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

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.

In article ,
"Danny Deger" wrote:

"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

Danny,

Go back to "Airplane Performance Stability and Control," by Perkins &
Hage, John Wilet & Sons, NY, London (1949).

"Orval Fairbairn" wrote in message
news
snip

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

How much power is a B-29 generating during a full throttle run-up before
takeoff :-)

Having said this, I agree now that the equation accurately calculates the
increased power required to fly the same IAS at a higher altitude.

Danny Deger

"Danny Deger" wrote

How much power is a B-29 generating during a full throttle run-up before
takeoff :-)

That goes back to the basic definition of HP. You have to do work on an
object, since HP is the amont of time required to lift an {whatever weight}
object to {whatever} height.

The noise and heat being produced, and the fuel being burned suggests
something else, though. :-o g
--
Jim in NC

I'd suggest a REALLY GOOD flight review next time you need one with a REALLY
GOOD instructor who knows the relationship between power, altitude, and true
airspeed.

Or did ..maniac assume a pseudonym?

Jim



"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."

Morgans wrote:

"Danny Deger" wrote

How much power is a B-29 generating during a full throttle run-up
before takeoff :-)


That goes back to the basic definition of HP. You have to do work on an
object, since HP is the amont of time required to lift an {whatever
weight} object to {whatever} height.

No, horsepower is not an amount of time. It is a rate of doing work. I
can lift 100 lbs 10' in one minute or I can lift 100,000 lbs 10' in one
minute. Same time, but vastly different amounts of HP required.

Matt

"RST Engineering" wrote in message
...
I'd suggest a REALLY GOOD flight review next time you need one with a
REALLY GOOD instructor who knows the relationship between power, altitude,
and true airspeed.

Or did ..maniac assume a pseudonym?

As you can probably see by the length of the thread, this one has gone on
for a while. I was talking about trying to explain the phenomenon of how to
keep the same IAS you need higher power as you increase altitude, but
without actually referring to concepts like IAS and TAS. I wasn't talking
about TAS increasing with altitude. I should have made that clearer in the
initial post. The whole thing started when a non-pilot friend of mine asked
how high planes could fly. I said that it depended on a few things, one of
them being the fact that engine power decreases with altitude because of air
density. At some point you'll hit an altitude where it can no longer
generate the power required for level flight. HOWEVER, even if you have an
engine that was turbocharged so well that it didn't lose power with altitude
all the way up to space, you'll still probably hit a limit (unless the
engine was extremely powerful) because the actual power required goes up
with altitude as well. Is there anything unreasonable in what I have just
said there? I don't think so. Where I went wrong is in not explaining my
question well enough, and people very reasonably thinking that I either
meant TAS or how open the throttle has to be.

As has been established, I am now certain that my understanding of power,
altitude and TAS was and remains correct. I was merely seeking a good layman
non-mathematical explanation of it. Hardly a major sin, I think.

"xerj" wrote in message
...

"RST Engineering" wrote in message
...
I'd suggest a REALLY GOOD flight review next time you need one with a
REALLY GOOD instructor who knows the relationship between power,
altitude, and true airspeed.

Or did ..maniac assume a pseudonym?

As you can probably see by the length of the thread, this one has gone on
for a while. I was talking about trying to explain the phenomenon of how
to keep the same IAS you need higher power as you increase altitude, but
without actually referring to concepts like IAS and TAS. I wasn't talking
about TAS increasing with altitude. I should have made that clearer in the
initial post. The whole thing started when a non-pilot friend of mine
asked how high planes could fly. I said that it depended on a few things,
one of them being the fact that engine power decreases with altitude
because of air density. At some point you'll hit an altitude where it can
no longer generate the power required for level flight. HOWEVER, even if
you have an engine that was turbocharged so well that it didn't lose power
with altitude all the way up to space, you'll still probably hit a limit
(unless the engine was extremely powerful) because the actual power
required goes up with altitude as well. Is there anything unreasonable in
what I have just said there? I don't think so. Where I went wrong is in
not explaining my question well enough, and people very reasonably
thinking that I either meant TAS or how open the throttle has to be.

As has been established, I am now certain that my understanding of power,
altitude and TAS was and remains correct. I was merely seeking a good
layman non-mathematical explanation of it. Hardly a major sin, I think.


I would just leave it that the engine looses power with altitude and not
mention the fact that for a propeller aircraft the power required also goes
up. Keep in mind for a jet the thrust required does not go up with
altitude. More output being required is strictly for props.

Danny Deger

"Danny Deger" wrote in message
...

"xerj" wrote in message
...
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.


This in not true. You will need the same power for the same IAS
regardless of altitude.


Don't listen to me. I was wrong. Same IAS at altitude requires more power.

Danny Deger