Bernoulli takes too much credit … and Coanda little!
Flying is something I enjoy – and although I am a recreational pilot now, I have been a theoretical pilot for far longer – in fact, since my teenage years!
I want to start my Aviation posts with a discussion of one of the biggest fallacies of flight – HOW airplanes fly. WARNING – The discussion ahead is for propeller-heads (pun fully intended) …
Most of us who remember our high school physics, will recall the theory of flight that we were taught – essentially that lift occurs due to Bernoulli’s principle. Recapping in a nutshell, the explanation was that the path for the airstream on the top of the aerofoil (the shape of the wing) is longer than that on the bottom. This means that for the air to meet at the same time at the trailing edge of the wing, (huge assumption – I’ll explain), the air stream needs to be traveling much faster over the wing than below it. According to Bernoulli’s theorem, this naturally results in a pressure differential and causes a force upward, thereby creating lift on the wing.
I have always wondered about this theory. When I was younger, it didn’t make sense intuitively – it wasn’t convincing – but my understanding of physics wasn’t advanced enough to dispute it. As I went through my engineering years in college, I found that my suspicion was indeed correct – and in fact, me and a few friends did experiments to rebutt that theory.
Here are some basic reasons why the popular explanation makes little sense.
First, the premise that air stream has to travel faster over the upper surface of the wing relative to the lower surface air stream is entirely based on the assumption that the two air streams need to meet at the trailing edge. There is no basis for that assumption. There is no reason why the air streams cannot separate – as they do in most airflow situations such as behind a car, at wingtips etc. So that premise is flawed.
Further, calculations show (trust me) that the velocity (as predicted by Bernoulli’s theorem) required by the wing relative to the air, to generate the lift required to take flight is far in excess of the actual rotation (lift-off) speeds of most airplanes. Indeed, the airplane I fly lifts off at 60 mph – and weights approximately 1000 pounds. According to Bernoulli’s formula, I would need to be at a speed in excess of 200 mph to generate 1000 pounds of lift! A 747 would need to take-off at over 280 mph!
Additionally, aerobatic planes fly upside down – as I am sure you’ve noticed. Unless the wings could somehow reverse their shape, the traditional Bernoulli’s principle-based explanation makes no sense – they would simply accelerate rapidly to the ground and crash!
Finally, look at plain old paper planes – they fly – and their wings are flat – no aerofoil. How do they fly – or float?
SO HOW DO PLANES FLY?
Airplanes fly because of the COANDA effect – the tendency of fluids to stick to surfaces. Look at the picture below – something quite intuitive and familiar – the stream of water sticks to the side of the bowl. For everyone who has ever tried pouring coffee out of a mug – isn’t it annoying how it runs down the side of the mug, instead of falling straight down vertically? That’s Coanda effect for you.
So here is how it REALLY works! Lift is created when the UPPER surface of the wing pulls air that is flowing over it downwards into a downdraft as a result of the Coanda effect – in effect the air sticks and follows the curved upper surface of the wing downward. Enter Newton’s third law – as the wing “pulls” the air downward into a downdraft, the wind exerts an equal and opposite force upward – and voila! – we have lift! One very important point to note – the UPPER surface of the wing PULLS the air down, as opposed to the lower surface pushing it … pushing the air down does help, but only contributes a small fraction of the total force causing the lift.
This means that the upper surface of the wing is the most critical surface for generating lift. Which explains why all the stuff hangs off the wing bottom (engines, armaments, undercarriage etc.) while the upper surface is clean and left all alone to perform the singular and fundamental job of lifting the plane into the air.
