Every now and then, there are a series of questions and answers regarding constant-speed propellers (henceforth known as CSPs), that pop up in the forums. I'm starting this thread for reference when theses questions come up.

I know personally, that CSPs can be as confusing as VORs until you get the big picture. Lets start with defining what a CSP is designed to achieve.

Everyone can agree that a fixed-pitch propeller is a compromise. For the sake of this discussion, we'll limit it to the two, common types; climb and cruise.

A climb propeller is like a low gear. Its less aggressive pitch is better suited for taking off from a dead-stop and then climbing. The compromise is in level flight where a more aggressive pitch would allow for higher cruise speeds. Just like a car... Low gears are for starting and accelerating, and higher gears for reaching higher speeds. If you own a C172 and fly mostly with passengers on short flights; your best choice might be a climb prop. It will get you up to altitude more quickly, especially on those hot days. If you fly light loads or long distances, a cruise prop might be your best choice.

A CSP gives you both of those, and everything in between.

To quickly put to rest the ideas that confuse most people when trying to understand CSPs; we must understand that a pilot does NOT select a blade-pitch. Consider this next paragraph carefully, and keep it in the back of your mind when confusion sets in.

We climb into our C177 and taxi out for takeoff.  With the prop-control pushed all the way forward, we apply full throttle and roll down the runway... off we go into a VY climb and at 4,000msl we level off and wait for the airspeed to increase and stabilize.

THERE ...  the CSP has just done it's entire job. During the takeoff and climb, it was constantly adjusting to be the ideal climb prop... and when we leveled of, it transitioned into a cruise prop.. and as the airspeed reached maximum, it became the ultimate cruise prop. All of this happened without us EVER touching the prop-control.

OK.. that's out of the way. Whenever you catch yourself asking a question about what happens when the pilot changes the prop-pitch, stop yourself and remember that paragraph. The pitch of the prop-blades at any given part of a flight are determined by airspeed and power setting (manifold pressure). All the prop-control does, is to decide at what RPM it will all happen.

Using a CSP

Since most of my CSP experience is in a C177RG; that's what we'll use for reference. The techniques apply to most all, normally-aspirated, piston-engined aircraft. Turbo-charged, super-charged and big, radial-engined aircraft will have different relationships between manifold-pressure and RPM, but the theories are the same.

Until you start flying CSP aircraft, you'll have been taught to reference your power setting, by RPM. The limitations become aparent, the second you apply full power for takeoff. I mean, that IS full power, but you'll not be anywhere near the highest RPM until you start picking up airspeed. Conversely, you'll soon see that you will get near max RPM while descending, when the throttle isn't anywhere near full. Without getting into the physics/chemistry/fluid-dynamics of what happens in a manifold and combustion chamber; we'll accept that manifold-pressure (MP), is an accurate indicator for the power setting. When flying a non-CSP aircraft, there are RPM settings for things like cruise, descent and approach. IN a CSP aircraft, it's all about MP, and the CSP makes it all more consistent. You won't get an artificially low/high MP when climbing/descending.

OK.. a complete flight and how MP/RPM are managed:

Takeoff.. Full throttle will yield about 28"MP (depending on density altitude) and max RPM should be 2700-2800.

As soon as a climb has been established, reduce RPM slightly (by the prop-control, not the throttle) to save on engine wear, but keeping the engine near the RPM where the most HP is generated throught the climb. This flight will be at 2500msl, so as we reach that altitude we'll pitch to stop the climb and wait for the airspeed to reach cruise speed (about 135kias).

Next, we reduce MP to aprox. 25". Then we reduce RPM to aprox. 2500. This does NOT increase prop pitch in a manner that will have it "biting" more air. The CSP will find that pitch all by itself. If we had left the RPM at 2700, the blades would already be "biting" as much air as they possibly could, given THAT RPM setting, and the current MP. What you're doing, is forcing the engine into a less-powerful RPM for the sake of engine wear and fuel consumption. Since you're forcing it into a less powerful RPM, you must FIRST reduce the MP...because too much MP for too low an RPM is like trying to start your car into motion using 3rd gear. It's damaging to every part of the system. That's where the old, "squaring" method comes from. Never let RPM/100 fall below MP in inches..   i.e... 25" / 2500RPM.. etc.

During cruise, the only use of the prop-control is for the sake of economy. If you're seeking maximum airpseed, you'd "let" the engine go back to its most powerful RPM and apply maximum MP. Various MP/RPM settings are for deciding what compromise between economy and speed suits the flight. An important side note, is that as you climb, full throttle will yield less and less MP. For example, a 24/2400 setting at 6,000msl will probably require full throttle. This where a CSP really shines. It gives you "full throttle" horse-power at high altitudes, without over-stressing the engine.

For a standard descent, ideally you'd plan it well enough to reduce MP by one inch per minute (this prevents shock cooling) to about 19". Again, the CSP will adjust the prop pitch for you. The only time you' force the RPMs lower, is when you have to reduce MP quickly. This would get into advanced CSP, not applicable here. Otherwise, you just go ahead and leave RPMs at a cruise setting until leveling off again.

During a landing.. the "P" in G.U.M.P.S. comes into play.

Gas on fullest tank / Undercarriage down / Mixture to best power / Props to highest RPM / Switches (lights) on ... so that in the event of a go-around, you're at max horse power.

There are more subtle things to touch on.. we'll get into them if/when the discussion warrants.

Take as many stabs as you like. This is pretty much what I do. I've long since stopped actual instructing, but I assist in ground school classes often.. Sometimes one-on-one. I remember how important it was to me, to get a grip on this stuff.

You look at this like I did. When I first flew a CSP aircraft, it drove me nuts that I'd pull the throttle back to descend, and the tachometer didn't budge, but the thrust definately evaporated..

As far as manifold pressure goes; it's the same deal with a car, motorcycle or chain-saw. It's really relative pressure.. as in less vacuum, not more pressure (unless of course it turbo or super charged). An airplane sitting with the engine not running has it's highest, possible MP (atmoshperic)(29+inches on a standard day). If you had a MP gauge in your car, it would tell you the same information. The less that the pistons have to "suck" the air into the cylinders, the more power the engine is generating, regardless of RPM. And just like a car, if you "bog" it down by trying to accelerate in too high a gear (letting prop RPM/100 fall below MP), it will let you know.

OK.. then.. The prop-control is a pretty much a differential valve. It determines how much of the hydraulic pressure that's being generated by the engine (in most planes the actual engine oil is the hydraulic fluid) is allowed into the hub to "force" the pitch coarser. This is all engineered so that any setting of that valve will take the proportion of the available HP needed to load the engine (via steeper pitch). It's like a catch 22. More power merely creates more load, so that moving the throttle has no effect on RPM (within operating ranges).

With more load producing a more aggressive prop, the whole thing works out nicely. It's a never-ending, give-n-take.  For example.. if you pitch the nose up to start a climb, the extra work of fighting gravity will slow the plane, which slows the prop. That obviously means less RPM so the "hydraulic pressure" lessens, so the blades are "allowed" to move to a finer pitch, and the RPMs don't decrease (they do for an instant, but it's negligible).

Another scenario..  If, in level flight, you simply apply more throttle, the extra thrust comes from the prop increasing its pitch. The tacometer doesn't budge, but the plane accelerates...

You can fly a CSP plane like a C182 all day long and leave the prop-control all the way forward... never move it. You'll get the full advantage of a CSP, as far as climbing power and top speed..  you'd just not be enjoying the economy and less engine wear at your disposal.... especially at higher altitudes.

My pleasure...

In the spirit of understanding what you're "driving"...  The equivalent of riding the clutch would be to not nudge the RPMs out of the red after takeoff. Granted, it takes you slightly away from the HP peak on the torque curve, but that 50 RPM or so, at high MP, adds up to lotsa engine life.

Also.. a pilot should know that in the event of low oil (or an outright mechanical failure of the CSP), the prop (on most GA planes), will "spring" to it's lowest pitch. It becomes the worlds best, fixed-pitch climb prop. At that point, you revert to flying by the tachometer and find a place to land.. In a C182 you'd be lucky to get 80kias while spinning the main bearings out of that poor engine 

Hey Brett, I got a few questions:

What comes into mind when choosing the number of propeller blades? Wouldn't 4 blades just be ideal for any plane?

And how does horsepower come into the equation, when talking engine output on CSP aircraft? It would seem, that a plane that can go up to 2700 revs with 500 hp, shouldn't go faster than one with 250.

Had another one, but I've forgotten it for now

OR...  try thinking of it tis way...

Instead of two different engines, think of it as two different power settings.

At 1/2 power (say 250hp) the CSP will set pitch much less aggressively in order to "let" the prop spin at 2700rpm..

As you increase throttle to full power (say 500hp), the prop STAYS at 2700rpm, because the CSP increases pitch accordingly ..  The prop-RPM doesn't change (constant speed prop    ), but the plane accelerates as the pitch increases.

OK now for turboprops you have the same basic system with the hydraulic pitch and so on.

Of course there are a few exceptions... the NK-12 family (which is a strange beast in the turboprop world already)
Usually your HP rating is based on sealevel measurements, and it decreases when the oxygen content of the air drops. On the NK-12 you have the rated HP rating up to about 20.000 feet... and blowoff valves.
As any turboprop is basically a jet engine with a gearbox on the main shaft... you can use the airflow inside for propulsion, either on the gearbox, or as exhaust gas.
The basic principle on the NK-12 is to switch to full turboprop mode as soon as the air does not have enough oxygen to provide power above what the gearbox can handle

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Yeah.. but try to think of it more like AoA, instead of drag. The prop blades are like little wings, and when there's more "airspeed" available, the CSP just asks them to do more work by increasing the AoA, and that in turn keeps their "airspeed" constant.

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No, it's accurate. A CSP can feather. Consider that a fixed prop cannot feather, that leaves only a CSP that can feather.. right ? (there are still a few controlable-pitch propellers in use, but they are rare and not worth discussing here)

The feathering is relative, too. On small, single-engine planes, it's not near as critical as it is on twins. The drag of a "dead" propeller is bad enough, but when it compounds the yawing problem of only having one engine running (say the left  engine quits.. you've got the right engine trying to yaw the plane to the left AND a dead propellor tryng to yaw it to the left) (AND throw in the fact that aside from being out on the right wing, a spinning propeller itself wants to yaw things to the left (p-factor and such), true feathering is required.

On most single engine airplanes, where your obvioulsy not worried about one-engine yaw, pulling the prop-control all the way back (lowest RPM meaning steepest pitch available), is near enough to a feathered setting to not really hurt the glide-ratio.. But on most light twins, pulling the prop-control all the way back WILL feather the prop, and help lessen the yaw problem (and reduce drag).

Some CSPs will actually default to feathered when an engine quits.. Just like they'll default to the highest RPM if the CSP mechanism fails.

Yeah..  it's a lot more fun to fly realistically..  You're quite welcome 

(all that FSX stuff is water long under the bridge    )

rox wrote on Jan 23^rd, 2009 at 1:00am:



Honestly ? Once you understand  how CSPs work, that's 99% of "complex" flying. Gear and flaps are something you had BETTER understand completely..  

Obviously (as mentioned) you need to think in terms of MP for power settings.. That's the biggest adjustment when you start flying complex. I mean,, you can't even count on you ears for feedback, because they're so tuned to RPMs, they can decieve you.

Once you get used to it.. it tougher to go BACK to fixed prop flying, than it was switching to CSP flying.. because your RPMs are all over the place. You get spoiled ..lol  With a fixed prop, your primary power reading is RPMs.. but during a climb or descent they're skewed. Not so with a CSP. MP is MP regardless of RPM.. and the CSP is always finding the ideal blade-pitch for you