Quickie
Colonel
Yes, that's correct. That's why I tried to explain it doesn't has to be Mach 2.9 at that altitude to cover that distance.
Aerodynamics thread
- Thread starter siegecrossbow
- Start date
Speed of sound more or less stabilizes past 10k, though. TBH, altitude actually matters a lot here, we're assuming it's 10k altitude, but air pressure drops by 75% between 10k and 20k, implying that drag drops by 75% as well. The SR-71, for instance, cruised at about 25k meters.
One other possibility is that for the J-20, the speeds being achieved are above 10k altitude; but I'm not sure whether we can say the J-20 is designed as a high altitude fighter. What is the probable coefficient of lift there?
One other possibility is that for the J-20, the speeds being achieved are above 10k altitude; but I'm not sure whether we can say the J-20 is designed as a high altitude fighter. What is the probable coefficient of lift there?
The speed of sound is faster the denser the medium. Most combat aircraft achieve speeds of greater than Mach 2 at higher altitudes, where the speed of sound is slower.
Quick question: is the F-22 draggier or less draggy than the J-20? The F-22 has lower wing sweep angles, and a lower fineness ratio of 1.4 vs 1.62. We know that with 320 kN of power, the F-22 can reach a maximum speed exceeding 2592 km/h.
Another point of comparison is the Su-57, which also claims a maximum speed of around 2600 km/h and is slightly finer than the F-22.
Another point of comparison is the Su-57, which also claims a maximum speed of around 2600 km/h and is slightly finer than the F-22.
Quickie
Colonel
There has to be a balance between air drag and jet engine power which have an opposite relation to air density. There comes a point when the rarefied air starts to be detrimental to speed. Mach 2.9 is not the only maximum speed that is possible. The speed of Mach 2.7 or Mac 2.8 at some lower altitude can still cover the same distance in the same amount of time, assuming air density hasn't start to pose a problem.
No one can say for sure in a comparison between the F-22 and J-20, but the F-22 is probably a bit more draggy than the YF-22. They relaxed the supersonic drag requirements because the engines were more than sufficient to meet target supercruise, so adjustments were made to help maneuverability at the lower end of the flight envelope.
Variable inlets aren’t absolutely necessary for speeds past Mach 2.0. There are multiple ways to attain sufficient pressure recovery past that speed. Fixed inlet geometries have gotten more advanced with improvements in computation, and an inlet designed with a system of bleeds can also manage subsonic airflow through the inlets. Furthermore, as engine design and control systems have gotten more advanced they’ve become more tolerant to shocks and irregular flows as well. The F-22, for example, has fixed inlets, and can probably go beyond Mach 2.2. The limiter on the F-22’s top speed isn’t its inlet, but surface heating of its composites.
When we are talking about maximum speed at altitude we are always talking about "true airspeed". As you climb, your indicated airspeed goes down on your analog airspeed gauge, and so you have to calculate "true airspeed"...
True airspeed is the actual calibrated speed through the air, that's why speed records are always calculated by using two speed runs on a reciprocal heading, you break the timer on a heading outbound to begin that leg and break another timer at the end of that leg, you then turn around and break the timer as you begin the inbound leg and finish your run on the inbound leg by breaking the timer that started your outbound leg.
You then take those two numbers, add them together and divide by two... for your true airspeed
That way you need not be concerned about air pressure and its effects. Both recip and turbine engines both lose thrust as they climb into less dense air, the supercharger and turbo are able to increase available horsepower at sea level by compressing the mixture, although as you compress that mixture you increase the heat production, hence higher octanes and intercoolers are necessary to support combustion and avoid detonation..
All turbines lose thrust as they climb, as most poster's probably realize you can play with variable inlet design to achieve a "ram-jet" effect, effectively increasing the amount of thrust available at a given altitude
The F-22 has a fixed inlet, whereas the J-20 and F-35 have DSIs. I'm not sure if DSIs can achieve the same efficiency at speed, and I'll happily admit I don't know enough about whether DSIs might have better supersonic performance at high altitudes (lower air pressure) and speeds.
@AFB: So we're in agreement? There is a claim made on Chinese state television that the J-20 can achieve 3000 km/h. There are factors arguing for (low drag characteristic of J-20), as well as factors arguing against (Stealth is ruined, airframe is ruined, engines are ruined, DSI inlet). We need more information before we can ascertain the reliability of the claim.
plawolf
Lieutenant General
The J20 doesn’t have a traditional DSI, the double hexagons near the inlet mouths are openings.
I have said for years that those are to allow for additional airflow to either be brought in, and/or bled out.
That should allow the J20 to vary the optimal speed of the intake in much the same way a traditional variable intake does, with the main difference being that the J20 uses airpressure to control the airflow inside the intakes, whereas a conventional variable intake uses a hydraulic ramp.
That would allow the J20 to achieve a wider speed range than would be possible with just a fixed DSI intake without the full weight and stealth penalities of a traditional variable intake.
I have said for years that those are to allow for additional airflow to either be brought in, and/or bled out.
That should allow the J20 to vary the optimal speed of the intake in much the same way a traditional variable intake does, with the main difference being that the J20 uses airpressure to control the airflow inside the intakes, whereas a conventional variable intake uses a hydraulic ramp.
That would allow the J20 to achieve a wider speed range than would be possible with just a fixed DSI intake without the full weight and stealth penalities of a traditional variable intake.