PA28: Difference in constant speed prop vs fixed pitch

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.

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.

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.
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.

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.

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.

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.

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.

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.