The Tejas Intakes          Prof.Prodyut Das

Indranil Roy has posted a lengthy thread on the above subject. Since he has directly addressed some of the queries to me I am publishing my initial response below.

The weakness of Mr. Roy’s presentation is that he has not explained why an intake buzz occurs nor does the paper mention anything specific but since the source quotes ONERA we must immediately believe that everything is fine. I may be wrong but I got the feeling he did not understand what an intake buzz is . Without that fundamental understanding of why a buzz occurs the danger I have been  pointing out in the Tejas intake layout cannot be understood. Below is a common sense understanding of the subject.

Intake Buzz:

An intake buzz is a highly destructive periodic oscillations caused by the shock wave attaching and detaching from inlet mouth compression surfaces;  the shock system moves violently and periodically along the inlet compression surface –which maybe external as in the MiG 21 and the Lightning -and the internal flow duct of the inlet suffers dramatic pressure oscillations.. Because of the high ( 2-4 atms) involved and the periodicity it is highly destructive to sheet metal structures of the region involved. To understand intake buzz ab initio we need be systematic

Step1. Wave Drag.

When a body moves through the air it pushes the air aside. At a speed  of around M 0.6 the air does not have the time to move aside and begins to compress in front of the body. At Mach M 0.8 the compression effect is so serious that the drag experienced by the body begins to rise steeply. At Mach 1 the compressed air form a “ sheet” or “wave” in front of the body and the body has to push the "sheet" to go ahead thus the thrust required increases rapidly which is called the transonic wave drag rise. At Mach 1 a “shock” is formed and behind the shock the air pressure rises suddenly. This can be beneficial if well exploited by the engine/airframe designer. In Ramjets- e.g. Brahmos- the shock pressure rise is so high it needs no other compressor.

Unfortunately this wave "sheet" shape does not remains static. With increase in velocity the wave begins to sweep or bend back the angle being given by Sinθ= 1/ Mach. Nr. So higher the speed the more this wave surface inclines back.  If the surface meeting the airflow is also swept back the Shock wave then forms a two dimensional shock complicating matters further

Step 2 Inlet Geometry

With the above as a primer now imagine a simple pitot inlet. e.g. MiG 17/19 etc.  Such an intake will generate shock wave at the lip. causing drag- which is why the rounded lip quickly was replaced by a "sharp" lip on fixed intakes e.g. F 5. It reduced drag. Now imagine if you put any surface ahead of that inlet e.g. the canard now being put on the Tejas Mk2. We have the situation where the canard will generate a shock wave and the inlet may also have a shock wave and these fronts will be moving and angling back. At a certain speed – which can be calculated easily if there was a scale drawing of the Tejas Mk2- the shock wave shed by the Canard LE edge for example  will impinge on the inlet lip . When this happens there will  be oscillating attachment and reattachment of the wave causing unknown but destructive vibrations "buzz" .This is trouble because the very high pressure acting on what is finally thin sheet metal structures.

In addition to the intake buzz in certain flight regimes the canard will shed its downwash into the inlet when when the canard is inclined to generate lift and yet again say during the 180-300 portion of a loop the canard will block or disturb the flow to the engine. How the engine behaves under such situations is unknown at the moment.

Step 3. the application

This is why in the Gripen , Viggen the Chief  Designers have the inlet extended ahead of the canard. The Rafale does that same thing more elegantly and the Typhoon is the best- actually using the canards to channelize the flow to the inlet whilst keeping the inlet away from the wave generated by the Canard though some people seem to have difficulty understanding the Typhoon's excellent intake layout. There is absolutely no possibility in these three cases of the waves interfering with the inlet flow which then becomes simpler to design reliably and test trouble free. 

Now think of the Tejas. It is a small aircraft. In a cramped space you are putting one and possibly two “wave generators” – the canard and the inlet not to mention the boundary layer slot and turbulence being created by the  forward fuselage which is significantly different in terms of all the above parameters wrt the others.  The canard’s wave will be moving in two dimensions inwards and backwards and begin interfering with the inlet .-and you think that development will be trouble free? The trouble  may occur briefly under certain flight conditions but that will be enough to hold up certification for years . I don't think the IAF will be too pleased.

This is what I call a failure of "Chief Designer- ship". It may be just possible that we trust the computers and somehow the intake buzz does not happen and the intake works trouble free. Good luck can always be handled. Imagine the alternate case. It turns out we have intake buzz. Getting that intake redesigned and building a new prototype will take at least three years which will hold up series production by that three years. Presume 2027 shifts to 2030. It will decimate the IAF. 

This is why the more mature engineers at SAAB, Eurofighter, Dassault have taken so much pains to keep the inlet ahead of the canards . This is why ADA needs to be brought under the supervision of genuine stakeholders who are anxious about the results. 

I had prepared some sketches but unfortunately am not able to load them but I think the explanation is clear. if you have an acquaintance with UG level Gas Dynamics.

I will not respond point by point because the bulk of the tweets by Indranil because i) I have not been given the original document on which Indranil is making his claims and ii) what is given does not relates to intake buzz. My conclusions as of now are

i)                 Indranil’s “ proof” and "facts"  does not prove anything about   Intake buzz. It is mainly with intake pressure recovery which is  mundane. Indeed the whole source is not being given and those who said it was an ADA document are  right -Indranil's statements not withstanding.

ii)               The burden of the “proof” is that ADA has gone to ONERA  I find nothing in the data as given which proves there will be no intake buzz.  There may be more  in the original document but I have not seen it..

iii)             The above explanation also answers Indranil’s question why the Mk1 “Eyeball”- with years of  reading and examining  matters aeronautical-  can be faster and more correct than Onera computers or irrelevant computer data be called out by common sense. We must remember that in 1989 the two IAF representative Air Cdre. Krishnaswamy ( later Air Chief)  and Wg. Cdr. Aggarwal CTP -none a Ph.D -wrote a 200 page dissent note - by gut feel quite possibly- since they did not have ADA's computers as to why the ADA proposal was unsound in several areas. The Chairman, IISc. Prof Roddam Narsimha- with zero Industrial or practical experience who was also the Thesis Examiner of the nominee Chief Designer  simply avoided the objections raised- quite possibly because at that point of time, without Raj Mahindra, there was no Chief Designer in ADA who could have tackled or challenged the IAF's points. He used his network skills ignoring the IAF's objections and passed the project as Technology Demonstrator. We are still paying for the original sin because the Mk2 is what should have emerged in 1989. People have protested but ADA has powerful backers for its non performance. The Nation is paying for this sin since then. 

iv)              The origin of the “proof” given by Mr. Roy is undoubtedly ADA. If indeed it is a peer reviewed journal I would like to review it myself to discuss further or else my belief is that it is a covert ADA effort to distract attention to its impending failure. watch out. It is a very great risk that no amount of cheerleading on behalf of ADA will mitigate the situation.

v)               Indranil has asked why I rely on my own judgement instead of the ONERA/ADA  document. I have been following the Tejas keenly for thirty years and have been subjected to much chest thumping  ( in the last century) about CAD/CAM/CAE/ Concurrent Engineering at many ADA sponsored seminars only to find very elementary mistakes had been made in every area during that period-mistakes that are being paid for not by ADA but the IAF and the country. This continues as far as I can see till today. It is safer to rely on one’s own judgment and not believe anything by any ADA official  Your experience may be different but I will wait till ADA /Tejas flies successfully. with that present layout. May happen if ADA is lucky. This new modified inlet extended by 90 cms is still too timid to be totally trouble free. It is often that the consultant will try out his riskiest solution first. he gets the data anyway. If the solution does not work he then negotiates another contract.