March 2020 | www.sayer.com

YRO invented all kinds

of gadgets, gewgaws and

chemical marvels. He was

often hired by Scrooge

McDuck to come up with

devices and ideas that would advance

Scrooge’s many businesses and help

acquire greater and greater wealth.

One of Gyro’s many inventions

really had me pondering. At a sale of his

inventions, Gyro was selling a light-bulb-

like device which was labelled a ‘dark light’.

Supposedly, this invention would work in the

opposite way an ordinary light bulb works.

Instead of spreading light when it is switched

on, it would spread darkness.

Of course, the fallacy inherent in such a

concept is that there is really no such thing

as darkness – there is only an absence of

light.

I suppose that it would be theoretically

possible to have a device that detects light

and emits light waves of the same amplitude

and frequency, but 180 degrees out of phase

– and thereby spread darkness – in much

the same was that popular noise-cancelling

headsets create silence. But that is a mental

exercise for another time.

In reality, darkness is just the absence

of light¸ silence is the absence of noise,

and vacuum is the absence of pressure.

However, we often think of darkness, silence,

and vacuum as ‘things’ in themselves,

particularly since they even have words that

describe them. However, all these terms

only describe the absence of other things.

Language can badly mislead our

thoughts. Take the word ‘empty’ as an

example. An empty box contains an innite

amount of ‘not-things’. That box contains

not-apples, not-stones and even not-Boeing

747s. The crazy thing is that we can prove

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CHRIS MARTINUS - PRESIDENT AIRCRAFT OWNERS & PILOTS ASSOCIATION – SOUTH AFRICA

When I was young I loved reading comics, just as all my friends

did. Back then, Disney characters were all the rage for us kids.

Mickey Mouse, Donald Duck, Goofy and the rest of the gang. But

my favourite of all was Gyro Gearloose, Disney’s anthropomorphic

chicken who was a prolic inventor.

www.sayer.com | March 2020

the most amazing ideas and theories if we

factor those ‘not-things’ into the equation.

That is the core of many fallacies.

WHERE DOES LIFT COME FROM?

In a recent Scientic American article,

author Ed Regis maintains that ‘No One Can

Explain Why Planes Stay In The Air’.

He quotes John Anderson, curator of the

Air & Space Museum, who says: “There is no

simple one-liner answer to this”. He goes on to

describe the various popular ideas based on

Bernoulli’s theorems, arcane mathematical

formulas, portions of Newtonian physics and

even some contemporary academic ideas

that try to use air’s viscosity and ‘circulation’

to explain the mysteries of something that

appears supercially so simple: merely that a

wing that moves through the air with an angle

of attack thereby generates this mysterious

upward force that we call ‘lift’.

There are various corollaries to

Bernoulli’s ideas, such as the ‘equal transit

time’ theory which supposedly explains why

the air ows faster over the top of a wing, thus

giving credence to Bernoulli’s idea that this

will cause reduced pressure (or increased

vacuum) thus ‘sucking’ the wing upwards.

Even Albert Einstein fell for this fallacious

idea and designed his infamous ‘cat-backed’

humped aerofoil that was an utter failure.

Some more modern theories also

propound various ideas about how low

pressure above a wing draws the wing

upwards. But all of these ideas hang onto

the absurd idea that ‘non-pressure’ above

the wing somehow ‘pulls’ the wing upwards,

as though there are tiny strings or rubber

bands holding air molecules together

which somehow attach themselves to the

top surface of the wing and yank the wing

upwards as those molecules go howling past.

These ideas are nonsense. A thin uid

like air has zero tensile strength. You can’t

pull anything with air. If you don’t believe

me, try sucking a candle out. However, it

is easy to blow a candle out. This simple

demonstration shows that those non-

words mislead our thinking. There is no

‘suck, ‘suction’, ‘pull’, ‘vacuum’ or even ‘low

pressure’. Those words merely describe the

absence or partial absence of pressure.

Here is another example: take two

people pushing on either side of a door. If

one stops pushing, the door will move in that

direction. But the door moves not because

the person who stopped pushing ‘pulled’ or

‘sucked’ the door in that direction. The door

moved because the other person continued

to push it.

Even the term ‘pressure’ is misleading.

Pressure is force times area. Sure, Bernoulli

describes differing pressures on differing

parts of the wing, but this is about as relevant

as analysing whether the people pushing

the door are exerting more force with their

ngertips or with the palms of their hands.

It doesn’t matter, because we are only

interested in the total force, which we call lift.

Yet another misleading thing is the way

textbooks describe lift. There are usually

pictures of an aerofoil showing lines which

depict the air owing over the wing. Similarly,

we see videos showing smoke streams

owing around a wing mounted in a wind

tunnel. But the reality is that air is static and it

is the wing that is moving through that static

air that is generating lift. It is true that this

is technically irrelevant, the relative speed

of wing and air being the generator of lift.

However, these depictions lead to fallacious

thinking about non-things rather than the

things that matter.

NEWTON AGAIN

Almost all theorists nevertheless give

grudging credence to Newton’s 3rd law,

stating that for every action there is an equal

and opposite reaction. Depictions of a

person jumping off the front of a trolley – and

the trolley rolling in the opposite direction are

usually used to demonstrate this important

law. The force of the person’s legs pushing

the person forward equally push the trolley

backward.

This is an example which most people

easily understand. Note that, unlike the

Bernoulli protagonists, we are not concerned

about the pressure in psi or kg per square

centimetre which may be exerted by the

jumper’s heels, toes or foot arch. We are

only interested in the total force.

This brings us closer to a simple

understanding of lift. Anyone who has

stood close to a rotating wing, a spinning

propeller or helicopter rotor can easily feel

that the static air has been accelerated in

one direction. Which brings us to Newton’s

2nd law, which states that force equals mass

times acceleration. Therefore, it follows that

the mass of air that has been accelerated

backward or downward by the prop or rotor

will create an equivalent force that moves the

aircraft forward or the helicopter upward.

This is quite simple, but it is also rocket

science. A rocket obtains forward thrust

equal to the force it uses to accelerate the

mass of exhaust gases out the back.

The fact of the matter is that the air is

very soft oppy stuff. You cannot stand on

it. It tends to get out of your way. But is does

have mass and accelerating that mass will

exert an opposite force.

HOW A WING ACCELERATES AIR

The Newtonian aspects are quite easily

understood and demonstrated, but how the

heck does a wing accelerate a mass of air

downward in order to get an upward lifting

force?

There are two aspects to this. The rst

aspect is accepted by most theorists. A wing,

even a at board, which is pitched upward

a few degrees, will deect the mass of air

ahead of it downwards as it moves forwards.

Nothing clever about this. The air simply

has nowhere else to go. The higher leading

edge of the wing ‘splits’ the air, and that

static mass of air ahead and below the wing

is accelerated downwards, thus producing

some lift.

But that is not the end of it. Because

the mass of air ahead and below the wing

has been displaced downwards, it leaves

a void above and behind the wing. No, the

Bernoullian ‘vacuum’ above the wing does

not ‘suck’ the wing upwards. Remember, air

has no tensile strength, nor is it particularly

‘sticky’.

What actually happens is that the

atmospheric pressure of the air above that

void forces that air downwards to ll that

void. The effect is that the mass of air

above the wing has also been accelerated

downward and thus seeks to exert an equal

and opposite force upon the top of the wing.

The air below the wing is still pushing upward

due to atmospheric pressure, but the air

above has gone, and the air above the void

is pushed and accelerates downwards, but

behind the trailing edge of the wing. If the

top surface was still in the way, the wing itself

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March 2020 | www.sayer.com

would block the downward ow. But the air takes a few moments to

accelerate and get to where the wing was, the wing has moved on.

So, the downward air ow blows past the trailing edge.

This is similar to the example of pushing a door and one person

steps aside and stops pushing.

To put it simply, the underside of the wing deects air downwards,

and then ‘sidesteps’ the air above the void it leaves behind and is

gone when the air above accelerates downwards to replace the

displaced air.

By the time the still air above has moved downwards enough to

equalise the atmospheric pressure above and the below the wing, the

wing has left and is deecting another mass of air downwards.

NEXT TIME

So, the nutshell explanation of lift is that the underside of the wing

accelerates air downwards and gets out of the way of the air above the

void it leaves behind before atmospheric pressure can ll that void.

There are many other effects and issues. The air under the wing

spills around the wing tips and will do the same at the trailing edge,

driven upwards by atmospheric pressure. This results in tip vortices

and stalls. Smooth aerofoil designs, clever wingtip designs and other

contrivances minimise these effects.

But that is now an area beyond the scope of this explanation.

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