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

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.

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.

xerj writes:

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.

Are you sure?

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

Are you sure?

Positive.

Here's backup:-

From http://www.av8n.com/how/htm/power.ht...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."

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

"xerj" wrote

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.
--
Jim in NC

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 ;-))

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."

This didn't really do the trick.

Can someone think of a better way of putting it without resorting to
mathematics and an explanation of IAS and TAS?

In a word, NO.

It is an issue of physics, and physics uses a lot of math.

To maintain the same TAS, she is right--untill IAS drops to the back side of
the power curve for the altitude at which she is then flying.

To maintain the same IAS, the power requirement will only increase linearly
in proportion to TAS with increasing altitude--until mach number becomes a
consideration (at some significant fraction of unity)

Therefore, within very finite limits, increasing altitude simply allows an
airplane to be flown at a higher TAS while holding the IAS within an
efficient range. That has the effect of only requiring the power to
increase linearly with speed--rather than as the square of the speed
increase.

I hope this helps.

Peter

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."

This didn't really do the trick.

Can someone think of a better way of putting it without resorting to
mathematics and an explanation of IAS and TAS?

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 - at least, not in any of the airplanes I've
ever flown. Take a look at the speed/power charts for a turbo and you'll
see what I mean - if you maintain 75% power the higher you go the faster you
go.

If you're talking about altitude effects on the power output of a
normally-aspirated engine, that's a different story. At about 8,000 feet a
normally-aspirated engine will probably be putting out around 75% power at
full throttle, and it will continue to decrease as you go higher.

BDS