Aircraft Engines
Unlike automobile engines,
aircraft engines run at high power settings for extended periods of
time. In general, the engine runs at maximum power for a few minutes
during taking off, then power is slightly reduced for climb, and then
spends the majority of its time at a cruise setting—typically 65% to
75% of full power. In contrast, a car engine might spend 20% of its
time at 65% power accelerating, followed by 80% of its time at 20% power
while cruising.
The power of an internal combustion reciprocating or turbine aircraft
engine is rated in units of power delivered to the propeller (typically
horsepower) which is torque multiplied by crankshaft revolutions per
minute (RPM). The propeller converts the engine power to thrust horsepower
or thp in which the thrust is a function of the blade pitch of the propeller
relative to the velocity of the aircraft.
Jet engines are rated in terms of thrust.
The design of aircraft
engines tends to favor reliability over performance. It took many years
before the reliability was established to fly over the Atlantic or the
Pacific Ocean. Engine failure at all stages in flight is a part of flight
lessons for student pilots. Forced landings without power are practiced
extensively over rural areas until the new pilot is proficient enough
to handle such a situation during a solo flight.
Long engine operation times
and high power settings, combined with the requirement for high-reliability
means that engines must be constructed to support this type of operation
with ease. The engine, as well as the aircraft, needs to be lifted into
the air, meaning it has to overcome lots of weight. The thrust to weight
ratio is one of the most important characteristics for an aircraft engine.
A typical 250 hp engine weighs just 15% of the total aircraft weight
when installed into a 3000 lb aircraft.
Aircraft engines also tend
to use the simplest parts and include two sets of anything needed for
reliability, including ignition system (spark plugs and magnetos) and
fuel pumps. Independence of function lessens the likelihood of a single
malfunction causing an entire engine to fail. Thus magnetos are used
because they do not rely on a battery. Two magnetos with two spark plugs
per cylinder are used in certified piston engines so that the pilot
can switch off a faulty magneto and continue the flight on the other—
dual spark plugs also provide improved combustion efficiency. Similarly,
for redundancy, a mechanical engine-driven fuel pump is often backed-up
by an electric one.
Another difference between
cars and aircraft is that the aircraft spend the vast majority of their
time traveling at high speed. This allows an aircraft engine to be air
cooled, as opposed to requiring a radiator. A few notable piston engines
of the past, however, such as the Rolls-Royce Merlin series have employed
liquid cooling, which, though efficient, added an extra level and complexity
and risk in that receiving an enemy bullet to the cooling system in
combat could cause coolant loss and engine seizure. In the absence of
a radiator aircraft engines can boast lower weight and less complexity.
The amount of air flow
an engine receives is usually carefully designed according to expected
speed and altitude of the aircraft in order to maintain the engine at
the optimal temperature. Just like overheating, too much cooling can
be a bad thing for an engine as well. Some aircraft employ controls
that allow a pilot to manually adjust the airflow into the engine compartment.
Aircraft operate at higher
altitudes where the air is less dense than at ground level. As engines
need oxygen to burn fuel, a forced induction system such as turbocharger
or supercharger is especially appropriate for aircraft use. This does
bring along the usual drawbacks of additional cost, weight and complexity.
While some countries require twin-engine airplanes for commercial passenger
transport, many, such as Canada, Australia, and the United States, allow
the use of single-engine aircraft for some commercial services, including
charter and sometimes scheduled commuter airline flights.
A second engine adds redundancy
so that the aircraft can stay in the air (or at least, descend more
slowly) if one engine fails, providing an important safety margin during
cruise flight over water or mountainous terrain; however, an engine
failure on a twin-engine piston aircraft can also cause serious handling
difficulties, especially right after takeoff, due to asymmetrical thrust.
A study of accidents in Australian air charter operations from 1986
to 1996 found that the overall fatal accident rate per hour for multi-engine
aircraft was more than triple that for single-engine aircraft, though
it did not isolate the accidents specifically caused by engine failure
and the multi-engine aircraft did not fly under identical conditions.[3]
According to the U.S. Air Safety Foundation, when an engine failure
leads to an incident (e.g. some damage or injuries), it has a 10% chance
of causing fatalities in a single-engine aircraft, but a 50% chance
in a twin.
At one time all engine
designs were new and there was no particular difference in design between
aircraft and automobile engines. This changed by the start of World
War I, however, when a particular class of air-cooled rotary engines
became popular. These had a short lifespan, but by the 1920s a large
number of engine designs were moving to the similar radial engine design.
This combined air-cooled simplicity with large displacements and they
were among the most powerful small engines in the world.
Both the rotary and radial
engine have the drawback of a very large frontal area (see drag equation).
As aircraft increased in speed and demanded better streamlining, designers
turned to water-cooled inline engines. Throughout WWII the two designs
were generally similar in terms of power and overall performance but
some mature-design radials tended to be more reliable. After the war,
in the USA, the water-cooled designs rapidly disappeared.
|
