Saturn AL-31F (Flanker) vs. AL-31FN (J-10)

[Deino]

Hi guys … again me with a probably stupid question, but since there’s a harsh discussion going on in one China-forum around that theory, I would like to get some facts …

o.k. here are the claims:

From what I know, the first J-10A flew using a modified spare engines that were meant for J-11.

Since no other Russian warplane is designed for the AL31FN, it is ludicrous to think that Russia will specially designed another engine just for China J-10A indigenous fighter.

Therefore the only logical step forward for Salyut is to modify an off-the-shelf product an AL31F with a switchable gearbox for the purpose based on China's own modification and improvision to their stock AL31F engine.

PLAAF 5719 Refurbishment at Chengdu has already mastered the AL31F engine. IMO I don't think relocation of the gearbox will be a great feat for them at this time.

All this indicated to us that China aviation engineers had most probably modified spare AL31F engine supplied around with the SU-27 to fit the smaller engine bay of the J-10 then later submit their requirement and specification to the manufacturer.

To admit I’m no engineer and have surely very little understanding in aviation propulsion, but from my understanding it is not possible or at least not that easy.

Following my understanding the FN is indeed a specialized, custom-designed version of the AL-31F designed on request of CAC/AVIC in order to power the J-10. Its main difference is the position of the gear-box, making a change/swap of engines from a Flanker to a J-10 or vice versa impossible.

Again I don’t know if at least in theory it is possible to relocate the gear-box but at least in operational use it is not done … my argument was a comparison – even if here to a different content – to the US F100 engine: There are certain reasons since an engine is a highly specialized item often optimized on the airflow of a certain aircraft. As such the USAF does not rework the F100’s used in the F-16 to fit them to the F-15 or vice versa.

Again that does not mean it is impossible … but is it that easy that it makes operational sense or logical ??
IMO not – especially in operational use – even if it would be technically possible.


Even more in mind the huge purchases that are well documented – in fact all but the first one – it is ridiculous to think the PLAAF would rework Fs to FN only for fun… otherwise what engine would then the J-11As are using ? Surely not FNs back-converted to Fs ?? And so far we have not seen any operational J-11A converted to a WS-10A.

So in summary my question is:
- is possible to convert an AL-31F into an AL-31FN “simply” by relocating the gear-box?
- and has anyone more detailed information about the first contacts to Saturn in order to develop the FN-version ? How large was that first purchase, how many engines were included ?


Thanks a lot in advance,
Deino

 

[elmayerle]

I'm nowhere near familiar with the airframes in question, but most current US engines, such as the F100, F101, F404/414, F110, & F118, do not have the gearbox mounted on the engine itself, rather there's a separate removable driveshaft extending forward to a separate Airframe-Mounted Accessory Drive (AMAD) (term used on F/A-18 and B-2, others may use a slightly different name but the idea is the same) gearbox to simplify engine changes and the F-16, for example, uses the same accessory-drive gearbox for both F100 and F110 engines. If China went to such an approach on the J-10 while the Su-27/J-11 has the gearbox mounted on the engine, that could explain it. The engines wouldn't be immediately interchangeable, but with a bit of (not easy!) conversion, one configuration could be switched to another.

I hope this helps.

 

[Deino]

Yes indeed ... thanks a lot !

 

[elmayerle]

Glad to be of assistance. If memory serves me correctly, one of the first usages of this concept was on the B-70.

If you look at the underside of Singapore's F404-powered A-4s, you can see where the structure and access doors have been modified to fit the AMAD gearbox in place of the engine-mounted gearbox on the J65.

 

[overscan (PaulMM)]

Photos of AL-31FN and AL-31F make it clear that the engine accessories and gearbox were relocated from the top to the bottom. This is the difference between the two. The reason is pretty obvious - most engines have them at the bottom for ease of maintenance. On the original design for the Su-27, the T-10, they were bottom mounted. They were switched to top-mounted to allow the nacelles to be reduced in depth, which reduced drag, improving performance. This works OK for the Su-27 design.

Now look at the J-10 - the top of the engine is a poor choice for the accessories location - it would create a bulge, interfere with the spine / fin. Not to mention, the J-10 design would have been originally laid out for an engine with bottom mounted accessories, so it would be hard to change without major work.

While would certainly not be beyond China's capability to relocate these parts for a prototype engine, it doesn't make much sense for them to do so. Salyut worked on the AL-31FN in 1992-1994, when the J-10 was still a long way from complete. Salyut engineers worked on integrating the engine to the airframe, intake alterations, etc, closely with Chengdu engineers. Moving the gearbox from the top to the bottom hardly needed "proving", it was always going to be possible, and I am pretty sure was one of the easiest parts of the process, so unless we are postulating a J-10 prototype flying before 1994, I don't see the point of it.

However, that doesn't mean it didn't happen.

 

[elmayerle]

Okay, I can see that. It's a non-trivial exercise to switch something like that, but far from un-doable. Having said that, I know practice, at least here in the US is, as I said, a remote-mounted accessory drive gearbox with just the driveshaft from the engine. Now if a design required that driveshaft on top instead of on the bottom, it would still take some engine redesign but it would not be an insurmountable task. All the designs I'm familiar with, though, have that accessory drive gearbox on the bottom for ease of accessibility.

 

[aam641]

On a side note. Would engine such as F404 function any differently, if it was mounted upside down?

 

[Deino]

Just another – hopefully not too stupid – question following a new rumour from a so called "big shrimp" concerning the J-20A's engine:

According to the leader of 毛发党, the WS-15 has just finished the ground testing, and it is ready to pass the test on the IL-76 platform. While the current J-20A uses the AL-31FM2, a special customized edition of the AL-31, also a joint development between China and Salut, just like the GT25000.

Therefore and like I said since so long: similar to the once and long still secret original AL-31FN-contract for the J-10, CAC chose a similar approach for the J-20. And since these are not exact AL-31FN Series 3, they were never mentioned or included in the number of engines reserved for the J-10B and J-10C. For the thrust - if it follows the data mentioned for regular FM2 – about 142kN should be assumed … or roughly the same as the 117S used in the Su-35.

I hope I’m not completely off since this Salut and Saturn division for the same AL-31-base is still confusing. Anyway I hope I’m correct, all AL-31FN for any J-10 are from Salut and in contrast the 117 & 117 are from Saturn.

Following the regular AL-31F/FN-development, both the 117 and later 117S from Saturn and the AL-31FM1/-2/-3 from Salut were competing upgraded/modified/developed versions of the original engine for the Su-35 and T50's engine and export upgrades as well … and for their domestic demand Saturn won.

And now I’m in theory-modus since IMO it would make perfect sense:

Since Salut was always CAC's preferred supplier of engines for the J-10A (AL-31FN) and later J-10B/C (AL-31FN Series 3) it was more than a logical decision that this loosing engine became the export-related product and since CAC was already an established customer, a new special tailored version - as just reported - based on the M2 became the J-20's powerplant !?

My point is; how long did it take that the AL-31FN for the J-10 was officially acknowledged ? The J-10 was long in operational service and for everyone with a slightest bit of common sense and understanding was clear that this engine is an AL-31-derivate. However both sides decided to keep the true nature of that engine secret as long as possible. Or have there been reports about a production of the AL-31FN before it was officially acknowledged?

And now we are in the nearly same situation: For everyone with a slightest bit of common sense and understanding is clear that the J-20 uses an AL-31-derivate, but all recent contracts known and officially acknowledged are only related to the J-10B (even the numbers fit quite nicely)... as such is it so much unlikely that history repeats itself and CAC again managed to arrange a still secret contract for a still secret customised version based on exactly the loosing contender???
Even more since the J-20 is at least by now not officially unveiled.

I know I might be far off with my theory and I know that much is still in the dark but I'm sure that the Sino-Russian military cooperation stands on much more solid ground than most expect, more than most are willing to accept or even more are able to officially confirm.

Deino

PS: By the way are there any info on how much both - Salut and Saturn - cooperate ? ... or are they by now so much different that also their developments are independent?

 

[overscan (PaulMM)]

I believe you are correct, though the exact designation of the engine is uncertain (to me) its a higher thrust AL-31F derivative probably roughly equivalent to 117S engine. It is possible the early prototypes had AL-31FM1 and the latest LRIP ones the AL-31FM2 (or equivalent). Salyuts' engines are installation compatible with basic AL-31F and relatively cheaper than 117S.

 

[Deino]

Do we know the external diameter of the AL-31F/FN's nozzle ???

 

[Trident]

IIRC the front flange where it joins the airframe engine nacelle on the Su-27 is 1.2m in diameter.