SDF Aerospace and Aerodynamics Corner

MiG-29

MiG-29

Banned Idiot
latenlazy said:
That lost in thrust applies to the loss of pressure recovery and is non unique to the DSI though. Even planes with variable compression don't have 100% pressure recovery at high mach, and so they are subject to the same phenomena.
Click to expand...

F-14 and F-15 have better pressure recovery, but here is the question how the J-20 achieves Mach 2.5?

why the F-35 achieves Mach 1.6 and the J-20 mach 2.5 if it is true having the same intake type DSI with same cowl lip?

The cowl lip is part of the boundary layer divertion, it does divert boundary layer away from the engine duct, on X-35/F-35 the intake cowl was modified so the bump was moved just to avoid the intake getting in boundary layer.

i only have one explanation either the DSI of J-20 is variable, which increases weight, RAM etc etc, plus imposes the need to modify the bump in flight or part of the intake duct and applies aerodynamic principles not published in the West or even known or the other possibility, that claim is totaly fake wrong and just a bluff.
Up to what i have read that is not possible so in my opinion the J-20 does not reach Mach 2.5 due to its intake design
 
Engineer

Engineer

Major
MiG-29 said:
a few details

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A rule of thumb often used is that 1% pressure loss reduces
thrust by 1%, but it became clear early on that the thrust loss
caused by pressure losses in supersonic flight increases
nonlinearly. For example, at a flight speed of Mach 2.2, a
typical engine losing 8% of the free stream total pressure
through the intake will suffer a reduction in thrust of 13% and a
5% increase in fuel consumption [1].

So your DSI at Mach 2 by losing 13% will lose around 20% of thrust and increase fuel consumption 9%; but that is not the only problem but creating supercritical states inside the engine duct, on mixed compression there is variable geometry not fixed.
Click to expand...
So? Pressure recovery ratio decreases on all aircraft as speed increases, and is not a unique phenomenon that only occurs on the J-10 or J-20. In the figure quoted in 2005, the pressure recovery ratio at Mach 2.0 is the same as that of F-4D. This means that F-4D loses 20% of thrust eventhough it has variable-geometry intakes. Nevetheless, the aircraft is still capable of going pass Mach 2.0 to reach Mach 2.2. There is no reason why an intake 0.03% better pressure recovery ratio won't work at the same or better speed.

Keep in mind that
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of 0.58 at MTOW.
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in similar condition is quoted as 0.98 (I assume this is quoted at MTOW). This means J-10 has more acceleration eventhough it suffers the same thrust loss at F-4D. In other words, at a speed like Mach 2.2, F-4D's engine won't allow the aircraft to go any faster while J-10 still can. Using
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as a reference, J-20 is expect to have a thrust-to-weight ratio in the range of 1.0~1.09. J-20 would have even more acceleration than J-10.

So what does this say? Same pressure recovery results in the same thrust loss in percentage. Yet, the remaining thrust still produces higher acceleration. There is no reason why J-10B and J-20 cannot fly faster than F-4D.

MiG-29 said:
and here is the question how J-20 achieves Mach 2.5?
Click to expand...

I do not believe J-20 can reach that speed, but being able to achieve something like Mach 2.1 would still be higher speed than Mach 2.0. You have no support that all DSI has an absolute top-speed of Mach 2.0 whereas there are evidences that suggest your claim being false.
 
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MiG-29

MiG-29

Banned Idiot
Engineer said:
So? Pressure recovery ratio decreases on all aircraft as speed increases, and is not a unique phenomenon that only occurs on the J-10 or J-20. In the figure quoted in 2005, the pressure recovery ratio at Mach 2.0 is the same as that of F-4D. This means that F-4D loses 20% of thrust eventhough it has a variable-geometry intake. Nevetheless, the aircraft is still capable of going pass Mach 2.0 to reach Mach 2.2. There is no reason why an intake 0.03% better pressure recovery ratio won't work at the same or better speed.

Keep in mind that
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of 0.58 at MTOW.
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in similar condition is quoted as 0.98 (I assume this is quoted at MTOW). This means J-10 has more acceleration eventhough it suffers the same thrust loss at F-4D. In other words, at a speed like Mach 2.2, F-4D's engine won't allow the aircraft to go any faster while J-10 still can. Using
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as a reference, J-20 is expect to have a thrust-to-weight ratio in the range of 1.0~1.09. J-20 would have even more acceleration than J-10.

So what does this say? Same pressure recovery results in the same thrust loss in percentage. Yet, the remaining thrust still produces higher acceleration. There is no reason why J-10B and J-20 cannot fly faster than F-4D.



I do not believe J-20 can reach that speed, but being able to achieve something like Mach 2.1 would still be higher speed than Mach 2.0. You have no support that all DSI has an absolute top-speed of Mach 2.0 whereas there are evidences that suggest your claim being false.
Click to expand...

General characteristics F-4D

Max. speed 1,320 mph 2124 km/h



that is not exactly Mach 2.2


F-4E Phantom II
Max. speed 1,485 mph 2390 km/h
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plus consider flame outs, consider F-4 has not supercruise and barely flew at Mach 1 most of the time J-20 supposedly will fly at mach 1.8 all the time and mach if needed, so tell me if pressure recovery is not important


Home Base: Houston, TX
Operation: Western, Central and Eastern USA
Model: F-4D
Wing Span: 38' 5"
Length: 63' 0"
Height: 16' 6"
Max Speed: 1,485 mph/M. 2,25
Gross Weight: 61,651 lbs
Power Plant: 2x General Electric J79-GE-15 turbojets
Thrust: 2x 17,900
Fuel Capacity:
Armament: One 20-mm M61A1 rotary cannon; Four AIM-7 Sparrow missiles or 3,020 pounds of weapons under fuselage; Up to 12,980 pounds of various weapons on underwing pylons.
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but let us say it achieves mach 2.2
it uses tuyrbojets which are more fuel thirsty and use afterburner more often
Electric J79-GE-15 turbojets
 
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MiG-29

MiG-29

Banned Idiot
Engineer said:
Designers of F-35 traded off speed for thrust by using an engine with a larger by-pass ratio. The DSI on the F-16 testbed allowed the aircraft to reach Mach 2.0.
Click to expand...

F-35 has more thrut to weight ratio so it carries more weapons has longer range and carries more fuel, plus allows for a stealth airframe
 
Engineer

Engineer

Major
MiG-29 said:
General characteristics F-4D

Max. speed 1,320 mph 2124 km/h



that is not exactly Mach 2.2


F-4E Phantom II
Max. speed 1,485 mph 2390 km/h
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Click to expand...

It is still Mach 2.0, achieved using intakes with pressure recovery ratio of 0.87 at that speed and an inferior thrust-to-weight ratio than a J-10.
 
Engineer

Engineer

Major
MiG-29 said:
F-35 has more thrut to weight ratio so it carries more weapons has longer range and carries more fuel, plus allows for a stealth airframe
Click to expand...

F-35 sacrificed speed to achieve that kind of thrust, and it does so using a bigger bypass ratio on its engine than F-22's engines.
 
MiG-29

MiG-29

Banned Idiot
Engineer said:
It is still Mach 2.0, achieved using intakes with pressure recovery ratio of 0.87 at that speed and an inferior thrust-to-weight ratio than a J-10.
Click to expand...

that is not the only problem if that was the problem then you would fly an AMX very lightly and make it fly at mach 2, but the real problem is engine damage or plain surge and compressor stalls

As the air is brought from free stream to the compressor face, the flow may be distorted by the inlet. At the compressor face, one portion of the flow may have a higher velocity or higher pressure than another portion. The flow may be swirling, or some section of the boundary layer may be thicker than another section because of the inlet shape. The rotor blades of the compressor move in circles around the central shaft. As the blades encounter distorted inlet flow, the flow conditions around the blade change very quickly. The changing flow conditions can cause flow separation in the compressor, a compressor stall, and can cause structural problems for the compressor blades. A good inlet must produce high pressure recovery, low spillage drag, and low distortion
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So it is not just thrust to weight ratio but surge and flame outs plus damage and increased maintainance, thus in the F-4D you have a fighter that usually does not fly at Mach 2 and most of its flight time is at Mach 0.9 this was the real reason it could have a relatively low pressure recovery but at mach 2.2 it will fly very few minutes and will guzzle a lot of fuel.

But on J-20 it supposedly flies at Mach 1.8 cruise speed and Mach 2.5.

To be economic at Mach 1.8 needs excellent pressure recovery and do not use afterburner means very high pressure recovery.

However DSI`s 0.91 pressure recovery is not higher than on F-14`s intake at mach 1.8 which is close to 0.98.
 
L

latenlazy

Brigadier
MiG-29 said:
F-14 and F-15 have better pressure recovery, but here is the question how the J-20 achieves Mach 2.5?

why the F-35 achieves Mach 1.6 and the J-20 mach 2.5 if it is true having the same intake type DSI with same cowl lip?

The cowl lip is part of the boundary layer divertion, it does divert boundary layer away from the engine duct, on X-35/F-35 the intake cowl was modified so the bump was moved just to avoid the intake getting in boundary layer.

i only have one explanation either the DSI of J-20 is variable, which increases weight, RAM etc etc, plus imposes the need to modify the bump in flight or part of the intake duct and applies aerodynamic principles not published in the West or even known or the other possibility, that claim is totaly fake wrong and just a bluff.
Up to what i have read that is not possible so in my opinion the J-20 does not reach Mach 2.5 due to its intake design
Click to expand...

Because external inlet compression isn't the only compression factor. There is also compression from the tunnels, as well as geometry from the fore body.

Besides, the F-35's speed limit has nothing to do with the pressure recovery of its inlet. In fact, top speed isn't related to thrust at all. Even if we stuck a variable geometry inlet that was tested for high pressure recovery on the F-35 it would not be able to reach beyond mach 1.6 because that's the speed limit of its engine. By going for a higher bypass design, the F135 engine slows the exhaust velocity. It is the exhaust velocity that determines speed limit.
 
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MiG-29

MiG-29

Banned Idiot
latenlazy said:
Because external inlet compression isn't the only compression factor. There is also compression from the tunnels, as well as geometry from the fore body.

Besides, the F-35's speed limit has nothing to do with the pressure recovery of its inlet. In fact, top speed isn't related to thrust at all. Even if we stuck a variable geometry inlet that was tested for high pressure recovery on the F-35 it would not be able to reach beyond mach 1.6 because that's the speed limit of its engine. By going for a higher bypass design, the F135 engine slows the exhaust velocity. It is the exhaust velocity that determines speed limit.
Click to expand...

What determines speed always is drag and weight versus thrust, however if your engine has a surge or a flame out it simply stops working, thus inlet design affects the whole design, you can have pre-compresson like in Mirage 2000 or F-16, however if you engine can not work it simply stops, surge and engine stall will determine the thrust.


MiG-25 has a low thrust to weight ratio but has variable geometry intakes, so their engines work fine at Mach 2.8, as the DSI flies at higher speed, pressure recovery worsens thus the posibility of an engine stall increases.

F-35 is a single engined fighter, same like J-10, so an engine failure may become fatal, same is JF-17, so speeds in the range of Mach 1.8 are the upper level of safety, Mach 2 could be okay, at the expense of safety, reliability, thrust, range and weapons load.

F-35 flies in the safe zone like JF-17, J-20 supossedly flies at, Mach 2,5 and supercruises at Mach 1.8, this sounds a bit technically speaking unfeasible and unlikely.
 
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