Flight fuel wastage?

Question

Now we all know the earth is round.
So apart from the air being thinner why do air craft fly so high?
You would think that because the closer to the earth you are the smaller the circumference therefore the shorter distance for the flight and thus saving fuel and emitting less carbons

Answer 1

The extra distance is negligible - if flying at, say, 6 miles (32k feet) up, the total extra distance on a trip to the other side of the world is only pi * h = 20 miles, or about one-sixth of one percent.

However, the advantage in reduced drag from flying in the thinner air is considerable. At the top of Everest (roughly equal to a jet's cruising altitude), atmospheric pressure is reduced by about 70%. Drag is proportional to air pressure, so is also reduced by 70%. Subsonic jets use about 5% of their power overcoming drag (the other 95% is used on lift), so you save about 3% on fuel by flying higher - and just as importantly, since time is money, you can also fly faster.

Answer 2

Actually, the air being thinner is the biggest reason. The thinner air offers less resistance and so less thrust is required to maintain cruising speed and thus less fuel is burned than if the aircraft is to fly the same distance at a much lower altitude.

So from that point of view, particularly more so in long haul flights, it is more fuel economic to climb to a higher altitude and maintain a specific cruising speed than to fly lower, even though more fuel is used to climb to that altitude and there is an increase in distance to be travelled (as you point out, due to being having to travel a larger circumference).

Answer 3

All aircraft have an optimum cruising speed and altitude. Jet planes are much more efficient above 30,000 feet. As far as the longer distance to fly? It is not a factor since the increases distance is miniscule and hardly measurable when you are talking about an earth radius of 3963 miles or 2 billion feet. Do you think that an extra 30,000 feet will make a difference?