In a world where we no longer actually need pilots to fly planes, we are still debating about the fundamental theory of how we actually get in the air in the first place. Ironic, huh?

m.smh.com.au/travel/travel-news/engineer...-20120126-1qjrc.html

I'm confused as to how Bernoulli's Principal can be used as a theory for the generation of lift? His study was conducted using venturis, which are closed tubes. A wing is not a closed tube. You can't observe the behaviour of a fluid in a closed tube, take away the tube and then expect the fluid to behave in the exact same manner.

This is by no means a shot at the AERODY book, I know you have to teach it because of CASA. It's just the whole concept seems a bit implausible to me. It's like whoever pondered using Bernoulli's principal as the basis for lift generation was just stumped and needed an easy answer.

This argument has gone on for years. Bernoulli's principle is of course quite valid but it's true that it doesn't explain the production of lift for an aerofoil in a free airstream. I have mentioned in the Aerodynamics book that the CASA syllabus specifically requires that the candidate is able to explain the production of lift in terms of Bernoulli' principle. That is why I have covered it in the book - the aim of the book is to get you through the CASA exam.

The so called 'coanda effect' is now favoured as a better explanation of the process of lift production. I suppose what really matters is that a pilot knows how a wing is expected to behave rather than the exact process involved.

G'day Bob,

Yeah, I completely understand why you teach it. I mean, you teach to the syllabus! I just think it's a little ironic that as far as aviation has advanced over the last 100 or so years there's still no agreement on how it all actually happens!

Did you see the video that engineer made?

Yes I did. I note that we all agree that the airflow over the top surface does speed up and the pressure it exerts on that surface is less than the pressure exerted on the bottom surface. Those pressures have long been measured in wind tunnel tests and there is no argument there. The main argument with Bernoulli is the assumption that the airflow over the top and bottom surface must take the same time to complete the journey.

Over the years, this argument has become so heated that some of the supporters of each side have developed an almost religious zeal about supporting their opinions. Some of the instructors I have met who support the coanda effect become almost hysterical if anyone mentions pressure as having anything to do with lift generation. That's nonsense of course, pressure has everything to do with lift generation. The eternal argument is simply about how the pressures are modified as the air passes over the wing.

A good video demonstrating the Coanda Effect

It's crazy how heated it can get!

I have a question on symmetrical aerofoils and excuse the question if it's a bit silly.

How do they effect the dynamics of normal straight and level flight? It seems fair enough that they're used to improve inverted flight performance, but wouldn't it sort of cancel itself out? If the bottom is producing lift in the same way as the top because of their symmetrical shape, wouldn't it be producing the lift in a direction opposite to the top of the wing? I.e. the top is producing lift towards the sky and the bottom is producing lift towards the ground, thus cancelling eachother out and produing a net value of lift of 0?

Cheers =]

Your logic is quite correct. A symmetrical aerofoil would produce no lift at zero angle of attack. The pressure distribution over the top and bottom surface would be symnmetrical and there would be no net force generated in any direction. However that is only the case at zero angle of attack. A symmetrical aerofoil is quite capable of making lift - and lots of it - but it always needs a greater angle of attack than a normal cambered aerofoil. That means more drag for any given lift value and therefore a reduced lift/drag ratio.

That's the penalty we must pay for better inverted performance. As I often tell my aerobatic students, a symmetrical aerofoil flies 'equally badly' either way. It will always be less efficient than a cambered aerofoil and you just have to put up with that if you want to fly upside down!