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

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

"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.

--
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.

On Feb 4, 12:11 pm, "Andrew Sarangan" wrote:


Great. But the poster said Denker had made some incorrect assumptions.
I am still anxiously waiting to hear what those assumptions are.

Here is a good way to look at it. Posters on this thread referenced
Denkers material several posts ago and yet we still dont have an
answer.Is everyone on this thread stupid, or was Denker not being very
clear.Can you explain what Denker was talking about?

I don't know Denker personally, but I have read the book, which he
gives to the world for free, and I have greatly benefited from his
insights.

I think the reason his book is free is because no one would pay for
it.Think about it, it is too tecnical for some, and for others it is
not tecnical enough (or accurate enough).I think he would have a
pretty slim market if he were to publish it.Another big detraction is
the way Denker jumps back and forth between aerodynamic theory, and
giving dual instruction.This iritating and I think he should stick to
one or the other.
As far as "benifiting greatly" from this book, I gotta ask you, are
you a pilot?Not that it really maters, but it would explain why you
dont understand peoples issue with this material.

If you think my asking him to provide an explanation is
an insult, then I don't know what to say.-

I think your sugesting I am a know it all is an insult.I think your
sugestion I go back to the basics is an insult.Why dont you START with
the basics.

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.

And you, I presume, are not a physicist, a mathematician, or an engine
mechanic.

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

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.

"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

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.

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.

Yeah, those damn eggbeaters hanging out the front make it all pretty
complicated. I most certainly DON'T have a Master's in Aerospace. I find it
slightly comforting that a guy that does says it's complex.

Thanks for taking the time to answer.

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.

Do you mean working back from TAS to get an IAS?

I looked up a Navajo information manual. There's a chart True Airspeed vs
Density Altitude. I chose the line for 260 BHP which is around 75% of the
350 BHP engines.

At sea level the TAS is shown as around 207 MPH (have to interpolate, it's a
grid that goes up in 10s). That is obviously the IAS as well.

At 20,000, the TAS is close to 250 MPH. The inferred IAS is 184.

Any thoughts?

"xerj" 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.

Yeah, those damn eggbeaters hanging out the front make it all pretty
complicated. I most certainly DON'T have a Master's in Aerospace. I find
it slightly comforting that a guy that does says it's complex.


Jets and rockets are actually much easier to do design work on than prop
planes. The jet produces thrust, which is the thrust used to propel the
plane. Calculate the thrust required then it is a simple step to calculate
fuel flow from the engine to get the thrust. With a prop, exactly what
happens as you convert rotation power into thrust is complex, complex,
complex.

Thanks for taking the time to answer.

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.

Do you mean working back from TAS to get an IAS?

I looked up a Navajo information manual. There's a chart True Airspeed vs
Density Altitude. I chose the line for 260 BHP which is around 75% of the
350 BHP engines.

At sea level the TAS is shown as around 207 MPH (have to interpolate, it's
a grid that goes up in 10s). That is obviously the IAS as well.

At 20,000, the TAS is close to 250 MPH. The inferred IAS is 184.

Any thoughts?


See my other posts. I stand corrected. For a given engine power, IASI
drops off with altitude. For a jet, IASI does not drop off for a given
engine thrust as the plane climbs. Maybe that is an inherent reason jets
are faster at altitude than a prop.

Danny Deger