I have read with interest the theories of lift and the arguments to support them. The first ever lecture I gave was on Helicopter Theory of Flight, in 1974, supported by Bernouli's Theorem. At about the same time in my life it was my job to stand underneath a hovering helicopter and, as you can well imagine, it was drafty! For many years until 2009 I would have supported the idea of the application of Bernouli's Theorem to produce lift from wing shape. Between these occurences I had also learnt to fly and sail. Without thinking I accepted the proposal that the sail was effectively a wing on it's side, producing a lateral force, also called lift.

In 2009 I was asked to lecture on aerodynamics and decided to brush up on my theory, expecting to find Bernouli's theorem everywhere. I was surprised and discovered a great controversy which made me re-evaluate my beliefs. I had meanwhile made the personal discovery that the principle that produces "lift" from a wing was the same as that for a propeller (ship's or aircraft) and a rudder (likewise). Also I was reminded that there is indeed a significant downwash from a hovering helicopter and, of course a rearward wash from a propeller. The problem I therefore have with the application of Bernouli's excellent and accurate theory on fluid flow and pressure drop to aerodynamics is that a sail doesn't have much of a difference in distance round the inner and outer surfaces - indeed it can be considered negligible. But it produces significant lift! Then I thought some more and considered that if the air below the wing is at a higher pressure than the air above the wing, the direction of airflow (high to low) would always be UP. How come air was coming down in significant quantities from the hovering helicopter? So, in conclusion, based on the fact that the travel distance of air round a sail is almost exactly the same and therefore the application of Bernouli's Theorem would be wrong and the fact that a propellor does the same job as a wing, I now believe that the correct application for lift must be equal and opposite reaction (Newton 3)caused by a change in momentum of air downwards. I have several books of the theory of flight and have spent hours online researching, but it is observation of personal experience that carries weight and as a sailor and a flyer, I agree with the camp that supports downwash and Newton.