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AZBrit

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AZBrit
Ok, first post here so hi to all of you POA'rs.

I have a question regarding constant speed props that I cannot find a definitive answer to.
I know when adding power, RPM, the blue lever, should be adjusted first and when reducing power, the black lever, controlling manifold pressure should be adjusted first.
What are the consequences of making the adjustments the other way around?

If this this has been covered previously, apologies, did search but didn't find anything.

That's it, thank you in advance for your helpful replies!
 
Welcome to POA. If you search the internet for Jim Deakin's articles on AvWeb (Pelican's Perch), you will find a great source of getting a better understanding of this.
 
AZBrit said:
What are the consequences of making the adjustments the other way around?
Click to expand...

Think of the blue knob as a gearshift. Full forward is low gear. You start in low gear to go uphill at high RPM.

Both levers are forward. Now you've finished climbing the hill. Shift to a higher gear. Pull the blue knob back.

Now you want to descend. Technically you can (in most aircraft) pull either one back, you'll be "coasting".

Now you want to add power and go back up. The vehicle is in high gear. You need to put it back in low gear first, then mash the throttle. If you mash the throttle first, you'll lug the engine on the big hill.

Landing, you'd think you want both back, but you don't. There's a possibility that you'll need to climb a big hill in a hurry (go around), so just downshift to low (push the blue knob all the way forward) ahead of time so if you need to go up the big hill in a hurry, all you do is mash the throttle.

The only time you might want both all the way back is if the engine quits. Less drag in high gear so maybe you'll make that grass strip over yonder.

And of course in most twins all the way back is not just high gear but fully feathered.

Make sense?
 
What Denver said. Also remember there is a mixture in the mix (so to speak), so the "standard" way to do it is mixture-prop-throttle to add power, and thottle-prop-mixture to reduce power, and most throttle quadrants are laid out in that order.
 
denverpilot said:
Think of the blue knob as a gearshift. Full forward is low gear. You start in low gear to go uphill at high RPM.

Both levers are forward. Now you've finished climbing the hill. Shift to a higher gear. Pull the blue knob back.

Now you want to descend. Technically you can (in most aircraft) pull either one back, you'll be "coasting".

Now you want to add power and go back up. The vehicle is in high gear. You need to put it back in low gear first, then mash the throttle. If you mash the throttle first, you'll lug the engine on the big hill.

Landing, you'd think you want both back, but you don't. There's a possibility that you'll need to climb a big hill in a hurry (go around), so just downshift to low (push the blue knob all the way forward) ahead of time so if you need to go up the big hill in a hurry, all you do is mash the throttle.

The only time you might want both all the way back is if the engine quits. Less drag in high gear so maybe you'll make that grass strip over yonder.

And of course in most twins all the way back is not just high gear but fully feathered.

Make sense?
Click to expand...
This is a great explanation, nice Nate!
 
The actual rule is "Avoid high manifold pressures at low RPMs." Advancing the prop first and retarding it last sort of makes it impossible to violate the rule but really unless you're going to full power while at a cruise RPM or you cut the cruise RPM while at full power you're not going to have a problem. Most of us execute go arounds by must jamming everything forward. On approach I don't really touch the blue knob until I'm established on the approach (and then the throttle has already been retarded).
 
Ryanb said:
This is a great explanation, nice Nate!
Click to expand...

It's a good start for anyone who's driven vehicles they needed to shift. I took a chance. Kids who've never done anything but mash on the gas pedal in an automatic, it doesn't make any sense.

Putting on my "practicing to be a CFI hat", I'd ask you...

What would the manifold pressure gauge read sitting on the ground with the engine off at the airport?

As a next intro to a further discussion about it.

Other questions:

What would it read if the engine weren't running at 10,000' MSL?
What keeps the prop at high RPM in flight?
Is that low blade pitch or high blade pitch? Why?
What moves it toward high pitch/low RPM?
Is that low or high pitch?
What moves it back to high RPM?

What's this airplane's maximum Manifold Pressure?
How much of a difference is that between when the engine is off and at takeoff/max power?

What happens during an engine failure / power loss?
(That one is tricky... all sorts of fun stuff to talk about there.)

If the aircraft is turbo-normalized....

Why can we overboost at sea level and not up here at about 12,000' MSL?

What does that mean we can do (with the throttle) on our way up from sea level to our final high cruise altitude?

What's a good method to do this smoothly?

(All leading toward that in most turboed airplane's, you'll lose about 1" of MP per 1000' of altitude gain, and without a turbo you can't do anything about it, but with a turbo, each 1000' you can usually push the throttle up another inch on the MP gauge to maintain takeoff power... to a point... where you can't anymore. Above that point, full throttle won't overboost the engine anymore.

Etc.

Learning not to push the throttle up and have high MP and low RPM is just the basics -- and has to be ingrained as automatic response. Your CFI will broaden out the topic for you with questions like the above.
 
Its considered poor practice to pull the power abeam the numbers and then push the rpms in and get an rpm surge. Let the power go all the way down to low mainifold pressure, like almost none, then push the prop in when it doesnt do anything except get the knob in for a possible go around. Hope you can figure out what I mean

As for consequences if you do it wrong. If you stay within 5 of each other; 500 rpm and 5" manifold, its not going to hurt anything no matter how you do it.
 
The consequences of pulling the blue knob first are overstated. If the urban legend was true, twin trainers would be replacing engines left and right after every prop feathering. The threshold at which this kind of engine lugging would damage it is nowhere near the operating regimes most people end up fiddling with the blue knob in the first place. More OWT for the uninitiated to promulgate.
 
coloradobluesky said:
Its considered poor practice to pull the power abeam the numbers and then push the rpms in and get an rpm surge. Let the power go all the way down to low mainifold pressure, like almost none, then push the prop in when it doesnt do anything except get the knob in for a possible go around. Hope you can figure out what I mean

As for consequences if you do it wrong. If you stay within 5 of each other; 500 rpm and 5" manifold, its not going to hurt anything no matter how you do it.
Click to expand...
Overstated. Once the throttle and airspeed is low enough that the RPM lowers on its own, pushing the prop knob in will make no difference. It does not take all that low manifold pressure to do this. On the 182s I fly, 18 inches or so will do it.

Nate, you need to qualify one of your questions... What does the MP read if the engine isn't running and the prop is stopped at 10,000? MP will be well under ambient with a closed throttle and windmilling prop. It will also help reduce drag in an engine-out scenario to pull the prop all the way out and push the throttle all the way in, to reduce pumping losses. Of course it will make no difference if the throttle is choked with something else such as carb ice or induction ice.
 
I personally don't even touch the prop lever after cruise (barring another climb) until I'm on the ground. But if I do need to do a go-around, it's not a big deal because I can just mash all three forward at once if I need to. For a vernier prop control, it's not a bad practice to set it for max RPM at some point in the pattern.
 
hindsight2020 said:
The consequences of pulling the blue knob first are overstated. If the urban legend was true, twin trainers would be replacing engines left and right after every prop feathering. The threshold at which this kind of engine lugging would damage it is nowhere near the operating regimes most people end up fiddling with the blue knob in the first place. More OWT for the uninitiated to promulgate.
Click to expand...

One doesn't typically start the feather process at full power... so there's no "lugging" whatsoever. Not to say students probably haven't pulled the WRONG blue knob before in most training twins, but the analogy is pretty weak sauce. It's not SOP to be banging along at 36.5" in the turbo Seminole and yank the blue lever back.
 
Sac Arrow said:
I personally don't even touch the prop lever after cruise (barring another climb) until I'm on the ground. But if I do need to do a go-around, it's not a big deal because I can just mash all three forward at once if I need to. For a vernier prop control, it's not a bad practice to set it for max RPM at some point in the pattern.
Click to expand...

I like my vernier in the 182... I just slowly twist it forward as a way to do it smoothly.

The controls in the Seminole with the long cables running out into the wings always seem to be less precise and bind up a little when you're moving them. Vibration also seems to move them a bit after you've stopped pushing or pulling due to that binding. I never set MP or RPM in that airplane without looking again in a couple of seconds because it'll always overshoot wherever I stopped making a control input by a small amount.
 
In all honesty, you can do whatever you'd like with either of the two levers in whichever order you'd like in a normally-aspirated airplane. The dangerous part is when you start throwing the red knob into the mix. That's where you can do a lot of damage.

As far as oversquare ops are concerned, I'll fly along at 28" and 2,100rpm all day long. Coming in over the numbers, my prop is usually only spinning 2,000rpm before I pull power to idle.

As someone else mentioned, look up John Deakin's articles as well as articles by Mike Bush. You have a lot to learn. Running a big-bore, contant-speed engine is about as difficult as it gets when done properly in today's world.
 
OWT. Running over square won't hurt the engines.
 
Key is to just follow the tables in the POH, some of which are "over square." If you just fly by the book you'll be fine.

A lot of people get confused by the blue knob because they think it directly controls the prop pitch. In most cases it doesn't... it's a constant speed prop not an adjustable pitch propeller. Ultimately the prop governor is doing all the adjusting so the prop is changing pitch a lot even while that blue knob stays still
 
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This is the article that was referenced above. http://www.avweb.com/news/pelican/186778-1.html a good read.

As has been mentioned above, in a normally aspirated plane it's hard to do damage with the two levers in either order.

As long as you're operating and staying within any limitations in the POH, you're going to be OK. Having said that I find the preferred order a little smoother and easier.

.... and welcome
 
rocketflyer84 said:
Key is to just follow the tables in the POH, some of which are "over square." If you just fly by the book you'll be fine.
Click to expand...

This. The DA40 POH says to go from 2700RPM down to 2400RPM yet stay at full throttle "above a safe height" (which I take to mean ~500AGL)
 
coloradobluesky said:
Its considered poor practice to pull the power abeam the numbers and then push the rpms in and get an rpm surge. Let the power go all the way down to low mainifold pressure, like almost none, then push the prop in when it doesnt do anything except get the knob in for a possible go around. Hope you can figure out what I mean

As for consequences if you do it wrong. If you stay within 5 of each other; 500 rpm and 5" manifold, its not going to hurt anything no matter how you do it.
Click to expand...
I talk about that as, "you are below the prop governing range" when referring to throttle position, so it is "safe" to adjust to high rpm without the WAANNGGG surge. For most aircraft that would be below 15-17in MP.

Blue forward as part of the GUMP check.
 
asicer said:
This. The DA40 POH says to go from 2700RPM down to 2400RPM yet stay at full throttle "above a safe height" (which I take to mean ~500AGL)
Click to expand...
But what is the MP at full throttle at sea level? It's been a while since I've flown the DA-40.

Could be "about" 25in MP and decreasing in the climb with full throttle.
Piper Arrow, 200HP, full throttle and prop for take off. At a safe altitude 25 squared. Throttle to 25MP prop to 2500RPM for cruise climb.

In the T-41B, full power, prop redlines at 2800. But we are departing from 2200MSL or higher DA. Fuel flow according to the table for DA at full throttle.
Leave throttle at full power, normally 25in MP or less and reduce prop to 2500RPM and mixture back to 12gph.
Cruise, 2350-2400 RPM, full throttle, normally 20-22in MP or less fuel flow about 9.5-10gph depending on EGT.
 
asicer said:
See post #7 :)
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No numbers there. I've had aircraft pull 25in, to 28in. I don't think I've ever had one pull static sea level pressure unless it had a turbo.

Seneca II, 40in MP for takeoff and climb, 30in MP cruise.
 
BillTIZ said:
I talk about that as, "you are below the prop governing range" when referring to throttle position, so it is "safe" to adjust to high rpm without the WAANNGGG surge. For most aircraft that would be below 15-17in MP.

Blue forward as part of the GUMP check.
Click to expand...
It is about 10" in my RV.
 
Thanks guys. Some really informative answers. Appreciate the time and effort put into the replys.
That's my weekend reading sorted!
 
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