h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

sparked by watching a wren flit through the trees near where I have a ciggie break at work:

Does air temperature have an effect on flight? By which I mean, cold air is denser than warm air. Therefore it is presumably more difficult to move through. But equally, being denser, it should provide greater lift when pushed? Or, for fixed wing aircraft, the differential above/below the wing should be greater?
So does the denser air provide greater lift and if so does this offset the greater energy requirements to move through it? I presume so since I think on of the advantages of jets cruising so high is very thin air and thus low fuel consumption?

I'm not an expert Ictoan but I can't imagine that it has an appreciable effect on such small creatures. Their predominate concern is getting enough energy to keep warm and although flight uses a significant amount of energy I'm not sure that a difference of less than 200 millibar will have an impact greater than the temperature itself. The difference between UK summer temperature of, say 22 degrees C and current winter temps of -10 to -15 degrees C will be the decider for survival. There may actually be a heat advantage in flying through colder, denser air.

Don't quote me though.

t.

oh agree with that - but what about on a 747?

>>So does the denser air provide greater lift and if so does this offset the greater energy requirements to move through it? I presume so since I think on of the advantages of jets cruising so high is very thin air and thus low fuel consumption?

You've got that the wrong way around, haven't you? Jets travel high because the air is thin there, so the greater lift in denser air must _not_ offset the greater energy requirements to move through it.

Should anyone bring the Ekranoplan (?) into the discussion?

http://news.bbc.co.uk/1/hi/7638659.stm

"oh agree with that - but what about on a 747?"

Do Wrens fly standard or business class?

t.

All the aerodynamic forces on a body are directly, linearly proportional to the density of the air.

Halve the density of the air, and you halve the lift. You also halve the drag. This is why airliners fly high - less drag = less fuel per mile.

Next relevant question - just how much does the density of the air change with temperature, at a given pressure?

Let's compare two temperatures: a cold day of -5C and a hot day of 30C.

On the cold day, at atmospheric pressure, air has a density of 1.3163 kg/m3.

On the hot day, same pressure, density is 1.1644 kg/m3.

That's a 12% difference. Seems quite significant. That's the equivalent of flying, on the cold day, to an altitude of just over 1300 metres (about).

I'd say it's enough so that if you were operating at the edge of your flight envelope - i.e. you could only just get into the air on a good day, like say those human-powered aircraft that seemed to be popular in the 70s and 80s, you'd notice the difference, and you'd be better off flying when its cold.

But most birds, including wrens, have wide performance margins, i.e. they've got lift to spare. I doubt they notice the difference, especially as I've pointed out the simultaneous benefit they're getting from the reduction in drag.

cheers Tiggy