Hi all,
I am sorry to raise such basic question here, but I really hope that you can help me clarify my understanding. When it comes to lift generation, there are plenty of theories. When I dive in with this topic, I am so confused because some widely accepted theories are proven wrong by NASA. This is my current understanding:
Bernoulli’s Principle states that the higher the dynamic pressure (speed of airflow), the lower the static pressure. Since the air flows along the upper wing surface in a faster speed relative to the lower wing surface, the upper wing has a lower static pressure and while the lower wing has a higher static pressure. Such pressure difference creates an upward force which is lift.
I acknowledge that equal time assumption has been proven wrong a long time ago. However, I am so confused about
why the air flowing along the upper wing surface is faster.
FAA’s PHAK explains this by a venturi tube, when the mass of air entering the tube needs to be the same as that exiting the tube, the airspeed must increase in the throat, therefore the pressure will decrease. First I am not sure how an airfoil relates to that, and second
NASA has also disprove this.
The book also explains the lift generation by “air stream strikes”. Here is the extraction (FAA, 2016):
As the air stream strikes the relatively flat lower surface of a wing or rotor blade when inclined at a small angle to its direction of motion, the air is forced to rebound downward, causing an upward reaction in positive lift. At the same time, the air stream striking the upper curved section of the leading edge is deflected upward. An airfoil is shaped to cause an action on the air, and forces air downward, which provides an equal reaction from the air, forcing the airfoil upward. If a wing is constructed in such form that it causes a lift force greater than the weight of the aircraft, the aircraft will fly.
I don’t know why there are two air streams striking two different positions of the leading edge, one is the lower section and the other is the upper. I learned that the relative airflow is just divided into two path at the leading edge, but not having a force striking to the wing. In addition, it seems to be proven as an incorrect theory here by
NASA.
Apart from the theories that explains lift generation by pressure difference, Coanda Effect and Newton’s Third Law seem to be the two that are widely accepted. The former explains why the air tends to stick to and flow along the wing surface, which should be the precondition of all lift theories; while the latter is used to prove that the air flowing along the upper wing surface is deflected downward (action), an upward force (reaction) is created as lift. Are these two still correct in explaining lift generation? Is my understanding correct?
Hope you guys can help!