söndag 12 augusti 2012

ALLSTAR Network: How Airplanes Fly: Incorrect

The Aeronautics Learning Laboratory for Science, Technology and Research funded in part by NASA describes How Airplanes Fly Level 3 for teachers of high school / lower division college students as follows in an article by David Anderson and Scott Eberhardt, authors of the book Understanding Flight: 
  • Almost everyone today has flown in an airplane. Many ask the simple question "what makes an airplane fly"? The answer one frequently gets is misleading and often just plain wrong. 
  • We hope that the answers provided here will clarify many misconceptions about lift and that you will adopt our explanation when explaining lift to others. 
  • So, how does a wing generate lift? To begin to understand lift we must return to high school physics and review Newton’s first and third laws.
  • The lift of a wing is equal to the rate of change in momentum of the air it is diverting down.
  • The amount of air pumped down for a Boeing 747 to create lift for its roughly 800,000 pounds takeoff weight is incredible indeed.
  • Pumping, or diverting, so much air down is a strong argument against lift being just a surface effect as implied by the popular explanation.
  • So how does a thin wing divert so much air? When the air is bent around the top of the wing, it pulls on the air above it accelerating that air down, otherwise there would be voids in the air left above the wing. Air is pulled from above to prevent voids. 
  • The natural question is "how does the wing divert the air down?" 
  • Why should a fluid follow a curved surface? 
  • The answer is viscosity; the resistance to flow which also gives the air a kind of "stickiness". 
  • Viscosity in air is very small but it is enough for the air molecules to want to stick to the surface.
  • At the surface the relative velocity between the surface and the nearest air molecules is exactly zero. 
  • Because the fluid near the surface has a change in velocity, the fluid flow is bent towards the surface. Unless the bend is too tight, the fluid will follow the surface. This volume of air around the wing that appears to be partially stuck to the wing is called the "boundary layer".
The message is that lift is generated by air being diverted down (which is a trivially correct), and air is diverted down because of some kind of "stickiness" of air (which is definitely not correct), as made clear on Text Book: Not Understanding Flight and on The Secret of Flight.

The teachers of high school/lower division college students are thus led by scientific expertise to present  trivialities and nonsense. 

I will now inform ALLSTAR about the New Theory of Flight and report the reaction. 

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