Constant Speed Prop question

nddons

nddons

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I'm getting checked out this weekend in a DA-40 with a constant-speed prop. I've only flown fixed pitch Cessnas, and so I'm interested in getting a good understanding about CS props before my flight.

I've read an AOPA article on CS props, but does anyone have any better resouces that you can refer me to? I just want to be as prepared as I can be.

Thanks.
 
nddons said:
I'm getting checked out this weekend in a DA-40 with a constant-speed prop. I've only flown fixed pitch Cessnas, and so I'm interested in getting a good understanding about CS props before my flight.

I've read an AOPA article on CS props, but does anyone have any better resouces that you can refer me to? I just want to be as prepared as I can be.

Thanks.
Click to expand...

Try these links:

http://www.mccauley.textron.com/prop/propframese.html

http://www.avweb.com/news/maint/185020-1.html


I may have another which I will post if I find it.

David
 
McCauley has an awesome graphic representation and explanation. None of my text books have as good material.
 
OK, here we go...

I saw this in the other thread linked above...

As for what I do (1971 182N, max RPM 2600): Wide open throttle. Prop full forward for takeoff, back to 2500 when practicable for noise abatement and used for cruise climb throughout the flight, and down to 2200 (bottom of green arc) for cruise. Pitch over for descent, pulling throttle a bit if I get too fast. Descent from TPA at 12" and put the prop back full.
Click to expand...
I've seen this in other posts everywhere, most notably Tom Gresham with his Bonanza. WOT for all operations until slowing to land. I wonder if this is only efficient with high horsepower slippery planes such as Bo's. First time I've really seen anyone make mention of it in regards to a 200-235 hp range airplane. (Actually with Tom's plane he runs it at a constant 30" MP, not sure if that is WOT on a TNIO-550 equipped Bo or not but same theory.)
 
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From a functional standpoint, just think of a CS prop as a continuously variable manual transmission -- first gear (low pitch/high RPM) for takeoff, second gear (mid pitch, mid RPM) for climb, and high gear (high pitch/low RPM) for cruise.
 
Ron Levy said:
From a functional standpoint, just think of a CS prop as a continuously variable manual transmission -- first gear (low pitch/high RPM) for takeoff, second gear (mid pitch, mid RPM) for climb, and high gear (high pitch/low RPM) for cruise.
Click to expand...

I explain it as trim along the rotary axis. Just like a plane will always try and find the speed you trim it to by adjusting the pitch, a prop will always try and find the RPM you set it to by adjusting its pitch.
 
Ron Levy said:
From a functional standpoint, just think of a CS prop as a continuously variable manual transmission -- first gear (low pitch/high RPM) for takeoff, second gear (mid pitch, mid RPM) for climb, and high gear (high pitch/low RPM) for cruise.
Click to expand...

EdFred said:
I explain it as trim along the rotary axis. Just like a plane will always try and find the speed you trim it to by adjusting the pitch, a prop will always try and find the RPM you set it to by adjusting its pitch.
Click to expand...

Thanks, guys. Those are great ways to visualize what's going on up front.
 
alaskaflyer said:
OK, here we go...

I saw this in the other thread linked above...

I've seen this in other posts everywhere, most notably Tom Gresham with his Bonanza. WOT for all operations until slowing to land. I wonder if this is only efficient with high horsepower slippery planes such as Bo's. First time I've really seen anyone make mention of it in regards to a 200-235 hp range airplane. (Actually with Tom's plane he runs it at a constant 30" MP, not sure if that is WOT on a TNIO-550 equipped Bo or not but same theory.)
Click to expand...

Seems like a reasonable procedure to me, and it is not airplane-specific. Cruise revs should be as low as possible.

Bob Gardner
 
alaskaflyer said:
OK, here we go...

I saw this in the other thread linked above...

I've seen this in other posts everywhere, most notably Tom Gresham with his Bonanza. WOT for all operations until slowing to land. I wonder if this is only efficient with high horsepower slippery planes such as Bo's. First time I've really seen anyone make mention of it in regards to a 200-235 hp range airplane. (Actually with Tom's plane he runs it at a constant 30" MP, not sure if that is WOT on a TNIO-550 equipped Bo or not but same theory.)
Click to expand...

Well, since I posted it...

This is obviously not a high-HP or slippery plane, it's a 230-hp Continental O-470 attached to a Cessna Drag-o-matic.

Many folks like to pull back to "25 squared" shortly after takeoff, and I fully advocate pulling back to 2500 or fewer RPM as soon as practicable to reduce prop noise and create fewer angry airport neighbors. However, the "don't run oversquare" is pretty much an OWT as it's dependent on the specific engine you're running. Get an engine operating manual and see what the maximum recommended MP is for 2500 RPM.

In this particular case, on the takeoff roll in this plane the MP is normally only about 27" (roughly 1000 MSL field) so it's pretty close to 25 by the time I pull the prop back.

Finally, cruising at a lower RPM (I usually cruise at 2100 or 2200) should be more efficient, I think. When your plane goes faster, drag goes up exponentially, and that is true of your prop spinning faster as well, although I'm sure there's more to prop efficiency than just that.


WRT learning about CS props, I've found that the traditional FAA material didn't do a good job of teaching me at all. "Throttle controls MP, prop controls RPM" yeah we all learned that for our private pilot writtens. However, they never explained what was going on, and how that worked. Well, it's pretty simple: When you add MP via pushing on the throttle, the engine develops more power but the RPM stays the same because the prop takes a bigger bite of the air. When you pull on the throttle to reduce power, the prop governor adjusts the prop to take a smaller bite of the air to keep the RPM up. There are limits, so when you pull the throttle far enough that the RPM is reduced, the prop has hit the low-pitch stop.
 
Thanks. Believe me I was throughly disabused of the "oversquare" OWT long ago.

I agree that the lowest RPM for any given MP allowed by the POH is probably the most efficient...my question is running WOT in cruise and cruise climb vs. some lower MP. The simplicity of pushing the throttle all the way in for takeoff and not touching it again until descending into the pattern, controlling power with the prop control only is intriguing. Of course in a normally aspirated engine at higher cruising altitudes you are going to be at WOT to achieve any sort of decent MP anyway, so I'm probably overthinking the issue.
 
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alaskaflyer said:
Of course in a normally aspirated engine at higher cruising altitudes you are going to be at WOT to achieve any sort of decent MP anyway, so I'm probably overthinking the issue.
Click to expand...

Bingo. Once you're up to 5,000 feet or whatever, MP's are quite reasonable at full throttle - And you're going to want full throttle on your climb up there anyway. My normal procedure is everything full forward on takeoff, pull prop back ~1000 AGL, adjust mixture as needed above 3000, pull prop again after leveling off and accelerating to cruise speed...

From there, one of two things. For a fast approach ("N271G, there's a jet up your tail, maintain maximum forward speed on the approach") I keep the cruise power setting until about 350 AGL, then it's throttle back, prop and mixture forward (the prop acts as a great speed brake here), then full flaps as soon as I get to the white arc.

For a normal approach, I will pull to 20" MP for cruise descent, 16" to slow to normal approach speed, and 12" for final approach. After pulling power to 12", I should be out of the governing range so I push the prop control forward.
 
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