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PA28: Difference in constant speed prop vs fixed pitch



 
 
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  #11  
Old October 8th 04, 04:19 PM
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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


--

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

  #14  
Old October 8th 04, 05:38 PM
G.R. Patterson III
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Nathan Young wrote:

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.


They usually do, for the reasons you stated. If you adjust the controls such that the
engine is producing 75% power in level flight and the CS prop settles into a coarser
pitch than the fixed pitch prop had, you'll see a faster cruise speed. If the fixed
pitch prop was pitched to produce the best cruise speed, the CS prop won't do any
better there, but will improve your ROC.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.
  #15  
Old October 8th 04, 05:45 PM
Nathan Young
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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.
  #16  
Old October 8th 04, 05:48 PM
PaulH
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Nathan Young wrote in message . ..
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.


The fixed pitch prop IS a compromise, but if the prop is pitched to
permit generation of 75% power, it doesn't matter what the angle of
the prop is. The only thing the CS prop provides at cruise is a
choice of RPM for the same power. I can cruise at 22" and 2400 RPM
and get 155 mph in my Arrow or 25" and 2100 RPM. They both generate
65% and my speed is the same. If you want to go faster, you have to
generate more power. with CS, I have a choice of increasing either or
both MP and RPM. With fixed pitch, you can only push the throttle
forward, and the fixed pitch will limit the maximum power you can
generate.

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.


Yes, the CS prop will prevent the overspeed so you have one less item
to watch in descent. And if you set max RPM and pull the throttle
back, you can descend at 2000 fpm while staying out of the yellow
airspeed arc.
  #18  
Old October 8th 04, 06:38 PM
Newps
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G.R. Patterson III wrote:

Nathan Young wrote:

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.



They usually do, for the reasons you stated. If you adjust the controls such that the
engine is producing 75% power in level flight and the CS prop settles into a coarser
pitch than the fixed pitch prop had, you'll see a faster cruise speed. If the fixed
pitch prop was pitched to produce the best cruise speed, the CS prop won't do any
better there, but will improve your ROC.


In the performance charts for my 182 a given percent power always
produces the same speed at a given altitude.

  #19  
Old October 8th 04, 06:41 PM
Newps
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Nathan Young wrote:




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.


Yes, but also the other way around too. Climb real high with a fixed
pitch so even when you're at full throttle you can only get say 2100 rpm
in your 172. In my 182 I can always get the full max continuous 2450
rpm, regardless of what the manifold pressure may be.

  #20  
Old October 8th 04, 06:43 PM
Newps
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Doug wrote:

With my Husky, I go fastest with the prop full forward. I am not sure
if this is true with all small GA aircraft with Constant speed prop.


Of course it is. Set whatever rpm you want. Add in some more rpm and
you will go faster.


The Husky has essentially the same prop/engine config as the Lycoming
0-360 Mooneys.
What the CS prop gives me is a smoother ride, better fuel economy and
another knob to turn.


Smoother ride? Never heard that one before.

 




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