The Mechanics of Flying! 

The mechanics of bird flight revolve around many of the topics we discussed in class, such as Newton’s 3rd Law, differences in air pressure, resistance, friction and conservation of energy.

Getting Off the Ground: 

Birds convert energy stored in their muscles to kinetic energy as they overcome the drag of pushing through air and lifting up to flying height.  We see this energy conversion when the bird flaps its wings up and down.  The bird obtains its initial lift as it pushes its wings up and down, converting energy, and encounters Newton’s 3rd Law, every action has an equal and opposite reaction.  The bird’s wing pushes against the air and the air pushes back with an equal and opposite force.  Unfortunately, with every down stroke there must be an up stroke, what goes up must come down.  If the bird had an equal upward stroke to its downward stroke, the net force of the bird’s wing flaps would equal zero.  The up stroke would also be subject to Newton’s 3rd Law and would experience an equal and opposite force from the air.  Therefore, the force of the up stroke would cancel out the force from the down stroke.  

 

 

Angle of Attack is the angle between a reference line and the current position of the bird's wing.

 

Basically it is the range of the birds wing against the equilibrium, at rest position.

Luckily, bird wings are hinged and they have the ability to change their wing angle which results in either a reduction or increase of surface area.  The bird alters its angle of attack to better create the most efficient movement.  On the up stroke, the bird angles its wings to decrease the surface area which results in a decrease in air resistance of the air against the bird’s wing.  Ultimately that meansText Box: The Angle of Attack is the angle between a reference line and the current position of the wing. Basically, it is the distance the wing covers as the bird flaps its wings! that the angled wing on the bird’s up stroke, induces a smaller force against the wind so according to Newton’s 3rd Law, the air matches the up strokes smaller force.  Therefore the force from the down stroke is greater than the force from the up stroke and the bird takes off!

Up in the Air:

 

    Once the bird has flapped its wings and reached flying height, a new method of flight is utilized.  Due to their wing form, called an airfoil, the birds can glide without flapping their wings.  An airfoil is a special type of wing that birds have as well as things like airplanes.  As the wing moves through the air, the air passes above and below the wing.  The wing's upper surface is shaped so the air of the top of the wing speeds up and stretches out.  The air below the wing moves in a much straighter direction and constant speed.  This creates a difference in air pressure above the wing and below the wing.  The air pressure above the wing is less than the air pressure below the wing.  Since high pressure moves towards low pressure, the air below the wing pushes upwards towards the air above the wing.  This movement of air from high pressure to low pressure takes the wing with it and gives the bird lift even when it is not flapping its wings!

 

        Unfortunately, the bird’s work is not over once it has made it up into the air.  The bird must fight against drag!  There are three main sources of drag, not counting the effects of gravity due to the bird’s weight.

1.    

  1. Frictional drag is the most obvious and straight forward source of backward momentum that the bird encounters.  Frictional drag comes from the force of the air against the bird’s body.  Bird’s have a streamlined body which helps reduce the frictional drag.
  2. Lift-induced drag occurs when an object redirects the airflow coming towards it.  Lift is produced by the changing direction of air flow around a wing, so it is clear that a bird really relies on changing airflow to stay airborne.  Wing-tip vortices are tubes of circulating air which are left behind the wing as it generates lift.  The wing-tip vortices create lift-induced drag by deflecting the air downwards and behind the wing.
  3.   Form drag arises from, obviously, the form of the bird.  Birds with large cross-sections have high form drag.  Also, the faster a bird goes, the more it is affected by form drag. 

Making it Easier:

                         Birds have developed lots of physical and mechanical techniques that help reduce drag and have a more efficient and productive flying experience.


The bones of a bird are hollow and lightweight which reduces the overall weight of the bird.  This reduced weight creates less of a gravitational pull on the bird.  Also, when the bird pushes against the air and according to Newton’s 3rd Law, the air pushes back with an equal and opposite reaction, a lighter bird will go a further distance then a heavier bird.

The feathers of a bird are well suited for effective flying.  They are very light and flexible but still quite tough.  The feathers are smooth and flat which reduces air resistance.  The vanes of the feathers have hooklets which are called barbules that zip the feathers together, giving them strength to hold the airfoil.

 

The skeleton's breastbone has also adapted into a large keel, which is an extension of the sternum (breastbone).  The keel provides an anchor to which a bird's wing muscles attach, thereby providing adequate leverage for flight.  The keel is also helpful in that it is suitable for the attachment of large, powerful flight muscles which the bird needs to take off.