[masraum]

Originally Posted by Seahawk

I could spend a day writing about this:

The Seahawk is powered by two jet engines, the GE-T700-401Cs. They each put out, depending, 1600SHP.

They were designed to operate as efficiently as possible at low altitudes (https://en.wikipedia.org/wiki/General_Electric_T700) since most military helicopters are not outfitted with oxygen for the pilots to fly above 10,000 ft msl.

After that it gets really complicated. Unlike most fixed wing jets, the 700's in the 60 series helicopters are turning mechanical linkages in order to rotate the main and tail rotors, which provide lift and directional authority. A high altitudes, the density of the air is less, impacting the ability of the main and tail rotors to do their job.

Here are two links that go into helicopter performance:

http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch07.pdf

Helicopter Lesson Guides, Darren Smith, Helicopter Instructor

Density altitude is the key!

OK, yep, yep, I knew they were running on jets, which I guess was why I wondered if a high (but not that high) altitude offered some improvement, but between the cold and oxygen at higher levels (bad for passengers) and even more importantly, the fact that there's a lot less air for the props to work with to create lift, it makes perfect sense that they would design them to work at a specific altitude.