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View Full Version : Why exactly does power required go up with altitude?



jrex
04-14-2008, 04:24 PM
I understand that power = thrust x TAS and know that it is true, but I'm having a brain spasm trying to work out the "plain english" reason.

For instance: when you go up the air is less dense, therefore the airframe moves through that medium easier. What's the similar, non-formula explanation why you need more power? Is it because the prop needs to fly at a higher AoA to create the same thrust in the thinner air?

Thanks in advance.

Treetop64
04-14-2008, 08:53 PM
At higher altitudes less air pressure makes its way into the intake manifold, thus you push forward the throttle to maintain power the higher you go, while at the same time pulling back on the mixture to maintain the proper fuel-to-air ratio in the chambers. The manifold and EGT guages are some of the most important instruments in an aircraft cockpit, especially when flying a piston turbo...

Turbo and super charging reduces power loss at high altitudes by force-feeding air pressure into the manifold, but only up to a point...

jrex
04-14-2008, 10:02 PM
That deals with one side of the matter: power loss from the engine. But the other side is that as you go up, you need more power to maintain a given CAS.

Thinking about it again, possibly the best way to explain it in plain english is that you need to move the plane faster through the air to maintain a given dynamic pressure. Going faster requires more power.

VW-IceFire
04-14-2008, 10:32 PM
Not an aeronautics engineer but...thinner air means less for the prop to hang on to and less for the engine to suck in to generate power in the first place. Jets have far less of a problem with this as I understand...but in particular...since a prop aircraft really needs the air to move forward...the less air...the harder it is to go.

Just how I understand it.