Wednesday, January 28, 2009

How did Captain Sully combine thermodynamics with flying?

We should actually thank Captain Chesley Sully Sullenberger for his incredible skills at gliding an engineless jet safely into the icy waters of the Hudson River without any loss of life or major injury to the passengers and crew. The skill Sully had mastered was that of gliding. Gliding entails a fixed wing object using the pressure of moving air to create lift to counter the force of gravity. The Chinese invented kites as early at 500 BC. In ancient Chinese and Indian cultures the kite was used to ward off evil spirits. Leonardo da Vinci developed a design for the ornithopter as a device to mimic the flight of birds. Leonardo believed humans had enough muscle power to take off and fly. Alas even to this day no person has accomplished this.

A German engineer Otto Lilienthal was the first true airman in history. He demonstrated in a repeated way that controlled glider flight was possible. When Otto was 14 he attempted to fly using Leonardo’s ornithopter design. After repeated failure Otto realized that flight could not be achieved merely by a flapping of wings. In 1889 Otto published a book titled “The Flight of Birds as a Basis of Aviation”. Otto determined that birds do not only gain thrust by flapping their wings but also from the propeller-like action of their primary feathers. Otto also calculated the size of a wing that is needed to support a certain mass. In 1891 Lilienthal built and flew the first truly successful glider in history. The hang-glider of today is based upon it. Otto made over 2000 flights from 1891 till 1895. His longest glider flight was a quarter of a mile at an altitude of 75 feet attained by having a running start from a 50 foot hill. Otto unfortunately died the day after his longest flight due to a malfunction of his glider when he fell down the very same 50 foot cliff.

The history of motorized aviation starting from the Wright Brothers to today’s jets draws on the experience and knowledge gained by folks like Lilienthal. Jets have very large carbon emissions due to their high speed, large mass and therefore very powerful engines. Without the thrust of their jet engines they are simply too heavy to stay aloft for any extended period of time. Sully did prove however that the engineless jetliner with a mass of 70 tons can be glided for about 5 miles beginning from an altitude of 3,200 feet. The probable speed of the jet on impact into the Hudson was only between 120 and 140 miles per hour. Sully accomplished this by extending the wing flaps to the maximum position, thereby slowing the plane as much as pos sible without putting it into a tail spin. In the past 50 years of jet aviation this is the only emergency water landing a fully laden plane that ended without any fatalities.

Sully is the hero of the Hudson and he proved that there is no substitute for experience and cool nerves using the laws of physics to control an un-powered flight. Lilianthal looking down at US Air Flight 1549 from up there must have remarked “I knew it could be done”.