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.

: What the CS prop primarily gives is better climb and increased drag in
: descent if you need get down in a hurry.

: I've often thought a CS prop would be very beneficial in long
: descents. I often cruise @ 8-11k feet, and during descent, it is easy
: to redline the engine, so I have to remove some power, which decreases
: the airspeed.

Impatient, I see... This is one area where the CS prop could help, but my
experience has been that a descent like that will get you well in the yellow arc if
you keep it throttled up. In cruising flight, the fixed-pitch will pretty much do
what you need (i.e. RPM will increase with altitude at about the same rate as the max
MP drops). Thus, you can get 75% at up to about 8000' DA. Above that, without a
turbo, you can cruise the prop at 2700 RPM, but you won't get 75%.

It's really about being able to get *full* (or almost full) power at takeoff
and during climb. Think about a sea-level takeoff in the fixed 60" 180hp... probably
get about 2400-2500 RPM at 90mph IAS. You're really only getting (roughly)
24/27th's of your 180 hp=160hp. CS lets you run it up to 2700 for the same
climb and actually make your rated 180 at sea level.

Besides, power does not give you speed. Power lets you climb and haul more,
but speed goes up as the cube root of power.... i.e. doubling the power gives you
2^(1/3) = 25% more speed for a given airframe. So a Cherokee 180 at 135 mph will go
to at most 170 mph if you could strap on a 360 hp engine. A more realistic comparison
is to change it to a 235 and go to 147mph.

If the plane's legs don't go up (and thus don't need to "shift from
3rd-5th"), CS prop just buys you climb and/or load.

-Cory


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************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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