Sukhoi Su-57 flight testing, development & operations [2012-current]

jamesfranchigia said:
2D (flat) nozzles don't "reduce" thrust.
The area ratio of variable geometry nozzles changes in flight so that the nozzle operates near perfect isentropic expansion, where the gross thrust coefficient (F/Fi) is almost 1 (given the unavoidable losses due to angularity, friction, leakages...). 2D nozzles behave like the axisymmetric ones (some shock pattern are different tbf), and both output 99% of the ideal thrust.
A reduction in 5% would be massive, a value you can encounter in conditions of moderate under/over-expansion.
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That’s cool but flat nozzles still definitely lose thrust. It been documented in various studies including ones done by NASA. Your argument is like claiming there is no ‘fire’ in a combustion engine…..it’s actually rapid thermal expansion where opposing force cancels out to create force transmission through a drive shaft.

 
Galaxy said:
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The paper you sent evaluates the nozzle divergence coefficient, aka angularity losses. That is a simple, unavoidable phenomenon caused by the jet at the exit not having the sole axial component. As the nozzle secondary half angle increases, you lose more forward thrust. If you make the nozzle longer, the angle decreases and divergence coefficient tends to one. This is for both axisymmetric and 2D.
On page 16 you can even see the trend of the coefficient, where it is clear that 2D nozzle has always less angularity losses (and that should be obvious since you have only two ramps moving, with the side walls keeping the jet axial).
Galaxy said:
That’s cool but flat nozzles still definitely lose thrust
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Again, the 2D nozzle behaves like an axisymmetric one, with almost the same losses (around 1%). Plug nozzles don't, and generally have lower efficiency.
You claimed that the thrust of the F119 would be at least 5% higher if a non-2D nozzle were used, a value you can’t achieve by simply using a different nozzle. The thrust you lose is due to friction, angularity, expansion, leakage and cooling air throttling loss, which are present in all nozzle configurations. Afaik there aren't phenomena that appear only in the 2D nozzle that make it "definitely lose thrust" more than in the axisymmetric. Once the area and pressure ratios are defined, the axisymmetric and the 2D nozzle will output almost the same thrust.
You can take a look at this paper https://ntrs.nasa.gov/citations/19800015775
In summary, for the test conditions of this investigation, the SERN and 2-D C-D nozzles generally produce higher and the wedge nozzle generally produces lower thrust minus-drag performance than the axisymmetric nozzle base-line configuration.
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The tricky part is the integration of the nozzle in the fuselage, that's when you have differences in aft-end drag and you may choose 2D over ax (speaking of performance only, not radar/IR signature).
 
The inherent design challenges of a 2D nozzle are the increased surface area vs a round nozzle (increased surface drag, more area to be cooled), and the structural inefficiencies of flat surfaces in a pressure vessel (pressure loads cause bending loads). However, a vectoring 2D nozzle has the inherent advantage of being able to control the convergent to divergent area ratio for optimum supersonic expansion at varying nozzle pressure ratios.

One of the practical challenges of the 2D nozzle is sealing leakage paths where the pressurized exhaust flow escapes instead of exiting the exhaust thru the nozzle. This reduces the discharge coefficient, reducing the potential thrust. A lot of effort was put into the F119 nozzle development to minimize flow leakage.
 
I would say yes, although the divergent flap transitioning from flat at the convergent throat to a shallow V at the trailing edge is different. The resulting Hexagonal shape of the nozzle, rotated 15 degrees to each side of the aircraft, does not seem to be optimal for LO surface alignment. There may be aero advantages to that arrangement.
 
It would appear that all 6 surfaces of the divergent section of the nozzle are aligned with neither wings, horizontal stabs, vertical stabs, nor airframe sidewall. They appear to generate reflections in 6 new directions.
 
jamesfranchigia said:
The paper you sent evaluates the nozzle divergence coefficient, aka angularity losses. That is a simple, unavoidable phenomenon caused by the jet at the exit not having the sole axial component. As the nozzle secondary half angle increases, you lose more forward thrust. If you make the nozzle longer, the angle decreases and divergence coefficient tends to one. This is for both axisymmetric and 2D.
On page 16 you can even see the trend of the coefficient, where it is clear that 2D nozzle has always less angularity losses (and that should be obvious since you have only two ramps moving, with the side walls keeping the jet axial).

Again, the 2D nozzle behaves like an axisymmetric one, with almost the same losses (around 1%). Plug nozzles don't, and generally have lower efficiency.
You claimed that the thrust of the F119 would be at least 5% higher if a non-2D nozzle were used, a value you can’t achieve by simply using a different nozzle. The thrust you lose is due to friction, angularity, expansion, leakage and cooling air throttling loss, which are present in all nozzle configurations. Afaik there aren't phenomena that appear only in the 2D nozzle that make it "definitely lose thrust" more than in the axisymmetric. Once the area and pressure ratios are defined, the axisymmetric and the 2D nozzle will output almost the same thrust.
You can take a look at this paper https://ntrs.nasa.gov/citations/19800015775

The tricky part is the integration of the nozzle in the fuselage, that's when you have differences in aft-end drag and you may choose 2D over ax (speaking of performance only, not radar/IR signature).
Click to expand...


Flat nozzles have more area given the same engine diameter, when you have more area you have more friction which causes more boundary layers and loss of thrust because that thermal energy equates to kinetic energy which will be lost due to the friction. However, that is one of many issues, as there is also flow separation from corners which causes uneven pressure especially in the divergent area of the nozzle. With round symmetrical nozzles you theoretically have uniform flow but your argument is that by installing a square nozzles you have almost no loss? This despite major disruption of gas around corners as well as the area of divergence which means more friction and boundary layers and uneven pressure.

And I am not even mentioning thrust vectoring which will cause even greater loss. One study cites 10% loss when the nozzle deflects and one of the issues is none uniform flow and divergence which also exists even when there is no deflection. The F-119 engine (or SU-57 engines) are slaved with the flight control computers which job is to keep the aircraft from falling out of the sky, that is the stabilizers and nozzles are constantly moving so yes there is definitely a loss in thrust not only in the inherent loss of efficiency of flat nozzles designs themselves but TVC deflection. I didn’t mince my words when I spoke about a 5% loss in thrust. I even used an AI tool and it constantly predicted flat nozzles lose 3% to 10% based on data it collected with likely 10% counting deflection or deflection plus flat nozzle design.

 
Yes, thrust vectoring will reduce the forward facing thrust by the sine of the vector angle, simple geometry. This is the same for both 2D and Axisymmetric vectoring nozzles. The total thrust is not decreased, just used in a different direction to provide the vectoring flight control, which is the whole reason to have thrust vectoring.

This effect is relatively small. At the full 20 degrees vector angle of the F119, the forward thrust loss is about 9%, while about 40% of the thrust is acting in the vertical direction. At a more common vector angle of 5 degrees, the forward thrust loss is less than 4%.

At supersonic speeds, a slight vector angle can be used to offset the nose down trim that occurs when the center of lift moves aft. This reduces the amount of download required on the horizontal tail, with the reduction in drag more than offsetting the reduction in thrust from the trim vector. The reduced deflection of the horizontal tail also allows for improved supersonic maneuverability in both both pitch and roll.
 
F119Doctor said:
I would say yes, although the divergent flap transitioning from flat at the convergent throat to a shallow V at the trailing edge is different. The resulting Hexagonal shape of the nozzle, rotated 15 degrees to each side of the aircraft, does not seem to be optimal for LO surface alignment. There may be aero advantages to that arrangement.
Click to expand...

Rotating the nozzles should not have any noticeable effect on RCS since the nozzles are at the very rear where the electronic energy (EM) energy will be scattered and redirected to the sides with an offset angle. The F-117 used the same methods but to a more extreme level where the EM scattered in dozens of directions at various areas of the aircraft ie: front, sides, top, rear, ect. Those faceted areas also created corner reflectors.

In fact rotating the nozzles actually lower the RCS from the side hemisphere for two reasons. Firstly the side nozzle itself it almost flat so if they aligned the engines at zero offset it would create a 90 degree angle; moreover, the nozzles themselves would create 90 degree corners at deflection or divergence (red wedge shapes in the background photo). It was absolute necessary to rotate the nozzles or have a large RCS from the 90 degree corners. The nozzles themselves also have perfect platform alignment with the rest of the aircraft ie: the green lines in the photo.

IMG_2256.jpeg
 

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