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

Description

When one of the engines on a typical multi-engine aircraft becomes inoperative, a thrust imbalance exists between the operative and inoperative sides of the aircraft. Overview Fixed-wing aircraft range from small training and recreational aircraft to Wide-body aircraft and military cargo aircraft. Thrust is a reaction force described quantitatively by Newton 's Second and Third Laws. 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.

Factors affecting engine criticality

Asymmetrical yaw

When one engine becomes inoperative, a torque will be developed which depends on the lateral distance from the center of gravity (C. A torque (τ in Physics, also called a moment (of force is a pseudo- vector that measures the tendency of a force to rotate an object about 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. Flight dynamics is the science of air and space vehicle orientation and control in three dimensions Due to P-factor, the right-hand engine typically develops its resultant thrust vector at a greater lateral distance from the aircraft's C. P-factor, also known as asymmetric blade effect and asymmetric disc effect is an aerodynamic phenomenon experienced by a moving Propeller with a high Angle of 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 propellers turning clockwise as viewed from the rear. A propeller is essentially a type of fan which transmits power by converting Rotational motion into Thrust for propulsion of a vehicle such as an On aircraft (such as the Beechcraft Travel Air) with counterclockwise-turning engines, the right engine would be critical. WikipediaWikiProject Aircraft. Please see WikipediaWikiProject Aircraft/page content for recommended layout

Aircraft which have counter-rotating propellers rotating toward the cockpit on the top side (such as the Piper Seneca) do not have a critical engine, while both engines are critical on aircraft with counter-rotating propellers turning away from the cockpit. Counter-rotating propellers, are found on twin- and multi- Engine, propeller-driven Aircraft and have propellers that spin in opposite directions WikipediaWikiProject Aircraft. Please see WikipediaWikiProject Aircraft/page content for recommended layout The Lockheed P-38 was an example of the latter.

The operating right-hand engine will produce a more severe yaw towards the dead engine, thus making the failure of the left-hand engine critical

Accelerated Slipstream

Lift can be roughly defined as an upwards force resulting from an airstream going over and under a wing. In the context of a Fluid flow relative to a body the lift force is the component of the Aerodynamic force that is Perpendicular to the flow On aircraft with propellers mounted on the wing, the propwash from the engine will accelerate the airstream over the portion of the wing directly behind the propeller. This results in greater lift behind the propeller than at other spots on the wing. From the P-factor effect, the right wing's center of lift will be further from the C. G. than the left-hand wing. While failure of either engine will cause a rolling motion towards the inoperative side, the rolling motion will be more severe with the right engine operating. Flight dynamics is the science of air and space vehicle orientation and control in three dimensions Thus, the failure of the left-hand engine is critical. Again, this example depends on both engines turning clockwise when viewed from the rear.

Spiraling Slipstream

Air forced backwards from the propellers tends to spiral as it moves rearward.

Non-aerodynamic criticality

On certain aircraft, hydraulic, pneumatic or electrical systems may be powered by one engine. This engine would therefore be critical in this respect.

References

(2004) Airplane Flying Handbook. U. S. Government Printing Office, Washington D. C. : U. S. Federal Aviation Administration, pp. 12-27 to 12-28. FAA-8083-3A.