SDF Aerospace and Aerodynamics Corner

Re: J-20... The New Generation Fighter III

MiG-29 said:

i see right away you do not know how an inlet woks because you lack understanding of what is supercritical states, is not the geometry of the inlet, because intake cones are 3D, but the intake size is to capture an specific amount of air, a bump is a body that generates multishocks of very small strength, however on a DSI as the one of F-35, JF-17and J-10B i am right, their fixed geometry means the higher the speed lower the pressure recovery as a result of a fixed area capture and unability to create the 4 shocks a 2d intake with ramps and by pass slot.

Having subcritical states means spillage and lower pressure recovery, in few words subcritical states mean the oblique shocks move forward and do not enpinge any more on the intake cowl lip, supercritical means the normal shocks go further inside the intake duct and decrease pressure recovery due to a stronger normal shock

Is not the DSI intake by it self but the turbine it self, that with low pressure recovery fails, increases burn fuel.

However the DHI seems to increase the ability of the engine to achieve higher pressure recovery or at least increase the ability to start the RAMJET at speeds of Mach 2.5-3, this does not mean the pressure recovery is the ideal for a fighter, but simply they have created a better RAM intake that allows starting the engine before the turbine fails or fuel runs out

Click to expand...

[/QUOTE]
1)Even variable inlets experience a drop in pressure recovery at higher mach. They also have a fixed capture area
2) You can't calculate how strong the shocks are just from looking at the bump.
3) There are always other features that are sources of shock and compression.
4) It's not about the use of the bump but the calibration of sub and super critical states. In other words, the optimization of air flow. You can have a bump geometry that is optimized for acceptable pressure recovery at a higher mach. There is no fighter that does not have reduced pressure recovery as they break mach 2.



Inlet cones are also not complex 3D shapes. They're interaction with the air flow is that of a 2D manifold.

Equation said:

I thought if a plane goes higher in altitude the air became thinner and temperature drops (colder), therefore less intake of air flow? (Just curious)

Click to expand...

aircraft have have auxiliary intakes to increase mass flow or bypas doors to get rid of unwanted mass flow, as the aircraft increases speed will require different capture area, the F-14 at slow speed has a very large capture area but at high speeds the area reduces and the bypass slot opens wider, F-111 was the same.

Re: J-20... The New Generation Fighter III

latenlazy said:

1)Even variable inlets experience a drop in pressure recovery at higher mach. They also have a fixed capture area
2) You can't calculate how strong the shocks are just from looking at the bump.
3) There are always other features that are sources of shock and compression.
4) It's not about the use of the bump but the calibration of sub and super critical states. In other words, the optimization of air flow. You can have a bump geometry that is optimized for acceptable pressure recovery at a higher mach. There is no fighter that does not have reduced pressure recovery as they break mach 2.



Inlet cones are also not complex 3D shapes. They're interaction with the air flow is that of a 2D manifold.

Click to expand...

this show you do not even know intake geometry, cones are 3D bodies, and they can have 2 slopes, F-111 when it expands has two angle slopes on the semicone
by expanding or collapsing it keeps the critical state and the oblique and normal shocks impinged on the intake lips cowls, without it they won`t impinge on the cowl lips and will create subcritical or supercritical states reducing pressure recovery

[video=youtube;_OBEQ_N49Ec]http://www.youtube.com/watch?v=_OBEQ_N49Ec&feature=relmfu[/video]

Re: J-20... The New Generation Fighter III

MiG-29 said:

this show you do not even know intake geometry, cones are 3D bodies, and they can have 2 slopes, F-111 when it expands has two angle slopes on the semicone
by expanding or collapsing it keeps the critical state and the oblique and normal shocks impinged on the intake lips cowls, without it they won`t impinge on the cowl lips and will create subcritical or supercritical states reducing pressure recovery

[video=youtube;_OBEQ_N49Ec]http://www.youtube.com/watch?v=_OBEQ_N49Ec&feature=relmfu[/video]

Click to expand...

Have you never heard of a manifold?

Also, keyword was complex 3D

Re: J-20... The New Generation Fighter III

latenlazy said:

Have you never heard of a manifold?

Also, keyword was complex 3D

Click to expand...

Look a F-111 or SR-71 are 3D intakes, you are there wrong, now the DSI bump is a multishock 3D object, that is true, the difference between the Cone of a SR-71 and the Bump of F-35 is the bump is a multishock generator due to its curvature.


Supersonic flow over a wedge surface has an attached, straight oblique shock wave from thenose. The flow downstream of the shock is uniform and parallel to the surface with a surfacepressure equal to the static pressure behind the shock, p2w. Note that the wedge flow is two-dimensional. The shock is a function of the free stream Mach number




A supersonic flow over a cone will also have an attached, straight oblique shock wave from thenose but since the cone is a three-dimensional body, the flow has a relieving effect. Aconsequence of this is that for the same body angle δ, and the same Mach number, the shock onthe cone is weaker than the shock for the wedge, it will have a smaller shock angle β


The problem on a F-111 is to control the mass flow to impinge the shock waves on the intake cowl lips and avoid supercritical or subcritical states.
This is done by moving the cone forward and opening the intake bypass doors


The same applies tp SR-71 that uses a mixed compression air intake with a translating cone and bypass doors.





BLEED BYPASS TURBOJET ENGINES LOCATED MIDSPAN ON
THE WING, EACH DEVELOPING OVER 32,000+ POUNDS AT SEA
LEVEL, STATIC CONDITIONS
– FOR COMPARISON, THE ENGINE PRODUCES HORSEPOWER EQUAL
TO THAT FOR THE QUEEN MARY
– THE PROPULSION SYSTEM CONSISTES OF THE INLET, FOLLOWED
BY THE J58 ENGINE, AND EXITS THROUGH AN AERODYNAMIC
CONVERGENT-DIVERGENT EJECTOR NOZZLE
• INLET HAS A TRANSLATING 26-DEGREE CONE WHICH ACTS AS THE
INITIAL DECELERATION OR COMPRESSION SURFACE PRODUCING A
SERIES OF SHOCK WAVES UP TO THE INLET THROAT
• THE SHOCK TRAIN ENDS WITH A FINAL TREMINAL OR NORMAL SHOCK
FOLLOWED BY A SUBSONIC DIFFUSER
• THE PURPOSE OF THE INLET IS TO SUPPLY THE AIR REQUIRED BY THE
ENGINE AT THE HIGHEST PRESSURE RECOVERY AND THE LOWEST
DRAG, WITH A MINIMUM OF DISTORTION. THIS IS NOT AS SIMPLE AS IT
SOUNDS! A TURBOJET ENGINE IS A CONSTANT VOLUME MACHINE. THIS
MEANS THAT REGARDLESS OF FLIGHT SPEED – FROM M = 0 TO CRUISE
SPEED – THE SPEED OR MACH NUMBER ENTERING THE ENGINE IS
RELATIVELY CONSTANT, BETWEEN M = 0.3 AND 0.5

THE SR-71 ENGINE AIR INLET IS A MIXED EXTERNAL AND INTERNAL
COMPRESSION, AXI-SYMMETRIC INLET, WITH GRADUAL ISENTROPIC
COMPRESSION APPROACHING THE THROAT
• THE BOUNDARY LAYER WHICH BUILDS UP ON THE CENTERBODY IS
REMOVED AHEAD OF THETERMINAL SHOCK THROUGH A POROUS
BLEED
• THIS BLEED PASSES OVERBOARD THROUGH LOUVERS AT THE ENDS
OF THE CENTERBODY SUPPORT STRUTS
• FORWARD BYPASS DOORS, LOCATED CLOSE TO THE THROAT MATCH
THE INLET TO THE ENGINE, BY-PASSING AIR OVERBOARD
• THE COWL BLEED IS A “SHOCK TRAP” RAM BLEED WHICH SUPPLIES AIR
THROUGH TUBES ACROSS THE BY-PASS DOOR REGION INTO THE
ENGINE SECONDARY PASSAGE, WHERE IT IS USED FOR COOLING, AND
FED THROUGH TO THE ENGINE EJECTOR
• AFT BY-PASS FLOW JOINS THE COWL BLEED FLOW AND PASSES
THROUGH THE ENGINE EJECTOR
• THIS BY-PASS WAS INSTALLED TO PROVIDE SUFFICIENT FLOW TO
PERMIT ENGINE OPERATION AT IDLE
• DURING ACCELERATION, AFT BY-PASS IS USED TO REDUCE THE
FORWARD OVERBOARD BY-PASS WITH ITS ATTENDANT DRAG



]• THE TERMINAL SHOCK POSITION IS CONTROLLED AND
REGULATED FOR FORWARD AND AFT BYPASS DOORS WHICH
MATCHES THE INLET AIR SWALLOWING CAPABILITY TO THE
ENGINE REQUIREMENT

• THE COMPRESSION RATIO AT CRUISE IS 40 TO 1
• AT CRUISE, EACH INLET SWALLOWS APPROXIMATELY 100,000
CUBIC FEET OF AIR PER SECOND – EQUIVALENT TO TWO

• FORWARD BYPASS DOORS ARE OPEN WITH THE GEAR DOWN,
CLOSE WHEN LANDING GEAR RETRACTS, BUT BEGIN TO OPEN
AGAIN AT MACH 1.4 TO DUMP EXCESS FLOW CAPTURED BY INLET


IF AN UNSTART OCCURS IN ONE INLET ABOVE MACH 2.3, BOTH
SPIKES ARE DRIVEN TO THE FORWARD POSITION AND THE
FORWARD BYPASS DOORS ARE OPENED TO OBTAIN A RESTART
AND MINIMIZE YAW TRANSIENTS

Re: J-20... The New Generation Fighter III

MiG-29 said:

Look a F-111 or SR-71 are 3D intakes, you are there wrong, now the DSI bump is a multishock 3D object, that is true, the difference between the Cone of a SR-71 and the Bump of F-35 is the bump is a multishock generator due to its curvature.


Supersonic flow over a wedge surface has an attached, straight oblique shock wave from thenose. The flow downstream of the shock is uniform and parallel to the surface with a surfacepressure equal to the static pressure behind the shock, p2w. Note that the wedge flow is two-dimensional. The shock is a function of the free stream Mach number




A supersonic flow over a cone will also have an attached, straight oblique shock wave from thenose but since the cone is a three-dimensional body, the flow has a relieving effect. Aconsequence of this is that for the same body angle δ, and the same Mach number, the shock onthe cone is weaker than the shock for the wedge, it will have a smaller shock angle β

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The problem on a F-111 is to control the mass flow to impinge the shock waves on the intake cowl lips and avoid supercritical or subcritical states.
This is done by moving the cone forward and opening the intake bypass doors


The same applies tp SR-71 that uses a mixed compression air intake with a translating cone and bypass doors.

Please, Log in or Register to view URLs content!

BLEED BYPASS TURBOJET ENGINES LOCATED MIDSPAN ON
THE WING, EACH DEVELOPING OVER 32,000+ POUNDS AT SEA
LEVEL, STATIC CONDITIONS
– FOR COMPARISON, THE ENGINE PRODUCES HORSEPOWER EQUAL
TO THAT FOR THE QUEEN MARY
– THE PROPULSION SYSTEM CONSISTES OF THE INLET, FOLLOWED
BY THE J58 ENGINE, AND EXITS THROUGH AN AERODYNAMIC
CONVERGENT-DIVERGENT EJECTOR NOZZLE
• INLET HAS A TRANSLATING 26-DEGREE CONE WHICH ACTS AS THE
INITIAL DECELERATION OR COMPRESSION SURFACE PRODUCING A
SERIES OF SHOCK WAVES UP TO THE INLET THROAT
• THE SHOCK TRAIN ENDS WITH A FINAL TREMINAL OR NORMAL SHOCK
FOLLOWED BY A SUBSONIC DIFFUSER
• THE PURPOSE OF THE INLET IS TO SUPPLY THE AIR REQUIRED BY THE
ENGINE AT THE HIGHEST PRESSURE RECOVERY AND THE LOWEST
DRAG, WITH A MINIMUM OF DISTORTION. THIS IS NOT AS SIMPLE AS IT
SOUNDS! A TURBOJET ENGINE IS A CONSTANT VOLUME MACHINE. THIS
MEANS THAT REGARDLESS OF FLIGHT SPEED – FROM M = 0 TO CRUISE
SPEED – THE SPEED OR MACH NUMBER ENTERING THE ENGINE IS
RELATIVELY CONSTANT, BETWEEN M = 0.3 AND 0.5

THE SR-71 ENGINE AIR INLET IS A MIXED EXTERNAL AND INTERNAL
COMPRESSION, AXI-SYMMETRIC INLET, WITH GRADUAL ISENTROPIC
COMPRESSION APPROACHING THE THROAT
• THE BOUNDARY LAYER WHICH BUILDS UP ON THE CENTERBODY IS
REMOVED AHEAD OF THETERMINAL SHOCK THROUGH A POROUS
BLEED
• THIS BLEED PASSES OVERBOARD THROUGH LOUVERS AT THE ENDS
OF THE CENTERBODY SUPPORT STRUTS
• FORWARD BYPASS DOORS, LOCATED CLOSE TO THE THROAT MATCH
THE INLET TO THE ENGINE, BY-PASSING AIR OVERBOARD
• THE COWL BLEED IS A “SHOCK TRAP” RAM BLEED WHICH SUPPLIES AIR
THROUGH TUBES ACROSS THE BY-PASS DOOR REGION INTO THE
ENGINE SECONDARY PASSAGE, WHERE IT IS USED FOR COOLING, AND
FED THROUGH TO THE ENGINE EJECTOR
• AFT BY-PASS FLOW JOINS THE COWL BLEED FLOW AND PASSES
THROUGH THE ENGINE EJECTOR
• THIS BY-PASS WAS INSTALLED TO PROVIDE SUFFICIENT FLOW TO
PERMIT ENGINE OPERATION AT IDLE
• DURING ACCELERATION, AFT BY-PASS IS USED TO REDUCE THE
FORWARD OVERBOARD BY-PASS WITH ITS ATTENDANT DRAG



]• THE TERMINAL SHOCK POSITION IS CONTROLLED AND
REGULATED FOR FORWARD AND AFT BYPASS DOORS WHICH
MATCHES THE INLET AIR SWALLOWING CAPABILITY TO THE
ENGINE REQUIREMENT

• THE COMPRESSION RATIO AT CRUISE IS 40 TO 1
• AT CRUISE, EACH INLET SWALLOWS APPROXIMATELY 100,000
CUBIC FEET OF AIR PER SECOND – EQUIVALENT TO TWO

• FORWARD BYPASS DOORS ARE OPEN WITH THE GEAR DOWN,
CLOSE WHEN LANDING GEAR RETRACTS, BUT BEGIN TO OPEN
AGAIN AT MACH 1.4 TO DUMP EXCESS FLOW CAPTURED BY INLET


IF AN UNSTART OCCURS IN ONE INLET ABOVE MACH 2.3, BOTH
SPIKES ARE DRIVEN TO THE FORWARD POSITION AND THE
FORWARD BYPASS DOORS ARE OPENED TO OBTAIN A RESTART
AND MINIMIZE YAW TRANSIENTS

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Click to expand...

They're still not complex 3D shapes.

Re: J-20... The New Generation Fighter III

latenlazy said:

They're still not complex 3D shapes.

Click to expand...

it seems you still do not understand why engines need bypass doors and variable geometry intakes.

THE TERMINAL SHOCK POSITION IS CONTROLLED AND
REGULATED FOR FORWARD AND AFT BYPASS DOORS WHICH
MATCHES THE INLET AIR SWALLOWING CAPABILITY TO THE
ENGINE REQUIREMENT

this means they control the air mass flow to avoid supercritical and subcritical states

• THE COMPRESSION RATIO AT CRUISE IS 40 TO 1
• AT CRUISE, EACH INLET SWALLOWS APPROXIMATELY 100,000
CUBIC FEET OF AIR PER SECOND – EQUIVALENT TO TWO

• FORWARD BYPASS DOORS ARE OPEN WITH THE GEAR DOWN,
CLOSE WHEN LANDING GEAR RETRACTS, BUT BEGIN TO OPEN
AGAIN AT MACH 1.4 TO DUMP EXCESS FLOW CAPTURED BY INLET

here you see as speed increases the excess of air mass flow is dumped and by passed
IF AN UNSTART OCCURS IN ONE INLET ABOVE MACH 2.3, BOTH
SPIKES ARE DRIVEN TO THE FORWARD POSITION AND THE
FORWARD BYPASS DOORS ARE OPENED TO OBTAIN A RESTART
AND MINIMIZE YAW TRANSIENTS
They reduce the capture area and dump excess air mass flow to weaken the terminal normal shock and avoid supercritical states and pressure recovery loses

This reflects why the DSI presented on the Chinese paper can not achieve higher pressure recovery, is not the shock it self, but the amount of air swallowed by the intakes what determines the pressure recovery.

The mixed compression of the SR-71 is even better than the F-14`s pressure recovery coefficient.

Now the paper paintgun has provided, is an interesting one since it claims the DHI overlaps the performance envelops of the turbojet and RAM jet.

Now the DHI intake is not the same DSI intake J-10B or JF-17 have neither F-35, but i will be honest perhaps the Chinese might have develop some type of intake that despite lower pressure recovery at mach 2.5 might allow the engine to fly at that speed.

Who knows, perhaps they`ve done it, but to me it seems the J-20 has a F-35 type of intake and it does not resemble the DHI of the 2005 paper.

Re: J-20... The New Generation Fighter III

MiG-29 said:

it seems you still do not understand why engines need bypass doors and variable geometry intakes.

THE TERMINAL SHOCK POSITION IS CONTROLLED AND
REGULATED FOR FORWARD AND AFT BYPASS DOORS WHICH
MATCHES THE INLET AIR SWALLOWING CAPABILITY TO THE
ENGINE REQUIREMENT

this means they control the air mass flow to avoid supercritical and subcritical states

• THE COMPRESSION RATIO AT CRUISE IS 40 TO 1
• AT CRUISE, EACH INLET SWALLOWS APPROXIMATELY 100,000
CUBIC FEET OF AIR PER SECOND – EQUIVALENT TO TWO

• FORWARD BYPASS DOORS ARE OPEN WITH THE GEAR DOWN,
CLOSE WHEN LANDING GEAR RETRACTS, BUT BEGIN TO OPEN
AGAIN AT MACH 1.4 TO DUMP EXCESS FLOW CAPTURED BY INLET

here you see as speed increases the excess of air mass flow is dumped and by passed
IF AN UNSTART OCCURS IN ONE INLET ABOVE MACH 2.3, BOTH
SPIKES ARE DRIVEN TO THE FORWARD POSITION AND THE
FORWARD BYPASS DOORS ARE OPENED TO OBTAIN A RESTART
AND MINIMIZE YAW TRANSIENTS
They reduce the capture area and dump excess air mass flow to weaken the terminal normal shock and avoid supercritical states and pressure recovery loses

This reflects why the DSI presented on the Chinese paper can not achieve higher pressure recovery, is not the shock it self, but the amount of air swallowed by the intakes what determines the pressure recovery.

The mixed compression of the SR-71 is even better than the F-14`s pressure recovery coefficient.

Now the paper paintgun has provided, is an interesting one since it claims the DHI overlaps the performance envelops of the turbojet and RAM jet.

Now the DHI intake is not the same DSI intake J-10B or JF-17 have neither F-35, but i will be honest perhaps the Chinese might have develop some type of intake that despite lower pressure recovery at mach 2.5 might allow the engine to fly at that speed.

Who knows, perhaps they`ve done it, but to me it seems the J-20 has a F-35 type of intake and it does not resemble the DHI of the 2005 paper.

Click to expand...

All that stuff about how the variable intake works is fine...but the whole point is that you can do all that with a fixed inlet given the right interaction between the inlet and tunnel geometry with the air flow. The reason that was not an effective solution before was because using such a method requires computation power that was not available until the 90s.

And btw, the paper paintgun cited, I cited in one of our previous discussions on this some months ago.

Re: J-20... The New Generation Fighter III

latenlazy said:

Unless you can get an equal or better result using a complex 3D shape...hence the DSI. And btw, the paper paintgun cited, I cited in one of our previous discussions on this some months ago.

Click to expand...

Relax, the DSI are not better than the SR-71 mixed compression results or F-14 2D inatke with ramps and by pass doors, you do not know if the DHI can be indeed applied to a fighter, and how it does work properly, the DHI paper is a methodology of how design a divertless hypersonic intake, i have not seen such aircraft now, it also mentions the design includes speeds from Mach 2.5 to 10 where the turbine does not work, i do not know if you understand that, but a turbofan like 117 or WS-10 or F119 need air flowing at Mach 0.3-0.6, however ramjets do not need that because the mass flow at those speeds is highly compressed so they do not need the engine and compressors to achieve it, so what the paper basicly says is not the pressure recovery is better but it allows the jet to start the ramjet earlier at mach 2.5 so the jet does not need to go to Mach 3.4 to start the RAMJET, it says the jet can start the RAMJET earlier.


So it only allows the RAMJET to take over earlier, as a RAMJET engine it will be better if it is true but that is not completly the only way to make ramjets, today EADS claims they can build a jet engine with hydrogen that will fly at mach 4 and the model they have has SR-71 cone intakes.

[video=youtube;N9nGURCYnLQ]http://www.youtube.com/watch?v=N9nGURCYnLQ[/video]

DSI or DHI still will ingest some boundary layer as the cone intakes of SR-71, but the intake seems to be better for fighters where the engines are buried on the fuselage

however read this until the desired cowl plane Mach number, flowfield uniformity, and geometrical constraints are achieved (step 91).
this means they have to optimise the intake to a cetain speed, it does not mean it is good for all speeds

this could mean as in the case of the EADS design it might have 4 engines, two turbofans and two ramjets with their respective individual separated intakes

In addition, step 97 may comprise reassessing shock positions, cowl plane Mach number, and capture area, and assessing boundary layer thickness and surface pressure gradients; and step 99 may comprise evaluating off-design conditions and augmenting with DHI forebody flow control.


In step 83, the method also may comprise performing a series of inviscid conical and two-dimensional shock calculations to determine viable shock system configurations, including a plurality of shocks and shock angles, to achieve the desired cowlplane Mach number; and designing a segmented, cone-shaped FFG required to generate the desired shock system configuration. In step 85, the method may comprise factoring an intended streamline cut position of approximately 10% to 40% of the cowl height

cowlplane mach number means the method is to design optimised intakes to particular hypersonic speeds, and what might work at Mach 4 might not work at Mach 6 or mach 2, they also have to optimise capture Area.

okay, let's agree on a few things :

1. it is possible that a diverterless intake design can remove boundary layer for any given mach number range it is designed for
2. it is possible that a diverterless intake design can maintain an effective pressure recovery ratio for any given mach number range it is designed for

now if the next argument is airflow mass control, please read this, it's a paper for SAAB stealth fighter project with many details, which also describes how DSI controls mass flow in different mach numbers by bump interaction with inlet cowl


will you agree to the next point that :
3. it is possible that a diverterless intake design can produce an effective mass flow control in any given mach number range it is designed for