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

"xerj" wrote in message
...
If you
can maintain constant power (turbo charging), you get better and
better performance with altitude.

The TAS will increase, but say you want to hold a specific angle of attack
and its attendant IAS (maybe for range), you will need more power to do
that as you get higher.


This in not true. You will need the same power for the same IAS regardless
of altitude.

Danny Deger

This in not true. You will need the same power for the same IAS
regardless
of altitude.

Same thrust, not same power.

"Danny Deger" wrote in message
...

"xerj" wrote in message
...
If you
can maintain constant power (turbo charging), you get better and
better performance with altitude.

The TAS will increase, but say you want to hold a specific angle of
attack and its attendant IAS (maybe for range), you will need more power
to do that as you get higher.


This in not true. You will need the same power for the same IAS
regardless of altitude.


Don't listen to me. I was wrong. Same IAS at altitude requires more power.

Danny Deger

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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Xerj,

I was trying to explain to a non-pilot why increased power is required with
altitude.

It is? I may have an idea what you mean, see point 2 below.

She said "isn't the air thinner up there so there isn't as much
resistance?"

Exactly right. Bright girl! That's why turbocharged piston airplanes like to
fly high.

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."

Uhm, not really. For a normally aspirated engine, the power output will
decrease during the ascent because of thinner air, which means fewer air
molecules per volume to burn. Thus, you need to increase the power setting to
make up for that (and you need to put less fuel in the cylinder, too, which is
achieved by leaning). At some point you will run out of throttle doing that.
There is an optimum altitude where the balance between loss of air resistance
and loss of engine power amounts to the best speed for the maximum amount of
power available. It is usually between 6000 and 8000 feet for what we fly.

--
Thomas Borchert (EDDH)