The problems experienced by transonic intakes will also include all those encountered by subsonic intakes. Aircraft that fly up to Mach 1.2 is normally equipped with pitot type intakes and operate with a normal shock wave positioned just ahead of the intake lip.
The lip of the intake made sharper to reduce the velocity of air flowing over the outer surface of the intake, thus reducing nacelle drag.
Problems can occur when the shock wave moves into the mouth of the intake in response to an increase in engine mass airflow requirements.
CRITICAL OPERATION IN TRANSONIC INTAKE
This is where the normal shock wave positioned across the mouth of the intake and the mass of air entering the intake matches the engine’s requirement.
Diffusion of the air occurs in two steps:
1. The velocity reduces to below Mach 1 through the normal shock and pressure increases.
2. Subsonic diffusion then occurs inside the intake reducing the air velocity to about Mach 0.5 at compressor entry and raising its pressure.
This occurs when the mass airflow requirement of the engine is less than the air delivered by the intake, for example, if the engine throttled back.
The normal shock wave pushed forwards by air spilling out of the intake.
This spillage increases nacelle drag which, if the intention of throttling back was to slow the aircraft, can be of use.
Diffusion now occurs in three steps:
1. Across the shock wave.
2. In front of the intake.
3. Then inside the intake.
This occurs where the engine mass airflow requirement exceeds the flow available. The pressure reduces inside the intake and draws the normal shock into the duct. The first section of the intake in front of the shock has Mach 1 flow in it.
Mach 1 flow will accelerate in a divergent duct and reduce the air pressure. Effectively, part of the intake length is now lost in a pointless expansion of the air before it reaches the shockwave. Diffusion occurs in two steps, through the shock and then normal subsonic diffusion up to compressor entry. This type of operation can occur when the engine accelerated to high power.