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
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
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.
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
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.
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.
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.
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.
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,
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
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