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The F-22 has square nozzles, while the Russian Sukhoi PAK FA has circular nozzles.

There are both 5th generation planes, but with different nozzles, why?

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  • $\begingroup$ have you looked at this? aviation.stackexchange.com/q/16268/1467 $\endgroup$ – Federico Sep 20 '15 at 12:24
  • $\begingroup$ @Federico It does not clarify the reason why Sukhoi has 3D-vectoring nozzles with 2 engines while Lockheed Martin decide to simplify. $\endgroup$ – R S Sep 20 '15 at 12:38
  • $\begingroup$ that boils down to different specifications. And to know why the specifications look like they do, you have to ask that question to whom has written them. $\endgroup$ – Federico Sep 20 '15 at 12:48
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Circular nozzles have these advantages:

  • They can be swiveled in any direction, so both pitch and yaw moments can be produced. Square nozzles only allow to create pitch forces, because their panels can only be moved around one axis. When moved differently in two-engined aircraft, limited roll moments are also possible.
  • Round nozzles are lighter. As long as exhaust gas pressure is higher than outside pressure, a square nozzle will be subject to bending loads across the width of its panels. This will make a square nozzle much heavier than a round nozzle.

Actually, square thrust vectoring nozzles have just one advantage: They can be made to reflect radar waves in a few well-defined directions, while round nozzles will scatter them all around. This makes square nozzles the preferred choice for stealth aircraft, regardless of thrust vectoring.

Engine nozzles of the F-22

Engine nozzles of the F-22 (picture source). Note the serrated rear edges of the thrust vectoring panels.

Engine exhausts of the B-2

Engine exhausts of the B-2. Since the B-2 uses four non-afterburing engines, fixed nozzles are used. Their location helps to protect the airplane from detection by infrared (IR) sensors, but makes thrust vectoring impracticable.

Engine nozzles of the YF-23

Engine nozzles of the YF-23 (picture source). The lower part is longer to shield the hot exhaust against IR sensors from below.

If the goal is to mostly reduce radar reflections in the forward hemisphere of the aircraft, round nozzles can be tolerated. Only when the aircraft needs to be protected from radar-guided missiles fired from behind will the square nozzles give a clear advantage. It seems that the people behind the Sukhoi PAK FA (and other stealth fighters like the Chinese J-20) do not worry so much about this detail.

Sukhoi T-50 from below

Sukhoi T-50 (prototype for the PAK FA program) from below (picture by Dmitry Pichugin)

Chengdu J-20 rear view

Chengdu J-20 rear view (picture source)

However, even Lockheed-Martin and Pratt & Whitney now place maneuverability and weight over stealth in their nozzle design for the F-135 engine of the F-35 strike aircraft. Note the serrated edges on all contour steps. This helps to reduce radar returns, but is much less effective than a square nozzle. The benefit is less weight and better maneuverability.

F-35 in flight

F-35 with the F-135 nozzle visible (Picture source). Detail view below:

F-135 nozzle

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  • $\begingroup$ Another reason the F-35 uses a round nozzle is because of the planned vertical thrust vectoring mode. The designers got the idea for its operation from an ordinary house ventilation elbow, which has angled rotating sections allowing a variety of configurations. The F135 engine variant in the B and C STOVL versions will rotate in a similar way, during flight, so the circular exhaust prevents unwanted thrust moments in odd directions during transition. $\endgroup$ – KeithS Sep 21 '15 at 15:07
  • $\begingroup$ @KeithS: Uhhh - the Yak-141 is not a ventilation elbow, as far as I know. Actually, this technology was licensed from the Russians for the F-35. And to use a rectangular nozzle on the non-VTOL variants would have been easy. $\endgroup$ – Peter Kämpf Sep 21 '15 at 15:09
  • $\begingroup$ Watch this Youtube, it shows the system in an actual F-35 airframe: youtube.com/watch?v=iRgcC9eqEJg. You can see the rotating sections in action as the nozzle transitions. The nozzle itself rotates through at least 45 degrees. $\endgroup$ – KeithS Sep 21 '15 at 15:39
  • $\begingroup$ As far as the technology, yes it was Russian-derived, but less licensed and more outsourced. Lockheed basically financed Yakovlev's continued development of the nozzle after the CIS cancelled funding, in return for full access to Yak's work for use on the X-35. $\endgroup$ – KeithS Sep 21 '15 at 15:58
  • $\begingroup$ As far as using a rectangular nozzle on the A-variant, the airframe was designed to be modular, allowing the STOVL systems to be incorporated into the same basic design as the CTOL A variant.. Using a different exhaust configuration on the A would have meant re-engineering the rear third of the aircraft, at least. $\endgroup$ – KeithS Sep 21 '15 at 16:01
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The type of nozzle used in the aircraft depends on the design decisions taken. In short, the designers of F-22 chose stealth over manuverability, while the Sukhoi PAK FA designers went the other way around.

Circular thrust vectoring nozzles have been used in Sukoi aircraft for quite some time. The main advantages and disadvantages of the two types of thrust vectoring nozzles are:

  • The circular thrust vectoring nozzles allow control in all the three axes. For example, the PAK FA's engines have a deflection of of 15 (or 16) degrees in pitch axis and 8 in yaw axis, and through differential movement, in roll as well.

Su 35 TVC

Source: defencyclopedia.files.wordpress.com

  • The 'flat' thrust vectoring nozzles used in F-22 allows control only in the pitch (and roll) axis. However, they do allow higher deflection compared to the axisymmetric nozzles, with the F-119 engine used in F-22 allows deflections upto 20 degrees.

F 119 Engine thrust vectoring

Source: pw.utc.com

  • Another point is that the PAK FA uses a variant of the engine used in Su-30, the AL-41F1. A new engine is still under deveoplement. So the final configuration can very well be different, though I doubt it as Russians seem to value manuverability above stealth.

  • The main disadvantage of using a axisymmetric nozzle is that it is inherently unstealthy. As far as stealth goes, the circular (or spherical) section is one of the worst design choices. The F-22's nozzle design is optimized for stealth, especially the edges of the thrust vectoring nozzles are carefully aligned to prevent radar reflection. This is clearly visible in the planform views of the two aircraft.

PAK FA F-22 Planform Comparison

Source: http://manglermuldoon.blogspot.in

  • There have been efforts to reduce the RCS of axisymmetric nozzles by incorporating design choices like serrated edges, geometrical shaping (like very low curvature nozzle petals), advanced cooling system, and special coatings on internal and external structures. These were fist tested on the F-16 Low Observable Axisymmetric Nozzle (LOAN) program and the resuls were applied to the F-35, another stealth aircraft with thrust vectoring nozzles.

F-16 LOAN

Rear view of a standard P&W F-100-200 in F-16 and the LOAN nozzle in the foreground (LMTAS photo), Source: F-16.net

However, according to ausairpower.net analysis of J-20 which uses an engine similar to PAK-FA,

The tail aspect sector is largely degraded in RCS performance by the use of axi-symmetric nozzles which introduce strong specular and diffraction returns

  • In general, a square nozzle provides improved stealth behind and below the aircraft. The F-22 nozzle has serrated shaping similar to that in LOAN nozzles, which further help in reducing RCS.

Serrated edges in F-22 Nozzle

Source: http://defence.pk

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