J-20 5th Gen Fighter Thread IV (Closed to posting)

[ahadicow]

Well, since a DSI does reduce drag and weight, an argument could be made that sacrifices in top speed could be made for simplicity's sake. However, this still would not address the other bad assumptions being made, such as the notion that fixed geometry inlets all share the same operation parameters.

The argument would be, if simplicity or weight and drag reducing is the goal, why not just use fix-geometry inlet instead? Example has been provided. The apparent paradox is that in MiG-29's presentation, a DSI inlet is just like a fix-geometry inlet, that it's optimizable only for a fixed airspeed, but apparently F35 and J-20 had both adopted DSI design. So US and Chinese engineers go through all the trouble to adopt a new design, for what?

 

[Engineer]

This is my last reply on this thred to you about this topic, the J-20`s DSI simply can not be used for several mach numbers as long is fixed, simply like that

That is purely your own opinion. DSI has no problem covering Mach 1.0 and Mach 2.0 that are two Mach numbers. "At Mach 2.0 the pressure recovery is 0.87" for DSI.

if you can not understand it read a few papers explaining why F-15 uses a variable geometry intake of external compression and XB-70 uses mixed compression on a variable geometry intake the reason is the air mass flow that changes with speed and that will require a different size intake.

Variable geometry changes for that the throat

You should do some reading yourself first since you don't seem to realize the concepts of compression has nothing to do with whether an inlet is fixed or not. What's more, you've got your facts wrong about the purpose of variable geometry. Mass flow self regulates because the normal shock can shift:

The purpose of variable geometry is to maintain control of the normal shock at an ideal location for maximum engine performance.

 

[latenlazy]

The argument would be, if simplicity or weight and drag reducing is the goal, why not just use fix-geometry inlet instead? Example has been provided. The apparent paradox is that in MiG-29's presentation, a DSI inlet is just like a fix-geometry inlet, that it's optimizable only for a fixed airspeed, but apparently F35 and J-20 had both adopted DSI design. So US and Chinese engineers go through all the trouble to adopt a new design, for what?

Apparently the construction for a DSI is lighter and simpler than for a regular fixed inlet because of the removal of the diverter. Similarly that's supposed to help improve drag characteristics (and given the orthogonal geometry of diverters, supposedly makes it easier to reduce RCS). You could easily make an argument for how this would be an improvement on inlet design without improving the aerodynamic performance of the inlet.

Mig does make this argument, which isn't necessarily wrong. However, he brings it up as justification that the J-20 must have a certain speed limit, which makes no sense because it is not the inlet type but the specific parameters of the design that helps determines the plane's performance.

Either way, Mig-29 seriously oversimplifies what's going on. He assumes that the pressure recovery drop off point is an absolute marker of a mach limit, when it is not. Engines don't just stop working when pressure recovery begins to drop off. They work less well and efficiently up to a point that is determined by the engine's performance. He also focuses purely on the type of inlet to deduce pressure recovery, when there is in fact many other factors such as the inlet tunnel geometry. Furthermore, he asserts a performance number that simply hasn't been corroborated and is based on conjectures from bad comparisons that ignore variations in design intentions and parameters.

In short he is drawing unfounded conclusions of an airplane's performance based solely on the type of inlet an airplane is using with zero regard for variations in design parameters. If only engineering were so straightforward.

 

[Engineer]

What's the source for that? Are we just to take them at their word.

Paul Metz, the test pilot for the F-22, states that the F-22 has fixed inlet (it obviously has speeds far in excess of Mach 2.0).

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Aviation Week Jan 8 2007, "Turn and Burn", D.A. Fulghrum and M.J. Fabey

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(Third page)

So should I believe Mr. Metz or some random Russian site?

I look forward to your concession.

Exactly! Allow me to explicitly quote from

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. First, the knowledge about F-22 exceeding Mach 2.0 comes from Lockheed Martin:

The maximum thrust is classified, though most sources place it at about 39,000 lbf (173 kN). Maximum speed is estimated to be Mach 1.72 in supercruise mode and without weapons; with afterburners, is “greater than Mach 2.0” (2,120 km/h), according to Lockheed Martin.

Next, the notion that fixed inlet cannot exceed Mach 2.0 is completely groundless.

The absence of variable intake ramps may make speeds greater than Mach 2.0 unreachable, but there is no evidence to prove this.

Finally, the top speed from someone who actually flew the F-22 is 2.42. Both Su-27 and MiG-29 have variable-geometry inlet, yet their maximum speed is only Mach 2.35.

Former Lockheed Raptor chief test pilot Paul Metz says the Raptor has a fixed inlet. Paul Metz has also stated that the F-22 has a top speed greater than 1600 mph (Mach 2.42) and its climb rate is faster than the F-15 Eagle.

Fixed inlet can perform just as well as variable-geometry inlet. A certain Russian fan boy simply doesn't want to see the reality because of his bitterness with the success that is J-20.

 

[latenlazy]

The purpose of variable geometry is to maintain control of the normal shock at an ideal location for maximum engine performance.

You might want to emphasize that "maximum engine performance" has more to do with efficiency and deliverable thrust than top speed, because I'm pretty sure he'll just construe "maximum engine performance" to mean top speed.

 

[Engineer]

The argument would be, if simplicity or weight and drag reducing is the goal, why not just use fix-geometry inlet instead? Example has been provided. The apparent paradox is that in MiG-29's presentation, a DSI inlet is just like a fix-geometry inlet, that it's optimizable only for a fixed airspeed, but apparently F35 and J-20 had both adopted DSI design. So US and Chinese engineers go through all the trouble to adopt a new design, for what?

Another way to put your argument is this: if DSI is simply another fixed inlet, its designers would have simply called it a "fixed inlet" and not DSI.

Also, I think "paradox" is the wrong term to use to describe MiG-29's arguments. A paradox would imply the use of logic, whereas MiG-29's arguments have none. He first formulated an opinion that J-20 has poor performance, then twist facts so that they match his view point.

 

[Engineer]

You might want to emphasize that "maximum engine performance" has more to do with efficiency and deliverable thrust than top speed, because I'm pretty sure he'll just construe "maximum engine performance" to mean top speed.

If he does that his post would be picked apart and his arguments destroyed.

 

[MiG-29]

MiG-29, you defy logic here. If DSI can only be optimized for a specific point of speed then how is it different than a simple fixed geometry inlet design? Why bother? why not just optimize the inlet geometry to suit the flight at specfic mach number?

The fact that inlet design went from fix geometry to varable geometry to DSI shows DSI is the result of sophsitication in aero dynamic modeling and design. Not backward, like you have suggested, to sacrifice aero-dynamic perfomance for the sake of simplicity or stealth.(there is no evidence that DSI would improve stealth whatsoever) The core idea of DSI is to use a single fixed geometry "bump" to modify the air flow at a wide range of airspeed. Percisely becuase of the need to optimize for different airspeed, This "bump" need to have very complex shape that requires precise computer modeling of the airflow over its surface at various speed. It is because of this, only a few country had mastered this technology.

But in your brilliant mind, US and Chinese engineers just went lazy. "Let's go simple and leave hard stuff for Russians" and so decided to gimp high mach performance on their latest and most advanced fighter jet. But the fact is, both have successfully designed varible-geometry engine inlet before and chose DSI for its superior aerodynamic performance. Russia had never designed a single DSI hence lack the experiance and would rather nor risk experimenting that on their most important project so they went old-school, plain and simple.

To start all modern aircraft use computer modelling so DSI intakes are not different to other intake modern types.


Now DSI intakes are not superior or inferior, but are optimized to solve specific parameters and specifications.

A DSI intake is basicly a solution to get rid of the boundary layer splitter and cheapen maintainance and contruction with good results in performance and Stealth.

The fact they are made fixed is to reduce moveable parts that will require higher RAM treatment and higher cost.


The price is the speed, but that is no problem for the F-35 with its Mach 1.6 max speed or JF-17 at Mach 1.6.
because stealth is gained at lower price.

Now if you do not believe that, i can not help you, DSI are fixed as such have a narrower speed range that variable geometry, but the advantage in price is worthed.

If you want to believe the DSI will allow J-20 to achieve Mach 2.4 it is okay with me, you can speculate but the reality fixed intakes have a speed limit of around Mach 1.8-Mach 2 and you can see it in the Ching Kuo, LCA, F-18, Rafale, Su-34, Su-24, Gripen or F-16.

If you do not believe it okay, its okay, you won`t change reality by not agreeing about the fact fixed intakes have those speed limits

 

[latenlazy]

To start all modern aircraft use computer modelling so DSI intakes are not different to other intake modern types.


Now DSI intakes are not superior or inferior, but are optimized to solve specific parameters and specifications.

A DSI intake is basicly a solution to get rid of the boundary layer splitter and cheapen maintainance and contruction with good results in performance and Stealth.

The fact the are made fixed is to reduce moveable parts that will require higher RAM treatment and higher cost.


The price is the speed, but that is no problem for the F-35 with its Mach 1.6 max speed or JF-16 at Mach 1.6.
because stealth is gained at lower price.

Now if you do not believe that, i can not help you, DSI are fixed as such have a narrower speed range that variable geometry, but the advantage in price is worthed.

If you want to believe the DSI will allow J-20 to achieve Mach 2.4 it is okay with me, you can speculate but the reality fixed intakes have a speed limit of around Mach 1.8-Mach 2 and you can see it in the Ching Kuo, LCA, F-18, Rafale, Su-34, Su-24, Gripen or F-16.

If you do not believe it okay, its okay, you won`t change reality by not agreeing about the fact fixed intakes have those speed limits

Sources for DSI's having a narrower speed range?

The price for removing the diverter is speed? But then you go to indicate that all fixed inlet planes (aka planes with diverters) have the same speed limit?

PS. The YF-23 had fixed inlets and its top speed was above Mach 2.2

 

[MiG-29]

Sources for DSI's having a narrower speed range?

The price for removing the diverter is speed? But then you go to indicate that all fixed inlet planes (aka planes with diverters) have the same speed limit?

PS. The YF-23 had fixed inlets and its top speed was above Mach 2.2

fixed inlet throat limits the speed limit, engine tolerance will determine max speed, but operational speed is not equal to max attainable speed.

MiG-25 could fly at Mach 3.2 even when its intakes had lower pressure recovery and its manual sets the speed limit to Mach 2.8, but the price is the engine maintainability and service life


J-20 or F-35 could fly perhaps higher speeds but the engine service life and maintainability will requiere lower speed than the max speed attainable.

F-35 probably can fly at Mach 1.8 but the engine will have lower service life than at Mach 1.6
If you want to continue the topic let us move on to another thread let us leave it here until either of us post new pictures or videos of J-20