The photograph (right) was taken near Maldon, Essex, in the UK, looking directly overhead. It appears to show the result of an aircraft flying through thin cloud and dispersing it along its flight path. If an aircraft was responsible it had long since passed when the picture was taken. Is this a common sight and what mix of conditions is required to produce the effect?

We were taught in school that aeroplanes fly because the wings are bulbous on top, so the air has further to travel, therefore does it faster, therefore at a lower pressure, so developing lift. I heard this explanation given by a teacher to a class at an 'hands on' demonstration in the Science Museum with an expanded polystyrene wing and a hair dryer - shure enough the wing convincingly shot upward when inserted in the air flow. After they had gone I tried it myself, and it became clear to me that the convincing lift came not from the Bernoulli effect but from the angle of attack - held level there was no detectable lift; the children were being conned.

And in any case, aeroplanes can fly upside down, can't they?

So for a Jumbo at cruising altitude in level flight what proportion of the lift is due to the angle of attack, and what proportion to the hump in the aerofoil?

I watched this show on the History channel that suggested that the ancient Egyptians had the knowledge of flight (like the modern airplanes).

The photograph (above) was taken near Maldon, Essex, in the UK, looking directly overhead. It appears to show the result of an aircraft flying through thin cloud and dispersing it along its flight path. If an aircraft was responsible it had long since passed when the picture was taken. Is this a common sight and what mix of conditions is required to produce the effect?

Neil Sinclair, Chelmsford, Essex, UK

Peter White, Cardiff, UK

Alan Glass, London, UK

While I'm sure personal human flight is much easier through fixed wings and some sort of rocket or jet propellant, however, these are not nearly as cool as a nice set of wings like a bird. I know humans are much bigger and heavier than any flying species of bird and are therefore not naturally built for flight at all. But surely this isn't beyond the capabilites of humans?

I saw this kittiwake flying upside down (see photo) in Norway's Svalbard archipelago - about 79 north - while we were stuck in the sea ice. This and other kittiwakes were feeding on polar cod (about 13 centimetres long) that had become uncovered as our ship broke through the ice.

What is it doing and why? How many other birds can do this?

Bill Reed, US

Jacque and Tara Lawlor, Chelmsford, Essex, UK

(Image: alitaylor, stock.xchng)