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topic of the day 045

Why airplanes fly so high?

What? Who? How? Why?

From my basic understanding (and peter's input from his question of course), of course the biggest public transportation needed to be as high up in the air as possible to clear the skyscrapers and to inflict as little of noise pollution as possible upon the citizens down below.

The main reasons they needed to set the cruising altitude at 33,000 feet instead of just right above 100 feet could be that higher altitude equals thinner atmosphere which means less resistance on the plane as it moves through the air, letting it able to almost come down to an idle and stay elevated and moving so it also helps a lot with efficiency. Although less resistance mean less lift, you just have to travel faster to generate the lift. Weather plays its part as well since planes don't have to worry about getting caught up in the lower atmosphere where things like rain clouds, turbulence and such form, limiting outside interference as much as possible. There's also the fact that terrain is marked by sea level and some terrains may be much higher above the sea level than the airport strip they take off from and they need to be able to clear those with a lot of room left over.

As mentioned by psyki, "my pilot friend had this to say: So, turbine engines are most efficient at hotter temperature differentials. At 33k feet it's -50 outside, and the engines are up around 600. The lower drag coupled with the lower oxygen means lower fuel burn. My engines (on a 737) burn around 1500 lbs per hour at idle at sea level. At cruise at 38k feet, it's around 2200."

There is also the matter of having a safety cushion. It really doesn't matter if a plane crashes from 5,000 or 30000 feet once you hit the ground. Dead is dead. However, when something goes wrong, falling from 30,000 feet gives you a lot more time to fix things than falling from 5,000. It's like how ships will often avoid land in a storm - the danger zone is where sky or water meet land, so stay away from that. 

gus555 asked, "but doesn't less resistance also mean less thrust as well since these are not rockets (don't they expel forward atmosphere backward)?"
kemb0 answered, "cool air expands more when heated than warm air. It is the expansion of the air that drives combustion engines. The second reason is the low density of the air. Low density causes low drag and therefore the aircraft flies much faster at high altitude than on lowaltitude when it is given the same thrust." I guess the considerably cooler air makes up for the reduction in density.
Innominate8 added, "they expel their own burned fuel too, not just incoming air. But yes, as altitudes increase thrust generally goes down. Aircraft make up for this by being able to suck in a lot more air at low altitude than they really need to. The net result is still greatly increased efficiency.

Then? So?

As cleared by a real pilot below, the biggest reason is on how the engines of the planes are designed for and of course the shape of the planes themselves, optimized to be the most efficient at certain elevation. If you went to the original reddit post, you'll find much more informative information there other than those I've posted here after cherrypicking some of them. It is safe to say that after decades of flying, they have designed the aeroplanes to be performing at an optimum level at a certain altitude. The more you know!

Kabatica explained, "pilot here,
We can start by forgetting about piston aircraft that don't have any great benefits going above 10,000 feet compared to say 5,000 feet. Turbo-prop aircraft (Q400 or ATR-72) usually cruise around 30,000 since they have a benefit of the prop biting into a bit of a thicker atmosphere vs. a higher and thinner atmosphere
Jet turbine aircraft (737, 320, Cseries) leans itself out as the go higher: air:fuel ratio becomes most efficient. A rich vs. a lean engine in a piston aircraft can go from a 12:1 air to fuel ratio to an 8:1 fuel ratio in a few thousand feet and usually cannot get better than that.
All other factors like greater fuel efficiency (fuel burns can be cut in half to 1/4 of lower alt. burns), drift-down time (Gimli glider), greater radio reception and radar guidance, obstacle avoidance, but mainly its turbine performance (concorde cruised at 60,000), not friction avoidance.
One misconception is the friction factor. A headwind of +5kt at a higher altitude will not outweigh the benefits of less friction at a greater altitude. Oxygen (atmosphere) drops off a lot after 12,000 ft. I've changed cruising altitude from FL 19,000 to 13,000 ft to gain another 30 kts."

~ Thank you to reddit users: Triforce0218, RadomirPutnik, psyki, tylerawn, spookmann, gus555, kemb0, Innominate8, Kabatica & Browncoat1221 for the input. ~

Lesson Learned:

They're not flying that high because they can, it's because it's the optimal thing to do.

source: r/askscience, by peterthefatman