Complex

This explanation comes from my online correspondent, a pilot with several decades of experience in command & instruction in military & civilian airline enviroments, with clarifications from an A&P who is also and instructor.

There is a bit of a transition to a complex aircraft, after all, what with having retractable gear, constant-speed props, and all that fancy electronic gear. You'll have to learn to fly profiles -- get the most performance you can out of the airplane, especially on takeoff and missed approaches. You'll have to study the configurations specified for the simulator, e.g., with approach flaps and gear down, 2500 RPM, it takes 23" of manifold pressure to hold level flight at 110 knots (at a total fuel burn rate of 80 gallons per hour). So when you're being vectored around for an ILS in between the heavies and the high-performance turboprops, you can keep right up with the best of them. Eh?

One very nice thing about an airplane like this is that in approach configuration, pulling power off gives you 100 feet per minute descent for every one inch of manifold pressure reduced. Make sense? Pull off 5" of MP, you'll descend at 500 fpm.

But isn't there just a tingle of excitement to all this new stuff for you?

Unlike the fixed-pitch prop you now fly, the constant-speed prop is mounted on, in basic terms, a hollow shaft: at one end is the powerplant; at the other end, there's a movable piston. Oil fills this shaft: the engine provides pressure to the shaft, which (you guessed it!) moves the piston.

Permit me to digress for a moment.

You don't have to understand how a mammary gland works physically and chemically in order to drink a glass of milk, do you? Well, you don't have to understand all the magic of hydro-mechanics in the constant-speed propellor in order to operate it. But see if you can follow the rest of this.

The base of each propellor blade is mounted to the movable piston on a cam,so that when the piston moves forward or backward, the blade angle is changed. Keep in mind that the piston is controlled -- moved -- by oil pressure! More pressure, the piston pushes forward; less pressure -- what happens then? Forward of the piston, between the piston and the end of the hollow shaft, is a large spring which resists the movement of the piston.

Oil pressure moves the piston forward, and when pressure is reduced, the spring moves the piston backwards. (In some airplanes, oil pressure assists the spring to move it rearward.)

Obviously, all this combines to control the blade angles. With me thus far?

When you're taking off, climbing, levelling off from the climb, or starting the landing approach, the propellor pitch is set to high RPM -- the most "bites" per minute. The propellor now has the lowest angle of attack to the relative wind, and the highest number of RPMs: it's operating at its highest efficiency.

In cruise, however, you want the propellor to take a bigger bite, right? This moves more mass of air rearward, and the mass of the airplane accelerates in the opposite direction as a result. You COULD move more air rearward at a higher RPM, but that's terrible fuel inefficient. Setting a lower RPM makes the propellor take a bigger bite, and it doesn't take a rocket scientist to figure out that a lower RPM means a lower fuel burn.

As you decrease RPM and the propellor takes a bigger bite, however, you're making the powerplant climb the energy hill -- like your heart, it's working harder in taking a bigger bite. As a result of this greater workload, the internal pressures in each cylinder climbs. It would be very difficult to measure and display these pressures in each and every cylinder, at least in some way that's meaningful to the pilot in terms of powerplant performance.

The pressure in the intake manifold is directly related and proportional to the internal pressures in the cylinders. Knowing what the intake manifold pressure is simply provides you a way of measuring the POWER that the power plant is producing. It is simply an indicator of what the engine is doing and must be combined with other information such as RPM, Exhaust Gas Temperature, outside air temperature, and fuel flow. Every airplane is different and to thoroughly understand the implications of the various values it is well worth the cost of a cup of coffee in a quiet spot with your favorite mechanic - find an older one that will appreciate the coffee, the rest, and the interest.

There's a very nice relationship, by the way, of RPM to manifold pressure (MP). Reduce RPM by 100 (without changing throttle position), and your MP climbs 1". Increase RPM by 100, and MP is reduced 1". Does this make sense to you?

It follows that by increasing RPM, you're reducing the load on the prop by making it take smaller bites -- thus the engine isn't working as hard. The converse is true: reduce RPM (make it take bigger bites) and the manifold pressure climbs.

To prolong engine life, most engine and thus aircraft manufacturers have recommend not letting manifold pressure exceed RPM divided by 100, e.g., 25"
MP, 2500 RPM; 22" MP, 2200 RPM.

The ONLY exception to this is when using takeoff power -- and then, there's often a five minute limit to this rule. Most airplane manuals specify maximum power for takeoff with a five minute limit or with some reduction from full throttle. Boosted airplanes (those with turbo chargers or super
chargers) have other restrictions and limitations.

For many years pilots have been thought not to let the MP exceed the RPM divided by 100, e.g., 25" MP, 2500 RPM; 22" MP, 2200 RPM. This was a good guideline 20 or 30 years ago given the technology that existed. Today it is possible and sometimes preferable to have one or two inches of MP greater than RPM. Again, this will depend on the specific engine, when it was overhauled, how much time is on it, what type of propeller you are suing, how the plane is generally flown, and under what conditions it is flown.

Now that cup of coffee is really gonna pay off. Higher MP settings can result in better (lower) fuel consumption, longer engine life, and a faster trip.

The danger of this is that if you were to suddenly have to miss the approach on landing (an airplane pulls out onto the runway when you're 4-5 seconds from touchdown) and you neglected to advance your RPM lever(s) to full forward (high RPM, or "fine" pitch, going to full throttle would crack or rupture the exhaust manifold and likely send one, several, or all the pistons flying through the cylinder heads. Not a good career move.

My idea of having fun doesn't involve having a very serious lack of power at the same time there's a substantial fire engulfing one or both engines.

So, there's a basic rule or two to follow. If you want the engine to operate at the highest power efficiency, for example, on takeoff, climb, stall recovery, and/or landing (and thus preparing for an unanticipated and very sudden missed approach), increase RPM to maximum before going to full throttle(s). Increase RPM first, then add power. Increase PRM first, then add power.

Whenever you want to decrease RPM, for example, transitioning to cruise flight, reduce manifold pressure first, then reduce RPM.

You're leveling off at 5,000' with takeoff power set (ATC asked you to expedite your climb to 5): you look at the engine instruments and see 2750 RPM and 30" MP. What should you reduce MP to, if you want to cruise at 25" and 2500 RPM? 2750 RPM - 2500 RPM = 250 RPM. That's how much you have to reduce RPM for a cruise power setting. But you have to reduce MP first, right?

My goal is to have 25" of MP for cruise, and I know that for every 100 RPM I bring the prop back, manifold pressure is going to climb 1". Hmmmmm. If I first reduce manifold pressure to 25" (less an additional 2.5"), to equal 22.5", when I pull RPM back from 2750 to 2500, manifold pressure should climb 2.5". I now have 25" and 2500 RPM! It sounds a little more complicated than it really is in practice.

One last consideration is fuel mixture.

Going to high RPM before going to full power you already understand. However, if you do that from cruise power (where you had the mixture leaned out for maximum fuel efficiency), the mixture will be too lean for full power performance. To avoid detonation when using full power, in a case such as a landing or approach go-around, FIRST move the mixture to full rich, then RPM to high, then throttle to full power. (Normally, you do this as a part of your landing checklist so that you you're ready for a go-around.)

Setting cruise power is just the opposite: reduce MP, then reduce RPM (watch the MP climb!). Last, you're ready to lean the mixture. There's a number of ways to do this, all utilizing common sense. Most airplanes equipped with constant-speed props have a fuel flow gauge that's valuable in approximating cruise power settings. This gauge is calibrated for fuel flow in gallons per hour and are usually very accurate if they're well maintained. In many cases, there are also markings for takeoff and cruise power settings, and within these two ranges there are additional markings for percentage of power (i.e., 45%, 55%, 65% power).

These markings will help you get into the neighborhood of where you want your power to be set.

The next step is to fine-tune the mixture, either for peak power efficiency or maximum fuel efficiency. This is usually done most efficiently by measuring exhaust gas temperature, or, less accurately (but with equal reliability) by measuring cylinder head temperature.

All this seems, perhaps, to be a bit much for someone getting into a simulator. However, a good instructor will go over this and help you to get the most out of your aircraft. It's a part of having a professional attitude about the art and science of aviating.

I understand how you must look at the additional workload and procedures: I'm not hearing you say that you don't want to keep learning, am I? Think of how much better you'll be at 105 KIAS -- or 80 KIAS -- if you can fly with proficiency at 180 KIAS or 250 KIAS. True, you do have less time at 250 or 180 than 105 or 80, but the equipment in the airplane capable of flying approaches at 180 KIAS makes you far more efficient and proficient -- and helps you to make more efficient use of your time. You can also create time for yourself -- you don't have to be rushed, you know. (That's one of the effective uses of a holding pattern: if you're not ready for an approach, do a few turns in the hold!)

The primary benefits of utilizing a simulator is (1) to pick up your scan rate; and (2) learn attitude flying. I am continually visiting the simulator for (1) and (2), both. And I fly 75-85 hours a month, with an average of 6 approaches (to minimums) and 2 hours actual instrument every month.

And, if (one day) you become an instructor, a solid foundation of knowledge and the experience of practice -- in all forms, whatever they take -- is one of the keys to being a good instructor.

Next: On Advanced Instruction

Judy & JJ
 
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