cenci
Posts: 11
Joined: Wed Feb 07, 2001 9:35 pm

### Why Fly At 36,000ft?

ok ok , so if a jet engine relies on mass air flow (density) to provide thrust why do aircraft cruise so high (36,000ft) where the air is thinner?

why not cruise at 20,000ft where the density is greater and the engine wouldnt have to work as hard?

Fly2HMO
Posts: 7187
Joined: Sat Jan 24, 2004 12:14 pm

### RE: Why Fly At 36,000ft?

You are right, density is less, BUT...

At 36000 feet the air is cooler, because the air is cooler, the engines operate more efficiently (less fuel is consumed) and produce just as much power (thermodynamics ring a bell?) I.e. You produce more energy by heating very cold air (say -40C) and turning it really hot (800C), than if you heated warm air and and turned it really hot (say 35C to 800C), therefore, less fuel needs to be fed into the engine to produce the same amount of (heat) energy.

So, if current commercial planes were capable of cruising at FL600, they certainly would do that often.

At least that's what I learned from turbines class, but ...

A) I'll let the engineers and mad scientists get into more detail...

or B) I'll be flamed by them for being even slightly wrong

]

[Edited 2006-04-14 22:42:19]

[Edited 2006-04-14 22:42:36]

hangarrat
Posts: 428
Joined: Wed Jul 06, 2005 12:24 pm

### RE: Why Fly At 36,000ft?

Funny, I was always under the impression that the efficiency gain from operating at a greater altitude was the decrease in air resistance or friction on the skin of the aircraft.

I agree, let's let someone drop the science.
Spell check is a false dog

Posts: 3697
Joined: Thu Nov 08, 2001 1:04 am

### RE: Why Fly At 36,000ft?

 Quoting FLY2HMO (Reply 1): You produce more energy by heating very cold air (say -40C) and turning it really hot (800C), than if you heated warm air and and turned it really hot (say 35C to 800C).

http://en.wikipedia.org/wiki/Heat_engine

The above link explains this well, the hotter you can heat the air, the greater the power output you get for a constant mass of fuel. Most engines are limited by combuster and turbine section materials and their lack of ability to withstand ultra-high temperatures (over 800 degrees C). Instead of pushing up the hot temperature, you can lower the cold temperature and increase thereby the power of the engine.

Currently you will find most modern engine turbine sections are made from Nickel Alloys but they are at their upper limit. Ceramics is the way forward however their strength and rigidity is nill...

 Quoting Cenci (Thread starter): why not cruise at 20,000ft where the density is greater and the engine wouldnt have to work as hard?

Because the air is denser @ 20,000ft, it makes the aircraft harder to push through the air, requiring a higher power setting, burning more fuel.

A Similar reason if you fly slower in an aircraft, the difference in power between flying at Mach .80 and say .75 isn't massive, however the extra time you spend in the air because you are flying slower IS and that is why its always good to fly at the economical speed.

[Edited 2006-04-14 22:54:27]
Whoever said "laughter is the best medicine" never had Gonorrhea

Skyslave
Posts: 44
Joined: Sat Oct 22, 2005 12:52 pm

### RE: Why Fly At 36,000ft?

You can fly faster in air thats less dense. I'm pretty sure its only because of the structual limitations. If you were to fly 500kts at 10,000ft you would probably cause some pretty harsh damage to the airframe. The engines are pretty powerful, and most likely wont notice a big difference (power wise) between 10,000ft and 35,000ft.

RichardPrice
Posts: 4474
Joined: Sat Apr 23, 2005 5:12 am

### RE: Why Fly At 36,000ft?

Actually as with increasing altitude, the falling density of the atmosphere means the net thrust produced also falls (this does not mean a lower efficiency) but because the air is cooler less fuel can be used to produce that thrust, and it also makes the entire airframe more efficient due to less drag due to friction. So its a little of both of the previous answers.

The specifics are very ... indepth.

Grbld
Posts: 349
Joined: Tue Dec 06, 2005 9:25 pm

### RE: Why Fly At 36,000ft?

 Quoting Kaddyuk (Reply 3):Because the air is denser @ 20,000ft, it makes the aircraft harder to push through the air, requiring a higher power setting, burning more fuel.

Nope, see below.

 Quoting Skyslave (Reply 4):You can fly faster in air thats less dense. I'm pretty sure its only because of the structual limitations.

Also nope. Here's why: You have to understand the difference between airspeed and groundspeed. What's important for forces acting upon the aircraft (drag, lift etc) is the airspeed, but what's important for your flight time, schedule and geographic planning is your groundspeed.

Now let's introduce a third: Fuel efficiency. This factor is dependent of the temperature. The colder it gets, the lower your fuel burn becomes.

As you go higher, air density decreases and you can fly more efficient. No, not because it's easier to push the airplane through the air mass (that would be called "drag") but because the air molecules are farther apart.

Think of it this way: In one yard or foot or meter, you have X amount of "air molecules" (not the scientific term) lined up. Now if the air becomes less dense, the amount of air molecules in your distance unit becomes less. However, since the jet engine ploughs its way through the same amount of air molecules, you actually go faster, compared to the ground.
To clarify even more: Say you have 100 molecules in a meter at 20,000 ft and you have 50 molecules in a meter at 30,000 ft. However, the jet engine sucks through 100 molecules per second, so at 20,000 ft it travels one meter per second, while at 30,000 ft, it will travel 2 meters per second (the numbers are just purely for the explanation).

When you climb above FL260 or thereabouts (depending on your max mach and max indicated airspeed, aircraft type), you'll start to decrease your indicated airspeed, which is roughly the speed at which you move through the surrounding air molecules. However, due to the air density decreasing, it's still possible for your ground speed to remain very high.

At FL260, you'll get right up at Mach .800 and about 320 knots indicated airspeed. If there's no wind, your ground speed will be something like 475 knots, because when you suck up 100 air molecules at that altitude, on the ground the same distance might have 130 air molecules. Above FL260, Mach speed will become limiting, and it goes down as you go up, so while keeping Mach .800, your indicated airspeed will go down. At FL390, it may be something like 260 knots, while your ground speed without wind can be 420 knots. The point where your Mach number and airspeed meet, is usually where you also have your highest ground speed.

The main reason to go even higher, even though you'll slow down a little, is because of the fuel savings. Ad hoc reasons would be because of a better tail wind, to avoid delays, to avoid being stuck behind a slower aircraft, to get out of turbulence, to avoid clouds or things like that.

Hope that explains it a bit!

Grbld

bri2k1
Posts: 952
Joined: Sun Dec 19, 2004 4:13 am

### RE: Why Fly At 36,000ft?

 Quoting Grbld (Reply 9):The colder it gets, the lower your fuel burn becomes.

I have to take issue with this explanation. The fact may be true, but not for the reason you state. If we're talking about maximum continous thrust, the limiting factor is a temperature ceiling. It takes less fuel to heat warmer air to that ceiling compared to cooler air. The reason it's more efficient is because cool air is more dense, so more air can be compressed into the engine for combustion.

 Quoting Grbld (Reply 9):Think of it this way: In one yard or foot or meter, you have X amount of "air molecules" (not the scientific term) lined up. Now if the air becomes less dense, the amount of air molecules in your distance unit becomes less. However, since the jet engine ploughs its way through the same amount of air molecules, you actually go faster, compared to the ground. To clarify even more: Say you have 100 molecules in a meter at 20,000 ft and you have 50 molecules in a meter at 30,000 ft. However, the jet engine sucks through 100 molecules per second, so at 20,000 ft it travels one meter per second, while at 30,000 ft, it will travel 2 meters per second (the numbers are just purely for the explanation).

I have to take issue with this, too. By this logic, a turbine engine would approach infinite efficiency in a vacuum, because it would have to travel so much farther to gobble that next air molecule. It's a jet engine, not Pac-Man!
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