PLANES CAN FLY UP SIDE DOWN

If you don't understand how planes can fly upside down then you will never understand how planes can fly right side up.  Yes the physics textbooks are wrong.  Next time someone tells you that Bernoulli's Principle is the primary reason planes create lift etc. Ask them -- HOW DOES A PLANE FLY UPSIDE DOWN?

Bernoulli's Principle

According to Bernoulli's Principle, for an airfoil (wing) with a convex upper surface, the air on the upper surface has to go faster than the air on the lower wing surface and lift is created in that manner.  If so then how do planes fly up side down? 


Newton's 3rd Law -- Action Reaction

Its Newton's 3rd Law, "For every action, there is an opposite and reaction".  This creates the major component of lift especially at higher speeds. According to conventional Bernoulli's Principle explanations, there are two air molecules at the leading edge of the wing moving in a horizontal manner.  One molecule of air goes over the top of the wing and one under. The molecule that goes over the top of the wing is typically labeled V1 and the one under the wing labeled V2.  

Supersonic Flow

In fact in supersonic flow, the air velocity for both surfaces V1 and V2 are exactly equal.  Look at the wings of an F-16, stunt plane etc. the cross section of the wings are symmetrical.  If so then how does an F-16 create lift?

See also articles on "supercritical airfoil" for later wing designs which attempt to minimize drag in supersonic flow.

When a plane with a non-symmetrical air foil is flying up side down, the angle of attack is just increased to overcome any effect of Bernoulli's Law.  Flying up side down with an unsymmetrical airfoil creates an unstable flow because two effects are fighting against each other.  Bernoulli's Principle is a valid principle it is just wrongly applied as the major factor for air foils in subsonic flow.

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Downdraft Behind the Wing

According to conventional Bernoulli's Principle explanations, there are two air molecules at the leading edge of the wing moving in a horizontal manner.  One goes over the top of the wing and one under. The molecule that goes over the top of the wing is typically labeled V1 and the one under the wing labeled V2. Both molecules reconnect at the tail edge of the wing and go horizontally to the right forever.  Because the top molecule has to go at a higher velocity due to its being forced to travel a longer distance in the same time, than the under foil molecule -- lift is created and the lift is proportional to V1 squared minus V2 squared.

Law of Conservation of Momentum

The hard fact of the matter however is that there is a downdraft behind all wings, regardless if the wing is symmetrical or non-symmetrical and whether in subsonic or supersonic flow.  The air molecules do NOT leave the wing in a horizontal manner but at some downward angle relative to the initial air stream orientation.  Again this is the only way that the Law of Conservation of Momentum can be maintained, L = mv where L is the momentum, m is the mass flow and v is the velocity of the mass flow. The plane goes up and the air goes down.  Nothing is life is for free.

This downdraft creates somewhat of a problem when trying to land the plane.  When the plane gets close to the runway, the downdraft becomes somewhat compressed and bounces off the runway beck to the plane. The pilot is forced literally to drive the plane into the ground in order to land it.


Mind over Matter

In 1963 during a Mechanical Engineering Fluids class we were shown films of the air stream over a conventional air foil (wing) with lines of smoke in a wind tunnel. Because I had been told in high school physics and college freshman physics that the air flowed horizontally when leaving the wing, my mind refused to accept what the film clearly showed:  that the air behind the wing was thrown downward.  It was only years later that I realized that I had missed (and so did the rest of the class as well as the instructor) that the air behind the wing was being thrown downward.  My mind had been programmed not to recognize the actual down draft.  After all in high school physics we were told about Bernoulli's Principle as well as in freshmen physics.  Now the same story was presented to us in Mechanical Engineering Fluids and the stroyh had not changed.  

​Curve Balls in Baseball
Ever wonder how and why curve balls can curve.  A curve ball is created by the pitcher who puts a rotational spin on the ball as he throws it towards the batter.  This is hard to illustrate absent a diagram. A moving curve ball has two components.  The primary linear velocity is indicated by big V and a rotational velocity indicated by little v.  As the curve ball is thrown the surface of the ball is a composite of the primary velocity (V) and the rotational velocity (𝝎) in radians/second times (actually a cross product (x)) the radius of the ball at any particular point on the ball's surface which is rx𝝎 or r𝝎 for short.
The opposite ends of the ball have velocities relative to the air stream as V + r𝝎 and V - r𝝎.  One would expect that the ball would curve towards the side of the higher air velocity because of the lower air pressure.  Laboratory tests show otherwise.  Actually the ball "throws" air due to momentum to the side of highest velocity and the ball moves in the opposite way.  Action, reaction.   This is the only way that the law of Conservation of Momentum can be applied and observed.  If air had no viscosity, it would be impossible to throw a curve ball.

Curve balls in Golf, Football and Tennis

I am sure that the same effects due to spinning of the ball occur in these other sports but that their effect is minimized for various reasons.

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