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

"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/a...83-25-2of4.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!

"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