Official Chengdu J-20 Discussion Thread

[MiG-23MLD]

What I'm saying is that given how the DSI works, having an movable inlet lip is still probably a much simpler and lighter solution than say variable shock ramps. Why would a solution like this be necessary? Probably because part of the requirements for the plane is to have it move at speeds beyond what a fixed DSI would allow, or perhaps it has to do with how capable the engine is.

I don't know how much more or less such a small change as a moving lip will increase RCS, especially if it's properly managed. Finally, the F-22 has a fixed inlet design with diverters. Perhaps a DSI offers better control of RCS (by controlling the direction of radar reflection entering the inlet?)

They can do a moveable lip that thing is no problem, however the real advantage for the DSI was its simplicity, there is not other advantage, it is simply simplicity, it diverts the boundary layer without the need of a splitter, on the F-35 it was added because it simply reduces weight and maintainace and therefore costs.
Adding moveable parts will cancel out its main advantage.
Is it the best type of inlet? no it is not, it is useful under some conditions, the F-35 only gets to Mach 1.6 so it does really pay off a DSI, same is the JF-17; the T-50 does not use it and on the YF-23 they used another solution and this was the wing leading edge was part of the inlet lip, they drilled some holes to take the boundary layer and they create a really simple inlet, the wing leading edge hardly generates a noticeable boundary layer so that was a pretty much smart solution.

 

[lizs]

Another advantage of DSI is being "stealth". It hides the engine fan blades very well.

If F-35 wants simplicity and light weight only, why doesn't it use a simpler F-16 style pitot tube inlet?

 

[SpudmanWP]

The DSI weighs less than the F-16 inlet.

Article : Code One Magazine

 

[latenlazy]

They can do a moveable lip that thing is no problem, however the real advantage for the DSI was its simplicity, there is not other advantage, it is simply simplicity, it diverts the boundary layer without the need of a splitter, on the F-35 it was added because it simply reduces weight and maintainace and therefore costs.
Adding moveable parts will cancel out its main advantage.
Is it the best type of inlet? no it is not, it is useful under some conditions, the F-35 only gets to Mach 1.6 so it does really pay off a DSI, same is the JF-17; the T-50 does not use it and on the YF-23 they used another solution and this was the wing leading edge was part of the inlet lip, they drilled some holes to take the boundary layer and they create a really simple inlet, the wing leading edge hardly generates a noticeable boundary layer so that was a pretty much smart solution.

I know all that. I'm simply saying that a DSI with a movable lip may not be as light or simple as a fixed DSI, but may still be lighter and more simple than any other variable inlet alternatives (of course we wouldn't know for sure). This would offer a compromise between the benefits of a fixed DSI and a variable inlet.

 

[MiG-23MLD]

Another advantage of DSI is being "stealth". It hides the engine fan blades very well.

If F-35 wants simplicity and light weight only, why doesn't it use a simpler F-16 style pitot tube inlet?

They do not use a DSI o the F-16 simply because the design is frozen, but here see the F-35 and JF-17 have lateral inlets that feed a single engine, so the inlet ducts are S shaped.
Now ask your self this following question why flat chines are stealthy and round forebodies such as the F-15 and Su-27 forebodies are not?
The DSI bump is not exactly stealthy, what happens it does give a relatively low RCS and lower weight and complexities with the same performance up to Mach 1.7

 

[MiG-23MLD]

I know all that. I'm simply saying that a DSI with a movable lip may not be as light or simple as a fixed DSI, but may still be lighter and more simple than any other variable inlet alternatives (of course we wouldn't know for sure). This would offer a compromise between the benefits of a fixed DSI and a variable inlet.

I do not think a moveable inlet lip will be better, it only will be a solution to keep a DSI inlet. i think what happens most people want to say the J-20 achieves Mach 2.2 easily.
In my opinion it is not as fast, perhaps it achieves Mach 2 with some thrust penalties, but it won`t go beyond Mach 2.
First you have to consider real problems, the bump of the inlet in the X-35 was coaxial at the center of the inlet and the inlet wall notch projected forward to generate a single shock wave.http://www.airforceworld.com/fighter/gfx/jsf/x35_1.jpg
On the F-35 now, the bump has been moved out of the axis of the inlet closer to the aircraft forebody chine, this was done because the inlet wall has been racked and the upper inlet lip is closer to the DSI bump than the lower inlet lip.http://www.militaryfactory.com/aircraft/imgs/lockheed-f35-lightning-ii.jpg
The current J-20 has the same inlet type.
So moving the entire inlet fairling might probably complicate the boundary layer ingestion by the inlet.

 

[latenlazy]

I do not think a moveable inlet lip will be better, it only will be a solution to keep a DSI inlet. i think what happens most people want to say the J-20 achieves Mach 2.2 easily.
In my opinion it is not as fast, perhaps it achieves Mach 2 with some thrust penalties, but it won`t go beyond Mach 2.
First you have to consider real problems, the bump of the inlet in the X-35 was coaxial at the center of the inlet and the inlet lip projected forward to generate a single shock wave.http://www.airforceworld.com/fighter/gfx/jsf/x35_1.jpg
On the F-35 now, the bump has been moved out of the axis of the inlet closer to the aircraft forebody chine, this was done because the inlet lip has been racked.http://www.militaryfactory.com/aircraft/imgs/lockheed-f35-lightning-ii.jpg
The current J-20 has the same inlet type.
So moving the inlet lip might probably complicate the boundary layer ingestion by the inlet.

Of course, we don't know if the J-20 actually has a movable lip. However, though the J-20's bump has been moved closer to the aircraft's forebody, its bump is not racked. Furthermore, its inlet lip has a different geometry. I don't know if that's significant, but it could indicate that the aerodynamic properties of their inlet designs aren't perfectly analogous.

Also, what makes you think it's not capable of speeds above mach 2?

 

[MiG-23MLD]

Of course, we don't know if the J-20 actually has a movable lip. However, though the J-20's bump has been moved closer to the aircraft's forebody, its bump is not racked. Furthermore, its inlet lip has a different geometry. I don't know if that's significant, but it could indicate that the aerodynamic properties of their inlet designs aren't perfectly analogous.

Also, what makes you think it's not capable of speeds above mach 2?

Let me explain you again
the bump of the inlet in the X-35 was coaxial at the center of the inlet and the inlet wall notch projected forward to generate a single shock wave.http://www.airforceworld.com/fighter/gfx/jsf/x35_1.jpg
On the F-35 now, the bump has been moved out of the axis of the inlet closer to the aircraft forebody chine, this was done because the inlet wall has been racked and the upper inlet lip is closer to the DSI bump than the lower inlet lip.http://www.militaryfactory.com/aircr...ghtning-ii.jpg
The current J-20 has the same inlet type.
so moving the entire inlet fairling might probably complicate the boundary layer ingestion by the inlet.
I never meant the bump was racked just the inlet lateral wall.

The position of the bump is very important to avoid the boundary layer getting in, also its shape, form, height and width, another thing is the inlet faring shape, this must create a single shock cone with the bump, that is the reason the X-35 has a notch projecting from the inlet wall.
On a regular aircraft with a splitter, the boundary layer is eliminated by positioning the inlet from the main fuselage a few centimeters away, but on the DSI this is more complicated, so i do not think it will work as smoothly as it is thought.
To get to Mach 2.5 the pressure recovery has to be high, so the engines need high pressure recovery, without a variable geometry inlet supersonic flow will simply enter the inlet flow reducing the thrust of the engine or simply stalling it, so without a variable geometry inlet, the J-20 won`t get beyond Mach 2 even having engines that at sea level and static thrust give 18 tonnes

 

[latenlazy]

The position of the bump is very important to avoid the boundary layer getting in, also its shape, form, height and width, another thing is the inlet faring shape, this must create a single shock cone with the bump, that is the reason the X-35 has a notch projecting from the inlet wall.
On a regular aircraft with a splitter, the boundary layer is eliminated by positioning the inlet from the main fuselage a few centimeters away, but on the DSI this is more complicated, so i do not think it will work as smoothly as it is thought.
To get to Mach 2.5 the pressure recovery has to be high, so the engines need high pressure recovery, without a variable geometry inlet supersonic flow will simply enter the inlet flow reducing the thrust of the engine or simply stalling it, so without a variable geometry inlet, the J-20 won`t get beyond Mach 2 even having engines that at sea level and static thrust give 18 tonnes

But if it does have a variable DSI, it might be able to generate high pressure recovery to reach higher speeds? Of course, we don't know if it does or doesn't right now, nor do we know which speed envelope the J-20's inlet design is designed for (which is very important for a fixed design). That doesn't preclude it from being either possible or feasible.

I perfectly understood what you said earlier about the placement of the bump. Indeed, except for the specifics regarding the F-35, you haven't told me anything I didn't already know. I was merely saying that given the different geometry of the J-20's inlet design from the F-35, it might be entirely possible for a moving inlet lip solution to work, despite the potential complications involved with how the bump handles the boundary layer. The one thing I've learned about aerodynamics is that there is no general answer for specific circumstances.

 

[MiG-23MLD]

But if it does have a variable DSI, it might be able to generate high pressure recovery to reach higher speeds? Of course, we don't know if it does or doesn't right now, nor do we know which speed envelope the J-20's inlet design is designed for (which is very important for a fixed design). That doesn't preclude it from being either possible or feasible.

I perfectly understood what you said earlier about the placement of the DSI. Indeed, except for the specifics regarding the F-35, you haven't told me anything I didn't already know. I was merely saying that given the different geometry of the J-20's inlet design from the F-35, it might be entirely possible for a moving inlet lip solution to work, despite the potential complications involved with how the bump handles the boundary layer. The one thing I've learned about aerodynamics is that there is no general answer for specific circumstances.

The DSI only works fine for speeds lower than Mach 1.6. regular commercial jets or the A-10 can work with simple inlet cowlings, fighters that fly beyond Mach 1 up to Mach 1.8 need a fixed inlet with a fixed shock generator in example F-18, Su-24 or F-5.
The F-16 works fine near Mach 2 but it can not use all its potential thrust, beyond Mach 2 you need variable geometry inlets, see both the F-15 and F-16 have the same engine but the F-16 won`t pass Mach 2 even having similar thrust to weight ratios(TWR), but the lower TWR MiG-25, F-111, MiG-21 and Mirage III will out run the F-16 by just having variable geometry inlets.

it is very difficult to harmonize the bump position with the inlet fairing so it can go beyond Mach 2.2 and still do not allowing boundary layer and supersonic flow into the engine, i do not consider the DSI is a variable geometry one on the J-20 for me it is very unlikely it is and i find it redundant in terms of complexity adding moving parts to the DSI fairing on the J-20

 

[SpudmanWP]

The DSI was tested up to Mach 2 on the F-16 without any problems. It is not limited to Mach 1.6.

Article : Code One Magazine

 

[MiG-23MLD]

The DSI was tested up to Mach 2 on the F-16 without any problems. It is not limited to Mach 1.6.

Article : Code One Magazine

same is its fixed regular inlet, regular F-16 also get up to Mach 2, but the engines can not use their full potential, there is a thrust loss by no adding a variable geometry inlet, the Mirage III will out run a F-16 even having an engine with half of the power of the General Electric F110-GE-132 turbofan and the F-16 won`t pass Mach 2 while F-15 does having the same engine

the DSI works fine at Mach 1.6 you can go up to Mach 2 but the engine won`t work in optimal conditions, there is a loss of thrust by not using a variable geometry inlet, the DSI works best up to Mach 1.6 later on as any fixed inlet drops in efficency until it simply stalls the engines beyond mach 2

 

[latenlazy]

it is very difficult to harmonize the bump position with the inlet fairing so it can go beyond Mach 2.2 and still do not allowing boundary layer and supersonic flow into the engine, i do not consider the DSI is a variable geometry one on the J-20 for me it is very unlikely it is and i find it redundant in terms of complexity adding moving parts to the DSI fairing on the J-20

It wouldn't be redundant if the plane is designed to effectively go beyond Mach 2.

 

[SpudmanWP]

I know, I was showing you that DSI works above mach 1.6.

btw, when was the last time any speed above mach 2 was ever used in combat?

 

[latenlazy]

I know, I was showing you that DSI works above mach 1.6.

btw, when was the last time any speed above mach 2 was ever used in combat?

Almost never . But it's necessary for quick dashes and getaways. If the suspicion that the J-20 is designed for deep strike and penetration roles, that dashing ability will be useful. It might also be useful in closing the plane against other 5th generation platforms to neutralize BVR.

 

[MiG-23MLD]

I know, I was showing you that DSI works above mach 1.6.

btw, when was the last time any speed above mach 2 was ever used in combat?

the DSI works fine at Mach 1.6 you can go up to Mach 2 but the engine won`t work in optimal conditions, there is a loss of thrust by not using a variable geometry inlet, the DSI works best up to Mach 1.6 later on as any fixed inlet drops in efficency until it simply stalls the engines beyond mach 2

The F-16 has no problem because it is a transonic fighter but the J-20 will only use it for supercruise as a modern tornado with supercruise.

 

[MiG-23MLD]

It wouldn't be redundant if the plane is designed to effectively go beyond Mach 2.

it is unlikely it is designed to fly beyond mach 1.6 in supercruise mode and beyond Mach 2 with afterburner.
i do not think the DSI has variable geometry on the J-20

 

[latenlazy]

.
i do not think the DSI has variable geometry on the J-20

Why not?

 

[MiG-23MLD]

Why not?

too much compromises, they have many variables to solve, on a Mirage 2000, the inlet cone does the work of wave shock generator at all speeds and the splitter keeps the boundary layer out of the inlet.

The DSI is different, it does both works, on the F-35. J-10B and JF-17 are fixed for sure and the Americans are experts in aviation, the main problem is for the bump to work it needs a position well stablished to get rid of the boundary layer, a simple change of position will allow the boundary layer getting into the inlet, that is the reason the F-35 when it changed the shape of its inlet cowling needed to change the position of the bump.

 

[SpudmanWP]

Why are you fixated on mach 1.6? Just because it's the F-35's publicly stated max speed? That has nothing to do with what the DSI can or cannot do. Also, the article says that it was used up to mach 2.0 without deviating from normal F-16 specs.

In other words, engine performance will be affected in the same way whether it's a DSI or conventional non-moving inlet. Btw, the article did say that the DSI provided slightly better subsonic performance (which is where any fighter will be spending most of it's time).