Transitioning to a constant speed prop

[RalphInCA]

Now that I have been signed off to fly our club's 172, I am beginning to think of my next step up in aircraft.

The club has several to choose from including complex aircraft.

Since I am the type of person who tends to overthink things, I am already worrying about the jump to constant speed prop.

What should I do to prepare for the jump? What do I need to think about, what additional piloting duties does the constant speed require?

 

[Henning]

You have the ability to refine your performance/economy curve. Don't over think it yet. Also don't worry about "what you can do to it", think about what it can do for you. You have two ways of reducing power with a CS prop, you can reduce it by torque with the throttle/MP, or you can reduce it by time using the prop/RPM. By reducing the power on the RPM side of the equation you keep the throttle open and torque values high getting the most work out of your fuel $$$.

 

[flyingron]

The constant speed prop is really not all that involved. You essentially can live in most aircraft with two modes: HIGH RPM for takeoff and climbs, A cruise setting any other time.

The biggest difficulty people have when going from something as simple as a 172 to an airplane with a C/S prop is that there's frequently more power, higher speeds, and less drag involved in that aircraft as well and getting a head of the need for making power changes is more likely to require some practice than the CS issues alone.

...and if you're not used to using trim, that's probably needed to be learned as well. A 182 or an Arrow or a Navion isn't something you just throw around where you want it and ignore the trim like the 152/172.

 

[Mistake Not...]

The constant speed prop is really not all that involved. You essentially can live in most aircraft with two modes: HIGH RPM for takeoff and climbs, A cruise setting any other time.

That's how I run it, but I thought I was missing something.

How does one pick the cruise setting?

 

[flyingron]

Take a look at the performance chart and pick one or two that give you the performance you want. Typically you'll have a "balls to the wall" takeoff and climb setting. Then you'll have a continuous climb setting if your plane has limitations on full power operation (for example, I bring my prop back to 2500 in my plane once I'm not going to hit anything). Then a cruise which for me is like 65% power (roughly 23"/2300 RPM). Then I have a cruise / approach descent of 18" that brings me down 800 fpm or so without gaining too much speed, and a "you need to get down to gear speed setting" of 14". Some where on approach I'll put the prop back up to 2500 for the extra drag and to the wall on short approach.

 

[Henning]

That's how I run it, but I thought I was missing something.

How does one pick the cruise setting?

Per the Lindbergh Doctrine you run the lowest RPM, highest MP combination that gets you the desired airspeed. For range IIRC the speed you're looking for is 1.3L/D Max with adjustment for head/tail winds.

As long as you keep both needles in the green arc and abide by any limitations given, you're going to be ok, so I suggest you experiment. You'll find most engine/prop combos will have a 'sweet spot' to them where everything seems to smooth out some and the engine takes on a solid but easy tone, not too loud, not too breathy. Play around with combinations and see how well you can do with leaning and keeping smooth as well. I found the IO-470s I ran had their sweet RPM around 2450, so even though I could pull back much further, I stayed there. My 7500' target speed was 180kts so I would leave the throttles WOT and lean back for 10.5gph per side. That would have the engines running smooth and clean. If I needed some power, I just add a bit of mixture.

 

[Mistake Not...]

FlyingRon, Henning, you guys are awesome.

First, I have a 100% power limitation of, I think 2 minutes. So I have been coming back to 24" mp after I reach 1000AGL (75% power). From the lindbergh heuristic, I should instead pick an rpm setting that gives me 75% at WOT. (I wondered about this after reading an article or two that maintains the engine is most efficient at WOT and to leave it there).

The only time in cruise (so far) that I haven't been at 75% is if I had a nice tailwind and would come back to 22/22, which loses 10 kts or so, but is a lot quieter and 2-3 gallons/hr cheaper. I will start playing around with that... new plane jitters and all. I have a lot more attention available now to play around than I did the first 100 hours.

The one thing I don't do is a descent setting. I've been "prop full forward, control descent with throttle" (more or less)... prop full forward, though. I need to get that figured out as well, because I understand I'll need it for the IR.

Thanks, guys. Probably have more questions after I go try this.

To sum it up though, it sounds like you could just treat it as a fixed pitch prop if you wanted, but additional efficiency is there (reduced rpm, increased mp) if you want / need it. And that has always been my problem with the constant speed prop... everyone (the books) go on and on about how flexible it is.. how much capability it gives you and then stops short of recommending how it should be used. I've been trying to come up with a mental model of it, and got nothin'.

 

[MAKG1]

Use your performance tables to make that decision.

It's seldom so simple as low RPM at high throttle, as that's a recipe for detonation if overdone.

To the OP, I suggest trying HP prior to complex. Most of those have CS props, too. But with retractible gear, that's going to be your focus.

A 182 will serve just fine for this. So will a 177 or 172XP (though those are neither HP nor complex) if you can find them.

 

[Graueradler]

It might help to think of the C/S prop as a constantly variable transmission. The fixed pitch prop is a single gear. It is like having a car with only second gear. You can select a pitch more towards "Low", a climb prop, or more towards "High", a cruise prop - but whatever you choose, it is a compromise. With the C/S prop - no more compromise.

 

[Mistake Not...]

ok.. that's the kind of advice I get a lot, but don't know how to apply.

Use my performance tables? How? What's the selection criteria? What goal am I seeking? What am I trying to minimize / maximize? I can post the tables if that'll help the discussion.

 

[Mistake Not...]

It might help to think of the C/S prop as a constantly variable transmission. The fixed pitch prop is a single gear. It is like having a car with only second gear. You can select a pitch more towards "Low", a climb prop, or more towards "High", a cruise prop - but whatever you choose, it is a compromise. With the C/S prop - no more compromise.

Right. That's what the books say, and then fail to give examples.

Again, I'd be happy to post a page or two from the performance section of my POH if you guys could walk me through a couple of scenarios (climb / cruise) and your thought process when you select settings.

 

[poadeleted20]

Since I am the type of person who tends to overthink things,

Glad we've established that up front.

What should I do to prepare for the jump?

First, read the section in the Airplane Flying Handbook on constant speed props. Chapter 11, pages 11-4 to 11-7, complete with nice graphics. Another very good read is John Deakin's "Those Marvelous Props" on AvWeb.

What do I need to think about, what additional piloting duties does the constant speed require?

Well covered in the references above. Once you've read them, come back with any questions you have.

 

[Gucci Pilot]

Use your performance tables to make that decision.

It's seldom so simple as low RPM at high throttle, as that's a recipe for detonation if overdone.

To the OP, I suggest trying HP prior to complex. Most of those have CS props, too. But with retractible gear, that's going to be your focus.

A 182 will serve just fine for this. So will a 177 or 172XP (though those are neither HP nor complex) if you can find them.

I agree. I was always cautioned against using low RPMs with high manifold pressures. Thats not to say the RPM value can never dip below the manifold pressure value(ex. 2400 RPM/26 MP). If your POH has values for it, it is approved for use. The Airplane Flying Handbook has a great chapter on complex aircraft and constant speed props. Fun Fact: The 172 XP has an STC that bumps it up to 210 HP. Even more rare than the 172XP itself but they are out there.

 

[poadeleted20]

FlyingRon, Henning, you guys are awesome.

First, I have a 100% power limitation of, I think 2 minutes. So I have been coming back to 24" mp after I reach 1000AGL (75% power). From the lindbergh heuristic, I should instead pick an rpm setting that gives me 75% at WOT. (I wondered about this after reading an article or two that maintains the engine is most efficient at WOT and to leave it there).

The only time in cruise (so far) that I haven't been at 75% is if I had a nice tailwind and would come back to 22/22, which loses 10 kts or so, but is a lot quieter and 2-3 gallons/hr cheaper. I will start playing around with that... new plane jitters and all. I have a lot more attention available now to play around than I did the first 100 hours.

The one thing I don't do is a descent setting. I've been "prop full forward, control descent with throttle" (more or less)... prop full forward, though. I need to get that figured out as well, because I understand I'll need it for the IR.

Thanks, guys. Probably have more questions after I go try this.

To sum it up though, it sounds like you could just treat it as a fixed pitch prop if you wanted, but additional efficiency is there (reduced rpm, increased mp) if you want / need it. And that has always been my problem with the constant speed prop... everyone (the books) go on and on about how flexible it is.. how much capability it gives you and then stops short of recommending how it should be used. I've been trying to come up with a mental model of it, and got nothin'.

None of this is necessarily quite true. Start by reading what it says in the AFH (see link above) then read the POH/AFM for your airplane, including the Limitations, Performance, and Systems sections. Then see how what is written there jibes with you wrote above.

 

[Henning]

For descent I would just pull back to 2200rpm and adjust the throttle to maintain MP as it came down. This keeps a nice rate of sink at 180kts.I do not go forward with the props until I bring the throttles back far enough that the prop drops off the governor, that way there is no surge or increase in noise when I bring them forward.

 

[warthog1984]

Right. That's what the books say, and then fail to give examples.

Again, I'd be happy to post a page or two from the performance section of my POH if you guys could walk me through a couple of scenarios (climb / cruise) and your thought process when you select settings.

Again, what are you trying to achieve? In climb, you want a fine pitch. In cruise, a coarser pitch. And somewhere along the way you'll find that the engine doesn't like certain extreme combinations, so you'll avoid those in the future.

 

[poadeleted20]

I agree. I was always cautioned against using low RPMs with high manifold pressures.

That's good to think about, but remember that the max MP for any given RPM should be stated either directly in the Limitations section of your POH/AFM or indirectly by seeing the top MP listed for any RPM in the cruise performance tables. Without those, you don't know what MP is too high for any given RPM in your aircraft/engine/prop combination, and there are no valid "rules of thumb" for this.

 

[poadeleted20]

For descent I would just pull back to 2200rpm and adjust the throttle to maintain MP as it came down. This keeps a nice rate of sink at 180kts.I do not go forward with the props until I bring the throttles back far enough that the prop drops off the governor, that way there is no surge or increase in noise when I bring them forward.

That may work well in Henning's 310, but I know planes where 2200 RPM is above redline, and others where you can't get that low even at idle. Read the book on your airplane/engine/prop combination and learn what to do you your plane.

 

[Gucci Pilot]

That's good to think about, but remember that the max MP for any given RPM should be stated either directly in the Limitations section of your POH/AFM or indirectly by seeing the top MP listed for any RPM in the cruise performance tables. Without those, you don't know what MP is too high for any given RPM in your aircraft/engine/prop combination, and there are no valid "rules of thumb" for this.

I'm talking about ridiculously low RPMs with full power. The rest of my post is basically exactly what you stated above.

 

[poadeleted20]

I'm talking about ridiculously low RPMs with full power.

I guess my point was that what is a "ridiculously low RPM" on one plane might be ridiculously high on another, so you really have to know the plane you're flying and fly it accordingly. Otherwise, you get silly stuff like "don't fly oversquare", which on some engines is the only way it runs while others might blow up before you get the MP in inches higher than the RPM in hundreds.

 

[Henning]

I'm talking about ridiculously low RPMs with full power. The rest of my post is basically exactly what you stated above.

You cannot have ridiculously low RPMs and full power, it is impossible. Full power comes at full rated RPM with full rated torque (gauged in MP). Power is a function of Torque * Time represented in MP*RPM. If you reduce the RPM, you cannot have full power without having a supercharger to take MP beyond redline.

With a Naturally Aspirated engine with CS prop it is not uncommon to go full throttle on take off and never reduce from there until descent for landing, just reduce RPM for cruise.

 

[Gucci Pilot]

You cannot have ridiculously low RPMs and full power, it is impossible. Full power comes at full rated RPM with full rated torque (gauged in MP). Power is a function of Torque * Time represented in MP*RPM. If you reduce the RPM, you cannot have full power without having a supercharger to take MP beyond redline.

With a Naturally Aspirated engine with CS prop it is not uncommon to go full throttle on take off and never reduce from there until descent for landing, just reduce RPM for cruise.

Sorry, high manifold pressure

 

[jordane93]

Glad we've established that up front.
First, read the section in the Airplane Flying Handbook on constant speed props

Free and helpful. Also, if you have a simulator with a throttle, try to learn the procodures. When I was transitioning into the Arrow I did a lot familarization with the gear and prop on MS Flight Simulator. Takeoff was prop full forward, throttle full. At 700 AGL, 25 squared. 25 inches, 25 rpm pitch for 90 kts cruise climb. Cruise was 23 squared, 23 inches and 23 rpm. It really helped me get used to the procedures once in the actual plane.

 

[JimNtexas]

The propeller is to your airplane as the transmission is to your car.

A C172 has a one speed transmission. This is exactly analogous to a 1969 VW Bug that is stuck in third gear.

An airplane with a constant speed prop has a multispeed transmission. You put it into 'low gear' (prop knob all the way forward) for takeoff and climbing.

In level flight you 'shift' to third, fourth of fifth gear by pulling the prop knob backwards.

If you pull the prop knob back too far it's exactly the same as trying to climb Pikes Peak in your 1969 VW in fourth gear. The engine will start to lug, overheat, and ultimately give up and stop running.

 

[brian]]

If you happen to get a chance to see an early Bonanza with a variable pitch (electric prop) - take a look and see if the owner will work the prop for you on the ground. Kinda cool

The CS prop used to worry me a lot - but it is not much. Just adjust as recommended by the POH (or during your checkout). As others have mentioned: watch the low RPM high MP settings. Other than that - enjoy going FAST!

 

[MAKG1]

If you pull the prop knob back too far it's exactly the same as trying to climb Pikes Peak in your 1969 VW in fourth gear. The engine will start to lug, overheat, and ultimately give up and stop running.

Rattle rattle rattle rattle rattle ....

It's worse at low altitude and low speed.

Unfortunately, you can't hear the detonation in an airplane like you can in a car.

 

[PPC1052]

A C172 has a one speed transmission. This is exactly analogous to a 1969 VW Bug that is stuck in third gear.

An airplane with a constant speed prop has a multispeed transmission. You put it into 'low gear' (prop knob all the way forward) for takeoff and climbing.

In level flight you 'shift' to third, fourth of fifth gear by pulling the prop knob backwards.

The thing that doesn't quite translate for me with this analogy is that in my POH, the fastest speed is always with the highest RPM setting, which might lead one to conclude that you go fastest in "first gear." But that certainly isn't true in your car, and the conclusion isn't true in your plane, either. In reality, your prop governor is constantly "changing the gear" by moving the pitch to allow the RPMs to match your RPM setting. Thinking of pushing the prop control full forward as akin to "first gear" and pulling the prop back as simply switching to a higher gear had me a little confused early because of this. The analogy is useful as a starting point, but not entirely accurate.

 

[Henning]

The thing that doesn't quite translate for me with this analogy is that in my POH, the fastest speed is always with the highest RPM setting, which might lead one to conclude that you go fastest in "first gear." But that certainly isn't true in your car, and the conclusion isn't true in your plane, either. In reality, your prop governor is constantly "changing the gear" by moving the pitch to allow the RPMs to match your RPM setting. Thinking of pushing the prop control full forward as akin to "first gear" and pulling the prop back as simply switching to a higher gear had me a little confused early because of this. The analogy is useful as a starting point, but not entirely accurate.

Let's get away from inadequate analogies. What the prop works on is load. It is actually closer akin to a torque converter than a transmission. A torque converter is exactly what it is and it uses torque from the crank on some wedges that deflect a fluid. That's what the prop does, it deflects air to the rear by swinging wings in a circle. With a constant speed prop, as load requirements change, the governor will adjust the angle of attack of these wings to maintain a desired set RPM.

It does this through the same basic mechanism James Watt invented putting the industrial revolution into high gear. There are a set of flyweights on a shaft with an adjustable counterbalance spring. That shaft is connected to a spool valve, which is actuated up and down through a linkage connected to the flyweights going in and out.

When the engine is turning the counter balance spring/prop control set speed, the spool valve is closed and the prop maintains its pitch. Now, if you pitch up to climb, you increase the load on the engine and it starts to slow down and the spool valve opens releasing oil from the prop hub and allows the spring to drive the prop blades back towards flat/fine pitch reducing the load and allowing the rpm to increase back to where the flyweights balance the spool weights closed.

Conversely, at the top of climb you put the nose down and start accelerating, reducing the load on the engine, now the flyweights go out and open the valve the other way driving oil pressure into the prop hub and driving the blades to a steeper/coarser pitch until the load evens out and the flyweights are back to their balanced setting.

Now some props will have slightly different different mechanism, but all hydraulic CS props work basically like that with a Watt style governor to regulate the prop pitch to RPM.

There are also non CS variable pitch props out there in hydraulic, electric, and mechanical designs.

 

[CTLSi]

Constant speed props let you adjust RPM and prop pitch from the cockpit.

A fine pitch prop has a low blade angle and will take a 'small' bite of the air. It requires lower power to rotate, allowing high propeller speed to be quickly developed, but achieving only limited airspeed. This is like having a low gear in your car.

A coarse pitch prop has a high blade angle. It will take a big 'bite' of the air. It requires greater power to rotate, limiting the propeller speed that can be developed, but achieving high airspeeds. This is like having a high gear in your car.

Fixed pitch props must be set to compromise for takeoff versus cruise once between ground adjustments if that capability is present.

 

[MAKG1]

The thing that doesn't quite translate for me with this analogy is that in my POH, the fastest speed is always with the highest RPM setting, which might lead one to conclude that you go fastest in "first gear." But that certainly isn't true in your car, and the conclusion isn't true in your plane, either. In reality, your prop governor is constantly "changing the gear" by moving the pitch to allow the RPMs to match your RPM setting. Thinking of pushing the prop control full forward as akin to "first gear" and pulling the prop back as simply switching to a higher gear had me a little confused early because of this. The analogy is useful as a starting point, but not entirely accurate.

Until your engine runs out of air, you WILL go faster in first gear.

If you want to climb a steep hill at high altitude (like, say, Sonora Pass, CA), you do it in first gear for the fastest climb.

If you want to get on the freeway the fastest, you do it in low gear. Almost any 5 speed can tolerate highway speed in 3rd, and some OHCs can take it in 2nd. Try it. It will be loud, and will consume fuel if it's in "passing mode" (likely), but it's the fastest way to get moving.

Henning, the torque converter analogy is even worse, as most of them these days (and indeed the past several decades -- even back to the old TH350s in the 70s) have clutches as well that engage almost all the time, generally in cruise. It keeps the TC from overheating the engine and burning out the transmission, a common failure mode from older transmissions.

 

[Henning]

Constant speed props DO NOT let you adjust prop pitch from the cockpit, CS props allow you to select an RPM that you want the engine/prop to turn. Variable Pitch props let you control the pitch.

 

[PPC1052]

If you want to get on the freeway the fastest, you do it in low gear. Almost any 5 speed can tolerate highway speed in 3rd, and some OHCs can take it in 2nd. Try it. It will be loud, and will consume fuel if it's in "passing mode" (likely), but it's the fastest way to get moving.

I am not talking about acceleration, I am talking about top speed. Given that you are limited in terms of how many RPMs an engine can handle, you will not reach top speed in cruise in first gear in your car, and you won't reach top speed with the props as their finest setting, which is why the prop governor increases the "coarseness" as you accelerate. If it were otherwise, you wouldn't need to have the constant speed prop.

Certainly, there may be torque or aspiration limits that prevent you from utilizing the efficiencies of the higher torque. But again, that's why you have the constant speed prop so you can accelerate, and then have the pitch change once you get up to speed.

My only point was that pushing the lever full forward does not exactly equal first gear, and pulling it back doesn't exactly equal 3rd, 4th, or 5th (take your pick.)

 

[brian]]

Henning has it... at least in my understanding..

Please note that you really can't think of "first gear" or whatever. For example: The early continental engines (like my E-185) make MORE POWER at higher RPMs. Some early 35 owners think the E-225 engine is 40 more HP than my E-185. HOWEVER, they are limited to RPM settings less than where the more powerful E-255 produces peek HP. Basically the power curve at 2050 (the RPM we 35 owners are supposed to cruise at - ok a little faster on the E-225) produces the same 165 HP on both the E-185 and the E-225. (The advantage of the E-225 is better climb and high altitude performance.)

Anyway, it is just a power curve for HP and you see this as a change in GPH and cruise speed. All the rest is just following the POH to make sure you don't break something or put undue stress on something.

(e.g. I can over speed the prop in level cruise and go from about 144KTS to a little over 150KTS. Not that I would do that though )

 

[JimNtexas]

Constant speed props let you adjust RPM and prop pitch from the cockpit.

A fine pitch prop has a low blade angle and will take a 'small' bite of the air. It requires lower power to rotate, allowing high propeller speed to be quickly developed, but achieving only limited airspeed. This is like having a low gear in your car.

A coarse pitch prop has a high blade angle. It will take a big 'bite' of the air. It requires greater power to rotate, limiting the propeller speed that can be developed, but achieving high airspeeds. This is like having a high gear in your car.

Fixed pitch props must be set to compromise for takeoff versus cruise once between ground adjustments if that capability is present.

Correct. And on both your VW Bug and your Piper Arrow when you put the transmission in 'high gear' the vehicle will travel further forward along the long axis of the vehicle for each revolution of the crankshaft , and will travel a shorter distance per crankshaft revolution in 'low gear'.

 

[Henning]

Stop with the gear analogies already!!! They don't fit. There is one gear, 4th, it's direct drive unless you have a geared engine then it's like driving in third where the engine spins faster than the output shaft multiplying torque. That is NOT what you are doing with a CS prop. On a car if you had to make an analogy it would be having a variable diameter wheel that maintains the engine at a set RPM regardless of speed or grade. There just is no car analogy, and creating them creates confusion and misunderstanding which leads to poor operations and old wives tales.

They are simple enough in and of themselves there is no need to add confusion. Load limited by fuel, governed to maintain RPM.

 

[PPC1052]

Stop with the gear analogies already!!! They don't fit. There is one gear, 4th, it's direct drive unless you have a geared engine then it's like driving in third where the engine spins faster than the output shaft multiplying torque. That is NOT what you are doing with a CS prop. On a car if you had to make an analogy it would be having a variable diameter wheel that maintains the engine at a set RPM regardless of speed or grade. There just is no car analogy, and creating them creates confusion and misunderstanding which leads to poor operations and old wives tales.

They are simple enough in and of themselves there is no need to add confusion. Load limited by fuel, governed to maintain RPM.

You are putting your finger on what I was alluding to in my first post above. The gear shifting analogy isn't exactly right, and does lead to confusion as you start thinking about what is really going on, and start flight planning for efficiency, etc.

 

[RalphInCA]

No wonder I was confused about this! It seems even the experts are confused. ;-)

I think I will go directly from a 172 to a jet. Skip the whole constant speed prop thing.

 

[Henning]

No wonder I was confused about this! It seems even the experts are confused. ;-)

I think I will go directly from a 172 to a jet. Skip the whole constant speed prop thing.

Turboprops for the most part are treated as jets, but the governor on a jet works the same.

 

[PPC1052]

No wonder I was confused about this! It seems even the experts are confused. ;-)

I think I will go directly from a 172 to a jet. Skip the whole constant speed prop thing.

Honestly, its really not hard. If you have driven a stick shift, then you had two controls-- a throttle and your gear shift, and you looked at your tach to determine when to shift. (Or maybe you just felt it, but go with me here.) You are constantly having to shift up and down to keep the RPMs in a range where your engine is happy.

In your plane, you have your throttle, and a neat control that sets the RPMs exactly where you want them. The governor does everything necessary to make sure the RPMs stay where your engine is happy so you don't have to switch gears. You don't really futz with it too much. Set if full forward for take off, and then pull it back to set it at something like 2300 or 2400 RPMs, and leave it there. Push it full forward when you approach to land just in case you need to go around. Easy, huh?

 

[Henning]

Honestly, its really not hard. If you have driven a stick shift, then you had two controls-- a throttle and your gear shift, and you looked at your tach to determine when to shift. (Or maybe you just felt it, but go with me here.) You are constantly having to shift up and down to keep the RPMs in a range where your engine is happy.

In your plane, you have your throttle, and a neat control that sets the RPMs exactly where you want them. The governor does everything necessary to make sure the RPMs stay where your engine is happy so you don't have to switch gears. You don't really futz with it too much. Set if full forward for take off, and then pull it back to set it at something like 2300 or 2400 RPMs, and leave it there. Push it full forward when you approach to land just in case you need to go around. Easy, huh?

If you explain it like that on your CP oral, you will fail.