From: Ray®	09/11/2001 17:18:21
Subject: AVIATION FAQ	post id: 490476
How does a plane's wing generate lift? [The Bernoulli Effect, Ski Effect, and Newton's 3rd law] From: Esselte Bernoulli accounts for very little of the lift produced by a wing. What a wing really does is displace downwards a weight of air equal to the weight of the aeroplane The lift of a wing is equal to the change in momentum of the air it is diverting down. Momentum is the product of mass and velocity (mv). The most common form of Newton's second law is F= ma, or force equal mass times acceleration. The law in this form gives the force necessary to accelerate an object of a certain mass. An alternate form of Newton's second law can be written: The lift of a wing is proportional to the amount of air diverted down times the vertical velocity of that air. Bernoulli effects account for a very small proportion of the overall lift, as I have already said. Take the case of a Cessna 172, which is popular, high-winged, four-seat airplane. The wings must lift 2300 lb (1045 kg) at its maximum flying weight. The path length for the air over the top of the wing is only about 1.5% greater than under the wing. Using the Popular Description of lift (Bernoulli), the wing would develop only about 2% of the needed lift at 65 mph (104 km/h), which is "slow flight" for this airplane. In fact, the calculations say that the minimum speed for this wing to develop sufficient lift is over 400 mph (640 km/h). If one works the problem the other way and asks what the difference in path length would have to be for the Popular Description to account for lift in slow flight, the answer would be 50%. The thickness of the wing would be almost the same as the chord length. A Physical Description of Flight