Transcript Animal Flight - Physics 420 UBC Physics Demonstrations

Animal Flight
By Lisa Acorn
Outline
• Start with a presentation about
animal flight
• We will build our own wings and test
mine
• Finish by calculating what percentage
of our body weights we’re able to
generate lift for
A little background
Four groups have evolved flapping flight
Vertebrates: pterosaurs, birds and bats
Invertebrate: insects
Invertebrates
• Insect wings
evolved
independently and
consist of non living
material and rods
• Can control
movement of wing
through pulling on
the rods
Vertebrates
• Wings amongst the
vertebrates
evolved through
the modification of
an existing limb
• They have muscles
that are able to
control their wings
Muscles
• As the size of an
animal increases,
the cross sectional
muscle to volume
ratio decreases
• The strength of a
muscle depends on
its cross sectional
area
Bird Muscles
The pectoralis muscle in birds is very large.
It can account for up to 35% of a bird’s
body weight
There are 49 other muscles associated with
wing movement
Physics
Air
• Air is NOT nothing!
• Air is composed of different types of
molecules: 78% nitrogen, 21% oxygen
and 1% others
• The density (mass/volume) of air is
1.2 kg/m3
• Air can create and react to forces
Forces
• Newton’s law states that a force is
equal to mass X acceleration
• An action on an object that can alter
the motion of that object
(move/speed it up, stop/slow it or
change its direction)
• Basically: a push or a pull
Forces
• Forces are vectors which means that
they have both a magnitude and
direction
• Forces in the same direction are
added together; those in opposite
directions are subtracted
• Pressure is defined as force per unit
area
Forces of Flight
• Flight is a balance of four forces : weight, lift, drag and
thrust (propulsion)
Thrust and Drag
• Thrust and drag are the forces that
act in or opposed to the direction of
movement
• Thrust refers to forces that are in
the direction of movement
• Drag are forces that act in the
opposite direction
Weight and Lift
• Weight acts in the downwards
direction and is equal to mass X the
acceleration due to gravity (9.8m/s2)
• Lift is the force that is perpendicular
to the flow of air
Producing Lift
Lift is produced when air is directed
down
Any shape will produce lift when moving
through air (hand out car window)
Airfoils are shapes that produce lift and
minimize drag
Airfoils
Generally have a rounded leading edge
and a sharp trailing edge.
Bernoulli and Lift
• Air moving over the
top moves faster
• According to
Bernoulli faster
moving air has a
lower pressure
• There is a net
force up
Common Misconception
• Due to the longer path length over the top
of the wing the molecules above have to
move faster to “catch up” with the ones
below
Reality
• Air does flow faster over the top of
the wing but it has nothing to do with
“equal transit times”
• The deflection of air downwards
leads to a partial vacuum above the
wing into which other air molecules
rush to fill
• So Bernoulli’s Equations still apply
Lift
Bernoulli’s equation:
P + 1/2 pv2 = constant
P = pressure, p = density
v = velocity of air
P + 1/2
2
pv
= constant
• Note that since adding the two terms
together yields a constant when one
variable changes the other must
change in a complementary way
• It is the square of the velocity that
matters, so a small change in velocity
will lead to a large change in pressure
Flapping Flight
• There are 2 phases
in flapping flight
• The downstroke
produces most of
the thrust and lift
• The upstroke also
contributes a small
amount of lift but
mostly produces
drag
Take Off
•During take off there is no lift
produced via air flow.
•Birds have to produce a large amount
of force to displace enough air to
achieve take off.
•There is evidence that birds rely
heavily on power produced by their legs
to achieve lift off.
•Some larger birds have to get a running
start.
Take Off
Flapping Patterns
• There are many different patterns
but the basic is: downstroke is
forward and down, while upstroke is
up and back
• Animals such as pigeons have a figure
eight pattern
• None simply flap them in the vertical
plane
Hovering
• Limited to small
birds and insects
• Power requirement
is much higher for
hovering than for
forward flying
Hovering Flapping Pattern
• The downstroke moves forward and
horizontally
• At the end of the downstroke, the
wing is rotated 180o
• The upstroke is also horizontal but
backwards
• The wing reverses the rotation and
this repeats
The Challenge!
The Challenge
• In groups of 4 or 5 build a set of
wings!
• Once you’ve completed your wings we
will use a force platform to measure
the maximum and average upward
forces that you’re able to produce
• The group with the best set of wings
will get a prize
What is the “best” set of
wings?
• Three factors will be considered.
– Results
– Concept/Theory
– Style
Bonus Challenge
• During your build time anyone who is
interested may try out the wings that
I’ve created
• The person who is able to generate
the highest average upwards force
will receive a prize
Consider the Following
•
•
•
•
Surface Area
Velocity
Weight
Flapping Pattern