Critical engine

The driving criticizes of a multi-engine Avion, is that of which the breakdown would cause the effects more penalizing for piloting of the plane and its performances.

Description

On a plane been driven by at least two engines (Bimoteur), when one two becomes inoperative, it occurs an imbalance of the Poussée exerted between the sides of the apparatus. This imbalance has negative effects in addition to the loss of pushed broken down engine. As it hereafter is explained, the left engine of a twin-engine with Hélice S traditional (i.e. whose two propellers turn in time direction) is generally regarded as being the critical engine.

Factors determining the criticality of the engine

Asymmetrical yawing moment

When an engine breaks down, a force (couple) appears whose importance is function of the side distance separating the Center of gravity (CdG) of the vector of pushed engine under operation, multiplied by the push of this last. The effect of couple (lace) tends to make turn the plane on the side of the broken down engine, tendency which the pilot must counter with the means of the orders of flight. Under the influence of the Factor P, the right engine develops a vector of thorough from which the resultant is further away from CdG than the left engine. The breakdown of the left engine will cause a more important yawing moment on the level of the right engine under operation than in the opposite case. So the pilot will have to exert a greater effort on (swing bar) maintaining the course desired. The breakdown of the left engine will be thus more prejudicial than that of the right engine and this one will be consequently regarded as criticism.

It is important to recall that this example applies to the engines whose propeller turns in time direction (direction of the needles of a watch). Those equipped with contrarotating propellers (as A400M) do not have a critical engine. On the planes whose propellers turn in anti-clockwise direction (opposite of the needles of a watch) the critical engine is the right engine.

Asymmetrical bearing pressure

The Portance results the various rates of flow of the air on each side of the wing (see also Intrados and Extrados). On the planes whose engines are assembled in front of the wing, the flow generated by the rotation of the propeller accelerates the flow on the portion of the wing located downstream, behind the engine. This generates consequently a greater bearing pressure in this zone. Because of effect of the Factor P, the point of maximum bearing pressure of the right wing will be further away from CdG than that of the left wing. The breakdown of any driving will cause a couple of positive Roulis in direction on the side of the broken down engine but the breakdown of the left engine will be more penalizing. There too, the left engine is regarded as criticism (always for an engine whose propellers turn in time direction)

Criticality other than aerodynamic

On certain apparatuses, the hydraulic, pneumatic or electric systems are pulled by one of the two engines. The breakdown of this engine would be critical as for the operation of these systems.

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