PA28: Difference in constant speed prop vs fixed pitch

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 *
************************************************** ***********************

wrote:


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.

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.

Not to mention the reduced useful load.

Newps wrote:


wrote:


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.


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.

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 *
************************************************** ***********************

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.

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.

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.

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.

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.

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.