Constant Speed Propellers..

[Brett_Henderson]

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.

[Zaphod]

OK I'm going to have another "stab" to see if I understand the very basic principal.

Please remember that all of my mechanical experience has been with small car engines and motorcycle engines so when you talk about manifold pressure I understand what it is but I don't think it has much influence with the sort of engines I'm familiar with.
Anyway, here goes.

The throttle controls power (MP) and the revs are determined/limited by how much resistence the prob generates with the air. The prop manages the AoA of the blades with the air to maintain the correct RPM for a given part of the flight.
Where I'm still confused is on what part of the setup does the prop lever act to alter the RPM. I'm talking on what Physical part. Is it on the engine or the prop?
For me anyway it's important to understand the mechanics so I can visualise whats happening when I operate something.

Around about now Brett you are beginning to realise what you have taken on.

If it all proves too much there is still the Magic theory to drop back on.

Best.
Zaphod

[Brett_Henderson]

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.

[Zaphod]

[quote]
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).


Another scenario..

[Brett_Henderson]

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  :o

[Brett_Henderson]

Bump.. per another thread ***

[Anxyous]

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

[Brett_Henderson]

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?


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

I looked into the number of blades thing, many mooons ago. It's less of an issue than you'd think, because it's not a NUMBER of blades issue;

[Brett_Henderson]

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.

[Anxyous]

Thank you very much, very informative answers

I'll see if I can remember the last one...

[Ivan]

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

[Anxyous]

So basically, once the prop lever is set, the prop maintains the set RPM, by increasing and decreasing blade pitch, thereby increasing and decreasing blade drag?

I got another Q:

I know what feathering is about, and what it does.

But in a mission in FSX, an engine on a Baron 58 stops, and you're told to feather the prop. The Baron 58 is a CSP plane, so is this just a screw-up on MS' part?

[Brett_Henderson]

So basically, once the prop lever is set, the prop maintains the set RPM, by increasing and decreasing blade pitch, thereby increasing and decreasing blade drag?

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.

But in a mission in FSX, an engine on a Baron 58 stops, and you're told to feather the prop. The Baron 58 is a CSP plane, so is this just a screw-up on MS' part?

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 ([i]say the left

[Anxyous]

Ah, I see.

Thanks!

[Brett_Henderson]

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

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