TYPES OF AIRCRAFT STALL

For the types of aircraft stall explanation, the single spool compressor only is considered.

LOW SPEED STALL

As explained earlier, the low compressor pressure ratio at low engine speed causes the air to choke in the rear stator vane stages and reduces the air mass flow.

This increases the angle of attack (positive incidence) on the front stage rotors towards stall. Thus, low RPM stalling occurs in the front stages of a compressor.

HIGH SPEED STALL

At high rotational speeds all the compressor stages are operating at a similar axial flow velocity.

If for any reason a back pressure were to form, for example in the combustion chambers, then the axial velocity will reduce initially more at the rear of the compressor than at the front taking the rear stage rotors closer to a positive incidence stall.

A sudden reduction in pressure rear of the compressor outlet, such as a deceleration would cause a rise in axial velocity at the rear of the compressor reducing the rotor angles of attack.

In the extreme, these could become negative. Thus, high RPM stalling occurs in the rear stages of a compressor.

Some off design conditions can also cause negative incidence stalls where the rotor blade angles of attack become zero or less.

Occurrences of negative incidence stalls are usually confined to rear-stage, high RPM, stall conditions.

ACCELERATION STALL

Too rapid an increase in fuel flow causes a back pressure against the turbine which is transmitted back to the rear of the compressor.

This may choke the rear stages and cause an effect similar to a low-speed stall where the front rotor stages approach stall.

Stalling then occurs in the front stages. If the combustion chamber pressure rises too high the backpressure may move forward to choke progressively earlier compressor stages.

ROTATING STALL

This type of stall occurs when one or a small group of rotor blades in a stage stall.

The turbulent flow off one rotor disrupts the air flow of its neighbour which then stalls and passes the effect on to the next and so on.

rotating stalls on a compressor blade

As each blade passes the effect on it becomes un-stalled so a small stall cell moves around the rotor disc.

The cell moves in the opposite direction to the rotation of the disc.

Rotating stall cells can be transient and merely die out or, progress to cause the whole stage to enter a hung stall.

They are usually caused by pockets of disturbed air such as vortices entering the intake from an external source.

 

 

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