Inlet cone
Encyclopedia
Inlet cones are a component of some supersonic
aircraft. They are primarily used on ramjet
s, such as the turboramjets of the SR-71 or the pure ramjets of the D-21 Tagboard and Lockheed X-7
. Some conventional turbojet aircraft including the Su-7 Fitter and the MiG-21 Fishbed also use an inlet cone.
engines, all airbreathing jet engine
s need subsonic airflow to operate properly, and require a diffuser to prevent supersonic airflow inside the engine. At supersonic flight speeds a conical shock wave, sloping rearwards, forms at the apex of the cone. Air passing through the conical shock wave (and subsequent reflections) slows to a low supersonic speed. The air then passes through a strong normal shock wave, within the diffuser passage, and exits at a subsonic velocity. The resulting intake system is more efficient (in terms of pressure recovery) than the much simpler pitot intake.
appears, emanating from the cone apex. Conical (and oblique) shock waves are akin to the bow wave on a ship. As the flight Mach number increases, the conical shock wave becomes more oblique and eventually impinges on the intake lip.
With increasing mach number the cone is not moved out of the inlet as it would make sense for the external compression, but inwards, because the cone sits in the cowl like a plug. By moving it inwards the path between the cone and the inlet narrows as needed for higher speed. The compression occurring in this path is called "internal compression" (opposed to the "external compression" on the cone) and can be subject to stall.
Care must be taken to prevent the normal shock wave, which lies behind the throat of the diffuser, coming forward through the throat, replacing the oblique shocks and reducing stagnation pressure
and leading to excessive inlet temperatures burning the compressor
.
To restore the engine after a surge the computer has to change inlet, turbine and nozzle geometry, fuel injection and counteract the yaw on a two engine airplane, by means of control surfaces and throttling back the second engine.
The boundary layer
on the cone is stretched as it moves up the cone preventing flow separation
, but for the internal compression and the subsonic compression the boundary layer still tends to separate and usually is sucked through tiny holes
in the wall. As a side note on the aerospike engine
the boundary layer gets thicker towards the end of the cone as needed for the greater speed difference between the air molecules just on the surface of the cone and the fully accelerated stream of air.
Some aircraft (F-35 Lightning II
, Mirage III) use a semi-conic centrebody.
Concorde
, F-15 Eagle
, MiG-25 Foxbat
, and the A-5 Vigilante
use so-called 2D inlets, where the nacelle is rectangular and a flat inlet ramp replaces the dual cones just described. These allow good pressure recovery without spillover through the full range of speeds by matching their angle to the mach angle, but have problems with seal
s at high speed that is high pressure and temperature (like in a diesel engine
).
Inlet ramps allow for swept inlet cowls (F-22 Raptor
, F-35 Lightning II
) to avoid shocks.
They are sealed by viton
or a metal sheet bend in direction of the higher pressure.
At least one supersonic and one subsonic ramp is used, but for improved seal multiple supersonic ramps can be used.
The boundary layer (something which the subsonic pitote inlet avoids by external compression) tends to separate
and the smaller boundary layer of the ramp inlet is an advantage compared to the inlet cone.
To avoid separation vortex generator
s are used, which mix the boundary layer with the free flow (or the boundary layer is sucked away through a porous surface, leading to drag).
After the fan the hot slow intermixed air is passed by the engine, while the fast cold air is delivered to the engine.
After the engine the comparatively cold bypass air is used as an isolation between the engine exhaust and the walls. Again two ramps can be used to form a variable supersonic nozzle
. Often a mirror-symmetric set-up is used with ramps on top and on the bottom.
There is one possibility for a stable, shockless supersonic to subsonic transition.
This is used in transonic wing
s and would ultimately mean to send the air into a loop, forming a vortex. Then the final shock to subsonic speed is oblique with the subsonic region moving from the outside of the vortex to the inside.
Many supersonic aircraft (Eurofighter Typhoon
, F-16 Fighting Falcon
) dispense with the conical centrebody and employ a simple pitot
intake. A detached, strong normal shock appears directly in front of the inlet at supersonic flight speeds, which leads to poor pressure recovery.
Also NASA adds a gap through the whole compressor. Supersonic flow jumps over it by means of ramps, while subsonic flow is able to turn and exit through the gap. In this way a stall is easier to remove http://pda.physorg.com/lofi-news-inlet-inlets-parametric_158.html. Also there are plans to measure the air in front of the inlet to detect turbulence and adjust the inlet in real-time.
behind a turbine runs with a stoichiometric mixture
like a ramjet
but at higher pressure and thus more efficiency than a pure ramjet.
It is claimed that an inlet at Mach 3.5 produces the same compression (44:1 http://encyclopedia.kids.net.au/page/je/Jet_engine) as the whole compressor of a jet engine at zero speed, so the turbine should be bypassed then.
Sound barrier
The sound barrier, in aerodynamics, is the point at which an aircraft moves from transonic to supersonic speed. The term, which occasionally has other meanings, came into use during World War II, when a number of aircraft started to encounter the effects of compressibility, a collection of several...
aircraft. They are primarily used on ramjet
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
s, such as the turboramjets of the SR-71 or the pure ramjets of the D-21 Tagboard and Lockheed X-7
Lockheed X-7
-See also:-External links:*...
. Some conventional turbojet aircraft including the Su-7 Fitter and the MiG-21 Fishbed also use an inlet cone.
Purpose
The main purpose of an inlet cone is to slow the flow of air from supersonic flight speed to a subsonic speed before it enters the engine. Except for scramjetScramjet
A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow...
engines, all airbreathing jet engine
Airbreathing jet engine
An airbreathing jet engine is a jet engine propelled by a jet of hot exhaust gases formed from air that is drawn into the engine via an inlet duct....
s need subsonic airflow to operate properly, and require a diffuser to prevent supersonic airflow inside the engine. At supersonic flight speeds a conical shock wave, sloping rearwards, forms at the apex of the cone. Air passing through the conical shock wave (and subsequent reflections) slows to a low supersonic speed. The air then passes through a strong normal shock wave, within the diffuser passage, and exits at a subsonic velocity. The resulting intake system is more efficient (in terms of pressure recovery) than the much simpler pitot intake.
Shape
The inlet cone is shaped so that the shock wave that forms on its apex is directed to the lip of the intake; this allows the engine to operate properly in supersonic flight. As speed increases, the shock wave becomes increasingly more oblique (the cone gets narrower). As a result, some inlet cones (for example, on the SR-71) are designed to move axially to maintain the shock-on-lip and allow efficient operation over a wider range of flight speeds. On MiG-21 this movement also makes the opening between the intake (diffusor) lip and the cone smaller, preventing the inside of the intake section from bursting open from dynamic air overpressure at the aircraft's highest speeds above Mach 2.Operation
At subsonic flight speeds, the conical inlet operates much like pitot intake, it sucks air. However, as the vehicle goes supersonic a conical shock waveappears, emanating from the cone apex. Conical (and oblique) shock waves are akin to the bow wave on a ship. As the flight Mach number increases, the conical shock wave becomes more oblique and eventually impinges on the intake lip.
With increasing mach number the cone is not moved out of the inlet as it would make sense for the external compression, but inwards, because the cone sits in the cowl like a plug. By moving it inwards the path between the cone and the inlet narrows as needed for higher speed. The compression occurring in this path is called "internal compression" (opposed to the "external compression" on the cone) and can be subject to stall.
Care must be taken to prevent the normal shock wave, which lies behind the throat of the diffuser, coming forward through the throat, replacing the oblique shocks and reducing 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...
and leading to excessive inlet temperatures burning the compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
.
- The cone is moved in and out of the pitot intake controlled by a computer with temperature and pressure sensors in the intake to match the throat to the mach number.
- With a ramjet, this occurs if excessive fuel is injected into the combustor, raising internal pressure too far. However, with a turbojet or turbofan, the problem arises when the engine is throttled back, causing a mismatch between intake airflow and engine mass flow. A trapdoor is needed to dump excess flow overboard. Or a variable geometry for the stators of the fanTurbofanThe turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...
is used.
To restore the engine after a surge the computer has to change inlet, turbine and nozzle geometry, fuel injection and counteract the yaw on a two engine airplane, by means of control surfaces and throttling back the second engine.
The boundary layer
Boundary layer
In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail. In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal...
on the cone is stretched as it moves up the cone preventing flow separation
Flow separation
All solid objects travelling through a fluid acquire a boundary layer of fluid around them where viscous forces occur in the layer of fluid close to the solid surface. Boundary layers can be either laminar or turbulent...
, but for the internal compression and the subsonic compression the boundary layer still tends to separate and usually is sucked through tiny holes
Boundary layer suction
Boundary layer suction is technique in which an air pump is used to extract the boundary layer at the wing or the inlet of an aircraft. Improving the air flow can reduce drag. Improvements in fuel efficiency have been estimated as high as 30%....
in the wall. As a side note on the aerospike engine
Aerospike engine
The aerospike engine is a type of rocket engine that maintains its aerodynamic efficiency across a wide range of altitudes through the use of an aerospike nozzle. It is a member of the class of altitude compensating nozzle engines. A vehicle with an aerospike engine uses 25–30% less fuel at low...
the boundary layer gets thicker towards the end of the cone as needed for the greater speed difference between the air molecules just on the surface of the cone and the fully accelerated stream of air.
Alternative Shapes
Some air inlets feature a biconic centrebody to form two conic shock waves, both focused on the lip of the intake. This improves pressure recovery.Some aircraft (F-35 Lightning II
F-35 Lightning II
The Lockheed Martin F-35 Lightning II is a family of single-seat, single-engine, fifth generation multirole fighters under development to perform ground attack, reconnaissance, and air defense missions with stealth capability...
, Mirage III) use a semi-conic centrebody.
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...
, F-15 Eagle
F-15 Eagle
The McDonnell Douglas F-15 Eagle is a twin-engine, all-weather tactical fighter designed by McDonnell Douglas to gain and maintain air superiority in aerial combat. It is considered among the most successful modern fighters with over 100 aerial combat victories with no losses in dogfights...
, MiG-25 Foxbat
Mikoyan-Gurevich MiG-25
The Mikoyan-Gurevich MiG-25 is a supersonic interceptor and reconnaissance aircraft that was among the fastest military aircraft to enter service. Designed by the Soviet Union's Mikoyan-Gurevich bureau the first prototype flew in 1964 with entry into service in 1970...
, and the A-5 Vigilante
A-5 Vigilante
The North American A-5 Vigilante was a carrier-based supersonic bomber designed for the United States Navy. Its service in the nuclear strike role to replace the A-3 Skywarrior was very short; however, as the RA-5C, it saw extensive service during the Vietnam War in the tactical strike...
use so-called 2D inlets, where the nacelle is rectangular and a flat inlet ramp replaces the dual cones just described. These allow good pressure recovery without spillover through the full range of speeds by matching their angle to the mach angle, but have problems with seal
Seal (mechanical)
A mechanical seal is a device which helps join systems or mechanisms together by preventing leakage , containing pressure, or excluding contamination...
s at high speed that is high pressure and temperature (like in a diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
).
Inlet ramps allow for swept inlet cowls (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...
, F-35 Lightning II
F-35 Lightning II
The Lockheed Martin F-35 Lightning II is a family of single-seat, single-engine, fifth generation multirole fighters under development to perform ground attack, reconnaissance, and air defense missions with stealth capability...
) to avoid shocks.
They are sealed by viton
Viton
Viton is a brand of synthetic rubber and fluoropolymer elastomer commonly used in O-rings and other molded or extruded goods. The name is a registered trademark of DuPont Performance Elastomers L.L.C.....
or a metal sheet bend in direction of the higher pressure.
At least one supersonic and one subsonic ramp is used, but for improved seal multiple supersonic ramps can be used.
The boundary layer (something which the subsonic pitote inlet avoids by external compression) tends to separate
and the smaller boundary layer of the ramp inlet is an advantage compared to the inlet cone.
To avoid separation vortex generator
Vortex generator
A vortex generator is an aerodynamic surface, consisting of a small vane or bump that creates a vortex. Vortex generators can be found on many devices, but the term is most often used in aircraft design....
s are used, which mix the boundary layer with the free flow (or the boundary layer is sucked away through a porous surface, leading to drag).
After the fan the hot slow intermixed air is passed by the engine, while the fast cold air is delivered to the engine.
After the engine the comparatively cold bypass air is used as an isolation between the engine exhaust and the walls. Again two ramps can be used to form a variable supersonic nozzle
De Laval nozzle
A de Laval nozzle is a tube that is pinched in the middle, making a carefully balanced, asymmetric hourglass-shape...
. Often a mirror-symmetric set-up is used with ramps on top and on the bottom.
There is one possibility for a stable, shockless supersonic to subsonic transition.
This is used in transonic wing
Swept wing
A swept wing is a wing planform favored for high subsonic jet speeds first investigated by Germany during the Second World War. Since the introduction of the MiG-15 and North American F-86 which demonstrated a decisive superiority over the slower first generation of straight-wing jet fighters...
s and would ultimately mean to send the air into a loop, forming a vortex. Then the final shock to subsonic speed is oblique with the subsonic region moving from the outside of the vortex to the inside.
Many supersonic aircraft (Eurofighter Typhoon
Eurofighter Typhoon
The Eurofighter Typhoon is a twin-engine, canard-delta wing, multirole combat aircraft, designed and built by a consortium of three companies: EADS, Alenia Aeronautica and BAE Systems; working through a holding company, Eurofighter GmbH, which was formed in 1986...
, F-16 Fighting Falcon
F-16 Fighting Falcon
The General Dynamics F-16 Fighting Falcon is a multirole jet fighter aircraft originally developed by General Dynamics for the United States Air Force . Designed as an air superiority day fighter, it evolved into a successful all-weather multirole aircraft. Over 4,400 aircraft have been built since...
) dispense with the conical centrebody and employ a simple pitot
Pitot tube
A pitot tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot Ulo in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy...
intake. A detached, strong normal shock appears directly in front of the inlet at supersonic flight speeds, which leads to poor pressure recovery.
Also NASA adds a gap through the whole compressor. Supersonic flow jumps over it by means of ramps, while subsonic flow is able to turn and exit through the gap. In this way a stall is easier to remove http://pda.physorg.com/lofi-news-inlet-inlets-parametric_158.html. Also there are plans to measure the air in front of the inlet to detect turbulence and adjust the inlet in real-time.
Ramjet
As the compression of the inlet raises with speed the compression of the first compressor stage is reduced accordingly. The afterburnerAfterBurner
The AfterBurner is a lighting solution for the Game Boy Advance system that was created by Triton-Labs.Originally, portablemonopoly.net was a website created to petition Nintendo to put some kind of light in their Game Boy Advance system...
behind a turbine runs with a stoichiometric mixture
Air-fuel ratio
Air–fuel ratio is the mass ratio of air to fuel present in an internal combustion engine. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich...
like a ramjet
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
but at higher pressure and thus more efficiency than a pure ramjet.
It is claimed that an inlet at Mach 3.5 produces the same compression (44:1 http://encyclopedia.kids.net.au/page/je/Jet_engine) as the whole compressor of a jet engine at zero speed, so the turbine should be bypassed then.