1
$\begingroup$

I just transitioned to a complex airplane (Piper Arrow II). While I get how to use the Manifold Pressure / Propeller levers, I still find myself trying to piece together how all the components would function in a given situation.

One such situation is with the prop lever full-forward. If you back off the manifold pressure with the prop full-forward, the RPM will drop because there isn't enough air / fuel for the engine to turn at a the request rate. Think coming in to land (prop full toward and start reducing manifold pressure).

My question is: Does the propeller flatten out (relatively speaking) more than in a fixed-pitch propeller aircraft in this situation or is it the same net effect as operating a fixed-pitch propeller plane?

I've heard that putting the prop full-forward and then just using the throttle is the equivalent of operating a fixed-pitch propeller plane. How accurate is this statement?

$\endgroup$
2
$\begingroup$

I believe an Arrow operates the same as my Cardinal.

With low manifold pressure, there is not enough oil pressure to drive the prop into anything other than flattest configuration no matter where the prop control is at. If you were take off with prop control not all the way in, then in theory the prop would start getting more twist as engine speed came up, and it would be like taking off in 4th gear or similar, when you want to start your acceleration in 1st gear and then shift up (think of how the RPMs bog down as you cycle the prop at 1700 to ensure the prop can twist).

So you start with the prop and controls all the way in to take off (balls to the wall - RPM, prop, mixture, carb heat - or levers if so equipped).

Once you get to altitude, you can start bringing them back, equivalant to shifting up and taking your foot off the gas (and leaning the mixture for increased). RPMs are dialed down, prop control is dialed out to increase pitch.

Then when you come from altitude and prep for landing, you play the game of reducing RPM, decreasing pitch, reducing RPM, decreasing pitch, until eventually you are in the pattern, prop is all the way to support a go-around, and RPM controls all the power, same as taking off.

$\endgroup$
2
$\begingroup$

Yes. The prop has a low pitch (fine/high rpm) and high pitch (coarse/low rpm) stop. The low pitch stop sets the blades quite a bit finer than you would have with a fixed pitch propeller, which is usually set at a pitch that will give more or less redline RPM while climbing at wide open throttle.

If a constant speed prop failed and the blades went to the fine pitch stop, the engine would overspeed at anything close to full power and you would have to throttle well back to keep the prop at redline.

Within the prop governor, wherever you set the prop control just applies a spring preload on a flyweight mechanism that regulates oil pressure to the prop to drive the blades within the operating range. It's a compact version of those spinny ball thingy governors you see on steam powered farm tractors, but with everything hidden inside the governor housing.

Like the steam tractor, the fly weights are driven off the engine. They operate an oil control valve that controls oil flow to the prop and are, obviously, rpm sensitive. The oil control valve has a null position that fixes the blades where they are, and moves one way to move the blades coarse, the other way to move them fine. When the prop is at the target rpm, the flyweights position the valve at null and the blades are fixed, hydraulically locked there. If the engine speeds up, the flyweights move out, operating the valve to drive the blades more coarse until the speed is back to the target setting, then back to null. And vise versa.

If you set the rpm to max, the flyweight governor spring preload is reduced to a point where the control valve null point is at redline rpm. If there is enough power being applied, the governor will adjust blade angle to regulate rpm at redline. When you reduce power, the governor will move the blades finer and finer to keep the rpm setting until they reach the fine pitch stop. Once at that point, if you reduce power further, the blades can't go any finer to keep the rpm up, and rpm starts to drop and you effectively, temporarily, have a fixed pitch prop set way too fine to be of any use as a fixed pitch prop.

The point at which rpm starts to drop because power is insufficient to keep rpm at redline, even with the blades as fine as they can go, is called "coming off the governor".

$\endgroup$
  • $\begingroup$ This is more of an operational question, but my understanding is that while coming in to land, your prop lever should be full-forward, but at the same time you've reduced power to the point where the RPMs actually are dropping ("coming off the governor" as you mentioned). So, all things being equal, in this configuration, 1700 RPM isn't really the equivalent of 1700 RPM in a fixed-pitch propeller plane because the blade is much more fine. Would this insinuate that you'd want to operate at a higher RPM with a constant speed prop since it is being less effective during this phase? $\endgroup$ – Ryan Griffith May 31 at 16:38
  • $\begingroup$ Just use whatever throttle you need to use to maintain the speed and glide path you are on and down worry about RPM, but it will be a bit higher than the same conditions with a fixed prop. Don't know if you've noticed it, but the blades going flat(-ish) trying to keep RPM up is why you get a much stronger braking effect when you suddenly remove power with a CS prop. There is a flip side to this but is a bit more advanced; if the engine fails, you want the prop to be as coarse as possible, at the high pitch stops preferably, since it's closer to being feathered. So you pull full low RPM asap. $\endgroup$ – John K May 31 at 16:47
  • $\begingroup$ If the engine fails, you lose oil pressure. So the prop should revert to same as the prop control being full forward. $\endgroup$ – CrossRoads May 31 at 20:40
  • $\begingroup$ If it's a counterweight prop (most small GA props) the counterweights move the blades coarse so when oil pressure is lost they will move to full coarse anyway if the prop is still rotating enough for the weights to work. Going to low rpm helps it get there faster and is less dependent on the counterweights. If it's a hydramatic (no weights), the only forces acting on the blades are oil pressure on each direction and blade pitching moment tending to fine pitch. If you get the blades close to coarse pitch with the prop cont, once the oil pressure is gone they will more or less stay put. $\endgroup$ – John K May 31 at 21:07
1
$\begingroup$

I haven't flown piston GA complex aircraft, but I can provide some perspective from the turboprops I have flown. Because they have been full feathering and reversible propellers though, this perspective could differ from your experience:

1.) Yes, they flatten out much more than a fixed pitch. At idle/beta the prop basically turns into a huge rotating speedbrake.

2.) I am guessing that they simply meant that you fly it with throttle only like a fixed pitch because you don't have to constantly think about or adjust prop RPM when changing power settings. If that is the case, then I agree with the statement. Otherwise I would ask for clarification from anyone making this claim.

$\endgroup$
0
$\begingroup$

Here are 2 references for operation of constant speed props: Google: How a Constant Speed Propeller Works, and Flying Magazine Constant Speed Prop Basics July 1, 2014.

The blue propeller knob controls blade pitch. At full forward, for takeoff, climb out, landing (preparation for Go-Around), the blade is set at a lower pitch relative to the direction of rotation, taking finer "bites" of the air. This is just like the lower gear of your car, providing lots of acceleration but poor fuel economy. Viewed from the front, yes, you could say it was "flatter", opposite of being "feathered" (blade turned in the direction the plane is going to reduce drag if prop is shut down).

"Putting the prop full forward and using the throttle is the equivalent of operating a fixed pitch plane", well, the real advantage of variable pitch is being able to cruise the plane in "overdrive" by throttling back (reducing manifold pressure), then opening the blade pitch (pulling prop lever back) so the prop takes bigger bites at lower rpm.

But in the stressful enviornment of landing, take-off, airport traffic patterns, etc, once your prop AOA is reduced (prop lever forward), you can think of it as a fixed pitch if it helps, as you are only using the throttle to control engine output.

Recommend reading those articles.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.