5
$\begingroup$

I think I understand the relationship between the power lever and the RPM lever in an aircraft with a constant speed prop thanks to this question. However, what'd be a general rule of thumb for flying a small aircraft (e.g. Cessna 170) that has a variable-pitch propeller?

Edit: I'm flying a Cessna 170 that has been modified to fit a constant speed prop. What I'm trying to determine are the general recommendations for flying with them. Things like: On takeoff you should use full power & full rpms, on cruise lower power to 75% but keep full rpms, etc. I'm not sure if there are general recommendations to begin with.

$\endgroup$
14
$\begingroup$

The answer to your question is actually really simple: do not exceed the limitations specified in the POH.

Really, that's it! Note that LIMITATIONS are not the same as RECOMMENDED PROCEDURES. Recommended procedures are just that: recommendations. Limitations (exclusively from the "limitations" section of the POH) are the hard and fast rules you must adhere to.

Full RPM in Cruise
As long as your POH does not impose a limitation on takeoff RPM (the Seneca II has such a limitation, for example), then there is nothing wrong with using max RPM from takeoff to shutdown. You won't damage the engine. If the POH provides a limitation on engine RPM (specifically in Section II of the POH), you must adhere to that limitation. It's common to see a 5 minute "max RPM" limitation on some turbocharged engines. Great, takeoff at max RPM then pull the engines back to the top of the green arc until you land. Easy!

Manifold Pressure & RPM
Mike Sowsun said: "You should always follow the manufacturer's procedures but a general rule for direct drive normally aspirated engines is to keep the manifold pressure less than the RPM for cruise power. (eg. 24 inches of MP at 2400 RPM)."

If you decide to apply "rules of thumb" to all phases of flight you will be voluntarily handicapping your airplane's full potential.

Some definitions:
Oversquare - manifold pressure > RPM (23" @ 2100 RPM)
Undersquare - manifold pressure < RPM (23" @ 2500 RPM)
Square - manifold pressure = RPM (23" @ 2300 RPM)

Do not embrace the idea that manifold pressure should never, ever exceed the value of the first two digits of the engine RPM (sounds pretty arbitrary when I put it that way, right?). It actually may be beneficial to run an engine "oversquare" in some cases! This is another example of why knowing your aircraft's "Section II" limitations is important. If 23" @ 2100 RPM falls within the limitations, go for it!! You won't hurt the engine. Same with 25" @ 2300 RPM or 26.756431" @ 2196.454634 RPM. I think you get my drift. As long as your manifold/RPM combination falls within the limitations set out in Section II of the POH, you're golden!!

Now, climbing out on a hot day with a garden variety Lycoming or Continental (like in 170)? 29" @ 2100 RPM probably isn't wise. 30" @ 2700 RPM? Yeah, that's better! 25" @ 2500 RPM isn't bad but it's no better or worse that 26" @ 2700 RPM or 30" @ 2650 RPM. Remember, there's nothing special about "square" manifold/RPM values. I'd routinely run the IO-520 in my Bonanza at max manifold pressure and max RPM from takeoff to cruise and simply leaning to an EGT target as I climbed (and watching CHTs, of course). Only when leveling off in cruise did I pull it back to an economical setting (sometimes oversquare, sometimes undersquare; always lean of peak). The engine ran beautifully with good CHTs and incurred no unusual wear. In fact, the engine's health improved. Exhaust and plugs became noticeably cleaner and compressions improved.

In cruise, set the power to any combination of manifold/RPM you desire provided that setting falls within the limitations in your POH. You won't hurt the engine with a valid manifold/RPM setting. It's possible to put undue stress on a motor with a bad mixture setting, though. POHs are notorious for giving poor guidance with regard to mixture settings. Mixture settings require a little more attention above ~65% power but that's another topic entirely.

I recommend that everyone with engine questions read John Deakin's "Pelican Perch" articles. He's THE source for fact based, accurate engine operation information. He happens to have touched on this "oversquare" myth here. See Myth #2.

Edit: I credited John Deakin with my linked article. I was mistaken. The author of the linked article is Linda Pendelton. She links to a Deakin article, though.

Mike Busch also addresses the oversquare myth here.

$\endgroup$
  • $\begingroup$ I accepted your answer because it's very complete. I wasn't aware it was possible to run your engine "oversquared" and I'll definitely check out the aircraft's limitations. I'm not sure if the Owner's Manual we have has a Section II (I believe POH's weren't mandatory at the time the aircraft was made). $\endgroup$ – Pepedou May 31 '16 at 14:14
  • 1
    $\begingroup$ The 170 manual I'm looking at has a limitations section (section IV). You'll probably have to find the STC info to determine if the CS changes the original limits. $\endgroup$ – acpilot May 31 '16 at 20:29
2
$\begingroup$

The general rule of thumb that I am familiar with is 2400RPM set by the governor control lever and 23in-Hg for manifold pressure set by the throttle lever. This is what we do in a Diamond DA-40, for example.

I have seen the same settings used in other small aircraft, but I am sure there are variations between models.

$\endgroup$
2
$\begingroup$

I think acpilot covered most of it. I'd like to add this little tidbit to assist in thinking about how to use your constant speed prop.

A constant speed prop is like a continuously variable transmission. The main settings to be concerned with are climb and cruise. In a climb scenario, particularly takeoff, it is desirable to have all power available, this is achieved by running your engine at a higher rpm.

In a cruise scenario, on the other hand, it is desirable to "upshift" by lowering the rpm at a cruise power setting. This has the effect of opening the pitch of the propeller. You may find additional efficiency at this setting.

I recommend, and it may exist in your checklist, to move the rpm all the way to the top end as part of your pre-landing checklist. While the effect on your descent and landing is not of great concern, this simplifies the steps necessary in the event of a go-around. You are already set up for max power, all that is necessary is to advance the throttle.

$\endgroup$
  • $\begingroup$ Thank you! Actually, my father is teaching me to fly that plane and that's exactly what he told me (and his father told him) about the prelanding procedure. $\endgroup$ – Pepedou Jun 1 '16 at 1:25
2
$\begingroup$

Most aircraft today either have a fixed pitch propeller, or have constant speed propellers where the RPM is controlled by a governor.

Some aircraft have ground adjustable propellers, or manually controllable, variable pitch propellers, but they are pretty rare these days.

Cessna 170's were all built with fixed pitch propellers.

It is possible to modify the C170 to a constant speed propellor but the engine would need to be changed, or modified, in order to supply oil pressure to the prop.

If you were to install a constant speed propeller in a C170 it would be operated just as any other light aircraft.

You should always follow the manufacturer's procedures but a general rule for direct drive normally aspirated engines is to keep the manifold pressure less than the RPM for cruise power. (eg. 24 inches of MP at 2400 RPM)

$\endgroup$
  • 1
    $\begingroup$ +1 for recommendation to follow the manufacturer's guidelines and square operating rules of thumb. Most engines can be operated "over square", within certain limits (most cruise-optimized fixed pitch props operate almost entirely over square during take-off, climb out, and cruise), but when in doubt, you're not likely to ever end up abusing the engine if you just stick to operating at square or below (i.e. MP less than or equal to RPM). $\endgroup$ – habu May 31 '16 at 3:16
  • $\begingroup$ "While a constant speed propeller is technically also a variable pitch propeller,"* No they are NOT technically the same! A variable pitch propeller is missing the governor that a constant speed propeller has. $\endgroup$ – jwzumwalt Jul 12 '18 at 16:59
  • 1
    $\begingroup$ @jwzumwalt: A constant-speed propeller is a subtype of variable-pitch propeller (as the blade pitches of a constant-speed propeller are not fixed to a single value). All constant-speed propellers are variable-pitch propellers, but not all variable-pitch propellers are constant-speed propellers. $\endgroup$ – Sean Apr 4 at 3:19

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.