Constant Speed Prop - Descent Settings

M

MJR Pilot

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MJR Pilot
I fly a Piper Arrow and am curious if others have views on proper prop RPM settings for descent? In cruise, I typically fly with RPM at 2400 and MAP between 22 - 24". For descents, I keep RPM at 2400 and pull MAP to 17-18" - combined with a little nose down, this gives me a nice 500-600 fpm descent rate. My question for the group is whether it is appropriate to keep RPM at 2400 during a descent - 2400 is the often stated level for cruise, but I've heard lots of different opinions on the best descent setting - some people have told me to push prop to 2500 for descent to increase drag and help descent rate, others have said to pull prop back to 1700-1800 to be "squared" with my MAP setting, and still others have said to keep it at 2400. Is there any "correct" answer or source of info on this? I can't find anything in the POH. Any views appreciated. Thanks.
 
I don’t think it really matters, as long as it’s in the green arc. I don’t love the idea of pulling it back to 1700 assuming that’s not in the green arc, but it probably won’t hurt anything for short periods.

I just leave everything where it is and pitch the nose down. Nice to regain some of the speed lost during climb. If it’s too bumpy to do that I’ll pull back the power, but the prop stays at cruise setting until GUMP.
 
I leave the prop where I had it during cruise. I don't usually do anything with it until the rpm starts to drop on it's own when I slow down to land, then it's prop full forward.
 
midwestpa24 said:
I leave the prop where I had it during cruise. I don't usually do anything with it until the rpm starts to drop on it's own when I slow down to land, then it's prop full forward.
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+1
 
midwestpa24 said:
I leave the prop where I had it during cruise. I don't usually do anything with it until the rpm starts to drop on it's own when I slow down to land, then it's prop full forward.
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+2

I’ve just started flying an Arrow, but I typically leave the prop where it is until it’s off of the governors. So far I’ve seen no reason to change it for this craft. My descent profile is a little different. I aim for 400-450 fpm with more power/speed. I try to keep the power in as long as I can so that I arrive at 2-3 nm at 17”.
 
Thanks all - this is consistent with how I've been flying as well. Appreciate the responses.
 
Pull power to bottom of green and leave prop speed alone.
 
I leave it at cruise setting and shoot for a 250ft/min descent unless ATC is asking me to descend. Adjust manifold and mixture as necessary to maintain about the same or better power setting during the descent.

BRain
 
Some pilots get a bit worked up over the concept of a prop you can adjust in flight. When I do constant speed prop transitions, I use the KISS principle. I teach there are only two (optionally three) settings one really needs to use on a normal flight.
1. Prop full forward (or just below redline for those prop tends to overspeed).
2. Cruise power (whatever your preference).
3. (Optional) if your manual limitations require an intermediate prop setting for climb which is less than full forward.

#1 is for takeoff, climb (except if you have #3), and landing.
#2 is for cruise (obviously) and descent.

MP? Depends. I tend to keep it at cruise for descent, although I incrementally reduce it to my approach setting as I get to approach altitude (especially in a turbo) so when I level off, I have nothing to do other than pitch and trim (I like to say I am efficient; my wife tells me I'm just lazy).
 
midlifeflyer said:
Some pilots get a bit worked up over the concept of a prop you can adjust in flight.
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I think it goes back to primacy, that RPM = Power setting. I had a pilot once come to me for high-performance and complex endorsement. He had done a bunch of home study before hand, too much. He was so hung up on changing prop settings, and the proper order of changing prop settings, as if the prop must be adjusted everytime anything else is touched. After we did the ground and got in the air, I showed him how's it done. His question then...that's it? He had built the constant speed prop up in his mind as this hugely complicated thing to operate, so much so he questioned why anyone would ever want one.
 
I guess I might fool with it on descent if I wound up high, though I don't think I ever have. My biggest problem with descent it keeping the Mooney out of the yellow on bumpy days. Sort of a nice problem to have, actually.
 
That’s my issue too: bumps. As much as I love to fly fast (who doesn’t), and always want to recover the speed given up in the climb, it just seems more likely that I’ll end up having to maintain Va due to bumpy conditions.
 
Depends on the descent rate you want. My 3-blade set to flat is a very effective speed brake. Down she goes.
 
Rgbeard said:
How long until the "squared" bull stuff goes away.

Oh wait.....
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If we’d always measured manifold pressure in Bar, it never would’ve existed. I wonder if it ever was valid in the engines of the early 1930s or whenever the idea was born.
 
I don't fly an Arrow, but I always pull the prop back for descent while I slowly decrease MAP. Saves gas, and I'm cheap. Prop all the way in when it's off the stops as some others have said.
 
midwestpa24 said:
I think it goes back to primacy, that RPM = Power setting. I had a pilot once come to me for high-performance and complex endorsement. He had done a bunch of home study before hand, too much. He was so hung up on changing prop settings, and the proper order of changing prop settings, as if the prop must be adjusted everytime anything else is touched. After we did the ground and got in the air, I showed him how's it done. His question then...that's it? He had built the constant speed prop up in his mind as this hugely complicated thing to operate, so much so he questioned why anyone would ever want one.
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You and I see the exact same behavior. Out interpretation is slightly different. We're probably both right to some degree. And we agree that they do get hung up and simplification is the answer :)
 
MJR...I also fly an Arrow 200 and do exactly what you do...prop stays at cruise setting and manifold pressure back to 17-18, just enough to keep the gear horn from sounding! A slow reduction in MP to avoid shock cooling, of course. By the time I turn final the prop control can go full forward without any change in rpm. Also is quieter for neighbors!
 
on descent - prop stays at 240. Like @EdFred said - leave in the power. Why slow down? Dontcha want to go fast???

Rgbeard said:
How long until the "squared" bull stuff goes away.
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Yes. On climb and in cruise you better have that thing squared up at 25/25 and 24/24 (respectively) or you are gonna blow that engine to smithereens and destroy all of the baby seals and puppies in a 10 nm radius. Running oversquare for even 10 seconds is one of the leading causes of global warming. Don't EVER do it.
 
..as other have said. I leave it alone, I keep MP below 22" but the RPM stays at the cruise setting, which is usually 2200. As I get close to the pattern it goes to 2400.. and somewhere on final it all goes all the way forward

I don't get the people who pull the power to 16" and crank the engine up to 2,500 at 7,500 when starting a descent. Why not make up the lost speed? Heck.. if it's smooth let it cruise up into the yellow!
 
steingar said:
keeping the Mooney out of the yellow on bumpy days
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WillFly4Food said:
That’s my issue too: bumps
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..what do we define as "bumps" ..?

I've sat right seat with people who hang in the yellow in "bumps" and everyone seems to have a different definition of occasional / light / moderate / severe

Mind you, I'm personally very careful as the planes I'm flying are old enough to be my dad! but.. I'm not slamming on the brakes either for one hiccup here or there
 
tspear said:
Make sure you also avoid shock heating. So no increasing the throttle faster then about fifteen minutes from idle to full power.

Tim

Sent from my HD1907 using Tapatalk
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And never ever do pattern work in any big bore Continental.
 
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