A complex aircraft is one which has manually or automatically controllable pitch propeller, flaps, and retractable landing gear.

I understand the use of flaps, and operating retractable landing gear seems easy enough, but I never did completely understand the use of controllable pitch props.

The prop control changes the RPM, and the throttles control the manifold pressure. Usually for cruise, I've been simply keeping the RPM and Manifold Pressure gauges in the green. Do procedures vary from aircraft to aircraft, or is there a 'general rule of thumb' procedure that is used with a lot of smaller complex aircraft?

Ozzy's correct. It's a bit of a strange concept at first but you'll get the hang of it quick. The Manifold Pressure (MP) represents the engine's power output and is controlled by the throttle. Now the tricky part is how controlling the pitch of the propeller plays into this. If you were to set the engine at a certain manifold pressure and adjust the pitch of the prop from coarse to fine, you'd notice the engine RPMs increasing. This is because as the pitch decreases, the prop is taking smaller bites of air which, in turn, reduces the drag of the prop, so it spins faster. You can sorta think of it like a gear shift in a car.

Now, most variable-pitch propellers today are of of the constant speed variety. These have a governor which moves the pitch of the prop (via. oil pressure). The governor will try to adjust the pitch of the prop to maintain a constant engine RPM. The blue prop leaver adjusts the set point (requested RPMs) of the governor. So, when the prop leaver is full forward, the set point of the governor is the redline of the RPM arch (usually around 2600-2700 RPM). As you pull the leaver back, you are moving the set point to lower RPMs.

Now, this makes for good flying in a number of ways. First, much like you want to be in lower gear when starting out of a stoplight in a car, you want to be at a fine pitch (high RPMs) when taking off in an airplane. Because the governor will control the pitch to maintain max RPMs, all you have to do for takeoff (in most aircraft, some may differ) is set the prop leaver full forward and give it full throttle. You if the governor is functioning properly, the engine will never overspeed.

As you you climb out, at 500 or 1000ft (depending on which instructor you're talking to), you'll usually pull back the prop and throttle a little (again, some planes may vary), 25^2 is the usual guidance. That means 2500 RPM and 25 inHg manifold pressure. As you climb into thinner atmosphere, your manifold pressure will begin to drop about 1 inHg every 1000 ft, so you need to bump the throttle up a little bit every so often as you climb to maintain 25 inHg until you hit full throttle.

When you get to cruise altitude, it's time to set the plane into high gear. The performance section in the POH will usually have a table of MP/RPMs for cruise settings but in the absence of the manual or when bombing around the local area 24^2 is a pretty common setting.

There's some more nuances to operating a variable-pitch propeller that you need to know about to fly the airplane properly, but it's late for me, so I'll digress to the other knowledgeable members around here.

It's been covered pretty well here, in the last few posts. And it does vary from plane to plane. The commom misconception that the prop-control selects pitch was put to rest.. and that a constant-speed prop is really just an infinately variable transmission, where the pilot selects optimum RPM, was pointed out.

While, in theory.. the prop-control does indeed change the blade pitch.. In reality, the throttle setting and the plane's airspeed have more to do with blade pitch. If you're flying along level.. and pitch the nose up (touching neither the throttle nor prop-control), the prop-pitch will lessen (become more fine) under the new load, in order to maintain the selected RPM setting.

The "squaring" for cruise method  (24X24 .. or ..25X25)   (24"mp/2400rpm ..  or 25"mp/2500rpm), is a good rule of thumb for most normally aspirated, or turbo-normalized, light piston airplanes.. and applies to the default Baron (and my Cardinal   ).  For turbo-charged piston planes (like the Default Mooney) it's a little different, because they're capable of higher than atmospheric, manifold pressure. And for larger turbo/super-charged, piston planes; it's a whole 'nother ball of wax... as they're capable of some VERY high manifold pressures and different, optimum RPM settings.

Where managing MP/RPM comes into play in a light piston airplane, is in the realm of fuel economy and engine wear.... and the biggest adjustment for the pilot, is to learn to fly by the MP gauge for power-settings, as opposed to the tachometer in fixed-prop airplanes.

I'll go through a takeoff/cruise/land scenario in a C177RG, as it translates well to most, constant-speed, light piston airplanes.

Takeoff with full throttle and max RPM (aprox. 28"mp / 2700rpm) and an optimized mixture setting (at or near sea-level, this is full-rich, and will be slightly leaned for density altitude (i.e.. on a hot day in Denver.. your mixture knob might be 1/2-way lean for takeoff.. AND, you'd be lucky to get 24"mp))... As soon as climout is established, back the RPMs up just a tad to get them out of the red zone on the tachometer. (note.. when concerned about obstacle clearance, maintain max RPM until clear, but the extra few knots don't justify the engine wear during normal takeoff).

During the climb you monitor and adjust mixture, by  EGT gauge and can lower the RPM a bit for economy (NEVER letting RPM get lower than MP ..  i.e.. 25"mp/2400rpm, as that's akin to trying to accelerate from low speed in a car, in 5th gear, and is hard on the engine). Since the whole idea is to get up to cruise altitude quickly..it's best to keep prop-rpm above 2600rpm during the climb.

If you know that your cruising altitude will be above 6000msl, you won't have to touch the throttle again until descent and landing... because the manifold pressure will fall off on its own, in the thinner air.

If cruising altitude is well below 6000msl; after leveling off, normal cruise (~75% power) will be, 25"mp/2500rpm. Economy or long-range cruise (~65% power) will be 24"mp/2400rpm... appropriately leaned.

If cruising altitude is above 6000msl, then simply use the "squared" rule. In other words; lower prop-rpm to match MP, but not lower than ~2300rpm. It can get a little tricky at high altitudes and requires some finesse by the pilot. There are fine lines between performance/economy and "bogging down" the engine. While in level flight at high altitude.. you might be tempted lower RPMs too much, because you'll see a reduction in fuel-flow. The big advantage to a constant-speed prop, is that you can avail the high, true airpeed (and low fuel consumption) at higher altitudes, without keeping your poor engine at high RPMs. This is where the art of piloting comes in. Manifold pressure has long become a non-issue (throttle is left wide open) so it's all about mixture and pop-rpm. And, it cannot be simply altitude specific, because atmospheric pressure can vary by an inch or two. Until you're intimately familiar with your plane.. you'll usually find the best compromise between fuel-consumption and true airspeed, when the prop-rpm and MP are perfectly squared...  With two key exceptions: Never let prop-rpm fall below ~2300rpm and NEVER let the prop-rpm fall below MP (via the squared method).

The first time a pilot flies a constant-speed prop and pulls the power back for descent; out of instinct, he'll glance at the tachometer and see no change to speak of. Up until this point, he'll have been targeting something like 2100rpm as a power setting for cruise-descent. Now he has to reference manifold pressure. It's a pretty intuitive thing. He'll get a similar sensation in lost airspeed and rate-of-descent, except now it's all about MP..not RPM. The trick now though, is that you have to time your descents better. Since the engine is still buzzing along at a higher RPM, power reductions can result in a dramatically altered mixture; which can result in shock-cooling. Ideally... you'd plan your descent so that you can reduce manifold pressure by 1" per minute. However, many times this isn't realistic, so it's good piloting to reduce the RPMs a bit during an accelerated descent in order to reduce shock-cooling. If all goes well.. you'll find yourself at pattern altitude, pattern airpseed, proper mixture setting and all set for the G.U.M.P.S. ( Gas [fuel on both or fullest tank], Undercarriage [gear down], Mixture [set for max power for the density altitude], Props [set to max RPM in case of a go-around], Switches [appropriate lighting switched on])...  The "P" in GUMPS is particularly important if you did indeed reduce RPM significantly during the descent.

OK.. I think that covers it...  I'll probably reference this thread when Sim Flight Training gets into complex and high-performance endorsements..

So, I've been practicing a bit of complex flying now, and I've got a few questions:

1.  I heard that you always move either the throttle or the prop control before you move the other one, but I can't remember which, does it actually matter?

2.  I've been flying the AOPA Cardinal, and in cruise I keep it at ~20 inHg and 2500 RPM (I can't follow the 25x25 rule because that would be almost full throttle, and cruise is ~75% throttle according to the manual), so if I wanted to slow down, would I decrease manifold pressure or prop RPM?  It seems like I just keep the prop RPM at 2500 RPM the whole time, even though the green arc on the tach goes from 1000 RPM to 2700 RPM.

3.  I'm a bit confused on prop pitch.  Increasing the prop pitch would increase the lift generated by each blade because you're increasing the AOA, but doesn't that mean that more drag is created with the increased lift?  And wouldn't this cause a lower prop RPM?  So why does a lower blade angle create more thrust? 

Thanks for any help.

***edit.. I was typing this as Splinter was responding.. just consider it a supplement to his accurate information   ***

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Excellent questions.. I'll try to address them in order.

1. More times than not; a change in either prop-rpm or power, requires a change in the other. The main concern is to never let prop-rpm fall below manifold pressure. Exceptions include; takeoff, where you'll reduce prop-rpm slightly without touching the throttle. This gets into gray area #1. On a cold day at sea-level, you'll be able to hold a MP slightly higher than max prop-rpm. As you climb, this will remedy itself, but if for some reason (like a very cold day and very high atmospheric pressure), the differential will be more than a few minutes.. you'll have to reduce MP slightly during the climb.

Now.. when leveling for cruise, you reduce power first. If you reduce prop-rpm first, it can fall below MP. When transitioning from cruise to climb; you increase prop-rpm first.. then MP (for the same reason). When descending; it's obviously power first, and then prop-rpm... and unless its' going to be an accerated descent, you don't need to touch the prop-rpm. If it is a quick descent, you'll want to reduce it slightly, because with the engine whizzing away at high rpm, and the throttle reduced, shock-cooling can set in (and of course mixture management plays a role... You can run extra lean, if need-be, to keep the engine temperature higher). A good rule is to at least touch both knobs every time you change one, as a reminder (also a good time to at least think about reaching for the cowl-flaps)(you can close them slightly after you first power reduction.. and close them completely at cruise.. BUT you'll need open them again a bit, if you go into a full-power climb).

2. This is all very much altitude dependent. If you're cruising at 8,000msl, you couldn't hold 25" if you wanted to. If you go from takeoff, right up to a cruising altitude of anything over 6,000msl, you'll just leave the throttle wide open and lower the prop-rpm to slightly higher than whatever MP it is that you have. When cruising at lower altitudes (where 25+ inches are still possible); a 75% power setting (not 75% of full throttle) would be ~ 25"/ 2500rpm.. probably very close to full throttle. A 65% power setting would be 24"/2400rpm (regardless of the throttle's physical position).

If you're getting anywhere near cruise airspeed out of 20" of MP at low altitude.. that model's flight dynamics are off.

3. Yes..It can be confusing. Just remember that you don't increase or decrease the prop pitch. All you do is select an RPM... and then the prop changes pitch according to airspeed and power setting in order to maintain the best thrust for that RPM/power setting.

When you takeoff.. you'll be at max RPM and Full throttle. As the plane first starts rolling; the pitch will be the "finest".. and as you gain airspeed, the pitch will "coarsen".. and if you never touch either control, and tranistion right into a full-throttle / max-RPM descent.. the prop-pitch will be full coarse. All these prop-pitch changes, and you didn't touch either the throttle OR the prop-control.

Man, complex aircraft are so much fun!  Especially when it takes 13 seconds for the landing gear to extend/retract!  Really makes you stay on top of your game when operating at short runways! lol

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Yes.. and I qualified that..

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