The Design Of The Das D-9 Hawk Moth

 

Developing the Tejas Mk 2 –a technical assessment               Prof. Prodyut Das

 

There were two interesting interviews related to the LCA project in November ‘21. One is a rather obsequious “interview” of the first Programme Director. The interview certified that ADA’s performance has been entirely to ADA’s own satisfaction. This is in contrast to the other interview by the new Air Chief who made no mention at all of the place of the LCA Mk II in the future re-equipment plans of the IAF. Given the trail of unkept promises of the LCA project this lack of mention in an important speech has triggered much speculation. One possible construct of the Air Force’s view, could be as follows: ADA has had an “order” for forty aircraft since 2003; about twenty have been delivered, just about sufficient to equip one squadron. The aircraft have serviceability and design problems resulting in an availability of around 46 hours/aircraft/per year as compared to the 225 hrs. /aircraft/year that is demanded from foreign vendors. The aircraft is overweight and lacks range. Now ADA has an additional order for 83 aircraft The Air Force’s view may be “let ADA get on with that and then the Air Force will see-no more hot air, no more “pie in the sky” but habeas corpus please”.

The progress on the project with the “new” Government has been much better than the laissez faire of the earlier regime but it too has not been steady, matters not there being helped by the fact that there has been five Defence Ministers in seven years. Continuity is vital. The order for 83 aircraft is an indication of the Government’s interest. This order actually “calls the programme’s bluff”. ADA now has to deliver; more than twenty years after the first flight there cannot be any more excuses. The Government is keen on local development and the keenness is bringing pressure on the organizations and it is beginning to show. In my view the recent “Balbo” of sixteen aircraft at Sulur and the interview alluded to above should not be viewed as unrelated and random unconnected events but reactions to this pressure by the Government.

Before I put down my views on the re-engineering of the design to avoid total repetition, I will remind the readers of my earlier writings on the LCA. “Wisdom and Courage” “The case for Simpler Fighters”, “Beloved aircraft or Lemon?” “The CAG report 2015” and “An inquest into the HF 24” which though on the HF 24 has interesting references to the destructive interactions between the HF 24 and the LCA programmes. These had appeared in various issues of Vayu between June 2009 and June 2021 and some of the later ones would have also appeared in my blog profprodyutdas2 .blog. https://prodyut7.blogspot.com/

The technical minefield

In developing the Tejas Mark 2 from the current Tejas Mk1/1A is not a straight forward development but more akin to a new program. The challenges are not technical. The problem lies in what I call the “Caritas” or “soft” part of Product Design.

The Design Team has to achieve the following:

1.      Increase the length of the fuselage by about 1.5 mts mainly to accommodate the new equipment that has become de rigeur over the forty years of development

2.      Add a set of canards/stabilizer to relieve the continuous tinkering with the flight control laws.

3.      Add certain amounts of equipment long pending.

4.      Introduce the F414 engine into the airframe.

5.      Reduce drag

6.      Reduce weight.

Requirements 1, 2, 3 and 4 are hostile to requirements 5 and 6. About forty percent of the design will have to be revisited. For ADA matters are not helped by the fact that the final product on offer must compare with the SAAB Gripen E of similar power and configuration.

The following is my assessment of the task:

Aerodynamically the fuselage needs to be increased by only half a meter between the forward pressure bulk head and the nose cone to get the requisite “C of F” and reduce drag. It is presumed that the remaining 1 meter being added for new equipment and fuel. By my estimates the designers have about 780 litres of additional volume after subtracting the volume of the inlet trunking. The weight of this fuselage length increase equipped will go up by about 450 kilos/ metre. The F 414 engine is about a hundred kgs heavier and will need additional fuel provision and that will naturally cut into the space and weight available for equipment. It is going to be a close-run thing between space, weight and fuel provisioning.

Canards

The HAL team has very few options but to put in canards if the problem is that rewriting the software with every major change is a time-consuming headache. However, Canards are misunderstood in their fine print. That it adds to the total lift is recognized but it is draggy and; careless detail design can wash away all the gains of the canard. The problem lies in two areas:

i)                 The canards and the wing form a “flow between two plates” situation where the resistance increases inversely as the cube of the separating distance. On a small aircraft like the Tejas the separating distance is much smaller and so the drag of the layout will be much more than an identical arrangement on a larger aircraft. To be sure there is CFD but if we go by the claim that the LCA Mk 1 was designed using CFD we are letting ourselves in for another round of uncertainty; we know what happened. Driving the CFD was maybe not a problem but knowing where to go certainly was.

ii)                The second problem is that the Tejas wing makes a feature of the “Levcons” which are used to generate additional lift under certain flight conditions. The downwash from the fore planes will affect this flow field significantly being located just forward and above ahead of the Levcons reducing their effectiveness. The flow to the engine will affected whether seriously we will not know as the CFD will not be able to guarantee. The question of what happens when the “levcons” are raised -as has been shown in some pictures-is something of interest -they will act as a dam.

iii)              Ideally the fore planes should have a lower aspect ratio so that they can maintain control authority when the wing stalls but generating lift from low AR surfaces does mean drag. One is reminded of a (much disputed!) comparison between the F 16 and the Viggen which were then competing for the NATO orders and it showed that whilst the Viggen had a 16% shorter take off the range of the F 16 was almost double. I mention this as an indication of the care needed in engineering.

The above is an incomplete list of the problem. The problems can be solved - but just how tricky the canard/inlet position in can be is revealed by examining the canards positions in three “in service” fighters with canards and the way those designers have worked to minimize the problems. The SAAB Gripen, has the inlet kept well forward of the canards same as its predecessor, the mighty Viggen, accepting the additional weight and drag as a price for uncluttered flow to the engines-engine behaviour in such flow being something that cannot be reliably predicted by CFD-especially if you do not develop your own engines.

The Eurofighter Typhoon has the best layout from this point of view. The broad under fuselage as with the F 16, acts as an induction plate guiding the airflow at high angles of attack with the slight anhedral of the canards acting as “endplates” to channel the flow in. The “disturbance effect” of the fore-planes is kept well away from the inlets and it is “long coupled” so the amount of lift it has to generate is less, low aspect ratio canards being “draggier” for a given amount of lift.

The Rafale has a very complicated arrangement but it is, elegant if not simple. Very long and curvaceous strakes separate out the airflow to the inlet well ahead of the canards. the “three dimensional” blended and rounded shape of the strakes assisting in the “channelization”. There is a slight overlap between the canard and the wing but given the size of the Rafale the drag penalty is insignificant compared to the same overlap, say, on the LCA. We are skating on thin ice. Meditation and quiet reflection “thinking first, looking around and then thinking some again rather than CFD” is the answer Trusting on CFD as an antidote to delays without a concurrent ability to flight test quickly will mean a repeat of the delays.

Compared to the above the ADA Tejas canard layout seems to be asking for trouble!

Digressing, ADA of course has a habit of making various claims/ promises, subsequently found unwarranted. There is an interesting anachronism and contradiction When the aircraft was designed in the period 1983 and 1995 i.e., till the roll out of the aircraft CFD, particularly in India, was extremely primitive.  The claims about extensive use of CFD during this phase is unlikely to be true. If true, the extensive-re engineering now needed indicates how unskilled the application was!  What the aircraft needs now as before, is not technology or funds- that is easy, but love and caritas which is much more difficult in supply.  That and political watchman ship. Much of the Defence projects were  “Kampani ka Maal- Darya mey daal” i.e.  the Company’s goods- toss ‘em into the sea”. Funds and difficult technology were not the problems they are made out to be; Öwnership"and "motivation" were; Project administration will significantly improve deliveries.

The Engine

 Much is made of the fact that the GE F 414 engine is a drop fit. A point of worry is the mass flow which is 77 kgs/sec as compared to the 66 kgs/sec of the F404 indicating an enlargement of the inlet trunking if one is not to put up with a 36% increase in duct pressure losses. Given reports that the LCA’s inlet system is faulty and does not allow full power even from the existing engine the new engine will be a convenient ( nice word for excuse!) occasion to redesign the inlet duct system though a 16% increase in flow area may eat into the strength of the former fuselage frames’ webs.

Quid deinde; What next?

It is not possible for any outsider to know what is ADA’s problem in developing to time. I can however share what I have observed from over three decades of developing advanced high-quality equipment in India in several fields of engineering.  It is possible that many of the LCA’s problems originate from the same mindset/culture and therefore may actually be easily solvable. These are:

i)                 The actual problem is rarely known.  Some junior engineer reports a problem and that goes up the organization. Seldom does some seasoned engineer go and make a first-hand survey of the problem. Yes, it happens- and everywhere; Trust me!

ii)                Even when the senior man goes and checks up, he is usually from the “management” streamside by training and is as clueless as the young engineer. All the “senior” does is to “manage” the junior and demoralize him by cursing him for being a clueless fool. This shortage of people with engineering capabilities is of course a by-product of the license permit Raj which made engineering and quality control “unprofitable” whereas “management” i.e.  churning out goods for a captive market was paying. The effective opening up of the economy post 1999 has created a cadre of trained development engineers at the middle management level and that is hopeful.

 

iii)              Finally, there were people with years of experience on the area who would swear very confidently that any change in the status quo in terms of standards, or tolerances or performance was not possible and yet experience shows it was often surprisingly easy and low cost to put in the necessary upgrades in performance. “It can’t be done” etc by senior people with years of experience need not be accepted as the gospel. In fact, “years of experience “may have been the root cause why prompt rectification was not done. Such people are often at the forefront of stymieing any effort at correction. It is a defensive reaction to avoid exposure of their bungling. This is why I say there must be a purge of the Tejas personnel before we can see progress. 

 

iv)              Given my personal experience and going by the CAG report of 2015 my analysis is thus.

i)                 At 7200 kgs. the aircraft is badly overweight and correcting the weight alone mainly in the airframe will transform the Mk1 aircraft into a useful weapon.

ii)                If we take the Marut airframe as a starting point then considering that the Marut was larger, over designed and in all metal the LCA Mk 1’s airframe with its extensive use of composites should weigh between 1900 and 2100 kilos.  Is this the case?- the fuselage alone apparently weighs as much!  Indeed, if one compares earlier Gripens (6850 kgs) with the similarly powered LCA Mk 1 then the empty weight of the smaller Tejas Mk1 and 1A should only be around 6200- 6400 kilos 

iii)              Examples of mis-design is difficult to come across in a” secret” project and ADA has always kept its card s close to its chest. Starting from fundamentals “fundas “in my college’s jargon, it is clear the weight of an undercarriage strut is a function of the landing weight, the length of the strut between the pivot and the axle and of course the landing speed. the LCA main undercarriage struts should weigh only about 80% of the MiG 21. However, and I confess I say this on the basis of a visual inspection of the undercarriage of a Bison and an early LCA Mk 1, the LCA’s undercarriage was definitely “battleship” in its propensities would not look out of place on the Tirpitz.  I tried to discuss this question with someone who has worked for a long time on aircraft certification but his reaction was typical. He quoted the rules citing the sink rates etc. The question is not the sink rates or the rules which usually have a firm basis. He totally missed the point. The "trick" is to first establish the comparative weights and see how close we can work to the limits. One hears that Bharat Forge has done an excellent job in weight reduction on the U/C struts but how did it that weight get on in the first place? Delays happened not because we were doing it for the first time- but because we did the job half cock the first time- and so we have to do every job twice- the haphazard location of many LRUs for example which affected serviceability and needed relocation. The fundamentally simple question is have we set and reached the limit? I suspect not and never; aircraft design is all about pushing to the limit but no “bureaucrat” will do that. I use “bureaucrat” because in my humble opinion holding an Engineering degree is merely a license to be allowed to walk on the shop floor.

iv)              The improvements in finish as compared to pre 2015 is noticed but again the question is had we set and reached the limits?  The finish of some composite aeroplanes such as the Diamond Arrow is noteworthy. The LCA’s finish is still nowhere near; indeed, the poor finish of the Hansa is anything to go by it is no wonder the LCA’s performance suffered because it appears that the first composites were fabricated by the same sources. That improvements in finish significantly affect performance need not be re-emphasised. 

These are three points where I have studied and it would be certain that many other details- The rigging of the aircraft has to be re-examined in case some very elementary mistakes are being overlooked, electric harness and hydraulic pipeline routing, structural “tricks” to improve weight- “simplificate and add lightness” was an old and much used phrase.

 It appears that the original weight declared i.e., 5500 kilos for the LCA MK 1 as given by the consultants was achievable and would have resulted in a very nippy and useful enough warplane though by the time the consultants design came along almost ten years had gone by since inception and additional equipment was inevitably demanded requiring extensions to the fuselage. Whilst much discourse is available about many technical wonders of the aeroplane the question as to why a 13.2 mts. fuselage was at all accepted from the consultants by the ADA when all other similarly powered aircraft were a least a metre longer is something carefully left unexplained so far. The same goes for the outrageously low aspect ratio.

In sum the situation appears to be like this: Introducing canards to the LCA is a possible solution but one that can run onto the reefs of weigh control and drag reduction especially we continue to show a lack of caritas. The proposed layout seen at shows is heading for trouble and require refinements along the lines discussed. In addition, there is a new engine to be introduced and that with a new inlet system; My estimate is about forty percent of the drawings will need to be changed which is close to another new design.

Weight control is a skill that ADA has yet to demonstrate but weight makes or breaks an aircraft. With weight of the LCA Mk 1/Mk1A brought down to around 6200-6400 kgs band the aeroplane will transform into a very reasonable and useful aircraft. Indeed, ADA must demonstrate its ability to control weight on the LCA Mk1/Mk1A along the lines of estimates I have indicated before being trusted with a Tejas Mk 2 or else we may go off on another technical “picnic”. Another point to watch will be the delivery rate of the aircraft on order. Good intentions are no substitute for hardware. Habeas Corpus!

 Finally, the Government must begin the process of encouraging the private sector to set up design Bureaux so that they can participate ab initio. In the recent tender for unmanned ground combat vehicles (UGVs) some 12 Indian contenders offered thirty-five proposals. Think of the ideas generated, think of the final product which the Army will now request trying to combine the best of every proposal. This can also happen in Aerospace. We can have new projects at one tenth the cost and one fourth the time of what the Government led effort has achieved. Who is stopping this? The cost to the country will be much cheaper.