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?

TIA

"xerj" wrote in message
...
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?

TIA


There isn't as much air for the prop to grab, so it slips more.

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

Just say it is "PFM"...

Or tell her that the squirrels under the cowling can't breathe as well
at high altitudes...

Personally, I prefer to just claim it all is just PFM though...

Here's my attempt at an easy layman explanation: "To stay up, the
plane must push down a certain quantity of air. A plane pushes air
down, by flying forwards. If there is less air, the plane must fly
forwards faster to push down the same total amount of air, so you must
add more power to do so."

On Feb 2, 7:18 am, "James Sleeman" wrote:
Here's my attempt at an easy layman explanation: "To stay up, the
plane must push down a certain quantity of air. A plane pushes air
down, by flying forwards. If there is less air, the plane must fly
forwards faster to push down the same total amount of air, so you must
add more power to do so."

Uhhhhh, I hate to bust anyones balloon, BUT, more throttle at higher
altitude does not mean you are getting more power than you were
getting with less throttle at lower altitude...

denny

The reason that explanation did not do the trick is because it simply
is not true. You don't need more power at higher altitude. You need
less power (for the same performance) because the air has less
resistance. Your non-pilot was correct.

Think about it. Your engine power (normally aspirated) drops with
altitude. But the airplane moves faster despite the power drop. If you
can maintain constant power (turbo charging), you get better and
better performance with altitude.




On Feb 2, 6:51 am, "xerj" wrote:
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?

TIA

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.

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

Denny writes:

Uhhhhh, I hate to bust anyones balloon, BUT, more throttle at higher
altitude does not mean you are getting more power than you were
getting with less throttle at lower altitude...

Indeed, if you work out the numbers, you'll find that you can fly with
less power at higher altitudes, even though the throttle may be set
higher.

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

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)

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