Fine power adjustment on constant speed prop

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flying_John

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flying_Joe
Hi all,

I'm getting interested in complex or high performance endorsement.

Biggest change from C152 seems to be power management.
I read some topic on internet how to adjust power.
But everything I read in focused on cruise setting or descend setting, and full forward in landing for go around.

How about fine adjustment for steep turn etc.?

Do you always prop first when increasing thrust first when decreasing and monitor combination MAP and RPM? That seems to be big workload to me.
 
It is multifaceted and can be complex. Up until now you have been flying with a fixed pitch prop and the only real power meter was RPM (maybe also a MP gauge) the only power controls were Throttle and Mixture, but at low altitude the mixture is always full rich, so it didn't matter much until high altitude cruise. Complex aircraft can change all that, depending on the type of flying you do.

Look up "Red Fin, Red Box, Lean of Peak" and read some of the articles presented. This will get you into understanding how some of these things are related. Basically, RPM and Manifold Pressure both affect airflow through the engine, and more airflow potentially means more power if you add enough fuel to make it. Most POH manuals specify best power mixture or close to it for cruise settings below 75%, and full rich above 75%. The Red Fin Red Box concept shows you how the engine internal stress is affected by power output and mixture settings. Below 61% power the mixture can be set anywhere Rich of Peak or Lean of Peak without harming the engine. Also, leaning to LOP drops the power by about 10% of the best power mixture setting, so if you were running at 70% ROP, then at LOP you'd be running at 63% (70-7=63) just by pulling back the red knob.

Good reading and good luck!!
 
Last edited:
flying_John said:
Hi all,

I'm getting interested in complex or high performance endorsement.

Biggest change from C152 seems to be power management.
I read some topic on internet how to adjust power.
But everything I read in focused on cruise setting or descend setting, and full forward in landing for go around.

How about fine adjustment for steep turn etc.?

Do you always prop first when increasing thrust first when decreasing and monitor combination MAP and RPM? That seems to be big workload to me.
Click to expand...
Here's how I run mine
-at 1000agl i reduce to 2500 rpm to reduce noise and be a little easier on the engine
-after accelerating to cruise speed, I set 65% power. I usually accomplish this by reducing rpm only, but if I'm low, I'll set it at 2200 rpm and reduce throttle.

I usually don't touch the blue knob again until I push it forward on short final. It really is that simple. If I need to climb I'll push it back up, and I sometimes increase rpm on descent if I need some more drag. If doing maneuvers like steep turns, I leave the rpm alone and just increase throttle by an inch of mp or so. Some instructors like to make it much more complicated than it really needs to be.

It seems like a lot at first, but you'll quickly get a feel for it.
 
Do any of you fly "over square" - Man Pressure exceeds RPMs?
 
Greg Bockelman said:
For something like that, you do not need to touch the prop control at all. Just an inch or so of manifold pressure would do.
Click to expand...
^^^ this. Listen to Greg.

OP quit reading internet garbage -- including some of the other answers in this thread claiming this is a complex thing -- and just go get in the airplane with an instructor and see it for yourself.

If the blue knob is too far back and you can't safely add power, you'll know it.

There's almost never any time you'll need any 'fine tuning" for something like a steep turn unless you're at very high altitude / DA and can't hold altitude without the prop at high RPM and the throttle wide open.

You won't be seeing that in a 182 unless you're flying steep turns at max gross above 10,000 MSL on a warm day.

Any cruise setting where you can still add power to climb is going to work just fine for airwork.

Don't let the extra knob get into your head. It's essentially like a car gearshift. Shift it into low before you stomp on the gas if you need the accelerator buried to the floor. Same deal.

You may be used to the smaller aircraft needing full power to hold altitude during the maneuvers. You'll enjoy flying something that doesn't need that with a much bigger engine. Especially lightly loaded.

Which is why Greg said it won't take much of a throttle change. An inch of MP or so is plenty when you have 230-ish horsepower depending on atmospheric conditions and altitude you're flying at today.

Needing to cram the throttle all the way forward on the big engine isn't usually necessary like maybe you've experienced in the little guy. :)

And when you truly need it, it's a gross movement not a fine tuning. Ha. Prop all the way up smoothly and throttle all the way up right behind it. I neeeeeeeed powwwwahhhh riiiiiight nooooowwww! Haha.
 
Some people love to make simple topics hard. Constant speed props are one of them.

All you gotta remember is max RPM to climb, reduce RPM for cruise, and keep MP below RPM.

That last one is what gets new complex pilots all befuddled and causes worry. It is a good practice for engine longevity, but your engine is not going to blow up if you forget and do it backwards once in a while.

IRL you take off at max prop, dial back to selected power setting to cruise, and return to max prop for landing. For maneuvering, there is no reason to diddle with it. Set it and forget it. For example, when I fly acro in my Super Decathlon, I set power to 26 squared (26 inches MP, 2600 RPM) and leave it alone.
 
Using the prop control isn't as complex in practice as it seems when you first start reading about it.

You really only have a couple of prop settings that you'll use.

Takeoff & Landing: Full increase
Climb: slightly less than full, often 2500 RPM *OR* Full increase when max climb power is needed
Cruise: Lower setting from the cruise performance chart for the desired cruise power setting. I like the lowest RPM that can provide the desired power setting and that is still smooth.
Maneuvering: High-cruise or the reduced climb setting so that a high-power setting is available quickly.
​
Beyond that, you're using the throttle for power changes and the manifold pressure gauge as your primary power setting reference.

The throttle is the gas pedal, the prop control is the gear shift.
 
WDD said:
Do any of you fly "over square" - Man Pressure exceeds RPMs?
Click to expand...
Sure, if the combination of MP and RPM is within those recommended in the manuals published by the airframe and engine manufacturers.

F'rinstance, the manual in my PA-32-300 has this power setting chart:

Screen Shot 2021-06-13 at 12.21.23 PM.jpg

So 27"/2200 rpm is an acceptable setting to yield 75% power at 2,000 feet.

The manual from my old K35 Bonanza had this graph to depict the acceptable range of power settings:

Screen Shot 2021-06-13 at 12.20.34 PM.jpg

I bet you've flown oversquare in airplanes with fixed-pitch propellers. But you didn't know about it because you didn't have a manifold pressure gauge, and there's nothing you could have done about it if you did. My Cessna 172N (180 hp) with fixed-pitch prop did have a manifold pressure gauge:

SAM_0198.JPG

It's just an unfortunate and confusing happenstance that the numbers of revolutions per minute in normal power settings are similar to manifold pressure expressed in inches of mercury x 100. Use different units. Express the manifold pressure in millibars and/or prop speed in revolutions per second, and there's no confusion.
 
WDD said:
Do any of you fly "over square" - Man Pressure exceeds RPMs?
Click to expand...
I've never been able to get my manifold pressure above 2200"

Like @Pilawt said, the whole "over square" thing is a myth. My standard cruise setting under 5000' or so is 25"/2200 rpm. Above that its WOT and enough RPM to get to 65%.
 
What gives you max speed? Max man pressure and max prop rpm?
 
Pilawt said:
I bet you've flown oversquare in airplanes with fixed-pitch propellers. But you didn't know about it because you didn't have a manifold pressure gauge, and there's nothing you could have done about it if you did. My Cessna 172N (180 hp) with fixed-pitch prop did have a manifold pressure gauge:

View attachment 97213
Click to expand...

That's very cool. (for teaching anyway...)

Why did it have the gauge?

If a picture is worth a thousand words, that's one of em.

Been interesting to see how many decades the world has been trying to kill the "oversquare" well-intentioned training technique that doesn't apply to every setup.

Cool photo.
 
WDD said:
What gives you max speed? Max man pressure and max prop rpm?
Click to expand...

The prop converts all (okay, most) of the hp put into it into thrust. More hp=more thrust=more speed.

Hp is mainly a function of how much fuel you can burn, which is a function of how much air you can pump. Higher mp and more rpm=more air moved through the engine=more hp.

Generally the engine & prop will be more efficient at lower rpms however.

If you didn't care about engine longevity & fuel cost, you'd just run the throttle & prop firewalled all the time. This is known as "full rental power".
 
WDD said:
Do any of you fly "over square" - Man Pressure exceeds RPMs?
Click to expand...
Yes. Can happen in fixed pitch props and in multiple POH configurations. Hell the T and TN Cirrus cruise much higher MP than RPM

It's a myth just like shot cooling. In my opinion.
 
denverpilot said:
Why did it have the gauge?
Click to expand...
My dad put it in when he did the engine upgrade in 1997. Somebody told him of the little trick to roughly calculate percentage of power.

Note the small placard to the left of the MP gauge. Add the MP to the RPM in hundreds (e.g., 22" MP + 2300 rpm/100 = 45). The sum corresponds to a % power. A sum of 42 is roughly 55% power; 45 ~ 65% and 48 ~ 75%. Not exact, but close enough in a 172. And it works at any altitude.

I liked having the MP gauge in the airplane. For loafing around at low altitudes I'd just pull it back to 21" and forget it, and it's not affected by turbulence or variations in airspeed. It also was the earliest signal of carb ice (yes, that airplane would get carb ice, even here in AZ). In the late '50s and early '60s a lot of Tri-Pacers and early Cherokees were delivered with manifold pressure gauges from the factory.

71CD7893-8DFD-4265-823E-9D28DD4ABD9A.jpeg
 
Pilawt said:
My dad put it in when he did the engine upgrade in 1997. Somebody told him of the little trick to roughly calculate percentage of power.

Note the small placard to the left of the MP gauge. Add the MP to the RPM in hundreds (e.g., 22" MP + 2300 rpm/100 = 45). The sum corresponds to a % power. A sum of 42 is roughly 55% power; 45 ~ 65% and 48 ~ 75%. Not exact, but close enough in a 172. And it works at any altitude.

I liked having the MP gauge in the airplane. For loafing around at low altitudes I'd just pull it back to 21" and forget it, and it's not affected by turbulence or variations in airspeed. It also was the earliest signal of carb ice (yes, that airplane would get carb ice, even here in AZ). In the late '50s and early '60s a lot of Tri-Pacers and early Cherokees were delivered with manifold pressure gauges from the factory.

View attachment 97219
Click to expand...
Really cool, thanks!
 
TThank you, everyone.
Now I've got ideas in practical operation of complex airplanes.

I also learned that an engine might be operated beyond the square even with fixed pitch prop.
As Denverpilot and Larry explain, pitch should be used gears in cars.
I drive stickshift and remember I wouldn't bother to shift down for small speed adjustment. It's very comprehensive explanation.

Thank you.
 
Pilawt said:
My dad put it in when he did the engine upgrade in 1997. Somebody told him of the little trick to roughly calculate percentage of power.

Note the small placard to the left of the MP gauge. Add the MP to the RPM in hundreds (e.g., 22" MP + 2300 rpm/100 = 45). The sum corresponds to a % power. A sum of 42 is roughly 55% power; 45 ~ 65% and 48 ~ 75%. Not exact, but close enough in a 172. And it works at any altitude.

I liked having the MP gauge in the airplane. For loafing around at low altitudes I'd just pull it back to 21" and forget it, and it's not affected by turbulence or variations in airspeed. It also was the earliest signal of carb ice (yes, that airplane would get carb ice, even here in AZ). In the late '50s and early '60s a lot of Tri-Pacers and early Cherokees were delivered with manifold pressure gauges from the factory.

View attachment 97219
Click to expand...
Jeff,

Your little trick works! I finally had time to check it against my POH in my 200 HP Super21. The Mooney POH gives the power at various altitudes but at the same MP the only difference is Temperature. Since Temperature goes down as altitude increases, power goes up if MP and RPM are held constant. It is a bit counterintuitive but it makes sense if you think about it. Here is what I found from my POH:

10°C 5°C 0°C -5°C
50°F 41°F 32°F 23°F
total 2500 Ft 5000 Ft 7500 Ft 10000 Ft
48 70 73 75 77
47 67 70 72 74
46 64 66 68 70
45 60 62 64 66
44 56 58 60 62
43 53 55 57 59
42 50 52 54 56

These numbers are based on best power mixture. For LOP power drops about 10%. I tried to adjust the formatting but it didn't work.

Thank you Jeff for sharing your trick!
 
flying_John said:
Hi all,

I'm getting interested in complex or high performance endorsement.

Biggest change from C152 seems to be power management.
I read some topic on internet how to adjust power.
But everything I read in focused on cruise setting or descend setting, and full forward in landing for go around.

How about fine adjustment for steep turn etc.?

Do you always prop first when increasing thrust first when decreasing and monitor combination MAP and RPM? That seems to be big workload to me.
Click to expand...

I touch the prop twice. Reduce RPMs on climbout, and on a go around. That's it.
 
I hate to point this out, but it is somewhat airplane and engine dependent. It's not hard but the POH rules. It's an extreme, but with a radial engine I couldn't get off the ground without being "oversquare."
 
ebetancourt said:
I hate to point this out, but it is somewhat airplane and engine dependent. It's not hard but the POH rules. It's an extreme, but with a radial engine I couldn't get off the ground without being "oversquare."
Click to expand...

Whoever come up with this over square crap should be shot. If MP was in millibars it would be easier to dispel this nonsense.
 
The over square misinformation I think comes from the days of radial engines, when you could more easily damage the engine by going over square. Like the guys are saying, it isn't all that difficult. Your POH or equivalent should have approved power settings from the manufacturer, even mine has that. What's difficult is getting a good lean mixture setting...
 
flying_John said:
Hi all,

I'm getting interested in complex or high performance endorsement.

Biggest change from C152 seems to be power management.
I read some topic on internet how to adjust power.
But everything I read in focused on cruise setting or descend setting, and full forward in landing for go around.

How about fine adjustment for steep turn etc.?

Do you always prop first when increasing thrust first when decreasing and monitor combination MAP and RPM? That seems to be big workload to me.
Click to expand...

yes, you enrich mixture, increase RPM, then increase throttle. It can be awkward at first and with time become second nature. You did say you wanted to fly complex/HP.
 
steingar said:
The over square misinformation I think comes from the days of radial engines, when you could more easily damage the engine by going over square. Like the guys are saying, it isn't all that difficult. Your POH or equivalent should have approved power settings from the manufacturer, even mine has that. What's difficult is getting a good lean mixture setting...
Click to expand...

I learned to fly behind a Wright R-1300, pretty old design. Like most Wrights it had an internal supercharger. T/O power was 44 inches for five minutes in the T-28A. Same engine in the Sikorsky H-19 was 44 inches continuous. I don't remember the RPM but I bet it wasn't 4,400.
 
ebetancourt said:
I learned to fly behind a Wright R-1300, pretty old design. Like most Wrights it had an internal supercharger. T/O power was 44 inches for five minutes in the T-28A. Same engine in the Sikorsky H-19 was 44 inches continuous. I don't remember the RPM but I bet it wasn't 4,400.
Click to expand...
Perhaps I am mistaken, just something I heard in passing. Still, modern aircraft have RPM and Manifold pressure settings in the POH or equivalent, so it shouldn't be that big a deal.
 
Ed Haywood said:
Bonus points for knowing what spaetzle is, double for using in a post unrelated to German food, and triple for spelling correctly.

Should have gone for 4x points with reference to spargel.
Click to expand...
ha! thanks

I grew up in a Hungarian household so spaetzle was a common Sunday meal for us.. however we called it 'sparga', actually 'spárga' to be exact

Speaking of, I tried buying a spaetzle maker a few years ago and all the ones I found where the crappy ones with the box that slides back and forth on the grid, it's not a good use of surface area. The one we grew up with was a tin funnel of sorts with a big crank, worked much better

something like this https://www.varagesale.com/i/hasymdj9pfgj3kxyd6zr2gcc-vintage-spaetzle-maker but I think our holes were bigger
 
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