J-20 5th Gen Fighter Thread V

[plawolf]

It was I who suggested that the hexagonal shapes are supplimentary inlet and outlets. The first pair are positioned such along the curved outer inlet housing that when opened, they can act as small intakes. The second pair could be opened to bleed out excess airflow.

In this way, they can allow the fixed DSI to enjoy some or all of the benefits of a true variable intake while retaining much of the weight and RCS savings offered by the DSI design over conventional splinter plates variable intake designs.

The diamonds on either side and on top of the radome are either apertures for 360 degree optical sensors, and/or cooling vents for the powerful AESA radar.

The things under the LERX are actually partial housings for the main landing gear wheels, the very tips of which actually protrude beyond the general profile of the fuselage. The rear half of those houses are actually part of the main landing gear doors. The front of those casing now house unidentified sensors.

The cigar shaped housings under the wings contain the actuators uses to control the flaps

 

[Air Force Brat]

It was I who suggested that the hexagonal shapes are supplimentary inlet and outlets. The first pair are positioned such along the curved outer inlet housing that when opened, they can act as small intakes. The second pair could be opened to bleed out excess airflow.

In this way, they can allow the fixed DSI to enjoy some or all of the benefits of a true variable intake while retaining much of the weight and RCS savings offered by the DSI design over conventional splinter plates variable intake designs.

The diamonds on either side and on top of the radome are either apertures for 360 degree optical sensors, and/or cooling vents for the powerful AESA radar.

The things under the LERX are actually partial housings for the main landing gear wheels, the very tips of which actually protrude beyond the general profile of the fuselage. The rear half of those houses are actually part of the main landing gear doors. The front of those casing now house unidentified sensors.

The cigar shaped housings under the wings contain the actuators uses to control the flaps

thanks Wolfie, I remembered someone had suggested that, and that should prove to all that I read and enjoy all your posts, I often like posts if they show that kind of perception, as it is difficult to get accurate information.

 

[b787]

. The first pair are positioned such along the curved outer inlet housing that when opened, they can act as small intakes. The second pair could be opened to bleed out excess airflow.

In this way, they can allow the fixed DSI to enjoy some or all of the benefits of a true variable intake while retaining much of the weight and RCS savings offered by the DSI design over conventional splinter plates variable intake designs.

A fixed intake is fixed, bleed doors only get rid of excess air, but variable intakes control the shock wave and the capture of air mass, the intake of J-20 is fixed, it can not control the sock wave as a variable one

 

[Scratch]

A fixed intake is fixed, bleed doors only get rid of excess air, but variable intakes control the shock wave and the capture of air mass, the intake of J-20 is fixed, it can not control the sock wave as a variable one

There is another point to what Plawolf is saying. It is not only about controlling the shock wave or cone. An intake may well be fixed to work best around "standard" flight speeds (of M0.7-1.5 perhaps??).
But in slow speed / high power demand situations, were "ram air pressure" is low (due to speed) the engine is actually using a considerable fraction of it's power to suck in air through the constricted intake. For these conditions it would best be really wide. To an extent it's impractical for the design & at speed.
Therefore, some types (airliners & fighter/bombers) do utilize so called "blow-in doors". In a simple form they are spring-loaded to the closed position. In high power / low ram pressure conditions, when there is a really low pressure in the inlet section, these doors are sucked open allowing additional air to flow in from the side.

Once you go faster, enough air is fed through the intake through speed. And engine power can focus on providing thrust, instead of sucking in air. Hence the wisdom "the faster you go, the faster you go faster". To a certain point of course.

It's just that I would have though that is too much moving parts and additional corners in a VLO design and would not have exspected them here.

Same for the alleged "bleed doors". I have heared of old aircraft, were engines had really long spool-up times, there was a vent to allow exhaust to bleed, so thrust could be reduced while keeping RPM up (I've spent almost an hour googling it, but didn't find a thing) e.g. for an approach.

I don't know of any contemporary fighter utilizing this and, for all I know, it corresponds with a very sluggish power-up reaction of the engine. Especially in the context of putting another set of moving parts & corners on a VLO design.

 

[plawolf]

In addition to letting and out air, the side intake especially, could be used to control shock waves.

How does a conventional variable intake control shock waves? If puts a piece of metal partially in the way of the air flow, thus changing the position and angle it hits the intake walls, and so shifts the location where the shockwave occurs.

The same effect can be done with a wall of all air rather than metal, which is what airflow coming into the main intake from the side side intakes will do, only in the opposit direction.

A variable diverted plat will push the point airflow hits the intake sides forwards, towards the intake mouth, which will cascade down to ultimately impact how many times the airflow 'bounces' in the intake and its final speed when hitting the fan face. So the faster you fly, the smaller your intake becomes.

Side intakes will push the first point of impact further backwards, towards the fan face. So it works in reverse, meaning the faster you go, the more you close the side intakes, so the shockwaves hits the intake wall further forwards. That happily also works in low speed situations, where as Scratch already mentioned, you will want more air mass to compensate for the low ram speed.

The bleed vents, in my view, come into play once airspeed has gone so high as to force the front side intakes to fully close, so are no longer able to help regulate intake internal pressures and shockwaves.

By opening them, you use the high pressure inside the inlet to vent air mass out, countering to a degree the excess ram effect from high speed flight.

I think the positioning would also be very significant, and its effect would be amplified if they are placed where the shockwave will first hit the intake walls after the front intakes have fully closed. So the effect cascades down to all subsequent shocks inside the intake.

I would think that when the front side intakes are fully closed would be the J20 hitting its supercruise speed limit, whereas it will hit its absolute speed limit when the side vent doors are also fully open.

 

[MastanKhan]

Same for the alleged "bleed doors". I have heared of old aircraft, were engines had really long spool-up times, there was a vent to allow exhaust to bleed, so thrust could be reduced while keeping RPM up (I've spent almost an hour googling it, but didn't find a thing) e.g. for an approach.

I don't know of any contemporary fighter utilizing this and, for all I know, it corresponds with a very sluggish power-up reaction of the engine. Especially in the context of putting another set of moving parts & corners on a VLO design.

Hi,

Sorry for off topic---Thank you for the post---reminded me of my grandfather's Belarus tractor of the late 70's. A russian tractor---because of the cold weather in rusia / poland---it had a very interesting feature----.

On the cylinder head---it had a handle---when pulled hard--it would push open all the valves on the cylinder head----so when you cranked the engine in cold weather---as there is no compression---the engine will have high cranking revolutions---and then you slammed the handle down to shut the valves and cold starting in freezing weather got easier.

Those who own diesel engines and live in cold weather would know what I am talking about---.

 

[Air Force Brat]

In addition to letting and out air, the side intake especially, could be used to control shock waves.

How does a conventional variable intake control shock waves? If puts a piece of metal partially in the way of the air flow, thus changing the position and angle it hits the intake walls, and so shifts the location where the shockwave occurs.

The same effect can be done with a wall of all air rather than metal, which is what airflow coming into the main intake from the side side intakes will do, only in the opposit direction.

A variable diverted plat will push the point airflow hits the intake sides forwards, towards the intake mouth, which will cascade down to ultimately impact how many times the airflow 'bounces' in the intake and its final speed when hitting the fan face. So the faster you fly, the smaller your intake becomes.

Side intakes will push the first point of impact further backwards, towards the fan face. So it works in reverse, meaning the faster you go, the more you close the side intakes, so the shockwaves hits the intake wall further forwards. That happily also works in low speed situations, where as Scratch already mentioned, you will want more air mass to compensate for the low ram speed.

The bleed vents, in my view, come into play once airspeed has gone so high as to force the front side intakes to fully close, so are no longer able to help regulate intake internal pressures and shockwaves.

By opening them, you use the high pressure inside the inlet to vent air mass out, countering to a degree the excess ram effect from high speed flight.

I think the positioning would also be very significant, and its effect would be amplified if they are placed where the shockwave will first hit the intake walls after the front intakes have fully closed. So the effect cascades down to all subsequent shocks inside the intake.

I would think that when the front side intakes are fully closed would be the J20 hitting its supercruise speed limit, whereas it will hit its absolute speed limit when the side vent doors are also fully open.

Great explanation gentlemen, using logic and reason, and as Master Scratch noted, the desired "operating range" of the J-20 is in the .7-1.5 range, these accomodations may be managed to benefit both ends of the speed spectrum, as Dr. Song made it very clear that the J-20 must be able to recover from post stall maneuvering with-out OVT, hence the fixed ventral fins, and the long coupled canards, while top end is almost certainly Mach 2+, thank you Wolfie in providing a logical explanation of how all these doors and vents would make that possible.

 

[Air Force Brat]

Hi,

Sorry for off topic---Thank you for the post---reminded me of my grandfather's Belarus tractor of the late 70's. A russian tractor---because of the cold weather in rusia / poland---it had a very interesting feature----.

On the cylinder head---it had a handle---when pulled hard--it would push open all the valves on the cylinder head----so when you cranked the engine in cold weather---as there is no compression---the engine will have high cranking revolutions---and then you slammed the handle down to shut the valves and cold starting in freezing weather got easier.

Those who own diesel engines and live in cold weather would know what I am talking about---.

Heh! Heh! Heh!, and my pet theory is the Russians used all that Belarus paint on the green wheels of their fighter aircraft.

 

[b787]

There is another point to what Plawolf is saying. It is not only about controlling the shock wave or cone. An intake may well be fixed to work best around "standard" flight speeds (of M0.7-1.5 perhaps??).
But in slow speed / high power demand situations, were "ram air pressure" is low (due to speed) the engine is actually using a considerable fraction of it's power to suck in air through the constricted intake. For these conditions it would best be really wide. To an extent it's impractical for the design & at speed.
Therefore, some types (airliners & fighter/bombers) do utilize so called "blow-in doors". In a simple form they are spring-loaded to the closed position. In high power / low ram pressure conditions, when there is a really low pressure in the inlet section, these doors are sucked open allowing additional air to flow in from the side.

Once you go faster, enough air is fed through the intake through speed. And engine power can focus on providing thrust, instead of sucking in air. Hence the wisdom "the faster you go, the faster you go faster". To a certain point of course.

It's just that I would have though that is too much moving parts and additional corners in a VLO design and would not have exspected them here.

Same for the alleged "bleed doors". I have heared of old aircraft, were engines had really long spool-up times, there was a vent to allow exhaust to bleed, so thrust could be reduced while keeping RPM up (I've spent almost an hour googling it, but didn't find a thing) e.g. for an approach.

I don't know of any contemporary fighter utilizing this and, for all I know, it corresponds with a very sluggish power-up reaction of the engine. Especially in the context of putting another set of moving parts & corners on a VLO design.

A fixed intake is used in designs with lower speeds than Mach 2,　bleeding air or additional intake doors are part of those types of intakes, for higher speeds you need variable intakes, my point was it can not replace variable intakes, the Harrier is an example, it can use fixed intakes but still uses additional intake doors, but it will not fly beyond Mach 0.9, the intake design is fixed in terms of the mach speed it will operate, you can not replace a variable intake with one fixed.

 

[b787]

In addition to letting and out air, the side intake especially, could be used to control shock waves.

How does a conventional variable intake control shock waves? If puts a piece of metal partially in the way of the air flow, thus changing the position and angle it hits the intake walls, and so shifts the location where the shockwave occurs.

it is more complex than that, but a fixed intake has a fixed throat area, true bleed doors also control the position of the shock wave, but not in the way you are suggesting, the capture area is controlled by the variable geometry of the intake it self, but there is a limit to the total capture area an intake has, that limit the max design speed, so no Fixed intake can operate beyond the speed it was designed to operate, the variable intake is slightly more adaptable.

Bleed doors or intake doors always will operate upon a max total air mass the intake design capture, in few words, all intakes work upon a range of speeds, fixed intakes are only effective only upon a speed range, while the variable ones expand only a little bit more that speed range.

you are confusing the max design speed an air intake has, with the functions bleed doors and intake doors have.

to exemplify, the harrier and SR-71 both use intake doors, but their design change upon the speeds they are designed to operate, one is fixed (Harrier) the other is variable (SR-71).

Any way this a bit off topic so i will cut it up here