Irans fighter/bomber jet Qaher-313. Skräp, medelmåtta eller värsting?

The other design has the engines embedded in the tail, where they ingest the low-energy boundary layer flow over the fuselage and reenergize the wake to reduce drag.

Embedded, aircraft boundary layer ingesting, distributed propulsion system: Ingestion of fuselage boundary layer air into the engine allows a potential reduced fuel burn. There is a trade between this gain and the losses due to the increased duct length needed for noise attenuation. Embedding the engines within the airframe implies a high degree of airframe and engine integration. Engine airflow is now affected by the airframe and, similarly, the presence of the engines alters the flow around the aircraft. There are several major challenges, associated with the non-uniform flow into the engine (distortion), which must be addressed for a practical aircraft configuration.

a 30% increase in fuel economy

Aerospace engineers believe they can reduce fuel burn by embedding an aircraft’s engines into these surfaces and ingesting the boundary layer air flow to propel the aircraft through its mission.

It sounds like a simple design change, but it’s actually quite challenging. Boundary layer air flow is highly distorted, and that distortion affects the way the fan performs and operates. These new designs require a stronger fan.