On Fri, 8 Oct 2004 15:19:52 +0000 (UTC),
wrote:

Nathan Young wrote:
: On 7 Oct 2004 13:02:37 -0700, (PaulH) wrote:

: At any altitude that permits you to develop 75% power with your
: current prop, a constant speed prop won't gain you an inch unless you
: want to fly at a higher power setting.

: Can you better explain this? My understanding is that a fixed pitch
: prop is typically a compromise in both takeoff pitch, and cruise
: pitch. Using the typical car driving analogy - a prop that is stuck
: in 3rd or 4th gear in a 5 speed transmission. So I would think a CS
: prop would net gains at both cruise and takeoff/climb.

I've got your identical plane (PA-28-180, 60" fixed pitch).

75% = 75%. All a CS prop does is let you have a variable speed "transmission"
for your airplane engine. You can get 75% power with an infinite combination of MP
and RPM. Say you run 75% at 24"/2400... if you have a fixed-pitch prop, 3000' DA will
give you 75% at, say, 25"/2300. If you climb to 7000', you can get 75% at, say,
23"/2500. That 75% will give you the same IAS no matter how you get it. Now, it
might be *slightly* (1-3%) more efficient to run oversquared due to less RPM-induced
engine drag, but it doesn't affect your cruise speed.

Great description - so to summarize - in cruise, the CS prop would
only benefit over a fixed pitch in cases where the fixed pitch prop is
redlined before running out of throttle.