Constant speed prop - throttle first or prop first?

At the altitudes I usually fly I can't hurt the engine no matte how far I pull the prop back. I see a max of 15" quite often at cruise.

I was taught to increase prop before increasing throttle and decreasing throttle before decreasing prop. I understand the reason for it. In a care it is usually called "lugging" the engine. Under very high power settings with significant horsepower, this makes sense. When flying at 6,000 ft and above, it is nearly a non-issue.

Like others said, the prop control is the same as a transmission in a car.

Joshua
 
I've always liked the "lugging the engine" or bicycle gear analogies as a loose explanation in the extreme for the SOP order in which to make changes when we are going to change both. One doesn't have to buy into the oversquare myth to adopt it.
 
My guess, "prop on top" or "over-square" rule, what ever you call it is simply a rule of thumb that got changed to a rule by some. Short answer is almost all flat piston aircraft engines can safety be run using this rule. So in one easy rule of thumb you can fly most any of these engines without ever looking in the book for acceptable power settings.

Now if you want to run more MP than RPM then perhaps you should look in the book to see what settings are acceptable.
might also come from pilots that fly a lot of different airplanes frequently.

Brian
CFIIG.
 
brcase said:
My guess, "prop on top" or "over-square" rule, what ever you call it is simply a rule of thumb that got changed to a rule by some. Short answer is almost all flat piston aircraft engines can safety be run using this rule. So in one easy rule of thumb you can fly most any of these engines without ever looking in the book for acceptable power settings.

.
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Stay out of aircraft with geared engines with that sort of attitude. This is what killed a lot of C-175 GO-300's. You can't even practically fly things like the GO-435/480 (and its variants) anywhere near "square"
 
brcase said:
My guess, "prop on top" or "over-square" rule, what ever you call it is simply a rule of thumb that got changed to a rule by some.
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Or, after watching various digitized versions of WWII and later military training films, some of these things were taught as more than just "rules of thumb" to so many people in mass quantity -- that it is only now starting to turn into "Just read the silly POH, would ya?" Culture creep.

There's some interesting "rules" in those old movies. Watching them nowadays, as nostalgia videos, we know those pilots were probably pounded by military instructors, and those rules went into hundreds of thousands of brains, but they may not match any sort of modern engine operation reality.

Kinda like the old coot I went to check out with to rent his Cherokee 180 many many years ago.

He was still flying WWII bomber approach and pattern speeds in the thing, and I was pre-warned by others back then, that I'd want to mimic that behavior, or he'd never rent to me.

(He did a personal flight with each prospective renter besides sending a CFI up with them for insurance reasons. He wasn't an instructor.)

The CFI I was going to use to check out in it, and two other pilots, warned me to just fly the thing around the pattern a couple of times at the speed of heat and the old guy would be happy. Everyone knew it was nuts, but not even the CFIs who checked HIM out in it for FRs could ever get him to slow down.

Full throttle around the pattern was pretty much the order of the day.

To him, it looked like what he did in bombers decades before, and that was what he was taught and was "right".

He complimented me on my balls to the wall runs around the pattern and said, "So many people come to rent my airplane and fly it around the pattern all slow and mushy. I like those flight controls to feel alive. No reason to be that slow! You're doing great!"

I wonder if he ever caught on that everyone was flying fast with him and normal speeds any other time they flew his plane? Haha.

Ultimately, I think that flight and the one with a CFI, turned out to be the only times I flew it, after all. Found access to a better deal on a different rental a lot closer to home and was flying other stuff back then.

Dude was set in his ways, and someone taught him to be that way.
 
Keep the prop and power at cruise setting when making your decent. You getting your time back! Ya know the time you spent dragging it up to "xyz" altitude....just don't bust Vne speed on the way down. In the mooney, in smooth air I can leave the prop and throttle where they were as long as I don't exceed 500-650fpm in the decent or else I'll bust Vne.


Sent from my iPhone using Tapatalk
 
In general,

Power up? Push them in right to left
Power down? Pull them out left to right
 
flyingron said:
Stay out of aircraft with geared engines with that sort of attitude. This is what killed a lot of C-175 GO-300's. You can't even practically fly things like the GO-435/480 (and its variants) anywhere near "square"
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You've got a point that geared engines require a different technique, but aren't they generally run well "under" square? Unless supercharged, you'd have a hard time generating enough manifold pressure in the GO-435 and 480 series to even get to "Square" unless you pull the props way back at low altitude. The recommended cruise power (prop) setting for the GO-series is 2600 RPM. Doesn't take much altitude for the engine to be unable to generate more than 26".
 
flyingron said:
Stay out of aircraft with geared engines with that sort of attitude. This is what killed a lot of C-175 GO-300's. You can't even practically fly things like the GO-435/480 (and its variants) anywhere near "square"
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I didn't/don't seem to have a C-175 or GO-300 Manuals in my collection to verify this. Online search didn't help much either.
Obviously since they run at higher RPMS the "rule of thumb" would allow higher manifold pressures. But I couldn't confirm it was possible to run them outside of recommended limits while adhering to the rule of thumb.

Brian
 
brcase said:
I didn't/don't seem to have a C-175 or GO-300 Manuals in my collection to verify this. Online search didn't help much either.
Obviously since they run at higher RPMS the "rule of thumb" would allow higher manifold pressures. But I couldn't confirm it was possible to run them outside of recommended limits while adhering to the rule of thumb.
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Most 175s had fixed-pitch props. Of almost 2,200 175 s and P172Ds built with the GO-300 engine, only the 1962 175C and the '63 P172D (182 units altogether) were built with constant speed props. But even with fixed-pitch, some customers were spooked by the high engine rpm numbers and tended to baby the engine, which didn't do it any favors.
 
New to Constant speed prop. Just got signed off on the 182 after years of flying 172's. I am STILL confused RPM to me means throttle and Manifold pressure is the props. But it seems this is backwards on the c182. ( Adj prop look at RPM window) Adj Power look at Manifold window.
 
When you turn the rpms back (by adjusting the prop of course), note that the manifold pressure INCREASES as you turn the prop back.

I'm at full throttle (25" and 2700 rpm (Im at 5000')) and I'm going to economy cruise. I reduce manifold pressure to 18". Then I turn the prop back to 2000rpm. The manifold pressure goes up to 20". I'm now running at 20/20.

Now I lean. I turn the lean knob and watch the rpm. Rpm will drop a bit when you lean so much you reduce power/engine stumbles. Richen it a bit so Im running as lean as possible.

Note the manifold pressure does NOT go up when rpm is reduced by leaning the mixture.
 
skippy101 said:
New to Constant speed prop. Just got signed off on the 182 after years of flying 172's. I am STILL confused RPM to me means throttle and Manifold pressure is the props. But it seems this is backwards on the c182. ( Adj prop look at RPM window) Adj Power look at Manifold window.
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Easy to solve. Just find a whiteboard and write 100 times,
"RPM means prop and manifold pressure is the throttle"​

Doesn't matter what is actually going on under the hood. Operationally, the difference between the 172 and 182 is that the power/thrust function has been decoupled. Where you had one control lever, you now have two. The result of small changes in prop pitch produced by the prop control are read on the gauge the powers-that-be chose to name "RPM" and small changes by the throttle are read on the gauge named "MP." The gauges could have been named "Dick" and "Jane." Accept it and be happy.

:cheers:
 
Fearless Tower said:
Where did the oversquare concept come from in the first place? I agree it is a myth, but who started it?

It clearly did not come from the radial engine world.
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I think it is based on the coincidence that the first digits of rpm are sometimes close to the first two digits of manifold pressure in inches. If we measured MP in Pascals nobody would try to correlate 2500 rpm with 84659.7 Pascals. Probably someone figured (incorrectly) that running their airplanes in high gear was somehow bad, and used this OWT to keep the kids in second or third gear.

My first car was a 1962 Ford Falcon with vacuum powered windshield wipers and the second was a 1969 VW bug. The relationship between RPM and MP has always be intuitive to me.
 
RalphInCA said:
I have just started training in a 172rg.

I know there is logic to which control (prop or throttle) needs to be used first when making power adjustments, but I don't (yet) understand which, and why.

Also, if I am just a little high during cruise, and I wanted to lose a few hundred feet altitude, how do I do it? In a fixed pitch prop aircraft, would pull power a little. Is it the same with a constant speed prop?
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Maybe a silly question but what are you paying your CFI for?
 
Go forward right to left..retard left to right.

Mixture, props, throttle if increasing...throttle, props, mixture if decreasing. Small adjustments to throttle or prop are ok individually as long as you stay within the operating parameters of the operating manual.

So all of this is general and we can't teach you the necessary info for small adjustments because that's what your are supposed to make yourself knowledgeable on before flying the airplane. All of this is just generalizations.

tex
 
Resurrecting an old thread with regards to a 421.

same question as the OP but in this aircraft I would think the opposite rule applies, particularly to descent. The 550 engines are prone and capable of being “prop driven” which is bad for this engine. So my question is, if you’re coming out of descent at say FL250. Wouldn’t it be better to adjust prop RPM first then throttle so that the power is always in front of the prop and you’re not running the risk of the props driving the engine?
 
Powering up, work the knobs right to left. Powering down, left to right.
 
USAF-LT-G said:
Resurrecting an old thread with regards to a 421.

same question as the OP but in this aircraft I would think the opposite rule applies, particularly to descent. The 550 engines are prone and capable of being “prop driven” which is bad for this engine. So my question is, if you’re coming out of descent at say FL250. Wouldn’t it be better to adjust prop RPM first then throttle so that the power is always in front of the prop and you’re not running the risk of the props driving the engine?
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How much do you need to reduce MP in order for the props to drive the engines?
 
Leave the prop in a cruise setting. That's where it's at a coarse pitch, and at lower power for descent it is less likely to reach the negative angle of attack that is necessary for the prop to drive the engine.

Once again, the POH/AFM is the source of wisdom for such stuff. If the 421 requires something different, it's POH will say so.

Here's a POH for the 421C: https://usermanual.wiki/Document/Cessna421CC421CGoldenEaglePOHAFM.3228283069/view

It doesn't mention any special settings for descent.
 
USAF-LT-G said:
Resurrecting an old thread with regards to a 421.

same question as the OP but in this aircraft I would think the opposite rule applies, particularly to descent. The 550 engines are prone and capable of being “prop driven” which is bad for this engine. So my question is, if you’re coming out of descent at say FL250. Wouldn’t it be better to adjust prop RPM first then throttle so that the power is always in front of the prop and you’re not running the risk of the props driving the engine?
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I used to fly a 421, and never touched the prop levers on descent until final. Why would you?
 
It's really not as complicated as people think it is. Think of it like a transmission, pulling the blue knob out is like going into a higher gear. So if you want to go up a hill you downshift then step on the gas. Going down a hill, just leave it in fifth and take your foot off the gas.
 
Silvaire said:
It's really not as complicated as people think it is. Think of it like a transmission, pulling the blue knob out is like going into a higher gear. So if you want to go up a hill you downshift then step on the gas. Going down a hill, just leave it in fifth and take your foot off the gas.
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Or as one DPE said to me, "you spent that extra time getting to altitude, get it back in the descent." Leave the throttle where it is, and add some nose down trim.
 
EdFred said:
Or as one DPE said to me, "you spent that extra time getting to altitude, get it back in the descent." Leave the throttle where it is, and add some nose down trim.
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Mooney has a high green arc, I leave cruise power settings in and come downhill at 185kts.
 
Bill said:
Powering up, work the knobs right to left. Powering down, left to right.
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I invented my own saying to help me remember: “pilots look down on their copilots. Copilots look up to their pilots“. So, starting with the pilot side, the levers go down towards the copilot side. Or, starting with the copilot side, the levers go up to the pilot side.
 
Funny how things stick with you. I was taught:
Throttle: T = take off (power)
Prop: P= put on (power)

That was over 40 years ago.
 
EdFred said:
I can only come down at 176kts IAS :(
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Bill said:
Mooney has a high green arc, I leave cruise power settings in and come downhill at 185kts.
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I don't pay attention to the speed (other than to make sure it isn't out of the green arc), but program the AP to a 600 or 700 FPM descent and let her rip. Makes for some SWEET ground speeds.
 
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