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PA28: Difference in constant speed prop vs fixed pitch



 
 
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  #21  
Old October 8th 04, 06:48 PM
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Newps wrote:
: 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.

: That formula has always been taken as gospel but the real world truth is
: if you could get a 360 hp engine under the cowl you would go a lot
: faster than just 170 mph. My buddy and I both have 182's. I have the
: standard 230 hp engine, he has the IO-550 300 hp engine in his. Even
: though he has more drag because he has much larger tires than I do as
: well as bubble windows on both front side windows he typically indicates
: 30 mph faster thah I do, and he lost 5-7 mph with the 3 8.50 tires over
: the 8.00x6 mains and 6.00x6 nose like I have. And while he goes
: virtually straight up he cannot legally haul more.

While I won't argue with your specific results, they really can't be
considered the norm. Even though two planes are identical models, they cannot be
directly compared from differences beyond control (mis-rigging, different weights, the
things you mentioned, etc). Take a look through a multi-purpose POH (e.g.
PA-28-150,180, or PA-24-180,250,400)... the numbers line up.

e.g. Book: Ratio^1/3 (relative to 250)
PA-24-180: 139ktas 140ktas
PA-24-250: 157ktas base
PA-24-400: 185ktas 184ktas

Awfully close numbers. Again, I'm not saying your numbers are wrong, just
that it holds remarkably well to published numbers. According to hp, he should
indicate 9.3% more speed than you. I doubt you're doing 323 mph in a 182...

-Cory

--

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

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  #22  
Old October 9th 04, 03:57 AM
Doug
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Well, I have heard from people SAY that they go faster with the prop
pulled back a bit. My experience is limited to the Husky (for constant
speeds). I've flown other CS prop planes, but not enough to have done
speed tests.

As for smoother, my engine runs smoothest at about 2450 rpm. Anywhere
else is more vibration. So there you go. You can choose the smoothest
rpm.

Newps wrote in message ...
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.

  #23  
Old October 9th 04, 04:00 AM
Doug
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Yes, and you will use less fuel at the setting with the higher
manifold pressure and lower rpms, per cent power and speed being equal
at both settings.

(PaulH) wrote in message . com...
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.

  #24  
Old October 9th 04, 04:19 AM
Newps
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Doug wrote:


As for smoother, my engine runs smoothest at about 2450 rpm. Anywhere
else is more vibration. So there you go. You can choose the smoothest
rpm.


But that is a factor of the engine, not the prop. Lots of planes are
like that.

  #25  
Old October 9th 04, 07:14 PM
G.R. Patterson III
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Newps wrote:

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.


I'm sure it does, but you would not see that speed if you replaced your CS prop with
a fixed pitch prop that has the compromise pitch that is commonly used on aircraft
with fixed pitch props. Most aircraft with fixed pitch props have a prop that has too
fine a pitch to get the best cruise speed at 75% power. Replacing that prop with a CS
prop will result in higher cruise speeds.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.
 




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