Follow up to monster thread below re IAS and TAS and power required...

[xerj]

Gettin' a bit confused here. (nothing new in that)

In the big sprawling thread I started down below, there's been a couple of
themes that have come up.

One is that I am pretty sure that for the same IAS (not TAS) at a higher
altitude, more power is required. However, one contributor to the thread has
stated that this is not the case:-

"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."

I'd always understood that power = thrust x velocity, hence the deduction
that it requires more power to go the same IAS at a higher alt. At the same
IAS the drag and hence the thrust is the same. Plug that into the equation
and you get the power required, which is more because TAS is higher at
altitude.

As for aircraft performance charts, they're for the most part in TAS, not
IAS.

However, the same author as the snippet above says:-

"The statement that power is drag time velocity is
incorrect."

Is it? I've seen that formula mentioned in almost every text on power that
I've seen.

Is there something I'm missing?

Not trying to be a PITA, just seeking clarification of something I was sure
was right. And I know that operationally TAS is much more important than IAS
except for, say, stall speed, best glide and the like. So it's a largely an
academic question, I realise. It was (sort of) started as a way of finding a
plain language non-mathematical explanation for the question "why does the
same IAS require more power at altitude?". I haven't found that plain
language explanation yet, but now I'm getting conflicting answers as to the
very definition of power.

Can someone clear it up?

TIA!

[BDS[_2_]]

"xerj" > wrote

> One is that I am pretty sure that for the same IAS (not TAS) at a higher
> altitude, more power is required. However, one contributor to the thread
has
> stated that this is not the case:-

Based on actual performance data for a Seneca II you are correct:

6000 ft
75% power (2400 rpm/32-in mp/26.3 gph)
IAS 157 kts
TAS 170 kts

16000 ft
75% power (2400 rpm/29.4-in mp/26.3 gph)
IAS 148 kts
TAS 187 kts


BDS

[Roger[_4_]]

On Mon, 05 Feb 2007 02:18:26 GMT, "xerj" > wrote:

>Gettin' a bit confused here. (nothing new in that)
>
>In the big sprawling thread I started down below, there's been a couple of
>themes that have come up.
>
>One is that I am pretty sure that for the same IAS (not TAS) at a higher
>altitude, more power is required. However, one contributor to the thread has
>stated that this is not the case:-

That would be true, but probably not doable.
Besides at altitude you fly a much lower IAS with out trying to boost
the power. You fly at a lower IAS due to lower air density at the
higher altitudes which is also the reason for the wider spread between
IAS and TAS.

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

[Danny Deger]

"xerj" > wrote in message
...
> Gettin' a bit confused here. (nothing new in that)
>
> In the big sprawling thread I started down below, there's been a couple of
> themes that have come up.
>
> One is that I am pretty sure that for the same IAS (not TAS) at a higher
> altitude, more power is required. However, one contributor to the thread
> has stated that this is not the case:-
>
> "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."
>
> I'd always understood that power = thrust x velocity, hence the deduction
> that it requires more power to go the same IAS at a higher alt. At the
> same IAS the drag and hence the thrust is the same. Plug that into the
> equation and you get the power required, which is more because TAS is
> higher at altitude.
>
> As for aircraft performance charts, they're for the most part in TAS, not
> IAS.

I found an aircraft performance chart and I stand corrected. At 75% power
the aircraft flies 140 TAS at sea level and 150 TAS at 8K feet. This is 140
IAS at sea level and 124 IAS as 8K feet. I am very surpprised, because I
always thought you would get the same IAS for the same power.

So for the same power, the IAS is less at altitude. I do know that the
calculations from engine power to thrust and power effect to the overall
system is more of an art than I science. I do stand by my statement that
for the purpose of using the fact that force times velocity is power , it is
not correct to say trust times velocity is power. The net force on the
airframe is zero, thrust equals drag so net force is zero. All of the
energy is going into the air, not the airframe. But it looks like due to
the higher velocity of the airstream at altitude (higher TAS) the power
requirement to move the air for a given IAS is greater. Keep in mind the
energy/power from the engine is going into moving the air, not accelerating
the airframe.

As an example of the problem of using thrust time velocity as power,
calculate the power being generated by engines doing a runup on the runway
before takeoff. Velocity is zero, thus "power" is zero. The equation
brakes down.

Live and learn though. IAS does drop with altitude for the same power out
of the engine. This effect may be one reason jets are inherently faster at
altitude. While thrust of a jet does drop with altitude, this drop is not
as bad on aircraft speed as the drop in power in a piston or turbo prop.

Thanks xerj. Even on old engineer can learn a few new things :-)

Danny Deger



>
> However, the same author as the snippet above says:-
>
> "The statement that power is drag time velocity is
> incorrect."
>
> Is it? I've seen that formula mentioned in almost every text on power that
> I've seen.
>
> Is there something I'm missing?
>
> Not trying to be a PITA, just seeking clarification of something I was
> sure was right. And I know that operationally TAS is much more important
> than IAS except for, say, stall speed, best glide and the like. So it's a
> largely an academic question, I realise. It was (sort of) started as a way
> of finding a plain language non-mathematical explanation for the question
> "why does the same IAS require more power at altitude?". I haven't found
> that plain language explanation yet, but now I'm getting conflicting
> answers as to the very definition of power.
>
> Can someone clear it up?
>
> TIA!
>
>
>
>
>

[Danny Deger]

"BDS" > wrote in message
t...
> "xerj" > wrote
>
>> One is that I am pretty sure that for the same IAS (not TAS) at a higher
>> altitude, more power is required. However, one contributor to the thread
> has
>> stated that this is not the case:-
>
> Based on actual performance data for a Seneca II you are correct:
>
> 6000 ft
> 75% power (2400 rpm/32-in mp/26.3 gph)
> IAS 157 kts
> TAS 170 kts
>
> 16000 ft
> 75% power (2400 rpm/29.4-in mp/26.3 gph)
> IAS 148 kts
> TAS 187 kts
>
>
> BDS
>

Thanks for the information.

Danny Deger
>

[Peter Dohm]

> One is that I am pretty sure that for the same IAS (not TAS) at a higher
> altitude, more power is required. However, one contributor to the thread
has
> stated that this is not the case:-
>
You are correct.


> "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.

That ignores a lot of technical niceties, and inefficiencies,
but is adiquate for our purposes.

> The power to move air to make the same thrust
> is the same regardless of velocity. Same IAS, same engine power

Wrong! That is the basis of most, if not all, of the misconceptions.
Power is proportional to thrust times velocity (speed)


> requirement. Look at some aircraft performance charts."
>
> I'd always understood that power = thrust x velocity, hence the deduction
> that it requires more power to go the same IAS at a higher alt. At the
same
> IAS the drag and hence the thrust is the same. Plug that into the equation
> and you get the power required, which is more because TAS is higher at
> altitude.
>
True.

> As for aircraft performance charts, they're for the most part in TAS, not
> IAS.
>
> However, the same author as the snippet above says:-
>
> "The statement that power is drag time velocity is
> incorrect."
>
> Is it? I've seen that formula mentioned in almost every text on power that
> I've seen.
>
> Is there something I'm missing?
>
You are correct. Given constant velocity, thrust and drag are the same.

> Not trying to be a PITA, just seeking clarification of something I was
sure
> was right. And I know that operationally TAS is much more important than
IAS
> except for, say, stall speed, best glide and the like. So it's a largely
an
> academic question, I realise. It was (sort of) started as a way of finding
a
> plain language non-mathematical explanation for the question "why does the
> same IAS require more power at altitude?". I haven't found that plain
> language explanation yet, but now I'm getting conflicting answers as to
the
> very definition of power.
>
> Can someone clear it up?
>
> TIA!
>
I still think that it's mathematical, but you seem to have simplified it
as much as practical.

[Peter Dohm]

> > One is that I am pretty sure that for the same IAS (not TAS) at a higher
> > altitude, more power is required. However, one contributor to the thread
> has
> > stated that this is not the case:-
>
> Based on actual performance data for a Seneca II you are correct:
>
> 6000 ft
> 75% power (2400 rpm/32-in mp/26.3 gph)
> IAS 157 kts
> TAS 170 kts
>
> 16000 ft
> 75% power (2400 rpm/29.4-in mp/26.3 gph)
> IAS 148 kts
> TAS 187 kts
>
>
> BDS
>
>
Thanks for posting the excerpt.

It's concise, and also gives a "feel" for the magnetude of the effect.

Peter

[Newps]

Danny Deger wrote:

>
> I found an aircraft performance chart and I stand corrected. At 75% power
> the aircraft flies 140 TAS at sea level and 150 TAS at 8K feet. This is 140
> IAS at sea level and 124 IAS as 8K feet. I am very surpprised, because I
> always thought you would get the same IAS for the same power.

Every pilot is taught that in private pilot training. Those of us that
live at higher altitudes really know this because we deal with reduced
power every day.

[Danny Deger]

"Newps" > wrote in message
. ..
>
>
> Danny Deger wrote:
>
>>
>> I found an aircraft performance chart and I stand corrected. At 75%
>> power the aircraft flies 140 TAS at sea level and 150 TAS at 8K feet.
>> This is 140 IAS at sea level and 124 IAS as 8K feet. I am very
>> surpprised, because I always thought you would get the same IAS for the
>> same power.
>
> Every pilot is taught that in private pilot training. Those of us that
> live at higher altitudes really know this because we deal with reduced
> power every day.

I remember being taught that TAS would be higher at altitude, but don't
remember what was said about IAS for a given power level as you climb. Most
cruise charts just have TAS (IIRC). I moved to jets in the Air Force and
for the same engine thrust the IAS is the same for any altitude in a jet.
Like I said, live and learn. That one reason I frequent this group.

Danny Deger

[Stan Prevost[_1_]]

"BDS" > wrote in message
t...
> 6000 ft
> 75% power (2400 rpm/32-in mp/26.3 gph)
> IAS 157 kts
> TAS 170 kts
>
> 16000 ft
> 75% power (2400 rpm/29.4-in mp/26.3 gph)
> IAS 148 kts
> TAS 187 kts
>

How come those are both 75% power? The high-altitude one seems like a
richer mixture, less power.

[Danny Deger]

"xerj" > wrote in message
...
> Gettin' a bit confused here. (nothing new in that)
>

Equating engine horse power to thrust times velocity is an
oversimplification, but it correctly calculates the increase in power
required to fly the same IAS as you climb. Using figure 9-31 in the
following:
http://www.faa.gov/library/manuals/aviation/pilot_handbook/media/faa-h-8083-25-2of4.pdf
it takes 55% power to fly 120 IAS at sea level. 120 IAS at 16k feet is 158
TAS. Using power as thrust times velocity we can predict it would take 73%
power to fly 120 IAS at 16K feet. This is exactly what the chart says --
73% power. P = T x V is a simplification, but it does capture for the same
thrust (IAS), power required from the engine is proportional to the velocity
(TAS). Thanks xerj for straightening me up on this.

Danny Deger

> In the big sprawling thread I started down below, there's been a couple of
> themes that have come up.
>
> One is that I am pretty sure that for the same IAS (not TAS) at a higher
> altitude, more power is required. However, one contributor to the thread
> has stated that this is not the case:-
>
> "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."
>
> I'd always understood that power = thrust x velocity, hence the deduction
> that it requires more power to go the same IAS at a higher alt. At the
> same IAS the drag and hence the thrust is the same. Plug that into the
> equation and you get the power required, which is more because TAS is
> higher at altitude.
>
> As for aircraft performance charts, they're for the most part in TAS, not
> IAS.
>
> However, the same author as the snippet above says:-
>
> "The statement that power is drag time velocity is
> incorrect."
>
> Is it? I've seen that formula mentioned in almost every text on power that
> I've seen.
>
> Is there something I'm missing?
>
> Not trying to be a PITA, just seeking clarification of something I was
> sure was right. And I know that operationally TAS is much more important
> than IAS except for, say, stall speed, best glide and the like. So it's a
> largely an academic question, I realise. It was (sort of) started as a way
> of finding a plain language non-mathematical explanation for the question
> "why does the same IAS require more power at altitude?". I haven't found
> that plain language explanation yet, but now I'm getting conflicting
> answers as to the very definition of power.
>
> Can someone clear it up?
>
> TIA!
>
>
>
>
>

[Al G[_1_]]

"Stan Prevost" > wrote in message
...
>
> "BDS" > wrote in message
> t...
>> 6000 ft
>> 75% power (2400 rpm/32-in mp/26.3 gph)
>> IAS 157 kts
>> TAS 170 kts
>>
>> 16000 ft
>> 75% power (2400 rpm/29.4-in mp/26.3 gph)
>> IAS 148 kts
>> TAS 187 kts
>>
>
> How come those are both 75% power? The high-altitude one seems like a
> richer mixture, less power.
>
>
>
Less back pressure at altitude, less MP required for the same %power.
Note the fuel burn is the same.

Al G