Have you ever wondered how birds and airplanes lift off the ground to fly? While researching the life of the snow goose for my last article in The Post-Journal, I learned why migrating geese fly in a ''V'' formation. Today, I will describe why and will describe how birds and airplanes become airborne.
Achieving flight for a bird or airplane is remarkable, but has a simple physical explanation. Lift is needed which is generated by air flowing over a wing. Forward motion moves air over wings; in the situation of a bird, flapping of wings pushes the bird forward while an airplane is moved forward by the thrust of the jet engine or propeller. The use of a handheld fan to cool ones face on a hot summer day helps one appreciate the force a bird's wing can exert on air to move itself forward. The top of a bird or airplane wing is rounded and the bottom is flat. This type structure is called an airfoil. If you imagine the top of the wing as having an exaggerated curve like the top of a long loaf of French bread, it is easier to understand that air travelling from the side over the top of the loaf travels a slightly longer distance from the front edge or side of the loaf to the back edge or other side of the loaf than air traveling under the loaf. Air traveling over the top of a wing meets the air traveling under the wing at the same time. Since the air over the top travels farther than air under the flat wing bottom but in the same time, it must travel faster than air under the wing. Faster moving air exerts less pressure down on the wing than the slower moving air under the wing. Less pressure on the top of the wing ''sucks'' the wing up and more pressure below pushes the wing up, creating lift! This concept is explained clearly by John Cronin in his short book ''Your Flight Question - Answered By A Jetliner Pilot'' (1998), when he compares air flowing over a wing to spraying water by putting ones thumb over the end of a running garden hose. Thumb pressure narrows the end of the hose so water speeds up spraying many feet away which children might do spraying their friends on a hot summer day. As water passes one's thumb coming out of the hose, it loses pressure from the municipal or rural water pump but gains speed. Likewise, air traveling faster over the top of the wing exerts less pressure on the wing so the pressure difference from wing top to bottom pushes the wing up. Physics students know this concept as the Bernoulli principle which states, ''the faster a fluid moves (air is a fluid) the less pressure it exerts,'' in this case against a wing.
The lift from the wing of a Boeing 747 jetliner must be extraordinary to carry the weight of 300 passengers and their luggage.
Article Photos
This taxidermy specimen of a snow goose in flight demonstrates the rounded upper surface and front edge of the wings which help generate lift for flight.
Photo by Robert Ungerer
Geese fly behind the wing of the bird in front of them because updraft of wake turbulence, also known as wing tip vortices, at the wing tip of the forward bird gives them extra lift.
An explanation of wing tip vortices is interesting and crucial to understanding airport safety. Stephen Dalton, in his book ''The Miracle of Flight,'' makes the point that air over the top of the wing has low pressure while air on the bottom has high pressure. Air wants to move to the low pressure on top of the wing, hence escaping around the tip of the wing. This creates the updraft of the wing tip vortice used by flying geese but, creates massive ''wake turbulence'' behind all landing aircraft. The turbulence of a large airliner can tip over a smaller plane landing minutes later on the same runway.
Thank goodness for the evolution of wings allowing a diverse number of beautiful birds to grace our planet and for man's ingenuity to invent aircraft which permit travel to experience lands and cultures far away from home.