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

Who cares about IAS? The question was does it take more power to go
faster, right? Any non pilot will think faster means true airspeed, not
indicated.

True, but the conversation got to how high a plane can fly. I said that
going higher did two things: limited the amount of power that an engine can
put out because of density, and that even if you had an engine that didn't
lose power, the power required goes up regardless.

Also, not that it would matter to a non-pilot, but IAS obviously matters for
keeping best range speed for instance.

"xerj" wrote

Also, not that it would matter to a non-pilot, but IAS obviously matters
for keeping best range speed for instance.


How so?
--
Jim in NC

I don't mean opening the throttle to make up for the engine power loss.
I
mean the fact that to maintain the same IAS you need more power as you
go
up.

Why the preoccupation with IAS?

At around 6,000 feet, the power of a non turbo piston engine is around
75%.
As you go higher, the power drops off, but the true air speed goes up.

Who cares about IAS? The question was does it take more power to go
faster,
right? Any non pilot will think faster means true airspeed, not
indicated.
--
Ok, I confess, I'd rather have an angle of attack meter to correlate more
directly with the best coefficients of lift and drag independently of
current weight. But IAS and a little math based on initial weight and fuel
consumed should work well enough for us cheap-skates.

Even if you are operating at a speed other than best L/D, which seems mostly
reserved for Glider Pilots and Jet Jocks, reference to IAS is about the only
way (that I know of) to keep the theoretical discussion understandable

Peter
Cheapest of the cheap ;-))

TAS increases with altitude for a given power setting due to less
aerodynamic drag at higher altitudes. It does not take more power to go
the
same speed at higher altitudes

It doesn't take more power to go the same TAS, but it does take more power
to go the same IAS.


The way most people fly, which is well above best L/D, the same TAS will
require less power with increasing altitude.

Peter

Also, not that it would matter to a non-pilot, but IAS obviously matters
for keeping best range speed for instance.


How so?

Best L/D occurs at a particular angle of attack. This corresponds fairly
well to indicated airspeed.

On Feb 2, 3:38 pm, "xerj" wrote:

Here's backup:-

Fromhttp://www.av8n.com/how/htm/power.html#sec-power-altitude

"Let's compare high-altitude flight with low-altitude flight at the same
angle of attack. Assume the weight of the airplane remains the same. Then we
can make a wonderful chain of deductions.

At the higher altitude:
a.. the lift is the same (since lift equals weight)
b.. the lift-to-drag ratio is the same (since it depends on angle of
attack)
c.. the drag is the same (calculated from the previous two items)
d.. the thrust is the same (since thrust equals drag)
e.. the indicated airspeed is the same (to produce the same lift at the
same angle of attack)
f.. the true airspeed is greater (because density is lower)
g.. the power required is greater (since power equals drag times TAS)
The last step is tricky. Whereas most of the aerodynamic quantitites of
interest to pilots are based on CAS, the power-per-thrust relationship
depends on TAS, not CAS.

This means that any aircraft requires more power to maintain a given CAS at
altitude. This applies to propellers, jets, and rockets equally."

What is interesting is that this author comes up with the right
answer, but he uses some false asumptions.Its obvious he hasnt spent
much time in a real airplane

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.
--
Transpose mxsmanic and gmail to reach me by e-mail.

What is interesting is that this author comes up with the right
answer, but he uses some false asumptions.Its obvious he hasnt spent
much time in a real airplane

What's false about the assumptions? He's talking about flight at the same
angle of attack at different altitudes.

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.

--
Transpose mxsmanic and gmail to reach me by e-mail.

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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