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Rod Machado’s Private/Commercial Pilot Handbook
3-28
Keeping Up To
Fig. 47
(Constant) Speed
Figure 47 shows how the constant
speed propeller system works on a typ-
ical single engine airplane. Oil pressure
from the engine provides the hydraulic
force used to increase the propeller’s
pitch. Moving the propeller control aft
sends high pressure engine oil to a pis-
ton/cylinder arrangement within the
propeller hub. This hydraulically push-
es the propeller toward a high pitch
position. Moving the propeller control
forward reduces oil pressure within this
piston/cylinder arrangement allowing
centrifugal force to return the propeller
blades to their low pitch (high RPM)
position. We cycle the propeller during
our runup (change the pitch from low
to high and back again a few times) to
make sure the system is working as
well as to purge cold oil from the pro-
peller’s hydraulic system.
RPM. Pulling the propeller control Since the tachometer tells you how of mercury—just like altimeters that
rearward causes the propeller to fast the propeller spins (its RPM), is we’ll discuss in Chapter 5).
attack the wind at a larger angle of there a gauge to tell you how much Manifold pressure is measured
attack (i.e., take a larger bite of air). throttle is applied? Yes. It’s called a downstream of the throttle valve, as
Propeller drag increases and engine manifold pressure gauge and it gives shown in Figure 49. When the throt-
RPM slows, as shown in Figure 46B. you an approximate measure of tle is closed, air outside the engine
engine power (Figure 48). (under higher atmospheric pressure)
At the beginning of this chapter, we can’t flow into the induction system,
said a vacuum is created in the induc- despite the vacuum on the engine
tion system as a result of pistons side of the throttle valve. Figure 50A
descending on their intake strokes shows a manifold pressure of 14 inch-
(Figure 49). With the throttle closed, es of mercury with a closed throttle.
the throttle valve in the induction The engine is sucking as hard as it
can but the outside air can’t get past
system prevents air (thus fuel) from
rushing into the cylinders and power- the closed throttle valve.
ing the engine. But what is it that Opening the throttle slightly caus-
forces air into the induction system es an increase in manifold pressure
in the first place? Yes, it’s the pres- as shown in Figure 50B. More air and
sure of the surrounding atmosphere. fuel are drawn inside the engine, and
Because atmospheric pressure is power increases. Eventually, as the
higher than the pressure within the throttle is fully opened (Figure 50C),
induction system, air flows into the the pressure downstream of the
cylinders. Simply stated, the atmos- throttle valve approaches that of the
atmosphere. In other words, the air
phere wants to push air into the
is being forced into the induction sys-
induction system (toward the suction
tem at the maximum pressure the
created by the downward moving pis-
tons). The amount of this push is atmosphere is capable of pushing.
measured by the manifold pressure
Did the
gauge (the gauge is nothing more
controller say,
than a barometric measuring device “Radar contact?”
Fig. 48 calibrated to read pressure in inches
