Critical engine
Encyclopedia
The critical engine of a multi-engine, fixed-wing aircraft
is the one whose failure would result in the most adverse effects on the aircraft's handling and performance.
imbalance exists between the operative and inoperative sides of the aircraft. This thrust imbalance causes several negative effects in addition to the loss of one engine's thrust. For reasons listed below, the left engine of a conventional twin-engine propeller-driven aircraft is typically considered critical.
will be developed which depends on the lateral distance from the center of gravity
(C.G.) to the thrust vector of the operating engine multiplied by the thrust of the operating engine. The torque effect attempts to yaw
the aircraft's nose towards the inoperative engine, a yaw tendency which must be counteracted by the pilot's use of the flight controls. Due to P-factor
, the right-hand engine typically develops its resultant thrust vector at a greater lateral distance from the aircraft's C.G. than the left-hand engine. The failure of the left-hand engine will result in a larger yaw effect via the operating right-hand engine, rather than vice-versa. Since the operating right-hand engine produces a stronger yaw moment, the pilot will need to use larger control deflections in order to maintain aircraft control. Thus, the failure of the left-hand engine is less desirable than failure of the right-hand engine, and the left-hand engine is critical.
It is important to note, however, that this example depends upon both propeller
s turning clockwise as viewed from the rear. On aircraft with counterclockwise-turning engines (such as the de Havilland Dove
), the right engine would be critical.
Most aircraft which have counter-rotating propellers
do not have a critical engine defined by above mechanism, however some such the Lockheed P-38 Lightning had both engines critical because the propeller rotation was purposely reversed for better gunnery.
Aircraft with center-line thrust propeller configurations (e.g. the Cessna 337) may still have a critical engine, if failure of one engine (either the front or rear) has more of a negative effect on aircraft control or climb performance than failure of the other engine.
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...
is the one whose failure would result in the most adverse effects on the aircraft's handling and performance.
Description
When one of the engines on a typical multi-engine aircraft becomes inoperative, a thrustThrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
imbalance exists between the operative and inoperative sides of the aircraft. This thrust imbalance causes several negative effects in addition to the loss of one engine's thrust. For reasons listed below, the left engine of a conventional twin-engine propeller-driven aircraft is typically considered critical.
Asymmetrical yaw
When one engine becomes inoperative, a torqueTorque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
will be developed which depends on the lateral distance from the center of gravity
Center of gravity
In physics, a center of gravity of a material body is a point that may be used for a summary description of gravitational interactions. In a uniform gravitational field, the center of mass serves as the center of gravity...
(C.G.) to the thrust vector of the operating engine multiplied by the thrust of the operating engine. The torque effect attempts to yaw
Flight dynamics
Flight dynamics is the science of air vehicle orientation and control in three dimensions. The three critical flight dynamics parameters are the angles of rotation in three dimensions about the vehicle's center of mass, known as pitch, roll and yaw .Aerospace engineers develop control systems for...
the aircraft's nose towards the inoperative engine, a yaw tendency which must be counteracted by the pilot's use of the flight controls. Due to P-factor
P-factor
P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller, that is responsible for asymmetrical relocation of the propeller's center of thrust when aircraft is at a high angle of attack.- Causes :When an aircraft is in...
, the right-hand engine typically develops its resultant thrust vector at a greater lateral distance from the aircraft's C.G. than the left-hand engine. The failure of the left-hand engine will result in a larger yaw effect via the operating right-hand engine, rather than vice-versa. Since the operating right-hand engine produces a stronger yaw moment, the pilot will need to use larger control deflections in order to maintain aircraft control. Thus, the failure of the left-hand engine is less desirable than failure of the right-hand engine, and the left-hand engine is critical.
It is important to note, however, that this example depends upon both propeller
Propeller (aircraft)
Aircraft propellers or airscrews convert rotary motion from piston engines or turboprops to provide propulsive force. They may be fixed or variable pitch. Early aircraft propellers were carved by hand from solid or laminated wood with later propellers being constructed from metal...
s turning clockwise as viewed from the rear. On aircraft with counterclockwise-turning engines (such as the de Havilland Dove
De Havilland Dove
The de Havilland DH.104 Dove was a British monoplane short-haul airliner from de Havilland, the successor to the biplane de Havilland Dragon Rapide and was one of Britain's most successful post-war civil designs...
), the right engine would be critical.
Most aircraft which have counter-rotating propellers
Counter-rotating propellers
Counter-rotating propellers, found on twin- and multi-engine propeller-driven aircraft, spin in directions opposite one another.The propellers on both engines of most conventional twin-engined aircraft spin clockwise . Counter-rotating propellers generally spin clockwise on the left engine and...
do not have a critical engine defined by above mechanism, however some such the Lockheed P-38 Lightning had both engines critical because the propeller rotation was purposely reversed for better gunnery.
Aircraft with center-line thrust propeller configurations (e.g. the Cessna 337) may still have a critical engine, if failure of one engine (either the front or rear) has more of a negative effect on aircraft control or climb performance than failure of the other engine.