Indeed. Mig-25 had a thrust to weight ratio of 0.4 but could still manage Mach 3.0 speeds if they really push it.
Chinese Engine Development
- Thread starter jackbh
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Indeed. Mig-25 had a thrust to weight ratio of 0.4 but could still manage Mach 3.0 speeds if they really push it.
manqiangrexue
Brigadier
I don't think that's true unless in a vacuum. Within any medium (air or liquid), the greater your velocity, the greater the frictional resistance force you must overcome. Your maximum speed is the equilibrium reached when the force from the thrust of your engines is equal to the force imparted by air resistance at that speed.
Yes but that aircraft was designed optimized to speed speed speed. Everything was sacrificed for speed so it was actually not a good fighter but an excellent long distance fast dash interceptor.
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Going by that logic the Concorde would never go supersonic.
Aircraft top speed is a very complicated matter because of two factors
1- Engine thrust changes with speed.
2- Drag coefficient changes with speed
3- In most aircraft structural, heat or control related problems are what limit the top speed. For example F-16 is limited by aeroelasticity
The SR-71 had to dive in order to break Mach 1. At full afterburner, it couldn't break Mach 1. But once it is at Mach 1.4 or so it could accelerate further without any problems without diving. At Mach 3.2 it was at low throttle and was limited by temperature.
The basic rule is as the aircraft goes faster the ratio of aircraft speed/to exhaust speed increases. This lowers the thrust in a linear way. If you accelerate to half of your exhaust speed you lose half of your thrust. This is called the velocity effect. The faster your exhaust speed is less thrust you lose per gained m/s velocity.
Another effect is the ram effect. The engine ingests more air at higher pressures as you accelerate. The mass increase is linear too (but only observed after some speed since the compressor sucks air in any way) but the pressure is exponential. This increases both the exhaust speed and mass. So after a point ram effect takes over the velocity effect and thrust starts to increase. Only low-bypass turbofans and turbojets ever reach this point. (Recent high-bypass engines reach this point too) Other propulsion types have too low of an exhaust velocity. For them, the velocity effect kills all the thrust and limits the top speed below the point the thrust starts increasing. The higher the exhaust velocity is earlier this point is reached.
Some aircraft reach a point where thrust starts increasing faster than the drag. This happens quite late and requires a good intake that recovers the pressure even at high speeds.
manqiangrexue
Brigadier
Why would you reach that conclusion by this logic? It can't be right that thrust is irrelevent to max speed; it's not right unless in outerspace. The Concorde is not a fighter; it is a passenger jet that doesn't have to worry about any of the aggressive flight envelopes that fighters so it's optimized to just speed like the FoxBat. And we saw from the Foxbat that you can achieve a lot of speed on relatively little power if you emphasize it in your design. My caveat for why J-20 would have such low top speed is if it were designed to prioritize speed last, sacrificing it for everything else.
An air breathing engine’s max speed is determined by how much additional work it can extract from the speed of the free stream, its ability to accelerate the air coming out faster than the air coming in. At some point the velocity of the free stream matches the velocity of your exhaust and you won’t be able to extract more work. The max thrust an engine can provide is not a fixed number. It’s usually the maximum at a specific set of flight conditions (mainly defined between an altitude and speed range). At highest speeds actual attainable max thrust goes down as the difference between your free stream velocity and your exhaust velocity shrinks. This is why the actual attainable speed limit in supersonic aircraft hasn’t gone up despite engines getting more powerful, and why some older turbojet powered planes actually have greater max speed than modern turbofan powered planes. It’s also why you can strap two GEnX sized turbofans onto a private jet sized plane and you’re not going to be breaking the sound barrier.
It’s also of course possible that the speed limit has more to do with other factors like preventing damage to surface materials. The F-22 supposedly could actually reach Mach 2.4 but those speeds could damage the airframe because of the amount of composites it uses on the surface. Some of those composites were stealth materials, but the point is speed limits imposed for preventing airframe damage has more to do with composites in general than stealth materials specifically. The J-20 might be able to go faster than Mach 2.0/2.2 but don’t expect that to be anything but an emergency speed. Even if you don’t permanently slag the airframe you might end up damaging the engine.
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Yes, it is. Max speed occurs when thrust equals drag. Drag is a function of speed.
Please read my post above yours.
manqiangrexue
Brigadier
Thanks. Learned a good amount from yours and @BoraTas posts.
I was under the impression for some reason, that the F-22's max speed was near Mach 3 though classified and that the Felon could do mach 2.8 or something. Compared to that, I thought that it would be kinda odd for J-20 to be limited to Mach 2-2.2 despite having the most powerful engines. Now I'm seeing that they're both performing at just over Mach 2 so it makes more sense. But WS-15 is a monster of an engine and for that reason I thought we'd get some pretty impressive speed numbers.
The maximum speed is determined by elementary Newtonian physics (thrust = drag). Jet liners don't go supersonic because the drag caused by the engine won't allow it.