Fighter Intakes: The Diverterless Supersonic Intake / Inlet
Most of the modern combat and civilian fixed wing aircrafts are powered by Turbofan or Turbojet engines. The design and performance of intakes dictate the overall performance of the air - breathing engine. The design of such air intakes is governed by the physics of the flow and in this post, we seek to delve deep into the supersonic regime with particular interest for the peculiar case of Diverterless Supersonic Inlets (DSI).
What exactly is the role of an Intake/Inlet at supersonic speeds?
The function of the air intake is mainly to prevent boundary layer ingestion and pressure recovery across various angles of attack. They accomplish this feat by separating the turbulent boundary layer of the fuselage from the air entering the compressor via different means and they control the location of shock waves formed in them.
The intakes are generally equipped with mobile cones (SR-71 Black Bird), Ramps (Concorde, F-14 and F-15) and Edges (F-106) to generate a weaker oblique shock and control its position which is essential, as combustion in gas turbines can only take place if the flow is subsonic. For aircrafts incorporating stealth, intakes play an significant role in reducing the frontal RCS (Radar Cross Section), and thus their detection.
But, why DSI?
The DSI is an impressive overall improvement. It’s origin dates back to the 1950s at NASA Langley by an Italian scientist Antonio Ferri. Their use was limited due to lack of CFD capability and computing power.
DSI’s are simple and thus cheaper to manufacture and much lighter as they have no moving parts and are effective in hiding the compressor blades, which are a very strong source of radar return, of the engine from the enemy radar thus, decreasing the RCS of the aircraft.
Due to their versatility, lower drag and better stealth profile than any other type of intake/inlet they have featured on various fifth generation platforms like the F-35 JSF, the Chinese J-20, J-31 and are expected to feature in futuristic combat platforms like the HAL AMCA (5th Gen), HAL AMCA Mk2(5+ Gen) and the yet to be named French — German 6th - Gen fighter aircraft.
Why the ugly bump?
The frontal surface of the smooth elongated “Bump” acts as a compression surface that is designed to generate an oblique and isentropic shock at supersonic speeds that forces the boundary around and away. It is to be noted that the generation of this shock is important as the flow past it is subsonic which is ideal for combustion.
The bump is accompanied by a slightly forward swept cowl to allow efficient bleeding away of the diverted air. The cowl and bump together divert the boundary layer away from the engine without the use of special ‘diverters’ such as splitter plates (Eurofighter Typhoon) and hence the name.
Are they all good?
Well not really, because the DSI has no moving parts and they provide most efficient pressure recovery at a particular Mach numbers, usually between Mach 1-2. The efficiency decreases with both positive and negative deviation from the sweet spot. Therefore, an intake with a mobile device such as a cone or a ramp can be efficient at larger range of Mach as they can mechanically move the mobile devices to control the position of the shock wave. Since modern fighters are not limited by speed/ thrust and rather emphasize on low observability, it is not surprising to see their application in a variety of combat platforms ranging from the Chinese-Pakistani JF-17 to the American F-35 Joint Strike Fighter.
A comparison of an F-16 Block 30 having a Normal Shock Intake and Lockheed Martin modified DSI testbed F-16 Block 30 which first flew on 11 December 1996. The experiment resulted in improved subsonic specific excess power and handling characteristics similar to the Block 30. The flight was demonstrated at Mach 2 which happens to be the maximum certified speed for the F-16.
