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

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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"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.

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.

--
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"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

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

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).