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of 225 thrust horsepower available for climb. Loss of
one engine would leave only 25 (200 minus 175) thrust
horsepower available for climb, a drastic reduction.
Sea level rate-of-climb performance losses of at least
80 to 90 percent, even under ideal circumstances, are
typical for multiengine airplanes in OEI flight.
OPERATION OF SYSTEMS
This section will deal with systems that are generally
found on multiengine airplanes. Multiengine airplanes
share many features with complex single-engine airplanes.
There are certain systems and features covered
here, however, that are generally unique to airplanes
with two or more engines.
PROPELLERS
The propellers of the multiengine airplane may outwardly
appear to be identical in operation to the
constant-speed propellers of many single-engine
airplanes, but this is not the case. The propellers of
multiengine airplanes are featherable, to minimize
drag in the event of an engine failure. Depending
upon single-engine performance, this feature often
permits continued flight to a suitable airport following
an engine failure. To feather a propeller is to stop
engine rotation with the propeller blades streamlined
with the airplane’s relative wind, thus to minimize
drag. [Figure 12-2]
Feathering is necessary because of the change in parasite
drag with propeller blade angle. [Figure 12-3]
When the propeller blade angle is in the feathered
position, the change in parasite drag is at a minimum
and, in the case of a typical multiengine airplane, the
added parasite drag from a single feathered propeller
is a relatively small contribution to the airplane total
drag.
At the smaller blade angles near the flat pitch position,
the drag added by the propeller is very large. At these
small blade angles, the propeller windmilling at high
r.p.m. can create such a tremendous amount of drag that
the airplane may be uncontrollable. The propeller windmilling
at high speed in the low range of blade angles
can produce an increase in parasite drag which may be
as great as the parasite drag of the basic airplane.
As a review, the constant-speed propellers on almost
all single-engine airplanes are of the non-feathering,
oil-pressure-to-increase-pitch design. In this design,
increased oil pressure from the propeller governor
drives the blade angle towards high pitch, low r.p.m.
In contrast, the constant-speed propellers installed
on most multiengine airplanes are full feathering,
counterweighted, oil-pressure-to-decrease-pitch
designs. In this design, increased oil pressure from the
propeller governor drives the blade angle towards low
pitch, high r.p.m.—away from the feather blade angle.
In effect, the only thing that keeps these propellers
from feathering is a constant supply of high pressure
engine oil. This is a necessity to enable propeller feathering
in the event of a loss of oil pressure or a propeller
governor failure.
Full
Feathered
90°
High
Pitch
Low
Pitch
Figure 12-2. Feathered propeller.
Change in
Equivalent
Parasite
Drag
Propeller Blade Angle
0 15 30 45 60 90
PROPELLER DRAG CONTRIBUTION
Windmilling
Propeller
Stationary
Propeller Feathered
Position
Flat Blade Position
Figure 12-3. Propeller drag contribution.
12-3
Ch 12.qxd 5/7/04 9:54 AM Page 12-3
The aerodynamic forces alone acting upon a windmilling
propeller tend to drive the blades to low pitch,
high r.p.m. Counterweights attached to the shank of
each blade tend to drive the blades to high pitch, low
r.p.m. Inertia, or apparent force called centrifugal force
acting through the counterweights is generally slightly
greater than the aerodynamic forces. Oil pressure from
the propeller governor is used to counteract the counterweights
and drives the blade angles to low pitch,
high r.p.m. Areduction in oil pressure causes the r.p.m.
to be reduced from the influence of the counterweights.
[Figure 12-4]
To feather the propeller, the propeller control is
brought fully aft. All oil pressure is dumped from the
governor, and the counterweights drive the propeller
blades towards feather. As centrifugal force acting on
the counterweights decays from decreasing r.p.m.,
additional forces are needed to completely feather the
blades. This additional force comes from either a
spring or high pressure air stored in the propeller
dome, which forces the blades into the feathered position.
The entire process may take up to 10 seconds.
Feathering a propeller only alters blade angle and stops
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