The Tejas Mk2 corrections Prof. Prodyut Das
The news is
that the Government will not fund development of the Tejas Mk2 without first
securing the ToT for the F 414 engine. The Government has a choice of three
suitors and would naturally like to drive a hard bargain. A subtle other possibility
could be that the Tejas Mk2 Layout as was supposed to have been final has a
serious flaw and getting a second opinion/confirmation is felt necessary. The
idea of holding up the project until an engine is finalized could be a mask. I
believe the hold is no bad thing because the project was driving towards a
precipice.
The Tejas
Mk2 project was started in 2007. One should not need funds to think and pipe
dream but the project was funded in amounts of Rs 500 crores in 2007 and 2500 crores 2017 so the usual “shortage of funds” was not an issue. A roll out was
announced for December 2022 or thereabouts.
It was here
that things began to de- rail. It so happened that an aircraft enthusiast wrote
in Vayu VI/2021 about a possibility of the Canards being in the wrong position
vis a vis the intake. He used examples of the Eurofighter, Rafale and Gripen to
illustrate his point of view. The problems anticipated were:
1. 1.There would be intake buzz at
certain Mach nos. Attached is a simple sketch of the problem and it shows that
the shock wave off the L.E. canard would impinge on the inlet between 1.1 and
1.3 Mach starting a buzz. It would continue beyond M 1.6 .
2. 2.The use of the canards to control inflight variations would disturb
the inlet flow and cause the engine to “hunt” unnecessarily. This could lead to
reliability problems in the normally well- behaved F 414 engine.
3. 3.The use of the canards as an
airbrake on landing could result in the engine occasionally flaming out.
This was in 2021 and the apprehension voiced was met with the usual ridicule and scurrilous insinuations. In a possible illustration that “The humble advice becomes palatable only when stale” the ADA announced in March 2023 that a Tejas team was in contact with the French ONERA and the Tejas Mk2 intakes would be studied there. The specific problem of the intake for which the consultations is sought is not mentioned.
We have over 7000 hrs of flight experience with the essentially similar GE F 404. The F 414 has slightly higher- 77 vs 72 kg/sec- mass flow. We should have mastered Y intake technology by now. A pitot intake as in the Tejas is 1950s "old hat" technology and would be a discredit if ADA needs consultancy so late in the day. So why the flurry at the eleventh hour? And don't fall for teh explanation that X.y.z countries use ONERA ; their population base is minuscule and they did not come after forty years.
Before I conjecture on the question as to what is wrong with the Tejas intakes let me fill in the history. For the record there have been misgivings reported
about the Mk1 intakes amongst many many other things; I think the early prototypes did not have blow in and dump doors. The Tejas Mk 1 does not achieve its design sea
level speed (M 1.15) being capable of just about M 1.01 and one reason is that
the F 404 does not develop its full thrust with the present intake system. It
could be the intake but given that remarkable short 13.2 fuselage length it
could also be the Co F or just poor build quality. If it is not any of those
problem then it probably the inlet canard interference that is now worrying the ADA gentlemen and ADA is trying to see
if some “fix” can be done by ONERA to make the canard inlet interference risk
free without showing the up the goof. Another possibility- we are dealing with an organization that has managed to get a FOC for the Tejas w/o gun firing being completed - is that the Tejas Mk2 is not really needed but simply a "stealth" way of getting more funds to clear the design errors which ADA knows is in the Tejas Mk1 & more funds will attract attention at least to its past incompetence. That is the thought.
The aircraft was supposed to be rolled out on
various dates in 2023 and this need for a consultancy at the 11th
hour is unsettling of “investor” (i.e. you and me) confidence. In my native
district Bardhamaan (Burdwan) there is a local saying that roughly translates
as “At the time of the ceremonies the bride wants Potty!” (We be not a couth people!) and the present need for consultancy at the last moment is
something like that.
If the need for consultancy IS the canard inlet
interference, then this is what ADA will get written in French & with
graphs and table i. e. the same what was predicted in 2021:
i)
The
inlet is likely to suffer inlet buzz between M 1.1 and M 1.3
ii)
The use of the canard as a pitch control will cause irregular flow to the F404 engine and this
may cause reliability issues. The French, I am told, imported some F 404s for
their M2K programme and may have some clues about this engine but
how accurate their predictions will be is not known.
iii)
The
use of the canard as an airbrake may cause engine flame out during the landing
roll. May be not every time but once in a while cannot be ruled out.
The consultancy is not only a
waste of time , money and effort , it has ominous portends. Perhaps the idea of the ONERA consultancy is to
use it as a talisman to get on with the project. If the report is even mildly
assuring- I would not have accepted it even if ONERA’s recommendation was
bound in red leather and lettered in gold - the project goes ahead. If problems does show up then ADA
can say that it was misled by wrong advice. many DRDO labs are fund hungry; the rest, perhaps mercifully, somnolescent.
The possible solution(s)
The solution is completely simple. The shock
wave forming at the L.E. of the canard will impinge with the inlet lip. As the
schematic sketch shows all that needs to be done is to extend the intakes ahead
of the canards L.E. by about 300 mm thus avoiding the interference between the
two. In the Viggen and Gripen it was done ab initio; On the Tejas it would look a bit crude but it would work and avoid all this "bobbery" ( baap rey!) going on.
Such a solution will be about 40 kilos heavier,
will have slightly more pressure loss but more even distribution at the compressor face and there will be increased
friction drag but these losses could be tempered by introducing more current
area ruling practice. The proposed fix is not an elegant solution; the channel
between the canard lower surface and the intake upper bulge forming a "super velocity" re-entrant channel like
in the TEBDF. However, work it will! There will be absolutely no interference problems.
This could be tried as an interim solution so that the trials can go on. Some 40-50%
of the existing structure and its tooling could be used, the rest of the parts
would probably have to be re-worked or scrapped. This again could be looked at positively. Much of the Tejas structure could anyway do with a relook with an aim to achieving a target weight of 5300 kilos for the Mk.1 versions.
This should be much better than proceeding with the present layout hoping that somehow ONERA advice will do the trick. It won’t.
The deluxe solution
The real deluxe solution I feel would be to take the crisis as an opportunity and re- model the entire front fuselage to avoid the problem AND reduce the radar signature. This will depend on the exact contours of this area so I can’t give a sketch or description but picture is worth a thousand words and the sketch of the Sukhoi Su 75 illustrates the basic idea i.e. extend the inlet and blend it with the contours to achieve what I would call a supersonic Grumman A 6 E. If you are smiling at that description you are yourself probably fit to do the job! I am serious! What aircraft design needs is a lot of "feel". In the process you could probably do away with the inlet diverters as you bring the inlet forward and the boundary layer does not grow quite so much. The proposal has merit.
I confine myself to saying that I would have done
both solutions; The first to get the programme into the air and on with the
testing and the second deluxe solution as early as possible thereafter so that when
after all the test flying the mods indicated by the tests are being sorted out.
i.e. Block 1 and block 5 or 10 the design can claim reduced radar signature.
The vex
ADA has disappointed unfailingly. ADA’s performance can be gauged from the fact that starting out with an aim to design an aircraft with an empty weight of 5350 kilos it has till date achieved a weight of 7000 kilos. That too after forty years.
If we delete the BOC items i. e. items over whose weights
ADA does not have any control e.g. engine, ejection seat, guns, radar etc the
picture is that the design is 46% over the target design weight. With these
figures one cannot expect an acceptable platform which is why all the Tejas Mk1 produced are parked at 26 ED along with the spares for Spitfires XVIs that 26 ED rumored to hold on stock. I fear that ADA will rest on its laurels if it achieves 6500 kgs for the Mk1A knowing that is sufficient to get the next project funded and keep the private sector out.
The only silver lining is that the extent of the weight overshoot is so high that it must be quite easy to make significant improvements without much knowledge effort because the basic layout done by Dassault does have potential. Without in any way endorsing the Rs100 crore ( 1989 value ) consultancy fee allegedly paid to Dassault, the Dassault team were professionals and gave a workable idea.
If we focus on getting the weight of the Mk1 down to it’s original
figure of 5350 kilos- and that is quite possible- the design is quite a sizzler
with a general 20% all round improvement over the present performance and perhaps a 30%
increase in range payload over the present. Quick calculations show it would have half the wing loading and the same thrust to weight ratio as the F 16A's in clean combat condition which should offset the handicap of the bad 1.8AR. The fix for that i.e.. low AR is the crank wing tweak of the existing wing proposed in an earlier article of Vayu in 2010.
The problem is not technical. We need an aircraft development capability that
will put out a proposal within a year and a prototype in 4 years with testing being completed in a further three years; that is the
qualifying standard or else we are producing obsolescence and junk. This is what must be organized.
It will be noted that the Tejas Mk2 project
started in 2007 and after all that time and effort it has come up with what to me is a “dud”! What hope is there for AMCA with this kind of
capability? A purge and change in management are necessary or ADA will continue de facto to support the Import Lobby’s efforts whilst absorbing precious funds and time.
It is stupidity to repeat the same motions and expect different results. The
corrections to the Tejas's problems lie not in Bangalore but Raisina.
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