The Tejas Tweets – Replies to Vina’s points                                          Prof.Prodyut Das

 

“Vina”  has very kindly sent a set of points for discussions. I have already replied to them –literally off the cuff-on twitter but, seeing the interest generated, I felt that I should post it on blog as a more easily accessible record for future use.  The link he refers to is an application for a patent by SAAB in 1963 and is given here. s://patents.google.com/patent/US3188

 

My comments are as follows:

The attached patent application (PA) is very interesting but is irrelevant to the topic which I restate: The location of the inlet as shown on the Tejas Mk 2 proposals models is, in terms of development, very trouble prone. It will hold up certification. 

2. Before I move on to the topic above I will mention that the PA gives improved pilot visibility as a reason for reducing the inboard sweep- item 16 in the diagram-and shortening the length of the fuselage.  Both reasons are valid but I suspect SAAB  has not mentioned the third reason- this being a patent application and would be subject to scrutiny by rivals- that the reduced inboard sweep also actually helps to improve the lift for a given size of wing. I have a theory for it. What PAs don't  reveal or talk about is often more interesting than what they say.

3. Most of the fears expressed in the PA has been laid to rest/ apply to close coupled Canards as applicable to legacy Fighters.. It is not that the theoretical reasons cited are invalid but, depending on the purpose of the design, aircraft have been designed and produced in numbers which have flouted these fears.  The above statement about the canards located far ahead of main wing is probably only true if one considers 1960s technology and naturally stable aircraft. The fact that Eurofighter/ - as Vina him/herself  has twitted- would indicate that the fears of 1963 are manageable now. The XB 70 Valkyrie  used long moment arm canards. Similarly canard at the same level as the main wing can be workable has been demonstrated in the YF 17 and productionized in the FA 18. The LERX is , de facto, a fixed canard and does pretty much the same thing i.e. generate favourable vortices at high AoA. Regarding the canard being below the wing which is a no-no as per the PA of SAAB several of the Rutan canards have the canard below the wing- the Quickie” being an example. There is no need to treat the PA’s views as a father’s unalterable curse.

4. What the patent application (PA) discusses is pretty basic stuff. Digressing on n a personal note I was reminded of a book from the 1960s   “Design for Aeromodellers” a MAP publications which was available at the “India’s Hobby Centre"  for Rs.5. That book discussed, in much less intimidating language, the same problems i.e. where to locate the canard in vertical and horizontal terms w.r.t. the wing using the same unit i.e. the chord of the wing as unit of length.  

5. Our problem is that the Tejas wants a canard not so much as for performance- which can be achieved even with the current wing being “tweaked” - as to avoid fussing with the FBW codes with every redesign. Therefore we have to use a movable canard possibly with flaps. It is difficult to find a suitable location in the small airframe.

6. Now coming to the core issue : The location of the inlet as shown on the Tejas Mk2 proposals models is, in terms of development, very trouble prone. ONERA  tunnel testing notwithstanding, the design is liable to intake buzz and other bothers. To the tiresome question “Do you know more than ONERA?  the short and obvious answer is “Yes- ” (or else why should I stick my neck out?)  and the longer answer I have given at 3 in the end.  It will hold up certification.  Let me list my apprehensions:

i) The layout is liable to intake buzz. I have given my reasons for this in other blogs.

ii) Every change in canard settings in flight will alter the airflow pattern and change the mass flow to the engine. The engine will continuously “hunt” and see internally thermal and aerodynamic variations. The reliability and TBO of the engine-which is now excellent- may suffer.

iii) In the case after touch down the canard will rotate through 90⁰ causing major flow distortions to the inlet; this may result engine roughness and sometimes flame out. Minor problem? Even in peace time the Air Force won’t be amused by aircraft stranded on the runway holding up the traffic “pattern”. I could go on with a few more but you get the idea.

Vina is confident about ONERA results. I am not so sure. To have even a modicum of confidence in the ONERA findings ONERA must have the full data for the F 414. I do not know if GE has obliged. Vina says that France used the F 404 for its Rafale and therefore has the data. My counterpoint is we bought more F 404s than France and I don't know if we have been given the data necessary to make meaningful computer simulations so why would the notoriously competitive Americans hand over data to ONERA etc.

7. Finally the above impasse (to me) is not at all technical but about wisdom and honesty. It seems to me that though the Mk2 project was taken up in 2009 ADA- thirteen years which as an organization  is far below par in project studies – has, as usual woken up rather late about the intake. It can honestly admit it has a major problem on its hand and go about correcting it. No one will censure. I suspect that organizational pride/covering up past incompetence  is getting in the way. My “eyeball” estimate is that the 90 cms. extension recently proposed is way too timid to solve the impending problem but the full extension will bring adverse publicity.

Now the wisdom part. Let us assume that ADA is perfectly confident and right but naturally whether they or I am right no one can tell. at this  moment. If I am wrong I will be trolled and if I am right I get nothing. Now if ADA is right it will get a tray of Padma awards but if ADA is wrong ADA will get off scot free -as with the past forty years- but it will be a national disaster. The correction will take 3 to 5 years after the flight trials e.g. 2032 at the current going rate. The IAF will have ceased to exist. The alternate would be to import which was precisely the contingency  to avoid ADA was superimposed on our industry. 

I note that whilst Vina is impressed about  the SAAB PA (Vina ,that is what I got the impression, honestly)  I have not failed to notice that he/she has failed to observe that the intake position is well ahead and clear of the canards wake which is precisely what I am advocating. Now the question to answer, without bravado or bluster , is : why did all three top design teams waste weight and drag doing that? Trolling on the media will not do. ADA must either have the answer ready or incorporate the feature for trouble free development. Note no alarm is being raised about  the TEBDF's  canards location.

This is my response to the technical points raised by Vina . I will now comment/ settle on two personal points that need settling raised by him/her.

1.     At 6 of his twitters he has said:

For that interference, the separation shouldn't be too large and in fact rather small, contrary to what you assume.

My response is: I would ask you to read again what I have written. I have NOT said that the gap should be large. Show me where I have written that. What I have said – if you re-read carefully- is that in the same set up of wing canard areas , overlap and gap, the smaller the gap the higher –as a cube of the gap- the drag will go up. For example, in the Viggen the gap between the two planes is – given the Viggen’s size- say about 90 cms. For the mid wing Gripen it is about 60 cms and in the shoulder wing Tejas about say 45 cms. So even if all else parameters are identical the Tejas, because of the smaller gap will have 8 times the incremental drag of the Viggen. This is first year fluid mechanics and I stand by it. BTW Canards are draggy- we may face disappointment wrt range.

2 . I sensed – I may be wrong- that Vina has more faith in E- Engineering (CAD/CAM/CAE) and the abilities of ADA. I will share my experience. It so happens that I worked for several years as a full time employee of a leading Indian Engineering Company in their e-Engineering Department. It had about 600 people ( i.e. the number is given to indicate the sample size) and I have no reason to assume that they were less capable than ADA people. I had the role of going around and examining the engineering aspects of the work. What I found was from top to bottom the ability to deal with the nitty gritty of engineering and the ability to interact at the monitor to solve a problem – let alone solve it quickly- was  bad. Most could drive the  software every well but could not visualize the problem and certainly not offer engineering solutions that would not tread on the toes of some other areas. The other noticeable thing was that they lacked the necessary confidence in engineering to interact with foreign customers- and they were world leader companies- as equals. The reason , and I digress because it is a passion, was the wrecking of all education by political interference and the Industrial Policy Resolution of 1956  but this hobby horse is for later.

3. The last line of the above brings me to my views on foreign collaboration. Foreign collaboration is useful only when we can treat them as servants (functionally, not otherwise!) and we the masters of the situation. The Consultant's should be the second opinion- not the first! The ADA experience refers. Twelve Dassault engineers worked at ADA for twelve month and gave the basis for the Tejas. It featured :

 

i)                 13.2 mts. fuselage length

ii)               Shoulder wing

iii)             Aspect ratio 1.82

iv)             Tail less delta configuration

v)               FBW

I have no doubt that the design as submitted would have met the specification as it was at that point of time. The defect in the proposal was that it was, as I have discussed elsewhere, the most difficult aircraft configuration to develop further. Unfortunately at that point of time the Chief Designer/ Accepting Authority lacked the knowledge, let alone the wisdom, to point out this defect and get the proposal rectified. This  was made worse by that "devil's package " being  sanctioned as a Technology Demonstrator. ADA was put on a developmental tight rope it is walking even today..

ADA has not demonstrated so far the required levels of skills at the higher direction of aircraft Development. Its programmes are not only taking three times as long as they  should it seems that they are spending about five times as much money doing so. It must be brought under new management but for that new management to be effective ADA must be purged of Legacy Influences.

Appendix

…. no improvements in numerical and experimental design tools are ever likely to dispose of the need for physical insight. On the other hand, a good understanding of the flow phenomena involved has led to successful designs even in the early days when the available design tools were still rather poor.

The above comment is by Dr. Kuchemann ,who needs no introduction. What he emphasizes is that there is a real need to be able to "visualize" the problem. Without this ability to "visualize" one does not have a navigation chart by which to move towards the resolution of any technical problem and explains why using computers does not mean the solution used would be successful.  

 

“Vina”s comments are given below for reference .

V 2. In that the first section talks about the canards located far ahead of main wing like Eurofighter & the severe control problems encountered. The Saab close coupled canard was the solution that solved a lot of those control issues, where the canard was places just ahead & above.

V. 3. The main wing. The patent also describes what should be the ideal spacing between the canard and main wing as "according to the present invention, the secondary wing 8 is located only a relatively short distance ahead of the main wing"

5 although the secondary wing should have a substantial spacing above the main wing, the projected distance between the wings should be less than one third of the root chord of the secondary wing"

 

6.The patent also talks about what the crank in the delta wing does for Viggen (increase the aspect ratio) Now Viggen, Gripen, Rafale and LCA MK2 use close coupled canards, which from the Viggen patent solves the control issues and increases lift because if the interference of the canard downwash and main wing upper surface.

For that interference, the separation shouldn't be too large and in fact rather small, contrary to what you assume.

7. As far as intake buzz goes, any intake design and indeed whole model would have gone through wind tunnel testing and performance verified. We have the wind tunnels for that here and access to facilities abroad. For design aoa and sideslip would've been verified.

8. So really don't see too much issues. No comments on the rest of your stuff of course on the internal politics, decision making, would've, could've etc. If I have time and look through your cost data re timelines, will, come back to you. Have nice day.

.

 

s://patents.google.com/patent/US3188