Montgolifer
LIFT
What is lift?
What opposes Lift?
• Gravity
– The force that opposes lift is the pull of
gravity.
– Gravity acts on all bodies on or near the
surface of the Earth.
– The resulting force that gravity causes on
some mass is referred to as weight.
The Force called Lift
• In order to rise into the air, a force
must be created that is equal to or
greater than the weight acting in
the opposite direction.
Back to the Balloon
– In the balloon example last class, the lift force
was caused by an imbalance between pressure
over the surface of the balloon and the weight
of the balloon.
– With the balloon and its contents being lighter
than the surrounding air, the imbalanced
resulted in a net force in the upward direction.
Airplanes
• Airplanes are heavier than the air that they
displace and must rely on other means to
develop lift.
– What device is employed to develop lift on an
airplane?
The Physics of Lift
The principle of lift discovered by a
Swiss mathematician is?????
Daniel Bernoulli
• He never heard of an airplane.
• His work had nothing to do with flight.
• He was trying to explain mathematically,
the variation in pressure exerted by a
moving mass of fluid ( a stream of water)
Bernoulli’s Principle
• His equation says that the pressure plus ½
times the density times the velocity
squared must always equal a constant
value.
• Pressure + ½ x density x velocity squared
= constant.
In other words
• Static pressure + dynamic pressure =
constant.
– Static means still (last lecture)
– Static pressure is used to describe the pressure
exerted by a static, or still, air mass.
– Dynamic pressure is the pressure that would
be exerted if the fluid were brought to rest.
Bernoulli’s Point
• That fluid only contains so much pressure.
• If a fluid is a rest, all of the pressure is
static pressure,
• however, if the fluid is in motion, then
some static pressure is traded off for
dynamic pressure.
Bernoulli’s principle
• As the air flow speeds up, there is an
increase in dynamic pressure,
• the static pressure must be reduced
because the sum of the two pressures must
be the same.
Example
• Static pressure + dynamic pressure= total
constant.
• 6+6 =12
– Now suppose the fluid speeded up.
– SP DP= TC
– 4+ 8 = 12
– Dynamic pressure increased so static pressure
must decrease so the end result is constant
(p.14).
Continuity
• The law of continuity or continuity
equation called the Law of Conservation of
Matter.
•
Applied to a moving stream,it states
that in a moving stream of fluid, the
density times the cross sectional area of the
flow times the velocity must be a constant.
– Q
x A V= constant.
Venturi Tube
• A venturi tube is a good example for
Bernoulli’s principle because it has a
restricted area of flow.
– Think about canoeing down a river. When is
the water static?
– When is the water moving rapidly?
Venturi
• A flow coming into a venturi tube will
have an increased velocity, due to a
reduced area, by the continuity law.
– If the velocity is increased, the pressure will
be decreased by Bernoulli’s principle.
• A tube connected at the reduced area will create a
suction. (principle behind gyro instruments, p. 16)
How airfoils create lift
Ahead of the airfoil, the air has a uniform
velocity associated with it across the entire
stream.
• When it reaches the airfoil, it must divide,
with part of the stream flowing over the top
surface and part over the bottom surface.
How an airfoil creates lift
• At this point the cross-sectional area of the
stream is reduced by the amount of space the
airfoil takes.
– According the the law of continuity, this reduction
must cause an increase in velocity.
– The end result is a greater lowering of pressure on the
top surface than the bottom, even though the pressure
is lowered somewhat over the bottom surface.
Next Time
• We will begin on airfoil terminology.
• If you have not read all of chapter 2 please
do so for next time.
Quiz on Lecture 2
Please take out a sheet of paper.
Include today’s date and your name.
Quiz on Lecture 2
• Please explain Bernoulli’s principle.
• What is the difference in static pressure
and dynamic pressure?
• Explain the Law of Continuity.
• How can a venturi tube create lowered
pressure?
• How does an airfoil create lift?