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

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

Peter Dohm writes:

In a word, NO.

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

Good physicists can explain any principle of physics without resorting
to math.

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

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

First, I stand by my remarks as mathematically accurate.

Second, you are technically correct that a given power (typically 75%) will
give a greater speed with increasing altitude. However, the increase in
speed will not be as much as many people seem to expect, but instead will be
very close to the square root of the optomists expectation.

The good news is that the graphs in the POH seem to be a good guide.

Peter

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.

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.

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."

"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

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.