Wings of gas
Question: When I look out of my kitchen window only 8 kilometres from
Heathrow Airport I can sometimes see a thick mist boiling off the top surfaces
of aeroplane wings. The mist starts out near the wing tips and forms two trails
of grey condensation, especially if there are a lot of low fluffy clouds in the
sky.
The effect is particularly spectacular with bigger aircraft such as Boeing
747 jumbo jets and Concorde. Why does this happen and why only occasionally?
Answer: The clouds of gas are actually plumes of rotating air called trailing
vortices or tip vortices. They are created as a result of the wing鈥檚
lift-generating processes.
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On a clear day, trailing vortices are often seen in the sky after the passage
of an aeroplane. The vortices form because the wing develops lift. That is, the
pressure above the wing is lower than that underneath it and, near the tips of
the wing, this pressure difference causes the air to move around the wing edge
from the bottom surface to the top. This generates a pair of counter-rotating
vortices.
In the sky, the vortex becomes visible when the air has a high humidity. The
velocities inside the vortex can be very high, and the pressure is therefore
quite low. The pressure decreases as you travel towards the centre of the
vortex, as in tornadoes and whirlpools. Because of the reduced pressure, the
water vapour in the air condenses as water droplets.
The strength of the vortex is related to the amount of lift generated by the
wing, so they become particularly strong in high-lift conditions such as
take-off and landing. They also increase in strength with the size of the
aeroplane, because the lift is equal to the weight of the aeroplane.
A large transport aeroplane such as a Boeing 747 generates vortices strong
enough to flip a small aeroplane if it gets too close.
Trailing vortices are therefore the principal reason for the time delays
enforced by the Federal Aviation Administration between takeoffs and landings at
airports.
Vahik Khodagolian
Gothenburg, Sweden
Answer: If the wings are short and stubby, like Concorde鈥檚, the wing tip area
is a large proportion of the total wing area, and there is more curl over into a
vortex. Concorde鈥檚 wings are highly swept back and the air curls over from
bottom to top along the length of the front of the wing, forming what is called
a 鈥渞am鈥檚 horn vortex鈥� on top of the wing. It is very strong by the time it
reaches the wing tips.
These condensation trails are entirely different to the vapour trails that
can be seen behind high-flying aircraft. Such trails are not behind the wing
tips, they are behind the engines, and are caused by combustion adding water
vapour to the very cold air at high altitudes.
Colin Richardson
Grantham, Lincolnshire
Answer: All aircraft at one time or another induce these clouds. I have seen
them form spectacularly from the propeller of my bush plane in Alaska.
A great twisting spiral resulted and demonstrated well why they used to call
them air screws. These clouds are most likely to form when you fly through air
that is just ready to form fog.
Ed Gauss
Fat Moose Flying Service (retired)
Niwot, Colorado
Answer: The same phenomenon is sometimes seen when saturated air blows over
static objects such as the corners of buildings or tree branches. A plume of
mist can be seen downwind and, if the air is supersaturated, the water droplets
may cause further nucleation, creating a cloud or even rain below the extended
plume.
John Etherington
Haverfordwest, Pembrokeshire
Answer: You can make your own clouds using a simple apparatus. Put about half
a cup of water into a large empty plastic fizzy drinks bottle, connect a bicycle
pump to the top and seal it with Plasticine or clay. Give the pump a few
strokes, swirl the water, then let the air out of the bottle quickly. Presto! A
home-made cloud.
Alf Cousins
One Tree Hill, South Australia
This week鈥檚 questions
Carrot brains: Why is it that when you repeatedly ask someone addition
problems that all add up to six (such as two plus four, one plus five) for a
number of minutes and then ask them to think of a vegetable, 90 per cent of
people will say carrot?
The person you are asking must have no knowledge of what you are asking them
or why. The questions should be asked rapidly, encouraging the person answering
to answer them quickly with little thought.
Adam Versteegen
Melbourne, South Australia
We have no idea if this is true or not but we would encourage our readers to
experiment on naive subjects and let us know the answer. As a control, we
suggest trying numbers that add up to other sums.
Do carrots just turn up everywhere, or do they turn up at random, or just in
Australia, or even more mysteriously, does every sum have a specific vegetable
associated with it? The Last Word eagerly awaits your experimental results to
see if we have a real question here鈥擡d
Flea market: How do fleas so easily find creatures to feed on? I once entered
a house that had been empty for almost a week. It was infested with fleas.
Within seconds, my feet and ankles were covered with the hungry little insects.
How did they know I was there, and that I was a suitable meal?
Gabriel Wizard
California
Headbangers: How is it possible for woodpeckers to peck so hard without
getting brain damage?
Pamela Brannon
Strongsville, Ohio
Ghostly voices: In 1980, my husband and I sailed from New Zealand to Hawaii
and back. Just the other day we confessed to each other that, on quiet nights in
the middle of the ocean when there was very little wind and hence very little
boat noise, we could hear voices coming from the mast. These voices would be
both male and female but you couldn鈥檛 make out what they were saying. We were
both quite sober and were not under stress.
Can anyone explain this? Our mast is aluminium and the stays are made of
coiled wire.
Jenny Pollock
Nelson, New Zealand