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 LiftLift 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

AirfoilsGenerally 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 pv2 = 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