Increasing power required with altitude.. what's a good plain english explanation?

xerj writes:

I was trying to explain to a non-pilot why increased power is required with
altitude. She said "isn't the air thinner up there so there isn't as much
resistance?" I said "yes, but the plane needs to fly fast enough for the air
over the wings to feel like it does down low. So the speed required goes up
you get higher. More speed need more power."

Actually, she's right. You need higher speed at higher altitudes in
order to maintain a given amount of lift, because the air isn't as
dense. However, you don't necessarily need more power, because thin
air presents a lot less resistance to the aircraft. Airliners fly
high in part because it requires less power (and therefore consumes
less fuel). That's why they are eager to get up to high altitudes.

You may need a higher _throttle_ setting, because the engines produce
less power in thinner air. However, the amount of power required
still diminishes. To climb from altitude A to B in an aircraft, you
may have to increase the throttle from 60 to 75, but at the same time
the power produced by the engine at a given throttle setting
diminishes by 30%, so in fact you are flying with less power at
altitude B.

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Actually, she's right. You need higher speed at higher altitudes in
order to maintain a given amount of lift, because the air isn't as
dense. However, you don't necessarily need more power, because thin
air presents a lot less resistance to the aircraft.

You need more power to maintain the same amount of lift as you get higher.
By "same amount of lift" I take that to mean angle of attack and the
resulting IAS for the same dynamic pressure. The formula is predicated on
TAS.

On Feb 2, 7:06 am, Mxsmanic wrote:
Actually, she's right. You need higher speed at higher altitudes in
order to maintain a given amount of lift, because the air isn't as
dense. However, you don't necessarily need more power, because thin
air presents a lot less resistance to the aircraft. Airliners fly
high in part because it requires less power (and therefore consumes
less fuel). That's why they are eager to get up to high altitudes.

MX, common misconception here about airliners.You need to look at the
fuel required to maintain a given level of thrust at altitude for a
jet engine.
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alice writes:

MX, common misconception here about airliners.

Hardly a misconception. The "sweet spot" for airliners is quite high,
and airlines like to be there in order to use the smallest amount of
fuel for a given distance.

You need to look at the fuel required to maintain a given level
of thrust at altitude for a jet engine.

I've looked that the fuel required to cover a given amount of ground,
and it's much lower at high altitudes.

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On Feb 2, 9:46 pm, Mxsmanic wrote:
Hardly a misconception. The "sweet spot" for airliners is quite high,
and airlines like to be there in order to use the smallest amount of
fuel for a given distance.

MX,
Duh.No one is arguing that a jet uses less fuel up high.It is the
reason why that is in question.You are making a HUGE misconception
about the reason why.In fact, it could be said that you are thinking
backwards.By your reasoning, A jet would never have a service ceiling!
Explain to us what a "sweet spot" is.Why is it that you feel the
airlines dont take into account TIME when doing the preflight planing.


I've looked that the fuel required to cover a given amount of ground,
and it's much lower at high altitudes.

OK MX, here is the "Given amount of ground" thing again.Think real
hard about what you are saying and why you seem to think time doesnt
factor into the equasion.If you have in fact looked into the cruise
performance charts on a airliner, what did it say in the thrust
required column.In other words, ignore the fuel for a minute and you
will have your answer.
KW


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alice writes:

Duh.No one is arguing that a jet uses less fuel up high.It is the
reason why that is in question.You are making a HUGE misconception
about the reason why.

Less fuel = less power.

By your reasoning, A jet would never have a service ceiling!

Jets have a service ceiling for several reasons. For one, eventually
the air is too thin to provide any lift, no matter how fast you are
moving. For another, eventually the air is too thin to support
internal combustion engines.

Explain to us what a "sweet spot" is.

Greatest distance covered per unit of fuel consumed, lowest wear and
tear on the aircraft (especially engines).

Why is it that you feel the airlines dont take into account TIME
when doing the preflight planing.

They do, but fuel costs more than time. That's why flights are longer
now than they used to be: airlines plan for fuel economy, not speed.

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On Feb 3, 9:42 am, Mxsmanic wrote:
Less fuel = less power.

MX, I am laughing now.I cant figure out if you are really that
ignorant or if you are just messing with me.The real reson jets fly
high is for speed.The higher they go, the faster they can go.Why do
you think jets measure cruise speed in Mach?What do you think happens
to Mach as a jet flys higher?Now explain how you can move a jet (Or
anything else for that mater, like a car) FASTER on LESS thrust?For
the 3rd time here MX, you have made a misconception and you are
thinking backwards.


Jets have a service ceiling for several reasons. For one, eventually
the air is too thin to provide any lift, no matter how fast you are
moving. For another, eventually the air is too thin to support
internal combustion engines.

MX, I am laughing even harder now!Can you explain the real reason
behind a jets service ceiling and what it is a function of?

Explain to us what a "sweet spot" is.

Greatest distance covered per unit of fuel consumed, lowest wear and
tear on the aircraft (especially engines).

??? I gotta ask you MX, what airline do you work for?Are you saying
they use LCR or CCR charts to determine power settings?I hate to argue
with you but when I was in initial training at my fist jet job, it was
explained (very compellingly) why we dont base cruise flight on
this.If you are doing something different, Id like to know the
reasoning behind it.

Why is it that you feel the airlines dont take into account TIME
when doing the preflight planing.

They do, but fuel costs more than time. That's why flights are longer
now than they used to be: airlines plan for fuel economy, not speed.

OK, Ill bite MX.Why are you claiming that speed is not a function of
economy.Has it not occured to you that the LONGER a plane is in the
air, the more wear and tear it is incuring?Also, the longer a plane is
in the air, the more fuel it is burning.Which of course means the more
fuel it has to carry.Which means it is heavier.Which requires more
thrust.Tell me how this saves fuel again?

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alice writes:

The real reson jets fly high is for speed.

The real reason they fly high is for fuel economy.

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alice wrote:
For
the 3rd time here MX, you have made a misconception and you are
thinking backwards.

The third time? You must be new around here.