ISAE has proposed a slotted inlet concept as a potential solution for enhancing the operability margins of a UHBR turbofan engine equipped with an ultra-thin and ultra-short nacelle.

Such a nacelle has low inlet contraction ratio and low highlight to maximum diameter ratio. This results in lower flow acceleration on the external side of the nacelle, which leads to lower wave drag, and lower flow acceleration along the inlet, which mitigates the speed overshoot before the fan face and thus prevents efficiency losses.

At low speed and high incidence conditions, however, the reduced inlet bluntness leads to excessive flow accelerations and possible flow separation, which damage engine performance and can compromise the stability margins of the fan.

The opening of the slot creates a convergent channel that provides an artificial increase in the aerodynamic contraction ratio, modifying the flow structure in a way that helps prevent separation and over-accelerations at the fan face region.