Operating Complex Aircraft

[BFMF]

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?

[ozzy72]

Your prop pitch is how much the prop bites the air. Fine pitch for take-off and coarse pitch for cruising. It affects things like range and acceleration.
Manifold pressure will vary from engine to engine not just aircraft to aircraft. Get the manifold pressure wrong and you'll shag the engine!

[Splinter562]

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.

[beaky]

"In the green" is a good starting point- there's a reason why such planes have a green arc on the engine gauges. But knowing the right numbers does make a difference.

Best thing, of course, is to find a decent flight manual for the plane- I have a few from Werner Schott; they're very good and available for download here at the V.

[Brett_Henderson]

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

[BFMF]

That makes a lot of sense. Thanks guys for explaining it all for me...

[Mobius]

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.

[Splinter562]

1) When adding power, increase the prop leaver first then increase the throttle leaver. When decreasing power, decrease the throttle leaver first, than the prop leaver. In Brett's post, he said that you should never let the prop RPM fall below the MP. Using this method will make sure that never happens.

2) Without a turbocharger, you manifold pressure at full throttle can never be more than the atmospheric pressure. That means that as you get past around 5000 ft, you cannot maintain 25 inHg anymore. At that point you just leave the throttle full open and take the best you can get. Take another look at the book, it should say "75% Rated" or "75% Power" or something like that. What this means is that you are using 75% of the engine's rated horsepower at sea level. So in a 200HP engine, you're using 150HP. The engine's available power drops off with altitude so you may need full throttle to get 75%, or you may not even be able to get that and have to go to 65% or 55%.

3) You are correct. And this is indirectly what the governor is doing. The governor trys to stay at the RPM you've set with the prop leaver. By increasing the pitch of the blades, they get more lift, but also more drag, so the prop slows down. And vice versa.

Trying to think of an easy way of putting this, but if you are familiar with the term Cl, lift coefficient, which is L = 1/2*[air density]*[velocity]^2*[chord lenght]*Cl. Seems complicated, but the only two things you're changing here are Cl and velocity (since the prop is spinning faster). So, what this means is that lift increases proportionally with Cl but exponentially with velocity. So less pitch at a faster RPM will give higher lift than more pitch at a slower RPM.

[Brett_Henderson]

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

[Mobius]

Thanks to both of you, that cleared it all up.

As far as the rated power thing, I was about 15 to 20 kts slower than what the manual said cruise would be, which is why I was so confused.

For the last one, that makes sense too.  I had a mental moment there for a bit.  I just finished my aerodynamics class in December, and I had Cl and Cd plots smashed into my head, so maybe I was just trying to block out the bad memories of long nights staring at them.

Thanks again, I'll keep practicing.  This adds a whole lot more to flying in FS.

[Boss_BlueAngels]

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

[Ivan]

BNrett... if he's flying a radial engined plane that formula doesnt work, but the author should have provided a MP/Rpm sheet with those anyway.

For An-2's (with a good FDE... not the Tim conrad one)
1800rpm @ 800mbar.

Yak-18 comes with a MP/RPM sheet, as does the Yak-12... and you better follow this sheet or the engine blows up

[Brett_Henderson]

BNrett... if he's flying a radial engined plane that formula doesnt work, but the author should have provided a MP/Rpm sheet with those anyway

Yes.. and I qualified that..

I Wrote:
[quote]The "squaring" for cruise method