Constant speed prop - throttle first or prop first?

RalphInCA

RalphInCA

Cleared for Takeoff
Joined
Jun 1, 2014
Messages
1,391
Location
McMinnville, OR
Display Name
RalphInCA
Home Airfield
KMMV
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?
 
For a couple hundred feet yeah, just leave the prop alone, honestly for how slow that plane is, I'd just pitch for 4-500FPM, and pull power back if needed.


I'd also read the POH, cover to cover.
 
In that case, just reduce throttle or trim it nose down a little. No need to adjust the prop.
 
Conventional wisdom is to reduce throttle before rpm and then increase rpm prior to opening the throttle. The reason given is that chances of detonation are reduced at higher rpm. I don't know how much difference it really makes with most of our relatively low compression engines.

As for your second question, minor power adjustments can be made with throttle alone.
 
RalphInCA said:
...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.
Click to expand...

think of it from a takeoff perspective. the last thing u do is mash the throttle. so, on takeoff, or increasing power, Mixture, Prop, Throttle.
decreasing power = the opposite: throttle prop mixture

RalphInCA said:
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?
Click to expand...

sheeeit, nose down, baby! YEEEEHAAAAWWWW!!! for real. but I think since you're asking from the constant speed prop perspective, just throttle back a bit. not much different from a descent from altitude to pattern altitude, either nose down/trim or reduce power, leave RPM where it is.

there are other ways, as people will chime in shortly and of course tear my method to pieces, but basically, ask ur instructor.
 
Is "Prop on Top" still taught? I think the MP should be greater than then first two digits of the RPM.

Could be on the OWT list.
 
Frankly if you log a couple flights, you'll know what to do just by feel, adding a fist full of power than letting the prop go, it ain't going to feel, or sound like a happy airplane.

There's a lot more to it than rote memory junk.

You'll also change your power about 1" per 1000ft due to density

Power can also be used to keep your target CHTs

Changing the prop can change your low power decent speeds

Etc et.


I don't have any real hard and fast rules, minus minim sustained and max sustained RPM (IO520), everything else is based off feel, CHTs, IAS, fuel flow, required climb or decent rates, etc.


Just go up with a CFI who's flown a CS prop for a few thousand hours for a living and play around with the plane, it'll all make sense.
 
Last edited:
For Cessna piston singles with CS props, you follow the "7" flow. Up the 7 to increase power, down the 7 to decrease.

Down the 7 is throttle, prop, mixture, cowl flaps. You can add trim and fuel selector if you want.
 
Ravioli said:
Is "Prop on Top" still taught? I think the MP should be greater than then first two digits of the RPM.

Could be on the OWT list.
Click to expand...

Oversquare is old school, at least from what I was taught.

Here's another reference:
http://www.flyingmag.com/technique/tip-week/constant-speed-prop-basics

"After initial climb when entering the cruise climb phase, now it's time to start moving levers. You'll start by moving the throttle lever back until the manifold pressure gauge reads the proper value; in the case of our 182 we're looking for 23 inches. Then you'll slowly rotate the blue knob until the rpm comes back to 2,400. This is the cruise climb setting for a Cessna 182Q.

Once you've reached cruise altitude, this is where the abundance of choices for throttle/mp setting makes having the POH nearby handy. You've probably been told at some point that flying "over square" can damage your engine; that is, allowing the manifold pressure reading to exceed the rpm reading can be harmful. This is also a holdover from days of yore and is generally untrue, although there are operating restrictions that limit certain mp/rpm combinations.

Looking at the POH for a 182Q, we can choose to cruise with the throttle set between 15 and 23 inches mp and the prop between 2,100 and 2,400 rpm. At 6,000 feet and standard temperature, as an example, pulling the throttle back to 20 inches mp and the prop back to 2,300 rpm will put us at 65 percent power burning 11.1 gallons an hour. In all, the 182Q's POH lists 17 different mp/rpm combinations to choose from at 6,000 feet."
 
MAKG1 said:
For Cessna piston singles with CS props, you follow the "7" flow. Up the 7 to increase power, down the 7 to decrease.

Down the 7 is throttle, prop, mixture, cowl flaps. You can add trim and fuel selector if you want.
Click to expand...

This is a good memory technique but it does tend to confuse the student as to why we just switched from a normal descent to emergency off field landing practice when they pull the mixture out to go down. (It works a little too well. Haha.)
 
denverpilot said:
This is a good memory technique but it does tend to confuse the student as to why we just switched from a normal descent to emergency off field landing practice when they pull the mixture out to go down. (It works a little too well. Haha.)
Click to expand...

You don't pull the prop all the way out to descend, either. You push it in for the before-landing checklist. Nor do you always open the cowl flaps fully for a climb or necessarily close them for cruise or descent. The flow isn't intended to tell you which way to adjust stuff, just which stuff to check. It's not a do-list.

And if you're at the top of descent, you really shouldn't end with an off field landing unless you're cruising really, really low. Emergency checklists do have you check the mixture.
 
MAKG1 said:
You don't pull the prop out to descend, either. You push it in for the before-landing checklist. The flow isn't intended to tell you which way to adjust stuff, just which stuff to check. It's not a do-list.

And if you're at the top of descent, you really shouldn't end with an off field landing unless you're cruising really, really low.
Click to expand...

Meh, that's not really making it sing though.

Pulling the prop back a little on a long decent will increase your forward speed and decrease cabin noise.

Best time to push the prop forward is when you pull power as you're runway assured, you shouldn't be able to hear and changes in the engine when you go prop forward (for possible go around or gusts) as the power should be far back enough to the point that the prop is more or less is "fixed pitch mode" if that makes sense.
 
James331 said:
Best time to push the prop forward is when you pull power as you're runway assured, you shouldn't be able to hear and changes in the engine when you go prop forward (for possible go around or gusts) as the power should be far back enough to the point that the prop is more or less is "fixed pitch mode" if that makes sense.
Click to expand...

This also helps keep the noise down for the airport neighbors...nothing like a 2-blade 185 going full fine 5 miles out. I mean I certainly love it of course, but...
 
Do you drive a stick shift car? You're driving on a flat road and approach a steep upslope. You don't just mash the throttle and lug the engine up the hill; you downshift and get more torque from the higher rpm. That's more or less what you're doing with the prop control; increase rpm first. Likewise, if you're reducing power from cruise, just reduce throttle and leave it in "high gear".

Ravioli said:
Is "Prop on Top" still taught? I think the MP should be greater than then first two digits of the RPM.
Could be on the OWT list.
Click to expand...
If that were the case, you'd be wrecking your engine in every low-altitude climb with a fixed-pitch prop. :)

rpm.jpg
 
Back in the old days when people drove standard-transmission cars, they knew what they had to do with RPM and power.
 
MAKG1 said:
You don't pull the prop all the way out to descend, either. You push it in for the before-landing checklist. Nor do you always open the cowl flaps fully for a climb or necessarily close them for cruise or descent. The flow isn't intended to tell you which way to adjust stuff, just which stuff to check. It's not a do-list.

And if you're at the top of descent, you really shouldn't end with an off field landing unless you're cruising really, really low. Emergency checklists do have you check the mixture.
Click to expand...

I know I know. I'm just channeling my CFI DPE who doesn't accept teaching things in a way that can be misunderstood if at all possible. (It's both fun to try to get phrasing exactly right around him and also frustrating as hell. Haha.)

He wouldn't accept the "sevens" thing at all. Well he would accept it but then he'd recommend that it not be used and offer up something better.

Pretty amazing at that, really, but he's both a professional writer and been teaching for goodness knows how long...

Mostly he just likes pointing out that there's always a way for a student to misunderstand any instruction that isn't 100% accurate.
 
Take a look at the Pilot's Handbook of Aeronautical Knowledge. It was some good information.
 
As long as you're at a cruise power setting you can pretty much do what you want with the knobs in any order.

The thing you don't want to do is shove the black knob all the way in with the blue knob out if you're anywhere near sea level DA (or you have a turbo).
 
Ravioli said:
Is "Prop on Top" still taught? I think the MP should be greater than then first two digits of the RPM.

Could be on the OWT list.
Click to expand...
Totally bogus old fable. If that were true, every turbocharged aircraft would be unflyable. We cruise at 31" and 2200 rpm.
 
Ravioli said:
Is "Prop on Top" still taught? I think the MP should be greater than then first two digits of the RPM.

Could be on the OWT list.
Click to expand...
Not exactly an OWT. Maintaining MP no less than the first two digits of RPM is a supercharged radial engine thing.

For engines like the R-985 and R-1340, for example, if you are running at 1800 rpm in cruise, you can use a manifold pressure 18-28". Big thing in the radial is not letting the MP get below the RPM in flight.
 
ZeroPapaGolf said:
Totally bogus old fable. If that were true, every turbocharged aircraft would be unflyable. We cruise at 31" and 2200 rpm.
Click to expand...
Not bogus at all, just doesn't necessarily apply to most flat engines.

What he posted works just fine for a turbo. The MP is going to be higher than the first two digits of RPM.
 
My takeoff power setting is ~29in and 2350rpm. I hope I'm not blowing up my O-200.
The "oversquared" myth is so ridiculously inappropriate for the O engines it's not even funny.
 
Fearless Tower said:
Not bogus at all, just doesn't necessarily apply to most flat engines.

What he posted works just fine for a turbo. The MP is going to be higher than the first two digits of RPM.
Click to expand...
I think he just wrote it down backwards. Everything I've seen about 'prop on top' is referring to always keeping the prop RPM higher than the MP.
 
I think of it in terms of load on the engine. Much like a standard transmission. The Throttle/MP being your gas pedal and the Prop being your transmission.
 
Here's the reality: it doesn't really matter. Big old radials aside, moving one before the other makes absolutely no difference unless you're a ham-handed moron. Oversquare is a myth. CFIs still teach this nonsense because most of them are just regurgitating what they were once told. Much of the "prop on top" nonsense is related to avoiding an oversquare situation. Because educated pilots know "oversquare" is a myth, that eliminates that need to move one lever before the other. The other reason for moving one before the other is to avoid a temporary prop overspeed (surge). This can be mitigated by not being a ham-handed moron and moving the controls with some finesse.

Honestly, most people way over-complicate the prop. There's rarely any reason to ever touch the prop control once you reach cruise. My profile:

Takeoff: full forward (this is 2850 RPM in my plane)
Climb: full power, prop back to 2700RPM (2850 has a 5 min limit). If my plane was a 2700 RPM limit or had no time limit, I'd leave the prop full forward in climb. I do this in Bonanzas, etc.
Cruise: unless I'm down low (under about 6000ft), full throttle and 2500 RPM
Descent: touch nothing until I absolutely have to for speed control, then only throttle

I don't bother pushing the prop full-forward before landing. For one, mine will surge until I'm on short final because I'm generally carrying enough power that the prop isn't yet on the low pitch stops. Second, if I need to go around, it's just as easy to shove the prop in along with the throttle. Third, the plane climbs just fine at 2500 RPM.
 
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.
Click to expand...

It's routinely blamed on the radial engine world, but there doesn't appear to be any real evidence that links the two. My guess is someone back in radial days made it up and it stuck and propagated, and because it originated in the radial engine world, that's where it gets blamed.
 
I learned about oversquare long before I flew a plane. Oversquare refers to the bore/stroke ratio. Why would anyone think it has anything to do with a ratio of manifold pressure to engine speed? Anyway, normalized and boosted engines run with higher manifold pressure than the first two digits of rpm most of the time with no particular ill effects. Frequently the normalized engines are identical internally to the naturally aspirated versions of the engine. The boosted engines tend to have lower compression ratios than naturally aspirated versions of the same engine.

More anyway, when it is someone else's engine run it the way they want it run. If you don't like the way they want it run and refuse to operate it that way then don't fly that aircraft. For your own engine read the POH and engine operation manual, apply common sense and decide how you want to operate.

A little story, when I bought the 'kota it really wanted to be run at either max rpm, at 2,400 or at idle. Just about any other engine speed produced excessive vibration no matter what was done with manifold pressure or mixture. After a fair bit of maintenance which included a prop overhaul, dynamic balance, a mag overhaul among other things it runs smoothly pretty much anywhere in the green arc. The POH has settings for a wide range of speeds and power outputs with some rather bizarre annotations that have best power and best economy at various power outputs. How in the heck do you have best power at 70%? It turns out Piper decided to define best power as rich of peak tit operation and economy as peak tit operation for any selected engine output. Tit=turbocharger inlet temperature. Anyway I wouldn't be surprised if a manufacturer decided to redefine oversquare in a similar manner as a sales or operation thing.
 
Clark1961 said:
I learned about oversquare long before I flew a plane. Oversquare refers to the bore/stroke ratio. Why would anyone think it has anything to do with a ratio of manifold pressure to engine speed?
Click to expand...

That's a completely different use of the term "oversquare" and has no real relevance here.
 
FastEddieB said:
That's a completely different use of the term "oversquare" and has no real relevance here.
Click to expand...
Actually it does. Did you read the entire post? I suggested that someone decided to redefine the term to suit themselves for some unknowable reason.
 
Clark1961 said:
Actually it does. Did you read the entire post? I suggested that someone decided to redefine the term to suit themselves for some unknowable reason.
Click to expand...

Read the post. Mentioning other meanings that the term has been used for historically shows off your knowledge while not helping the OP one iota.

IMHO, of course!
 
Ryanb said:
What are you flying?
Click to expand...
Not so much, what are you flying, but how high are you flying. Most non-turbo aircraft are full throttle above 8500 MSL with the MP decreasing as you continue to climb. 1inch of MP per 1000ft.

Normal to cruise at 10.5msl with 20MP and 2300RPM.
 
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.
Click to expand...
If I remember correctly, J. Deakin mentioned it was some kind of flow from guys who went from flying big radials to flat engines. Something about they couldn't fathom being able to run an engine at full power its entire life and made up their own "METO" power for the small engine.

I don't know what kind of merit this theory holds, but it's one that I've read.

Clark1961 said:
Anyway I wouldn't be surprised if a manufacturer decided to redefine oversquare in a similar manner as a sales or operation thing.
Click to expand...
Weren't most POH's written by marketing?

I cringe when people refer to the POH as the final source, legality aside.
 
You must log in or register to reply here.
Back
Top