A_F_2015_01_02_

ou would think

after 111 years of

flight we might have a

succinct answer for “What

makes aircraft fly?” but

instead we are plagued

by persistent myths that

seem to have a life more

robust than the zombie undead. In

fact, despite having the stake of logic

regularly driven through their hearts,

they seem to be reborn in science

classes and flying schools.

In science classes and on

YouTube, you can find spurious

explanations, solemnly given

without apparent awareness that

they are clearly nonsense.

However, that basic aerodynamics

was not well understood or that

completely erroneous beliefs were

held, came as a bit of a surprise to me

while teaching aerodynamics to 2nd

and 3rd year university students.

These students were training to

be pilots. Some already had a PPL

and were on their way to a CPL and

a few of the distance students were

already airline pilots.

Standing in the auditorium facing

my students in the first lecture, I

would tell them that we were going

to start off with some easy stuff and

would ask them to explain to me

how wings worked.

There was usually a moment

of silence and then some keen

individual would come forth with

an explanation that sounded

something like this:

“The air particles start off at the

leading edge of the wing and must

meet up at the trailing edge and

because the distance over the wing

is further than the distance under

the wing, the air going over the top

cambered surface must go faster, and

from Bernoulli’s Theorem, increased

speed means lower pressure so the

wing sucks itself up into the sky.”

I would then ask. “Is that correct?”

and wait for more answers.

As the silence descended some

would shift uncomfortably in their

seats until another brave soul would

offer the alternative “A wing works

like a venturi in that the upper

surface is curved like half of a venturi

and because of the Bernoulli Effect

as the air flows over the curved upper

surface it speeds up creating a low

pressure and hence lift”. When I

would ask why the air follows the

curved surface and the answer was

usually “The Coanda Effect.”

At this point I would whip out a

newly-constructed paper plane and

throw it across the room and as it

soared in graceful flight, I would ask

“Well how does that work? It has

flat wings ... same distance top and

bottom and no camber.”

Silence. I could almost hear the

myths exploding. In reality, unless

you are an aerodynamicist fiddling

around the edges of the very small

decimal points lift is a fairly simple

concept, so I won’t spend any more

time on the myths.

Next I would show them a

74-second video from Cambridge

University that forever banishes the

nonsense explanations to the domain

of the Tooth Fairy and Santa Claus.

As this frame from that video

shows, the streamlines never get

close to meeting. See for yourself at

http://www.cam.ac.uk/research/news/how-

wings-really-work

As for venturis? They are a closed

system and once you have half of one

it’s not a venturi any more.

So how do wings work?

NASA tells us “Lift is a force

created by turning a fluid.” If you

have ever stuck your hand out of a

car window and inclined it to the

air flow, you have experienced it in

simple clear terms. Angle your hand

and it goes up. Keep it parallel to the

airflow and it stays put. No change

of direction of the air-no lift.

Also, unless you are an

aerodynamicist, you don’t need to

know anything about Bernoulli, as

his formulae were based on Newton’s

Second Law: Force = mass x

acceleration. So what about all of the

“...air speeds up/accelerates over the

upper surface of a wing” stuff? Bunk.

Watch the video and see the air

slowing down as it comes in contact

with the wing. The wing’s lower

surface pushes the air forwards

and downwards in its direction of

travel. One component of lift. The

pressure wave (bow wave) pushes

the rest of the air up and over

the wing. As it rapidly changes

direction, the pressure drops

momentarily (streamlines close

together) and the air continues

following the angle of the upper

surface-(not Coanda just reacting

to static pressure and viscosity)

changing direction and creating the

second part of the lift force. The

pressure differences are a result of

lift creation – not the cause.

And as for wing shape? Hang

gliders, para-foils and sailboats

all have single-surface aerofoils.

Aerobatic aircraft have symmetrical

wings and missiles and other high-

speed aircraft have flat-plate wings.

All work very well and to top it

off, all modern airliners a have

supercritical wings; curved on the

bottom and flatter on top.

Oops, there goes another myth!

Lift Mythconceptions

Short Final DAVID BENTLEY

University lecturer David Bentley explodes some

hoary old ideas about how lift is generated.

82 australianflying.com.au

AUSTRALIAN FLYING January - February 2015

At this point I would whip out a

newly-constructed paper plane and

throw it across the room.

LEFT: The lower surface of the

wing pushes the air forwards and

downwards, and the bow wave

forces the rest of the air over the

upper surface.

RE-ILLUSTRATED FROM A CAMBRIDGE UNIVERSITY VIDEO