SECTION 7
CESSNA
AIRPLANE & SYSTEMS DESCRIPTIONS
MODEL 172RG
induction air. After passing through the filter, the induction air enters an
airbox which is mounted on the carburetor inlet. From the airbox, the air
enters the carburetor and is ducted to the engine cylinders through intake
manifold tubes. In the event carburetor ice is encountered or the intake
filter becomes blocked, alternate heated air can be obtained from a shroud
around number four cylinder exhaust riser. This heated air is ducted to the
airbox and passes through a valve controlled by the carburetor heat
control on the instrument panel. Heated air from the shroud is obtained
from unfiltered air inside the cowling. Use of full carburetor heat at full
throttle will result in a loss of approximately one inch of manifold
pressure.
EXHAUST SYSTEM
Exhaust gas from each cylinder passes through riser assemblies to a
muffler and tailpipe. The muffler is constructed with a shroud around the
outside which forms a heating chamber for cabin heater air.
CARBURETOR AND PRIMING SYSTEM
The engine is equipped with a horizontally-mounted, side-draft, float-
type, fixed jet carburetor mounted below the engine adjacent to the
firewall. The carburetor is equipped with an enclosed accelerator pump, an
idle cut-off mechanism, and a manual mixture control. Fuel is delivered
from the fuel system to the carburetor by gravity flow, the engine-driven
fuel pump, and/ or auxiliary fuel pump. In the carburetor, fuel is atomized,
proportionally mixed with intake air, and delivered to the cylinders
through intake manifold tubes. The proportion of atomized fuel to air may
be controlled, within limits, by the mixture control located on the center
area of the switch and control panel.
For easy starting in cold weather, the engine is equipped with a 3-
cylinder manual primer. The primer is actually a small pump which draws
fuel from the fuel strainer when the plunger knob is pulled out, and injects
it into the engine intake ports when the knob is pushed back in. The plunger
knob is equipped with a lock and, after being pushed full in, must be rotated
either left or right until the knob cannot be pulled out.
COOLING SYSTEM
Ram air for engine cooling enters through two intake openings in the
front of the engine cowling. The cooling air is directed around the cylinders
and other areas of the engine by baffling, and is then exhausted through
cowl flaps on the lower aft edge of the cowling. The cowl flaps are
mechanically operated from the cabin by means of a cowl flap lever on the
right side of the control pedestal. The pedestal is labeled OPEN, COWL
FLAPS, CLOSED. Before starting the engine, and throughout takeoff and
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1 July 1979
CESSNA
SECTION 7
MODEL 172RG
AIRPLANE & SYSTEMS DESCRIPTIONS
high power climb operation, the cowl flap lever should be placed in the
OPEN position for maximum cooling. This is accomplished by moving the
lever to the right to clear a detent, then moving the lever up to the OPEN
position. Anytime the lever is repositioned, it must first be moved to the
right. While in cruise flight, cowl flaps should be adjusted to keep the
cylinder head temperature at approximately two-thirds of the normal
operating range (green arc). During extended let-downs, the cowl flaps
should be completely closed by pushing the cowl flap lever down to the
CLOSED position.
A winterization kit is available for the airplane. Details of this kit are
presented in Section 9, Supplements.
PROPELLER
The airplane has an all-metal, two-bladed, constant-speed, governor-
regulated propeller. A setting introduced into the governor with the
propeller control establishes the propeller speed, and thus the engine
speed to be maintained. The governor then controls flow of engine oil,
boosted to high pressure by the governing pump, to or from a piston in the
propeller hub. Oil pressure acting on the piston twists the blades toward
high pitch (low RPM). When oil pressure to the piston in the propeller hub
is relieved, centrifugal force, assisted by an internal spring, twists the
blades toward low pitch (high RPM).
A control knob on the center area of the switch and control panel is
used to set the propeller and control engine RPM as desired for various
flight conditions. The knob is labeled PROP RPM, PUSH INCR. When the
control knob is pushed in, blade pitch will decrease, giving a higher RPM.
When the control knob is pulled out, the blade pitch increases, thereby
decreasing RPM. The propeller control knob is equipped with a vernier
feature which allows slow or fine RPM adjustments by rotating the knob
clockwise to increase RPM, and counterclockwise to decrease it. To make
rapid or large adjustments, depress the button on the end of the control
knob and reposition the control as desired.
FUEL SYSTEM
The airplane fuel system (see figure 7-5) consists of two vented
integral fuel tanks (one in each wing), a four-position selector valve, fuel
strainer, manual primer, engine-driven fuel pump, electric auxiliary fuel
pump and carburetor. Refer to figure 7-6 for fuel quantity data for the
system.
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