Originally Posted by
Matthew Parsons
I won't disagree with what you said, Deemar, but having the precise definitions about what the author in the NASA article (I don't have it) says could clarify things for us.
Well if anyone does have this article, then I'd be interested to see how it explains things. This is certainly one of the most debated topics amongst amateur aerodynamicists.
Originally Posted by
Matthew Parsons
I have heard the theories broken down to Bernoulli being the portion of lift due to the difference in pressure and the remainder being due to the deflection of the free stream. You can describe the latter with coanda effect if you wish.
Which misses the point soemwhat, they aren't two different effects, they are one and the same effect. If there is a deflection of the free stream, the there MUST be a difference in pressure across the lifting surface to cause this. If there is a difference in pressure across a lifting surface then there MUST be a deflection of the free stream. Two ways of looking at it, but one physical effect underlying it.
Originally Posted by
Matthew Parsons
The point is, most of the descriptions of Bernoulli's theory don't discuss what happens to the air aft of the trailing edge, which as Nick points out is the lift that we can comprehend easily, and it is the majority of the contribution of the total lift according to the spectral NASA report on "incorrect theories".
You are correct in that proponents of the different theories will only talk about their favourite aspect of the physical process that is lift generation. I guess my previous post was an attempt to correct that gap.
It is also worth noting that there are incorrect explanations of some of these effects. One in particular is the theory that the air on top of an airfoil goes faster because it has a longer distance to travel and it needs to "keep pace" with its "brother" air particle that travels along the bottom of the airfoil.
Originally Posted by
Matthew Parsons
If we only look at the deflection of the airflow, or more precisely the change in momentum of that airflow, then we miss the small contribution to lift due to the pressure differences.
So without seeing the NASA report, I can understand why someone may look at the theories in such a way as one being a small contribution and another being the most significant.
Actually, if you look at the lift created by the change in momentum, it will exactly equal the lift created by the pressure difference. This is because they are one and the same lift force.
Originally Posted by
Matthew Parsons
In the end, it doesn't matter too much to the average pilot. I disagree that line pilots don't need to know the lift equation. Understanding what your aircraft is going to do when conditions change is important, and I believe the lift equation is a good teaching tool to help us develop some anticipation. How you derive it doesn't matter too much because the formula that we use works well enough for the majority.
Fair point,
I guess if I was able to convince everyone then that would be one less conversation to have when you're not flying.
In terms of understanding what the aircraft is going to do as conditions change, then you are right. An understanding of the lift formulas, along with the Cl and Cd curves is probably a good thing for a pilot to have.
Daniel (I hope you all had a good Christmas)