How do planes fly upside down?

F-16 aircraft from the US Air Force Thunderbirds perform during a exhibition air show. Photo/AFP 

By Daniel Ondieki

 

 

In the one year since I started writing aviation articles, I have not answered what is perhaps the most fundamental question of all. How do planes fly?

Most people have a rough idea of the physics behind it. Any heavier object than air such as a plane or a bird is able to stay suspended in the air thanks to the production of a force called lift which primarily counters the weight of the object.

While lift is produced by many surfaces in a plane, it’s the wings that are responsible for most of the useful lift.

The theory goes that the upper surface of the wing is curved and consequently that air has to travel faster to arrive at the trailing edge of the wing at the same time as air that went around the bottom of the wing.

This increased velocity also a result of the wing acting as a venturi similar to pinching a hose pipe with flowing water, causes a reduction of pressure on the top part of the wing thanks to the Bernoulli effect. The pressure differential across the bottom and top parts of the wing produces lift.

This explanation was not only taught in school but repeated in all the principles of flight texts that I have read.

It turns out this is wrong. It cannot explain how planes with conventional wings can fly upside down, or how symmetrical wings produce lift.

The problem it turns out is that production of lift is a very complex subject that cannot be distilled into a simple convenient explanation. That being said, it hasn’t stopped people from trying.

A Nasa website clearly designed in the 90s and never updated to reflect modern times, informs us that the air taking the longer path at the top of the wing actually arrives at the trailing edge well ahead of the air at the bottom with an added downward direction. This new downward motion imparted on the air as per Newton’s laws of motion must be counteracted by an equal and opposite reaction which is lift.

Other explanations involve complex mathematical formulae derived by long deceased physicists.

It turns out Bernoulli is still involved in the race. Once the velocity of the air around the wing has been determined using better methods, Bernoulli’s formula can be used to calculate the actual pressure at each point around the wing. A knowledge of this pressure field can be used to calculate the lift. In other words the popular explanation of lift has the cart before the horse.

At this point we may wonder whether it even matters. The popular explanation of lift certainly doesn’t seem to have harmed generations of pilots despite being wrong.

Having the most accurate base knowledge helps us better understand how our planes fly in our day to day work. And no one requires us to know anything more detailed than the straightforward explanation.

The finer details of the Navier Stokes equation can be left to the aeronautical engineers.

Dr Ondieki is a pilot with an international airline