It's so damn easy, sheer complacency! Yanks think they are best in the world!
J-20 5th Generation Fighter VII
- Thread starter siegecrossbow
- Start date
- Status
J-20 landing in Dingxin.
Very unlikely currently.
I have heard more than one sources say J-20 top speed is mach 2.8, including one other time by Xi Yazhou who mentioned it in context of PLA telling Russians about it and they were surprised.
Frankly speaking - you can reasonably guess speeds from intake and sweep angle. It's just plain physics.
Intake is against high Vmax, but sweep angle is really for very high Vcombat.
Principles of Flight 101:
- Mach is a function of air density
- The lower the altitude, the denser the air, the higher the indicated airspeed required to get to Mach 1 and above
- e.g. Assuming ICAO standard weather conditions (at sea level, outside air temperature of 15 degrees, barometric pressure of 1013hPa), a plane would have to fly a calibrated airspeed (indicated airspeed calibrated for instrument error) of ~660kts to reach Mach 1... which is stupidly fast, fighters enter the pattern on the overhead break at about 300 to 400kts indicated depending on how big an asshole the pilot is when they come in for landing (think of Maverick buzzing the tower). Unless you're at an airshow, nobody ever flies that fast that low to Earth.
- The higher the altitude, air becomes less dense, the same indicated airspeed at a lower altitude would equate to a much higher true airspeed at altitude, and as a result the indicated airspeed required to reach higher Mach numbers is lower
- e.g. Again assuming ICAO standard weather conditions - a plane would reach Mach 1 if it flew a calibrated airspeed of:
- ~312kts at 40,000 feet
- ~248kts at 50,000 feet
- ~196kts at 60,000 feet
- Just some figures to compare those numbers to:
- A plane the size of a fighter probably takes off at about 120 to 130kts indicated depending of weight and loadout
- Commercial aircraft are limited to 250kts indicated below 10,000 feet, and cruising at Mach 0.80 would give an indicated speed of ~300kts at 30,000 feet and ~280kts at 35,000 feet
- e.g. Again assuming ICAO standard weather conditions - a plane would reach Mach 1 if it flew a calibrated airspeed of:
- The lower the altitude, the denser the air, the higher the indicated airspeed required to get to Mach 1 and above
- I don't know what the official service ceiling of the J-20 is, but given the standards of where modern technology is at (regardless if you're a western or Sino/Russian designed jet) cruising above Mach 2 at FL400-600 really isn't that mind blowing. It is also important to factor in the good chunk of gas it would take to climb to those altitudes anyway, especially considering in the amount of afterburner used to climb to those altitudes... afterburner would probably even be needed to sustain that high a Mach speed.
- I feel like the question to ask isn't about top speed (as impressive as it may be), but rather fuel economy of the jet and more importantly its ability to supercruise. What's the point of flying stupidly fast if you can only sustain it for a couple of minutes before running out of gas?
I doubt the top speed is all that hot. Airplanes with fixed intakes are typically designed for a given Mach range.
Even the Su-57 does not reach Mach 2.8 and that is with higher power engines, lighter weight, and variable intakes.
If J-20 reaches Mach 2.0 at all would be a good result I think.
Combat aircraft are often designed with Mach 1.2-1.8 in mind.
Mach 2.8 is like the speed of a MiG-31. At those speeds you need heat treatment of airframe and cockpit or else it melts due to air friction. In the MiG-31's case they used nickel plated steel in it. For the SR-71 the US used titanium. And for the Valkyrie the US used titanium-stainless steel honeycomb metal matrix composite materials I think.
Even the Su-57 does not reach Mach 2.8 and that is with higher power engines, lighter weight, and variable intakes.
If J-20 reaches Mach 2.0 at all would be a good result I think.
Combat aircraft are often designed with Mach 1.2-1.8 in mind.
Mach 2.8 is like the speed of a MiG-31. At those speeds you need heat treatment of airframe and cockpit or else it melts due to air friction. In the MiG-31's case they used nickel plated steel in it. For the SR-71 the US used titanium. And for the Valkyrie the US used titanium-stainless steel honeycomb metal matrix composite materials I think.
Last edited:
The hexagonal side intakes on the J-20 might be spillage doors, which allow for some variability with intake pressures and thus some variability of pressure recovery at different speeds.
(Huitong thinks they’re heat exchangers based on what I thought was a pretty shoddy post by someone on cjdby visually comparing those inlets to features on the F-35, but their location makes absolutely *no* sense for heat exchangers because you’d have to dedicate a whole lot of unnecessary volume to secondary tunnels, through a very confined amount of space on the side walls of the inlet that should already be dedicated to the *primary* tunnel for the engines as well as the side weapons bay, to connect them to any other part of the fuselage that would be generating the heat that you’re trying to transfer out. That’s a lot of unnecessary piping eating up valuable real estate that could be averted simply by putting those kinds of inlets closer to wherever the actual sources of secondary heat inside the fuselage would be).
put other things aside, i seriously doubt su-57 is lighter weightered given what Russian showed on how it was constructed.
the chinese has claimed multiple times that J-20 has 3d-printed mainframe where su-57 is still built in traditional rivet that requires a lot of additonal parts therefore weight
and no, that was not a variable intakes of su-57, a variable strake is not a variable intake
despite su-57 has a similar sweep angle to j-20 (47-48 degree if my memory is correct), but consider su-57 has much wider and the central lift body, the overall wetted surface is much larger therefore the overall drag especially in high speed would be much larger
so I seriously doublt su-57 has the similar or even close performance to j-20 at supersonic
Last edited:
that was the cruising speed they are talking about
the reason why they mentioned about the mach 2.8 is bacause it is a critical speed for most aircraft in terms of heat resistance, above that you either go SR-71 or mig-31 or some more advanced heat treatment methods. Below that the heat is generally still able to be handdled by regular aluminum.
- Status