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

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.

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.