19-11-2012, 03:38 PM
propulsion
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Introduction:
What is propulsion?
• The word is derived from two Latin words:
• pro meaning before or forwards and
• pellere meaning to drive.
• Propulsion means to push forward or drive an object forward.
• A propulsion system is a machine that produces thrust to push an object forward.
On airplanes, thrust is usually generated through some application of Newton's third law of action and reaction.
Turbo propeller engine working principle:
Propulsion is the net force that results from unequal pressures. Gas (air) under
pressure in a sealed container exerts equal pressure on all surfaces of the
container; therefore, all the forces are balanced and there are no forces to make
the container move.
If there is a hole in the container, gas (air) cannot push against that hole and the
gas escapes. While the air is escaping and there is still pressure inside the
container, the side of the container opposite the hole has pressure against it.
Therefore, the net pressures are not balanced and there is a net force available
Types :
There are two basic types of turboprop engines:
1. Single shaft
2. Free turbine
The main difference between single shaft and free turbines is in the transmission
of the power to the propeller.
In the majority of turboprops, the fuel pump is driven by the engine. This is
known as "direct drive.” In some older types of engines, the fuel pump is driven
by the propeller, which can affect proper response to an engine failure. Refer to
type-specific procedures.
Single Shaft. In a single-shaft engine, the propeller is driven by the same shaft
(spool) that drives the compressor. Because the propeller needs to rotate at a
lower RPM than the turbine, a reduction gearbox reduces the engine shaft
rotational speed to accommodate the propeller through the propeller drive shaft.
Free Turbine. In a free-turbine engine, the propeller is driven by a dedicated
turbine. A different turbine drives the compressor; this turbine and its
compressor run at near-constant RPM regardless of the propeller pitch and
speed. Because the propeller needs to rotate at lower RPM than the turbine, a
reduction gearbox converts the turbine RPM to an appropriate level for the
propeller.
fuel system problem:
Fuel leaks:
Major leaks in the fuel system are a concern to the flight crew because they may
result in engine fire, or, eventually, in fuel exhaustion. A very large leak can
produce engine flameout.
Engine instruments will only indicate a leak if it is downstream of the fuel
flowmeter. A leak between the tanks and the fuel flowmeter can only be
recognized by comparing fuel usage between engines, by comparing actual
usage to planned usage, or by visual inspection for fuel flowing out of the pylon
or cowlings. Eventually, the leak may result in tank imbalance.
In the event of a major leak, the crew should consider whether the leak needs to
be isolated to prevent fuel exhaustion.
It should be noted that the likelihood of fire resulting from such a leak is greater
at low altitude or when the airplane is stationary; even if no fire is observed in
flight, it is advisable for emergency services to be available upon landing.
Inability to shutdown engine
If the engine fuel shut-off valve malfunctions, it may not be possible to shut the
engine down by the normal procedure, since the engine continues to run after the
fuel switch is moved to the cutoff position. Closing the spar valve by pulling the
fire handle will ensure that the engine shuts down as soon as it has used up the
fuel in the line from the spar valve to the fuel pump inlet. This may take a couple
of minutes.
Fuel filter clogging
Fuel filter clogging can result from the failure of one of the fuel tank boost pumps
(the pump generates debris which is swept downstream to the fuel filter), from
severe contamination of the fuel tanks during maintenance (scraps of rag,
sealant, etc., that are swept downstream to the fuel filter), or, more seriously,
from gross contamination of the fuel. Fuel filter clogging will usually be seen at
high-power settings, when the fuel flow through the filter (and the sensed
pressure drop across the filter) is greatest. If multiple fuel-filter bypass
indications are seen, the fuel may be heavily contaminated with water, rust,
algae, etc. Once the filters bypass, and the contaminant goes straight into the
engine fuel system, the engine fuel control may no longer operate as intended.
There is potential for multiple-engine flameout.