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

"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

"xerj" wrote in message
...
To fly the same IAS requires the same power.

You mean the same *thrust*. The same IAS at a higher altiitude will be a
higher velocity, but the same thrust. The same thrust will give the same
dynamic pressure, which is basically what the ASI shows calibrated in
speed. However, thrust does not equal power. Power = thrust x velocity.

Power is net force time velocity. Thrust equals drag, net force is zero.
The energy change of the airframe overtime is zero. All energy from the
engine is going into the air. The power to move air to make the same thrust
is the same regardless of velocity. Same IAS, same engine power
requirement. Look at some aircraft performance charts.

Danny Deger


The drag curve (which is the same as the thrust curve in straight and
level flight) shifts to the right. The power curve shifts to the right AND
up.

To fly the same TAS, requires less power. Because the air is thinner,
you need a higher throttle setting to get the same power out of the
engine. Maybe you are getting throttle setting confused with power.

No, I'm not talking about how open the throttle is. I'm talking about the
effect above. I was trying to think of a way to explain it without
neeeding to refer to IAS and TAS and power curves. Still not sure how to
do that.

xerj writes:

That being: power = thrust x distance/time, i.e. p = t x v.

You might want to double-check the strict definitions of work, power,
and thrust. As I recall, thrust is a force. A force acting through a
distance is work. The rate at which work is performed over time is
power. So you are confusing work with power. Thrust times distance
is work. The rate at which work is performed is power.

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

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?

You might want to double-check the strict definitions of work, power,
and thrust. As I recall, thrust is a force. A force acting through a
distance is work. The rate at which work is performed over time is
power. So you are confusing work with power. Thrust times distance
is work. The rate at which work is performed is power.

I'm not confusing work with power. Yes, force times distance is work. But
force times distance over time is power. That is why thrust horsepower is
thrust times velocity, which is distance over time.

Danny Deger writes:

The power is the net force time velocity.

Power is the rate at which work is performed per unit of time. Watts
and horsepower are examples of power.

The total force on the airframe is zero because thrust cancels drag.

Thrust and drag are both forces, as you correctly surmise.

When a force acts through a distance, it performs work. And the
amount of work performed over a given period of time is power. Power,
work, and thrust are all different. Thrust is a type of force (so is
drag).

The thrust is the same at high and low altitudes for the same IAS, if you
look at the power required to spin the propeller to generate the same thrust
at the high and low altitude, you will find that the power to spin the
propeller is the same. Same IAS, same power.

That seems logical. IAS decreases with altitude for a given throttle
setting because engine power diminishes. At the same time, however,
TAS can increase because air density diminishes. For a given
aircraft, there is some altitude at which the variables combine to
provide best performance (highest speed over the ground in still air),
and there is also an altitude at which variables combine to provide
best fuel economy (lowest fuel consumption per mile). These two
altitudes are not generally the same.

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

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.

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

xerj writes:

I'm not confusing work with power. Yes, force times distance is work. But
force times distance over time is power.

Yes, agreed. I misunderstood you.

That is why thrust horsepower is thrust times velocity, which is distance
over time.

"Thrust horsepower" is a misnomer. Horsepower is power, thrust is
force. That may be the source of some confusion. It's kind of like
saying "newton watts."

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

"Mxsmanic" wrote in message
...


That seems logical. IAS decreases with altitude for a given throttle
setting because engine power diminishes. At the same time, however,
TAS can increase because air density diminishes. For a given
aircraft, there is some altitude at which the variables combine to
provide best performance (highest speed over the ground in still air),
and there is also an altitude at which variables combine to provide
best fuel economy (lowest fuel consumption per mile). These two
altitudes are not generally the same.

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

NOTICE!!!!
Mxsmanic is NOT a pilot, has NEVER flown an aircraft and is NOT qualified to
issue competent information regarding any aspect of the operation of any
aircraft.

Casey Wilson writes:

NOTICE!!!!
Mxsmanic is NOT a pilot, has NEVER flown an aircraft and is NOT qualified to
issue competent information regarding any aspect of the operation of any
aircraft.

Experience in flying real aircraft is irrelevant to this discussion.
Reading a physics textbook would be a lot more useful than flying an
aircraft for this topic.

If you feel compelled to point out who is flying a simulator and who
is flying a real aircraft, at least save yourself some trouble and
mention it only when it actually has a bearing on the discussion at
hand.

Some of the people in this discussion are right, and some are wrong,
and frankly I don't see any correlation between who is right and who
is wrong and who is a pilot and who isn't.

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