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

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

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.

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

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.

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Mxsmanic,

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


Jeeze, and now you're a physicist, too? This is such obvious BS.

But go ahead, explain quantum physics to us without math. You coud
actually make A LOT of money writing a book about it that way.

--
Thomas Borchert (EDDH)

Thomas Borchert writes:

But go ahead, explain quantum physics to us without math.

I'm fairly good at vulgarizations, but others are better. Try reading
Richard Feynman's lectures, or Issac Asimov's many vulgarizations of
complex topics that include physics. Einstein could also explain
things well if needed. Hawking does well in some of his work for the
general public. Many run-of-the-mill physicists are lost when asked to
explain things, however--presumably they lack the intelligence to do
so.

The reality is that people who actually understand physics can explain
it without resorting to math. The ones who use math are those who
have learned only the math, and have no intuitive grasp of the
subject. They are all too common these days.

You could actually make A LOT of money writing a book about it that way.

Some people have made a fair amount of money, although physics for the
masses isn't a hot topic. I'm not really interested in writing a book
at this time, although I've had stuff published in magazines. I have
some essays available for free download on my site on various topics
(not physics, currently, though).

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Mxsmanic,

your delusional qualities never cease to amaze me.

Oh, and in case you're wondering, yes, I do have a masters degree in physics, so I know what I'm
talking about. You don't. As usual.

--
Thomas Borchert (EDDH)

"Peter Dohm" 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?

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)


No, same IAS, same drag, same thrust, same power requirement from the engine
to generate the thrust. The statement that power is drag time velocity is
incorrect. That is the point where the error is made.

Danny Deger

No, same IAS, same drag, same thrust, same power requirement from the
engine to generate the thrust. The statement that power is drag time
velocity is incorrect. That is the point where the error is made.

All of the definitions of power that I have seen have been along the lines
of P = T * V, or something that equates to that.

For instance:-

"The formula for Thrust Horsepower (THP) is:
THP = D x V"

from http://selair.selkirk.bc.ca/aerodyna...nce/Page4.html.

That is wrong?

xerj writes:

All of the definitions of power that I have seen have been along the lines
of P = T * V, or something that equates to that.

Yes. I may have misread your previous post as "distance/time" meaning
"distance or time" (not distance over time).

Force * distance = work
Work / time = power
Thrust = force

A constant IAS requires constant power to maintain at any altitude. A
constant TAS requires constant power to maintain at only one altitude;
if the altitude increases, the power required diminishes, and vice
versa. The power produced by most powerplants diminishes with
altitude; the thrust they can maintain at a given IAS varies directly
with the power.

I think I have that right. It's easy to get confused.

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