The J-XX was not the first fighter was designed as DSI(or Chinese call it “Bump” inlet). Despite those X-35 and F-16DSI, only J-10B and JF-17 was fitted same DSI seemingly, not for detail but only lips. Exhausting all of so-called DSI pattern, we can put them in three categories by the shape of lips: one is X-35 alike, two is JF-17 used, and three is F-35 LightningⅡ. One against me may claim categorizing them by some way else, but due to the cause of the Bump on design, which we have known and presumed as only way, were forced moi insist my method until guys you point out various way to design this DSI.
When I discussed DSI with friends on ICQ or WLM, most them disagree that VG DSI has been used on J-XX also some Chinese bigoted the advantage of DSI due to single reason which is divertless. The funny is many Chinese well-know and pointed the principle of shock-wave-ride as the advantage here totally forgot object making from same principle could be and mostly are completely different thing and use in various place. I have to explain it is the disadvantage of DSI compare to CARET inlet cause the DSI necessarily to be VG inlet, then losing all of advantage from fantasy and return to be a normal inlet.
The inlet was never such important till aircraft being jet era because tremendous air flow needed by turbo and air compressor, but before sonic flight, we didn’t face any big problem on inlet design, now, we have two. The one is boundary layer, the other is shock wave.
The boundary layer is a low speed air flow stick on the surface of any part of aircraft, rubbing with high speed air flow upon them give to the aircraft not only drag but also heat. This is totally rubbish for flight. This is also why we can see many measures and devices used want to remove it. Traditional inlet was set dam-board to divide boundary layer from air flow which shall be led into inlet. The boundary layer on the surface of inner inlet will be absorbed by tiny bore, then flipped out of airframe. This is still an inactive way till the theory of wave-ride created.
Being the creation of wave-ride theory, many knew the sock-wave can be used as lift, few knew another awareness was the shock-wave could be used to blow the boundary layer, this was the very primeval trial to test on F-105’s inlet. Although this could be a sort of active way to blow boundary layer up, however, since the shock-wave leading by curve board, its swept angle changing cause nowhere should be directed, then it to be a pentant direction. Contemporaneously, inactive way to remove boundary layer was developed parallelly on F11. The forebody of inlet was raised like brae to accept the boundary layer stably into enormous bores that drilled on the surface beforehand. Like nowadays, the tiny-bores on ramp board, the bores are same function but the ramp surface are little bit different. Previous one this fixed, modern ramp-board are movable for swept angle of shock wave adjusting. The interesting problem here is, it is the very movement of swept angle of shock-wave not only influenced the effect of blow boundary layer up but also could be adjusted by ramp angle. No matter the area the shock wave directed are disparate, the ramp where create shockwave could be possibly known. If the shockwave is the nature bane to the boundary-layer, then the place the shockwave is born, where the boundary-layer disappear naturally.
So the problem must solving by creating infinite shockwave on the surface ahead of inlet. There are many trials but none perfect, you can see segment of ramp-board on MiG-29 used isentropic principle but still needs some tiny-bores drilled at the end of part. Therefore, the ramp should be a kind of shockwave alike, making no boundary-layer stand on for any moment. This is a logical source of “bump”. The “bump” are capable to blow boundary-layer because its profile is made due to the surface which can create shockwave, all of these walk around how to press the boundary layer out of the surface, none of business to creating lift, conversely the “bump” bring benefit to reducing drag meanwhile bring trouble to the air-flow for engine.
Someone here must be an expert who calculate how many airflow needed by jet engine during various speed with proper altitude. This is not the point we concerned so far, the point we caught here is must be different flow under different condition. Variable profile or angle or any other shape to suit various air-flow or intuitively say speed and altitude has never being successful on any aspect up to now. If the curve of DSI made is suitable to this, it has already been successful on MiG or early aircraft. Isentropic inlet was not a new study on aviation. Now we can see why there are three type of lip coordinate with “bump”.
Since we already explained the profile of “bump” is made not for lift but only boundary-layer, regardless what advanced principle it used by, so we can image it not concerned so many to air-flow. If the curvature of “bump” could create multi-shockwave for adjusting airflow into duct, it won’t be three types of lip present. We all knew that ramp surface will stimulate shockwave, if the shockwave is not keen enough to blow all of boundary layer, then that’s why we saw still amount of bores being on the bump of Thunder fighter. If the shockwave raised by “bump” is so strong that can be a conical cover the lip, to compress the engine pretty well. The cuspidal lips on F-35 showing its well covered and remain the wide route for boundary-layer aside, but according to subsonic needs wide section of air duct, the “bump” still is a problem. If the top of the “bump” being too wide, it will break the supersonic flow, if too narrow, then, engine lacking airflow during subsonic. Here is nothing to do with boundary-layer but cross-section for sonic flow.
Now we can see J-XX use VG DSI to changing cross section of air duct. The movable part must consist of a two-dimensional surface. This inner surface just like another “bump” inside, and adhering inner surface of lips, when the triangle part stretch out, the inner “bump” goes closer to the outside “bump”, making cross section narrowly to adapt supersonic flow, while subsonic flight is making, the triangle tongue draw back.