杏吧原创

Pressure situation

Question: We are all familiar with the popping ears associated with takeoff
and landing in an aeroplane. This is caused by changes in pressure, but because
the aircraft cabin is artificially pressurised, why isn鈥檛 the internal pressure
maintained at one level throughout the journey?

Answer: For reasons of fuel economy, large civil transport aircraft have to
fly at altitudes far in excess of those capable of sustaining life. Whereas 5500
metres is about the maximum altitude at which a person can live for any extended
period, a subsonic passenger jet has the best fuel economy when flying at around
12 000 metres.

Aircraft manufacturers, therefore, have no choice but to pressurise the
interior of a passenger aircraft. This poses huge technical problems. At 12 000
metres, where the pressure is about one fifth of that at sea level, the pressure
inside is trying to burst the fuselage apart. This pressure has to be contained
and all the stretching and flexing of the fuselage during a flight has to be
kept within safe limits. It is far easier to do this鈥攊f the pressure
differential between inside and outside is kept to a minimum, a cheaper and
lighter fuselage structure can be used.

For civil airliners this means that the pressure inside during cruising is
kept at the lowest possible safe level鈥�2500 metres. This is about the
maximum altitude which a normal healthy person can be subjected to without ill
effects. Even so, unfit people, those with respiratory illnesses and those who
have sampled a few too many duty-free drinks might still feel ill, even at this
altitude.

There is another problem: all airfields are not at the same altitude. In an
extreme case, a flight from Heathrow in England to La Paz in Bolivia would
entail going from sea level to around 5200 metres, where the air pressure is
about half that at sea level. Under these circumstances it is just not possible
to maintain the same pressure throughout the flight. Imagine what would happen
if the pressures inside and outside were not the same at the time the doors were
opened: the effect would be quite spectacular and most undesirable.

As for the ear popping, nowadays, 鈥渇or your safety and comfort鈥�, the internal
pressure is imperceptibly reduced, all under computer control, as the aircraft
climbs. It is gradually increased (or, in the case of La Paz and other high
altitude airports, decreased) during descent so that, as the aircraft is coming
to a stop on the runway, the pressure inside and out is the same. This is
normally sufficient for your ears to adjust, but if all else fails, pinch your
nose and gently but firmly increase the pressure in the nasal cavity until you
feel the pressure equalise.

Terence Hollingworth

Blagnac, France

Answer: An advantage of flying by Concorde is that the fuselage has to be
especially strong to fly at very high altitudes, so the cabin pressure does not
have to be reduced below that experienced at 900 metres.

Arthur Cox

Alton, Hampshire

Mighty Moths

Question: We are told that the moth Cactoblastis has cleared the
prickly pear cactus from 25 000 hectares of land (Letters, 1 March, p 53). What
does the moth live on now?

Answer: The Argentinian moth Cactoblastis cactorum is quite
specific to plants in the genus Opuntia (the 鈥減rickly pear鈥� cacti),
although it prefers some of these species to others and will hardly attack some
at all.

Several North American species of these cacti were an enormous problem in
Queensland and New South Wales. When the moth was released, much of the cactus
was destroyed and hoards of hungry caterpillars starved without suitable
food.

Later the cactus recovered, but the few remaining moths built up in numbers
again, and once more devastated the cactus, starting a long series of
oscillations in which the cactus grows, the moths catch up, the cactus dies and
the moth numbers fall. The cycle goes on to this day.

Clearly, the cactus never dies out completely, nor do the moths. If they did
the biological control would fail. What does Cactoblastis eat, then? It
eats prickly pear cactus, which remains present, although in low density,
throughout its former range.

Clyde Wild

Griffith University Gold Coast, Queensland

Free-fall falconry

Question: According to the Audubon Society, the peregrine falcon can dive at
speeds of up to 270 kilometres per hour. However, I have also seen publications
that state a speed of anywhere between 320 and 450 kilometres per hour. Is there
not a terminal velocity that limits diving birds鈥� speed through the
atmosphere?

Answer: A free-falling object will accelerate under gravity until it reaches
a speed where the aerodynamic drag force is equal to its weight. This speed is
the so-called terminal velocity. To work out the aerodynamic drag force of a
given body and hence its terminal velocity, a relatively simple formula can be
used. It states that at a given altitude, drag is proportional to the
streamlining of the body (the drag coefficient), the cross-sectional area of
the body and the square of its speed.

Depending on the weight of a given bird, and using engineering estimates of
the drag coefficient and cross-sectional area for a diving posture, I calculate
the terminal velocity of a diving peregrine to be in the range of 250 to 400
kilometres per hour. If anyone can lend me a stuffed bird, I would be happy to
do the wind tunnel tests required to get a more definitive answer.

Bill Crowther

University of Manchester

Answer: I remember reading in an American bird-watching magazine that the
story of peregrines travelling at tremendous speeds stemmed from an observation
made by a Second World War fighter pilot, who swore that while in a steep dive a
peregrine overtook him. To test this, a parachutist trained his pet peregrine to
follow him while he was in free fall. The peregrine achieved speeds of between
130 and 150 kilometres per hour before chickening out, if you鈥檒l excuse the
expression.

Bruno Frenguelli

University of Dundee

This week鈥檚 question

Plank syndrome: Several flies were gathered on a plank of wood in my garden
this summer. Strangely, they were all facing in the same direction. If one of
them took off and flew around for a while it would return to land near its
neighbours and, on landing, realign itself. At first I thought they were
aligning themselves with the Sun, but they didn鈥檛 alter position as the Sun
moved across the sky. Later, a larger fly of a different species landed on the
wood and faced the same direction too. Why did they all do this?

Bunny Mitch

Sheffield, South Yorkshire