Intake ramp
Encyclopedia
An intake ramp is a rectangular, plate-like device within the air intake of a jet engine
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...

, designed to generate a shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

to aid the inlet compression process at supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...

speeds. The ramp sits at an acute angle to deflect the intake air from the longitudinal direction. At supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...

flight speeds, the deflection of the air stream creates an oblique shock wave at the forward end of the ramp. Air crossing the shock wave suddenly slows to a lower Mach number
Mach number
Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...

, thus increasing pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

. However, shock waves are non-isentropic, so there is a reduction in total, or stagnation pressure
Stagnation pressure
In fluid dynamics, stagnation pressure is the static pressure at a stagnation point in a fluid flow.At a stagnation point the fluid velocity is zero and all kinetic energy has been converted into pressure energy . Stagnation pressure is equal to the sum of the free-stream dynamic pressure and...

relative to that of the free air stream. Although there are reflected oblique shock waves and a weak normal shock wave generated downstream, this form of intake is more efficient (i.e. higher total pressure recovery) than the equivalent pitot intake, particularly at high supersonic flight speeds.

Ideally, the oblique shock wave should intercept the air intake lip, thus avoiding air spillage and pre-entry drag on the outer boundary of the deflected streamtube. For a fixed geometry intake at zero incidence, this condition can only be achieved at one particular flight Mach number, because the angle of the shock wave (to the longitudinal direction) becomes more acute with increasing aircraft speed.

Some supersonic intakes feature two or more ramps, operating in series, to generate multiple oblique shock waves. Each downstream ramp is steeper in inclination than the previous ramp. These intakes are usually more efficient than single (oblique) shock wave inlets.
For a fixed geometry it is feasible to use curved intakes without any shocks before the final normal shock.
Modern fighter jets (like the F-22 Raptor
F-22 Raptor
The Lockheed Martin/Boeing F-22 Raptor is a single-seat, twin-engine fifth-generation supermaneuverable fighter aircraft that uses stealth technology. It was designed primarily as an air superiority fighter, but has additional capabilities that include ground attack, electronic warfare, and signals...

) have engines so powerful that top speed is not limited by them,
thus spillage at the intake is acceptable and a lightweight fixed geometry chosen. The nacelle has a round outer lip to defect spillage back into the longitudinal direction. The nacelle thus gets a very large cross section with more volume than needed for the engine
and in the F-22 the nacelles are combined with the fuselage to create a large weapon bay, place for fuel, and a wide body, which generates some lift.
Modern jets have a horizontal ramp thus spillage at least produces some lift.
In case of the F-22 the inlet has two ramps placed in a diamond shape, thus for most Mach- and thrust-conditions spillage occurs only at the aft / lower corner of that diamond.
In case of the F-22 the leading edges of the wing become supersonic at Mach 1.8 and certainly are an as large source of wave drag as the inlet.

Variable geometry intakes, such as those on Concorde
Concorde
Aérospatiale-BAC Concorde was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...

aircraft, vary the ramp angle to focus the oblique shock wave/s onto the intake lip.