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

"Mxsmanic" wrote in message
...


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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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.

"xerj" wrote in message
...
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 can certainly define a term called Thrust Horse Power as thrust x
velocity. And this link definition of Brake Horse Power is correct (torque
times RPM). But there is no reason to think these terms are equal in an
aircraft. A great deal of the power out of the engine (all of the power if
in steady state level flight) goes into the air and not the airframe. It is
my understanding that for a given thrust at a given IAS (actually Equivelant
Air Speed, EAS, is the better term), the engine power requirement is
basically the same for different altitudes. I wish I had a good aircraft
performance handbook to confirm this.

Danny Deger

"Danny Deger" wrote in message
...

P.S. I have a Master's in Aerospace and have worked in the industry for many
years. I will admit most of my schooling and experience was with jets and
rockets -- not pistons and props. But I do recall the equations and
techniques to calculate engine horsepower required for various flight modes
of a prop plane was VERY complex. I am CERTAIN equating thrust horsepower
(thrust times velocity) to brake horse power (torque time RPM) is wrong.
Anyone have an aircraft performance chart to look at the IAS for 75% power
at sea level and at altitude?? I am not going to say it will be exact, but
I think it will be close.

Danny Deger

In article ,
"Danny Deger" wrote:

"xerj" wrote in message
...
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 can certainly define a term called Thrust Horse Power as thrust x
velocity. And this link definition of Brake Horse Power is correct (torque
times RPM). But there is no reason to think these terms are equal in an
aircraft. A great deal of the power out of the engine (all of the power if
in steady state level flight) goes into the air and not the airframe. It is
my understanding that for a given thrust at a given IAS (actually Equivelant
Air Speed, EAS, is the better term), the engine power requirement is
basically the same for different altitudes. I wish I had a good aircraft
performance handbook to confirm this.


That is incorrect! A classic problem in sophomore aero engineering is to
determine the maximum altitude at which an aircraft will fly,
simplifying the problem by assuming turbosupercharging to allow constant
power and discounting compressibility effects, given its stall IAS and
lift/drag curves.

At very high altitudes a plane will fly very fast at low IAS (min porew
required speed/alpha.

The power = speed*thrust is valid and is a basic tenet of aero
engineering.

"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


To fly the same IAS requires the same power. 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.

Danny Deger

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.

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" 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.

On Fri, 02 Feb 2007 11:51:34 GMT, "xerj" wrote:

I was trying to explain to a non-pilot why increased power is required with

Increased power is not needed and not normally obtainable at higher
altitude with a normally aspirated engine. It takes less power to
maintain speed at altitude compared to lower. If you just maintain
power you go faster than you do down lower.

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?

"I think" you are confusing the difference between IAS and TAS at
altitude versus power at altitude, or as Dennis already suggested,
throttle position compared to power.


TIA

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

Increased power is not needed and not normally obtainable at higher
altitude with a normally aspirated engine. It takes less power to
maintain speed at altitude compared to lower. If you just maintain
power you go faster than you do down lower.

TAS most definitely increases. In a round about way, I was talking about
IAS. My understanding, and I'm pretty sure of it although I've been told
otherwise here, is that to maintain the same IAS (and thus dynamic pressure)
at a higher altitude, you need more power. I don't mean throttle position --
for the sake of the argument I am leaving density effects on engine power
output aside.

TAS most definitely increases. In a round about way, I was talking about
IAS. My understanding, and I'm pretty sure of it although I've been told
otherwise here, is that to maintain the same IAS (and thus dynamic pressure)
at a higher altitude, you need more power.

X, I hate to sound discouraging, but you may not find an answer here.I
looked on two websites and referenced the book Aerodynamics for naval
aviators, and they kinda contradicted each other.I think you are
looking for a real world answer to a hypothetical situation.The IAS or
dynamic pressure on a plane WILL decrease with altitude.Take a look at
a typical plane doing 300 KIAS at 10 thousand.The TAS will be within
about 40 KTS of this.Now climb up to FL350 and the KIAS will be about
230 with a TAS of about 475 (Roughly). Now you do need more power but
the point about IAS is mute (Or hypothetical) because you cant
indicate 300 KTS at 350.The part about maintaining the same AOA isnt
gonna happen either.I hope someone can explain this better.