The T53 Series Engines are designed to avoid conditions that allow a surge to develop, but the possibility of a surge still exists in engines that are improperly adjusted or have been abused.
Engine surge is usually brought to the pilot’s attention by its characteristic loud popping noise, accompanied by wildly fluctuating engine instruments. Repeated surging from the engine can cause damage to the airframe components and must be avoided. Surging indicates that engine operation is not normal. An investigation should be made to determine the cause and steps taken to correct the condition.
Engine surge is caused by a stall on the airfoil surfaces of the rotating blades or stationary vanes of the compressor. The stall can occur on individual blades or vanes, or simultaneously, on groups of them. To understand how this can induce engine surge, it is first necessary to examine the cause and effect of stall on any airfoil surface.
Airfoils are designed to provide lift by producing a lower pressure on the convex (suction) side on the airfoil than on the concave (pressure) side. A characteristic of any airfoil is that lift increases with an increasing angle of attack, but only up to some critical angle. Beyond this critical angle of attack, lift falls off rapidly. This is due largely to the separation of the airflow from the suction surface of the airfoil. This phenomenon is known as a stall. All pilots are familiar with this condition and its consequences as it applies to fixed-wing aircraft. The stall that takes place on the fixed or rotating airfoil sections of a compressor is similar to the stalling phenomenon of a fixed-wing aircraft.
It may be surprising to learn that the angle of attack of compressor blades and vanes can change, even though they are firmly fixed in the compressor rotor disc or stator shrouds, but such is the case. The change in angle of attack takes place when the relationship between the velocity of the air moving through the compressor and the rotational speed of the rotor is changed. To understand this relationship more clearly, it is customary to represent the air velocity and rotational speed by arrows (vectors) whose length is proportional to the magnitude of the air velocity or rotational speed.