Remember, TAS is your airspeed (ignoring the winds) in relation to the
ground. IAS is to the air around you. The higher you go, the less drag
on the fuselage and same as a fixed wing, less drag on lift prducing
surfaces mean more efficiency, which means a little better airspeed.
That IAS maybe lower due to less power, but for that amount of power
you are more efficient. That all adds up to higher TAS and lower fuel
burns. In the turbine I fly, I see up to a 5-6 gallon an hour less
burn at 5,000'.



On Fri, 29 Oct 2004 13:56:01 GMT, "Steve R."
wrote:

"B4RT" wrote in message
...

I usually use about 2000 agl for medium cross-countries and 45-55 hundred
for long ones. (My turbine works really good here + I get a free TAS
increase from the altitude) The other reason to fly higher on cross
countries is that you'll have less worry about towers & wires.

Bart

Ok, I'm a bit confused here. So Bart, or Kevin, or anyone who cares to put
in their 2 cents worth, have at it. :-)

My issue is with Bart's statement that there's a free TAS increase in his
helicopter with altitude. I'm sitting here, thinking about how I want to
word all of this and I'm starting to think that I'm about to answer my own
question so I'll put it to you kind folks and see what comes back!

I understand that fixed wing aircraft gain efficiency with altitude. The
simplified explanation is that as the aircraft climbs to greater altitudes
(ie: thinner air), there's less frictional drag on the airframe and the
aircraft achieves a higher TAS (true airspeed for those who may not know) as
long as you're not flying so high that you're no longer able to pull
standard cruise power from the engine. Assuming the same power settings
from the engine/engines, you'll achieve a higher cruising airspeed (TAS) at
altitude than you will at sea level. All of this happens automatically.

As for helicopters (any rotorcraft), the airspeed of the "wings" (ie: the
rotor blades) is limited to the maximum rpm that the rotor system can
sustain. Because of this, as the helicopter climbs to altitude, the rotor
blades can't see the kind of TAS increase that a fixed wing aircraft enjoys
because the speed of the rotor blades is tied to the max rpm of the rotor
system. Consequently, as the air thins with increased altitude, the
helicopter has to fly with ever increasing collective settings to maintain
the same lift/thrust levels. When you can't increase airspeed (rotor rpm),
you have to increase AOA (collective) to maintain a given lift/thrust
level!? One negative side effect of this is that the helicopters Vne speeds
lower as the altitude rises. What I've always been told is that because of
this, it doesn't generally pay for a helicopter to go to altitude on a
x-country flights unless there are sufficient favorable winds (ie: tail
winds!) to make up the difference. (?)

Now to the part where I might be answering my own question. Just as the
fixed wing aircraft sees in increase in TAS at altitude as long as it can
still maintain cruise power (generally 65 to 75% power), I'm starting to
think (I hadn't really considered it this way before!) that as long as the
helicopter isn't pushing it's Vne limits at altitude (due to higher
collective settings), the fuselage will see an increase in TAS also, even if
the rotor blades themselves are not, because of rpm limitations on the main
rotor system. Of course, if the fuse is seeing a higher TAS, then the rotor
blades will also see in increase, at least on the advancing
side..........but I don't think I want to get into that too deeply! ;-)

Am I getting this anywhere near right? I hope this makes sense. Thanks for
any replies! :-)

Fly Safe,
Steve R.