Developing Stealth Technology - Lessons from the American XST/F117 programme

Prof. Prodyut Das M.Tech, MIE, PGCGM M.AeS.I.

Forty years ago the Americans developed a very serviceable stealth aircraft , the world’s first, in  a remarkably short time and at very modest costs. As India discusses its own AMCA programme, the honesty, simplicity and common sense that guided the American programme is worthy of review.

 The Americans, geographically isolated, has always had an unique requirement of “air superiority over hostile airspace”. The Air war over North Vietnam showed that this requirement could no longer be met by the traditional US combination of “superior” aircraft and ECM. North Vietnam war was a static defensive action. The Ramadan/Yom Kippur War showed that the same  carefully co- ordinate defense system using low technology mobile AAA, tracked SAMs and aircraft could also be used for offensive operations. The American requirement of air superiority over hostile airspace was clearly under threat.

The Americans, despite considerable skepticism, identified stealth as a possible solution to the challenge. Stealth meant reduction of Radar, Aural, IR and visual signatures. Since the threat came mainly from radar and radar guided weapons the reduction of radar cross section (RRCS) was given primacy. The details of the American programme to develop stealth technology is of interest because an urgently needed technology was developed to meet an identified requirement at low cost to a tight time scale- a management skill that will be of interest to Indian Aerospace if the AMCA programme is pursued.

The Stealth development Programme; Step1. Establishing the basics

The bare bones of the American Stealth development programme are as given below. The focus on short steps, strict timelines, low costs and the verification and securing of each technology step before taking the next steps is inescapable.

Within a few months of the Yom Kippur/Ramadan War, as soon as the Israeli loss patterns had been analysed, Ken Parko of DARPA requested submissions from the top  five American fighter manufacturing companies. The DARPA focused on just two parameters.

What were the signatures thresholds for an aircraft to be essentially undetectable at an operationally useful radar range?

 Did the company have the capability to design and produce an aircraft with the necessary low signatures?

The approach at this stage was exploratory. Was stealth feasible?

The eventual winner, Lockheed, was not even contacted because Lockheed had not produced a fighter since the Starfighter of the ‘60s .The emphasis on experience because of the urgency of the programme is to be noted.

Of the five companies contacted two declined and one proposed continued emphasis on sophisticated platforms and advanced ECM. Two companies-Northrop and MacDonnell Douglas showed a grasp of the problem .In the closing months of 1974 i.e..twelve months after Yom Kippur the two companies were funded $ 100,000 each to conduct further studies. For convenience of Indian readers the $ values have been converted to  rupees at he then exchange  rates and the local purchasing power parity (PPP) factor of about 3.5 is being also given because the cost of an Indian Engineering man hour is much less than US. Thus we have $100,000/ 12Lacs/3.5 lacs. The small amounts released, even by 1974 prices, is noteworthy.

At this stage Ben Rich, President of the Lockheed “Skunk Works”  (secret prototype workshop) noted that Lockheed’s earlier SR 71 Blackbird and D 21 drones had shown low RCS capabilities and he approached the CIA-which had  financed these programmes – for permission to discuss with DARPA the practical experience that Lockheed had already gained on RCS.On the strength of these studies Lockheed was permitted to join the programme and submit a proposal but without any funding.

In early 1975 Lockheed formed a three man project team comprising of the following:

Dick Scherrer-             Project Leader

Denys Overholser       Radar Expert

Bill Schroeder             retired mathematician

The sub team Overholser and Schroeder concluded that a faceted shape based on simple geometric optics would be giving the required results at the lowest risk. They collaborated to set up a mathematical model and asked for six months. This was brought down to three months and in fact the team had developed RCS prediction software of reasonable accuracy, the ECHO 1 in five weeks.

The simulation software was immediately validated by experiment .The Lockheed Company funded $25,000 / Rs3 lacs/ Rs 85,000) for two metal foil covered1/3 scale wooden models- one for RCS anechoic chamber and the other as a wind tunnel model. The RCS test model was a checked at the Lockheed Radar Anechoic Chamber in 1975. The anechoic chamber was fairly simple. It was found that there was a reasonable but not perfect match between the spikes as predicted by the ECHO 1 software and as seen in the anechoic chamber tests. The main problem was the diffraction due to the edges. At this stage the ECHO 1 software was modified to take benefit of the work done a decade earlier by a Russian physicist Pyotr Ufimetsyev Chief Scientist, Moscow Institute of Radio Engineering, whose 1962 work “Method of edge waves in the physical theory of diffraction” permitted the prediction of RCS using curved surfaces and therefore a more advanced airframe could also be considered. Indeed there were was a powerful group amongst them the formidable legend Kelly Johnson who would have liked to see the Ufimetsyev equation applied for a more efficient airframe but this lobby was firmly over ruled in the interest of programme certainty. There was so much skepticism over the faceted design that the programe was nicknamed in typical American manner as the “Hopeless Diamond” the hopeless being for the predicted/presumed flying quality and the “Diamond” referring to the faceted shape.

   The model was then sent to the Mojave Desert where the flat featureless terrain was like a giant natural anechoic chamber. Amongst the many discoveries that comes from an experiment driven approach the engineers found that designing a non reflecting pole to mount the model for radar reflection studies was almost  as difficult as designing the aircraft shape! The Mojave tests further boosted confidence in the ECHO 1 modified software.

Based on its own experiments Lockheed submitted its proposals to DARPA for an experimental survivable Test vehicle (XST) in the summer of 1975- i.e. about 18 months from DARPA’s first enquiries.

DARPA now had three proposal from which to select for funding. The two from Lockheed and Northrop were quite similar .The MacDonnell Douglas team were the first to establish the minimum level of stealth required but according to reports they were not able to give a suitable submission-possibly by being  up to their eyebrows in the F 15 programme!

The Lockheed Northrop (LN)  XST technology menu

The LN XST focused on maximizing total stealth   i.e. Radar, IR, Aural and visual. Everything else was compromised to achieving this end. The logic was that stealth was a substitute for conventional good performance which in any case was and still is unachievable with total stealth.

Given below is the menu of stealth features in the programme.

High wing leading edge sweep. The XST had a LE sweep of 72^oand the F 117 had a LE sweep of 67^o. By comparison the highly swept MiG 19 and Su7 wings had 57 ⁰  LE sweep. Modelled on geometrical optics, the extraordinary high LE sweep reduced radar returns. Lockheed/Northrop’s previous experience in lifting body dynamics gave the team the confidence. High LE sweep is aerodynamically inefficient and current LE sweep is around 40^o.

Sharp edged LE sweep. Chosen for low frontal area this feature gives a  “sharp” stall. Current practice is not known to the author.

Faceted Aerofoil   Pictures of the F117 shows an aerofoil that appears to be made up of three flattish segments each that make up the upper and lower curved surfaces. This would give poor lift and uncertain handling. Current designs do not appear to use this feature.

Hopeless Diamond faceting   The American team  chose  this ‘inferior’ technology despite knowing its problems of uncertain handling and despite knowing of Ufimetsyev’s work in the interest of timely completion of project and because it “worked”. Current designs, particularly the B2, use increasing amounts of Ufimetsyev’s theory.

Inlet meshing gave excellent stealth of the inlet which is a stealth “hot spot”. However it is poor in terms of propulsive efficiency and is prone to blockage in icing conditions. Current designs use DSI ((Diverter less Supersonic Intakes) or use prominent ‘S” intakes.

Fishtail exhaust   Excellent in terms of IR and Aural stealth it is propulsion wise bad with a rumored twenty percent loss in efficiency. This was last used on the F 22 Raptor and current design go for as high a BPR turbofan as possible along with exhaust shielding by the empennage.

The resulting aircraft, the XST/F117 had excellent stealth characteristics but the current thinking is that like Variable Geometry the solution is as bad as the problem! Full stealth is “mission profile sensitive i.e. it is good for long range subsonic aircraft but not possible for fighters.

Step 2 proving the basics .The XST

DARPA studied the submissions of Lockheed and Northrop and based on their grasp of the problems as shown by their proposals they were asked to collaborate to produce the following quanta of advance:

1.      A large scale ground test model i.e. a structural test model.

2.      The construction of two flight models

The funding for the above was released on 26 April1976. The amount was $10.4 M/12.5 crores/3.56 crores.  The aircraft produced would be used to validate the following:

Validate in flight the four stealth features designed-Radar, IR, Aural, and Visual.

Demonstrate acceptable flying qualities.

Demonstrate that accurate computer modeling of stealth has been achieved.

It will be noted that the task requirement was focused on just stealth, handling and confirming a maturity of the RCS prediction and stealth engineering capabilities. Confirmation of the predictions at the earliest opportunities was the persistent aim. The XST was not required to have ANY operational capabilities. The XST prototypes were practically hand built- in metal (sic) –stealth being achieved by shape, faceting and RAM. There was no attempt at “optimization” which would be a waste at this stage of the development. The aircraft was “cobbled” together using many standard aggregates off the shelf. For example the engines GE J 85  4 came from the T 2 Buckeye Trainer, the landing gear was salvaged from a written off Northrop F 5 and the flight controls came from Lear Siegler and the cockpit instrumentation came from a F 16. The time saved by such an approach was reflected in the fact that the prototype flew on 1^st December 1977 i.e. within 18 months of funding approval which had come on 26 April1976. The “cut and try” approach can be seen from the fact that the aircraft had to be grounded to fit bigger fins which had been found necessary. This prototype was lost on 17 January 1978 on its 36^th flight. The second prototype was flown on 20 July 1978 and lost on 20 July 1979 on its 52^nd sortie due to systems related problem. It was found that the prototypes’ handling and flying qualities were so bad that it was officially reported that “the only (bad) thing that the XST does not do is to tip on its tail when parked! Despite such adverse comments the programme had generated sufficient confidence and data as to go in for the actual combat version, the F 117, was a substantially different machine from the XST technology demonstrator.

Step 3 Converting to a weapon:  XST to F 117

The project had demonstrated that the limited objectives set out above had been clearly achieved the Americans moved on to the next stage. They funded on 1^st November 1978 $ 340 M (Rs. 408 crores/ Rs.117 crores)  for 5FSD airframes, support equipment etc with a target date of 21 months to first flight i.e. July 1980. At this time the  design specified that maximum stealth was to be in the “Head on - +/- 25 degrees” segment” .i.e. during the penetration phase. There was no attempt even at this stage produce “all aspect stealth”. The IOC was to be March 1982. The F 117 used the ‘full stealth” treatment already proved on the XST. There is now increasing evidence that these features were overkill and later designs have moved away from ‘full stealth” in the interest of restoring aerodynamic efficiency.

The XST/F 117 programme management policy.

The management policies followed for the programme laid down fourteen rules covering programme management, organization, contractor customer relations, documentations, customer reporting, specifications engineering drawing, funding, cost control, sub contractor inspection, testing security and management compensation. The following expansion of some of the rules showed the result oriented approach.

1. It is essential that the programme manager have authority in all aspects of managing the programmes.

2. The Customer programme manager must have similar authority to the prime contractors managers. Incidentally the Customer was an equal financial stake holder with one third of the stake.

3. The number of people involved must be reduced to a vicious minimum i.e. between 10 to 25% of “normal”. In development programmes mediocre numbers is no substitute for talent. You either have the “right stuff” or you only have a high wage bill.

4. Very simple drawing and drawing release systems with great flexibility to make changes.

The People

The profile of Dick Scherrer, the team leader who was 54 when he took charge of the project is typical of the people involved. He was a BS in Engineering and from 1942 to 1959(he was at NASA Ames Wind Tunnel and had much practical experience in Tunnel testing. From 1955 to 1959 he was also working on the side on the Disney land “rides” such as “Dumbo the flying elephant”. In 1959 he moved to Lockheed California making project studies for about 5 or 6 important programmes including winning Lockheed submissions such as the P3 Orion, The S2 Viking and the Tristar. Though by our rules he would be “low” on formal qualifications the emphasis on hands on experience is to be noted.The degree of ‘hands on” experience can be gauged by the fact that when the Lockheed shop floor workers went on strike during the assembly of the prototype the engineers were confident enough  to do it themselves.

Analysing the management of the programme

Rene Descartes (1596- 1650) had observed “If you begin with certainties you will end up in doubts but if you are content to begin with doubts you will end up in certainties. The Americans were unconsciously following Descartes.

Their starting point was not an aim or an identified Technology but a search for what technology – at that point unidentified- would enable the USAF to survive and dominate hostile air space. Having identified Stealth as a possible solution they then tried to establish a minimum level of stealth required to survive. The technological difficulties were respected as only engineers and practical men can and no impossible goals were set.

Recognizing that “stealth” would have an adverse effect performance they focused on getting to know the Stealth technology. The XSF was a pure technology demonstrator with no combat capability and even the final F 117 was decidedly subsonic and of limited war load capability.

The Americans at the start, focused on just two common sense questions:

a)      Is it possible to reliably predict RCS /stealth characteristics?

b)      If so, would the resulting aircraft be flyable?

In spite of their enviable depth of experience , or perhaps precisely because of it , the Americans lost no time in backing each theoretical projection with hard data n matter how “crude” the data was at that point of time. The Americans, again because of their depth of experience, avoided making the mistake of developing the combat F 117 in one go. The Table 1 shows how different the XST was from the F 117. Translated in to India it would mean that the LCA TD 1should have been not KH 001, the LCA prototype , but a HF 24 or even a HJT 16  or MiG 21 but having the four technologies – composites , FBW, BVR and the Glass cockpit needed in the LCA programme. The resulting collapse of “to flight” times and the refinement hard data would have brought to the LCA programme can be imagined. In the case of the AMCA it would mean a subsonic aircraft using aggregates from concurrent programmes but incorporating all that we have learnt so far on stealth airframes so that all our detail design is based on proven technology and not on hypothesis and validated before we proceed to the infinitely more difficult job of designing a supersonic stealth aircraft. It is the problems that we cannot know theoretically that need to be unearthed at the earliest. No amount of computer studies can solve problems that one cannot foresee and can only be seen in flight. 

The very short lives of the  two XST prototypes – 52 and 36 sorties respectively shows the emphasis that has to be placed on generating data to validate the theoretical projections. The prototypes were there to generate data and they were hard driven. It may be mentioned that the equally skilled Russians had ten crashes in their SU 24 programme-four apparently due to airframe related problems and six due engine related problems. Crashes are an occupational hazard in developmental flying and there is no merit in a crash free programme. The risk taking informal approach can be seen by the fact that the F 117 prototype was test flown after just 4 taxi tests.

The Americans achieved remarkable fiscal and time control by slicing up the development into  salami slices. At no point was the development Target beyond the time horizon. Since DARPA officials stay on the job for about two years before moving out most of the horizons were in months and we in India can do so too.

Whilst the differences between the American and our way of doing things is obvious they Americans were not doing anything  that is impossible to do  in India-save the way we think. The HJT 36 and the HTT 40 programmes underline that we can put up a prototype in 36 months and these figures can improve. The only correction will be to slice up a project, no matter how complex, into a one, two, or maximum three years slices. Any project with a target ten years away will, to use a current phrase, go BVR. No one- the “funder” and the “fundee” will be around to answer or pick up the pieces when the time to call to account comes ten years hence! This is the fundamental weakness in our management.

Lessons for our AMCA programme

Given the above our AMCA programme should be reviewed thus:

1. Stealth technology is important enough for India to work on this field. The caution is that advances in RAM may radically change the way stealth aircraft are designed. Thus timeliness of completion is of even more importance than is usual. 

2. At the proposal stage it is essential to have more than one submission even if it means releasing 1-2 crores each to various selected (private sector) vendors. Multiple competitors generate ideas which can all be included in the final winner.  Multiple entries will also avoid “the only son” performance syndrome which solitary organizations have shown.

3. Developing a stealth AMCA “in one go” over ten years as is being proposed is a bleak prospect.

4. The first step should be a stealth technology demonstrator an AMCA-ST a subsonic stealth aircraft which will feature everything that we think we have learnt about stealth aircraft. This like the XST will maximize use of aggregates- engines systems undercarriage etc from the existing programme so that the only major design load is the stealth technology airframe.

5. The target time to flight should be thirty months and about three hundred crores. If the team cannot meet this target it will also not be able to do meet a ten year, ten thousand crore plan.

6. The AMCA XST should complete its flight tests – focusing mainly on stealth signatures and handling, in about a year.

7. If the programme turns in encouraging results it should lead to a small subsonic nocturnal intruder – not a supersonic AMCA as a first aircraft. A detection range of one tenth of the Canberra could be a possible target.

Small steps, tight control, continuous Government Interest (which seems now to be happening) can work wonders. Funding big plans and distant promises will be folly repeated.

Table 1

Parameter                              XST                                                    F117

Wing span                              6.58                                                     13.21

Length                                    14.41                                                   20.09

Wing Area                             35.86                                                   105.9

Empty Weight                       not known                                          13,154

MTOW                                   5670                                                    23,181

Engines                                   GE-J85-4A                                         GE F404F1D2

                                                2x13.15kN                                          2x48.51kN

Vmax                                      not known                                          M0.9 approx.

The difference between the tech demonstrator and the Final product is obvious.

All dimensions in Mts. Sq.Mts, Kgs,                                   

Prof. Prodyut Das

9007434226

Kolkata

26 August 2016

Appendix 1 Note on the sketches D 7 & D9

1. Stealth is a subject we cannot ignore and yet it may prove to be another dead end in Airframe design like VG wings. 

2. If stealth s useful it’s most likely application is in subsonic bombers and intruders. A stealth supersonic fighter is a technical contradiction.

3. A stealth supersonic fighter like the proposed AMCA is at the moment beyond our capabilities, certainly in terms of time and possibly in terms of our capabilities.

Given the above our focus should be on stealth aircraft like UAVs or an Intruder.

4.A stealth intruder with 30% more range than a Canberra B(I)58 with one tenth the detection range of a Canberra represents both a “do-able” challenge to our present engineering capabilities and also the resulting warplane would be a useful addition to our  armoury.

5. The two sketches are provisional but show the emphasis on “robust” (i.e. non RAM) stealth

Stealth is dependant largely on small size, high L.E. sweep, prominent “s” -ing of the inlet etc. The IR signature is reduced by fairly high BPR turbofans i.e. 2 to 4 BPR and masking of the exhaust by the empennage in the D7 layout. The Flying wing design is naturally stealthy but both layouts-“tailless”-“ V-tailed” need to be considered carefully as the Flying wing has its CG limitations and cannot be “grown”. The wing sweep of the Flying wing can be varied on the ground for stealth research.

6. The small size of the aircraft notwithstanding the bomb bay size of 4.00xmts 1.8mtsx 0.60 mts  will accommodate  three of all current IAF 500 kgs stores with LG kits. We do not want a situation when the service introduction is delayed by having to develop especial SDRMs

7. The aircraft is a two seater. Human stresses of flying long missions in strict radio silence are not known to us and a second crewman may be useful. The flying wing has tandem seating and the V tail design has side by side which despite stealth difficulties has better crew co-ordination. Programme wise it is easier to take out a cockpit than to introduce a stealth cockpit.

8. The design empty weight varies between 5500kg to 6500 kg with an MTOW of around 13,000kgs. Two Turbofans of around 30.00kN are proposed e.g. CF 700, TFE 731,3xFJ44 etc. The HTF 2500 can be a prospective candidate freeing dependence on a “foreign” engine. The NIS (I) –XST can use the Adour or the AI55 or the unreheated F 125.

9. The management of the programme would like the XST programme be in short steps.-i.e. stealth models, stealth anechoic chamber results, an XST and then a final design based on experience.

Note: First published in Indian Defence Review, 2016

A cat amongst the pigeons – an Indian concept of an universal light fighter.

The techniques of Market Research results in products which meet the customers’ needs to a greater degree. When it comes to combat aircraft the application of MR is conspicuous by its absence. The West prefers to sell their complicated specifications as the universal standard. The other reason is sophisticated specifications act as an entry barrier to Asian competition.

Sufficient statistical data exists to have a scientific approach to combat aircraft specifications. With changes in the Government policy the Indian Private sector will now have an opportunity to move into combat aircraft development and they start with handicaps. Therefore the importance of Market Research and a statistical approach to design appropriate fighters rather than “just like” fighters is imperative.

The air arms of India and Pakistan flew around 11,000 fighter sorties in the wars of 1965 and 1971. This involved mainly 2^nd and 3^rd generation Fighters and is extremely valuable a resource base because we own every bit of it. The Israeli Air Force flew about 14,000 fighter sorties in the 1967 and 1973 wars. The Arab Air Forces flew at least as many. If we now add the Sinai Clashes of 1967-1973, the Iran Iraq wars, the Vietnam wars and the various Gulf Wars we are looking at a data base of around 100,000 sorties. Much of the above, except the Syrian AF’s very interesting experience is known and much  can be gleaned.

These were sorties flown by 2^nd and 3^rd generation fighters operated by Air Forces with wide variety of training, operational doctrines and traditions. The combatants thus varied from well matched to so mismatched as to be almost asymmetric warfare. Operating conditions varied from clear infinite visibility over Sinai to sub continental haze to Vietnam monsoon and clouds. There is obviously a good comprehensive statistical data base on which to formulate what should be an ideal requirement. The West is not interested in such a study. They are at a disadvantage in producing to low cost. An Indian initiative in the direction of a “zero base” fighter may set the proverbial (lightweight) cat amongst the pigeons.

What are the real requirements?

The actual requirements that emerge will go against brochure wisdom.

The dogfight is not dead. Fighter aircraft will be designed to win the air to air. However these constitute  only perhaps ten per cent of all sorties (and combat losses) flown.

The main task of combat aircraft is close support and strike duties in VFR conditions. This task also sees the biggest losses- about 60-70% to low cost defences - but scant provisions are paid at the design stage to surviving this task or minimize losses.

There were perhaps a total of 10 cases, if that, in 100,000 sorties when a single seat radar equipped fighter located engaged and destroyed a hostile aircraft at night using its own radar. Work out the implications.

Even when opposing aircraft were capable of all-weather/Mach2/20,000mts altitude performance thousands of clashes occurred where the starting parameters were 450kts/3000mts/ VFR conditions which then wound down to WW2 parameters until one had to break out and run. Tongue in cheek, one would say that internal fuel capacity is more of an “outcome decider” than the max. AOA and other such performance consuming abilities.

BVRs/CCMs/Cannon

BVRs are not new. In Vietnam they showed a strict impartiality in what they knocked down so much so that soon the SOP was one flight went ahead to “visually identify so that the other flight could launch. This compromised the “surprise” element which is a BVR feature. The impartiality of BVRs about the “enemy” continues till this day.

BVR advantages are accepted but it is also important to know also the following.

1.      What were the numbers of BVRs that were launched and missed and under what parameters?

2.      Numbers that did a “blue on blue”?

3.      BVRs are heavy and “draggy”. How many had to be jettisoned at the beginning of a combat to “clean up” the aircraft?

Though CCMs are combat proven and definitely useful nobody is making the ‘sixties mistake of deleting the gun.

Sauce for the Gander?

If we transpose the above findings to a map of the performance envelope of the various generations we get Fig 1 which plots design speed and altitude capabilities of the various generations of fighters and matches it with what has been used in combat. The BVR effective ranges with altitudes is also marked. It will immediately be noted that some of the 2^nd generation Fighters were in every way nearer and better placed in terms of performance, size and handling for where the fight will take place rather than the 3^rd, 4^th and 5^th generation fighters. Combat experience vouches for this. More comprehensive equipment adds ‘global” versatility which is irrelevant to the Afro Asian Latin customer we are focussing on.  Starting from this we then have the conjecture what would happen if some of the 4^th and 5^th generation technologies were selectively read across to redesigned 2^nd generation airframes.It will be seen that a redesigned 2^nd gen airframe with later systems both has a Market exciting products can be developed at low cost. Having handled several system upgrades programmes a n approach of airframe plus systems upgraded is the logical next step.

Why “copy”

There are sound economic reasons for doing so. It certainly makes sense to any Engineer. Wise copying provides the “shoulders of a Giant” on which to stand and see further. Like the Chinese the Indian Industry will in the beginning base its designs on “copy”. Tales of “Makkhi ( squashed fly on drawing) to Makkhi” copy exists in every language in which engineers talk but actually it is rarely possible or indeed useful to do so. Copying has pejorative connotations but actually it can be an art form. The T 34 tank so shocked the Germans (surely, after the Russians, the “Herrenvolk” of Tank design) that they actually proposed to copy the T 34 “makkhi to makkhi” until it was realized that the German Industry would find it difficult w.r.t . supplies of aluminium required for the B2-34 Diesel Engine’s crankcase. The other nicer story, though probably apocryphal, was that it was pointed out that the “finish” of the T34 was so poor it would not pass the QC standards of the Technische-Amt-Heer! Ha! What the Germans did was actually the highest form of copying. They studied the T34 absorbed its philosophy and concepts and adopted it for their industry’s ecology. The resulting superb KPzW V “Panther” had the sloping armour and the “overhung” gun of the T 34 (something the “West” was long hesitant to introduce) but retained the peculiar interleaved road wheels the Germans were so fond of- doubtless for track metallurgy constraints. Between “makkhi to makkhi” and “philosophy” the possibilities of low cost product development are endless and exciting.

For the Project Engineer/Director the reasons for “copying” in Aeronautics are many and pressing.

Calculations and wind tunnel data are always optimistic. Having an actual aircraft as a near basis e.g. Folland Midge/Gnat or BAC 221/Concorde is reassuring and is wiser than going off the deep end and end up red faced and spluttering.

Aerodynamics is literally skin deep. Even a tiny aircraft like the Gnat had 10,000 parts. Each has to be conceived, located, stressed, detailed and specified in terms of metallurgy and production. All this takes time which is saved even by copying broadly.

The production details-manufacturing, jigs tools and fixtures, QC, heat treatment all can be that much more rapidly decided.

Every design has its faults. It is easier to correct a known fault in the “basis” aircraft than identify a potential fault which will cunningly wait till flight testing before it reveals its horns. I have in mind the fin and tail plane layout vis a vis the HJT 36 which is now requires more time and “fiddle” to correct.

Finally certification authorities are conservative to the point of being superstitious! It helps if they have can see something that has worked well before.

You will have noticed that copying is not about “lacking in knowledge” Copying is all about saving time. One can’t /won’t copy if one does not know or have the knowledge or the wisdom. It is certainly not “brainless”! The Designer has to have the brains- indeed wisdom- at every step to decide if the Muse is relevant to his ecology.

Does one need the drawings of the original to copy? No. Modern e –engineering tools and old fashioned engineering “horse” sense can obviate much of such need. What will be needed is one or two of the muse airframe.

Why the Gnat?

My ideal Light Fighter would be a twin engine having the F 86 Sabre’s pilot’s visibility with the MiG 17 wing (AR 4!) modified to have the MiG 19’s wing structural stiffness, may be the Su 7s wing section with its rounded L.E. which gave it superb low level manoeuvrability, the Gnat’s forward fuselage married to a twin engine rear fuselage from the MiG 19 and a “flak vierling” gun layout a la HF 24 all somehow blended with something of the Hunter’s grace and immense strength. Of such stuff as dreams are made on and it won’t cost the  earth! Sponsor for studies anyone? Of course the HF 24 can be the basis of an entire Air Force (AJT, LIFT, Strike and interceptor) all in that one basic air frame!

It has to be emphasised that any of the best of 2nd and 3^rd generation airframes suitably modified will respond to “re-systeming”  with current systems to create a “best” or “most useful” fighter at low cost. The Gnat is being used only as a convenient example of the nature of the effort required and  the metrics of the improvements that can be expected.

The advantages of the Gnat were:

It was naturally stealthy very difficult to see and even to hear which is very important in conflicts where the Mk1 Eyeball and Ear model A continue to be important sensors.

It is a superb fighter-small size, high T/W. high AR, low power burn per “G”

It was naturally area ruled. Formally area ruled aircraft e.g. Blackburn Buccaneer suffer excessive drag when operating at “off design “speeds- which fighters do all the time. The reports of the HF 24 “coming alive” at high speeds low level was probably partly due to the area ruling “kicking in” as it were.

It was brilliant structurally and very well engineered.

The aircraft was modular. Quite considerable surgery can be done on the airframe without involving a total redesign.

Despite the good points the Gnat had some grievous faults and these are listed below.

Lethal/ Totally unacceptable faults:

Poor ejection seat reliability

Poor ejection seat performance envelope

Unreliable Tail plane actuation

The windscreen to rear pressure bulkhead distance did not permit pilots taller than 5’10” unless they agreed to leave their knees behind in case of ejection. Interestingly several Gnat fanatics nevertheless flew the thing.

Unacceptable to very unacceptable faults

Cluttered Instrument panel made worse by the very cramped cockpit.

Gun stoppages in combat

Cracking of gun mounts in full fire out.

Brake parachute would scrape the ground and collapse causing “uncertainty” about reliable operation.

No antiskid

The very poor air- conditioning. The cockpit temperature was 40^o C with full “cold’ at low level summer 400 kts. In the Ajeet we improved it- to 37⁰ C! The very low bleed pressure (65psia) and the bad location of the CAU - in the engine bay so that the brake fan’s output cooled the engine bay- was to blame.

Idiosyncratic faults:

The severe nose up trim change on retracting undercarriage at take-off.

Generator MFO too high resulting in high taxiing speed or battery going flat.

Operational shortcomings

Inability to catch a 3^rd Gen Fighter or stay in the firing bracket long enough to bring it down

Inability to accommodate a CCM system within the existing Gnat airframe.

Equipment &specifications

The engineering approach would be to avoid a thorough redesign to get a golden standard and instead aim to have a reasonable solution to the problems

Seat and Safety

Introduction of the MBMk 16 or KVD 36 seat. In extremis- if cockpit space is a constraint- the old GF4 modified to have a rocket pack.

MDC

Larger Battery and Hydraulic reservoir with fully duplicated Hydraulics and electrical systems.

Redesigned tail plane actuation system replacing the Hobson system. Good conventional systems are available for “copying”

Windscreen –bulkhead distance increased to suit a larger population percentile but not necessarily to “International standards”. I expect the Aeroplane would sell mainly in plus minus 30 degrees earth latitude and we should design to such a population. I cite the example of Imperial Japanese war equipment was tailored to the Japanese population and was ‘snug” to Western pilots and Tank crews who tried them.

Glass cockpit

Redesigned gun feed as per the the Ajeet mods. which reportedly worked.

Strengthened main forgings to take Aden /Gsh 30 recoil which used to crack on full fire out- which incidentally was discovered after 10 years of service. One can never rest tranquil in this business!

The use of two (smaller) parachutes for braking a la Su 7.

The introduction of a “trimming” airbrake to avoid change of trim on selecting air brakes or “up” u/c.

Redesign of Generator/ECU to tune MFO of the generator with the taxiing speed.

What is the additional/ replacement equipment required?

 Seat & Safety: Martin Baker Mk 16/KVD 36/ GF4 Mod seat.LOX or OBOGS*, GPWS*APU

Engine : RR Adour 811/Honeywell 125 N/ Motor Sich 222-25

Fixed Guns and Weapons:2X Aden 30mm/ 2x Gsh 30 AO 65 for Ground attack /4 x Single NS 23  Flak Vierling layout/ 1x 23 mm GSh, 1x GSh-30-2  CCM.

Should there be ejector pylons? A lighter solution is to have Soviet style ejectible stores or our own Arrow pods- they are a lighter solution.

Should IR decoy flares be fitted? Maybe! This needs customer discussions because given the small size of the aircraft there will be situations when the IR flares may actually help locate the aircraft to the defenders!

Cockpit & Displays: HUD, HOTAS, 5xMFD, GPWS *, Lightweight Radar or Laser ranger*, RWR, RAM, Health and usage monitoring system (HUMS)*Radio altimeter,

Avionics: WAC, ADC, HMS, Mission computers*, Radio Compass,Ring laser Gyro*,Digital Map*,IFF, Auto pilot*,Self Protection Jammer.*

Alert readers will immediately note that these are standard equipment fitted to the Hawk and the Jaguar Darin 3 standards. Items marked*are only for select versions of the aircraft.

The airframe would have to be modified by lengthening and deepening it to accommodate the HMS, the new engine and the ejection seat. The wing has been slightly increased in area whilst keeping the same section plan form and twist etc. The sketch (fig2) shows the Adour Gnat and the Folland Gnat profiles to the same scale as a comparison of the sizes. Table 1.includes a “target volume” i.e Length x span x height which is an approximation of the target the aircraft will present to the enemy. This uses the Folland Gnat as a unit volume. The small target made by the Adour Gnat is noticeable. The dimensions are provisional and the project engineer will have to work like a bespoke tailor with his nips and tucks. The aircraft shown has about 900 litres additional space in the centre fuselage and about 80 litres in the forward avionics bay.

The Adour Gnat (1987)

The Adour Gnat (1987) is built to philosophy of “cheerful pessimism”- things are simple as possible but if they don’t work we will find out and fix them.. The layout is conventional so FBW problems don’t hold up the programme i.e. required it can be flown without FBW until that is debugged.. It also follows the philosophy of “de-optimize locally to optimize globally”; It uses standard supplies and rotables e.g. it uses the Jaguar/ Hawks engines hydraulics, brakes ,wheels, ECS and electrical etc perhaps as Government issue supplies to save time and money. Needless to say because the design would closely follow the Gnat’s  the development cost and time would be low. The prototypes would be built using “knife and fork” methods and standard metal rolled sheet and extruded stock. Forgings could be replaced by “machined from solid “items or open die forgings or even crankshaft quality SG iron where possible. Naturally all tooling and production methods would follow the Gnat /Ajeet’s practice. One nice thing is that like the Gnat the biggest of the sub- assemblies are small in size which would reflect on the tooling costs. This aircraft will be the trials aircraft rather like the Folland Midge and would be similarly used for concept evaluation. The (1987)?  It means that this aircraft could have been on the flight line by 1987 as it uses only technology that was available in India in 1983 but just look at the performance figures! The basic empty weight would be 2850 kgs as compared to the 2160kgs of the Folland Gnat. Today the relatively simple ‘analogue” would be used to check out the “Goods” and reassure the shareholders!It would provide hard test data and a list of mods that would be needed.

The Adour Gnat (1993)

Provided the Adour Gnat (1987) attains 90-95% of its predicted performance and customers are interested the next step would be to design an improved version using optimized made to order accessories where indicated e.g. brakes wheels tyres etc. This would also be the time to see if composites can be brought in. Composite alone can save about 180 kilos on an airframe of this size but the strategic implication of this material has to be understood. About FBW one has to be even more circumspect. In a small fighter like this the weight of the system has to be carefully assessed against the gain in trim drag. For example the pitch inertia would be one seventh of the LCA’s which itself is a small aircraft! A compromise which is interesting is the fitting of a single channel FBW which would take inputs from the Air Data Computer, the G meter, the ASI and select strain gauges embedded in the airframe to let the pilot know where he is on the V-n diagram. This aircraft could have been ready in 1993.

The Honeywell Gnat F125

This is the version that could be built today using the Honeywell F 125 IN engine. The other alternative engines are the latest Adour or that dark horse the Motor Sich AI 222-25 though RU/Ukraine politics are a factor. It may be interesting to develop three proposals each using US. UK and Russian /Ukrainian aggregates so that the customers can get equipment whose “philosophies” they are used to thus easing his logistics. The weight savings due to the Honeywell engine gives rise to the possibility of developing “heavy and light” versions of the Fighter. The terms are relative. The “Heavy series “would be optimized and equipped for Frontline close support and interdiction raids behind enemy lines. This would be equipped with MiG 27 style cockpit armour, fuel tank and lines “inerting” , heavy GSh 30 or GSh 30-2 cannon, armoured control and fuel lines which would give it very relevant protection and slash aircraft losses to  low cost air defence systems..The figure shows the side view which also emphasises the commonality between the variants. The variants will weigh in between 2400 kilos for an AD version with a single underbelly 23mm GSh to about 2850 for a dedicated “Shturmovik with armour protection and using Aden 30mms.The version using the GSh 30-2 would  be in between as that gun is lighter than a pair of Adens by about 90 kilos.

An assessment of the aircraft.

The term Gnat F 125 is being used to describe the generic version irrespective of which set of equipment and engines is used as the differences would be marginal. The table is self-explanatory and I will confine myself to the discussion in which the Gnat F 125 is compared with a F 16 as a typical threat.

The table shows that the Gnat F 125 will be a challenge to the F 16 even in on to one. In 4 vs 4 or 2 vs 2 the advantage will be with the smaller aircraft. In straight line performance the F 16 will gain but the price is the A/B to below “bingo” levels.

The  Gnat F 125 ‘s acceleration levels from M 0.6 to M 0.9 whilst retaining CCMs  will show startling improvement over the Gnat enabling the type to prevent a 3^rd or 4^th generation fighter from breaking off at will. The Folland Gnat’s M limits were 0.91 at LL and 0.98 at higher altitudes so it is reasonable to expect M 1 at “all altitudes” though if it is genuinely required (and I be a doubting Thomas!)  the wiser course would be to design a 6% wing. Fortunately such a change is easy thanks to the Gnat’s “modular” structural design.

General flying characteristics will still be very much “hot rod” like original Gnat though the Gnat F 125 will not have the “hair trigger” feeling of the original because of the  increases in damping and the inertias in all axes but  it will still very much remain a Gnat!. 

In the strike role the Gnat F 125 will carry up to 3000 kilos typically 4x 500 kgs plus 125 rounds/barrel for the 30mms and 2 into CCMs over a 180-220n.m. radius lo-lo-lo with the usual reserves.

At high altitudes, 10,000mts head on BVR case at 100 kms apart the F 16 will have a clear advantage in reach. Given the small size and signatures of the Gnat the F 16s detection range and the AMRAAM Kp will, however, be about 10% that of conventional size fighters.

At low altitude head on the firing window of the missile will be reduced .Chances of a BVR kill will be small to the extent the BVR may have to be jettisoned.

In ground attack the F 16 will have twice the range payload but will suffer four times the hits from SA/AAA of the same intensity. The Gnat’s life cycle delivery will thus be more than the F16’s because the Maths apart the pilot’s efficiency and aggression will increase with his improved survival.

Compared the LCA the Gnat 125 would be cheaper and just as good in most tactical situations with better close combat abilities. The LCA will be somewhat better in all weather profiles which is numerical not a high demand.

There is no magic. The Gnat is merely optimised for its most probable role and thus excels. It shows that there is a place for such a “concept warplane” in the inventories of even fairly sophisticated Air forces.

 A short note on the Development effort required.

Aircraft development does not require huge resources ,production may. One of the reasons why the Gnat has been selected for this study was that it required very little resources to develop. The following details of Folland Aircraft will be of interest. It was founded in 1940 and had never built a complete production aircraft before let alone a Jet aircraft.Its resource were:
Total area 53 acres. Covered Area including Office and Factory: 46,000 sq.mts.

Personnel: About 800 of which the design strength at its peak was 150 people yet the Gnat flew within 4 years of go ahead using prototyping technology of about 70 years ago.

With the use of Modern CAE, and CAM and the use of sound engineering and project management methods the manpower and capital requirements could be even further reduced.

The time line for development could be approximately as below

0-15 months – Project studies studying alternatives – Adour Engine with British Accessories, Honeywell 125 engine with US accessories and Motor Sich AI 222-25with Russian Ukrainian accessories and about 7 alternatives combinations of engines, seats and guns and various combinations of dimensions, weights and performances.

Tentative project costing, possible vendor /subcontractor identification and talks with suppliers of rotables for project partnership.

16-50 months Prototype in metal as “Company demonstrator”.

51-54 months: Flight trails and reviews of data and analysis

55- 70 months: Definitive pre- production machines with progressive introduction of most relating to “Heavy” and “Light” series fighters.

Price of the Cat

The price of weaponry, as with prices of drugs, has no relation with the actual cost of producing it. In India the true cost of production have been masked by the PSUs operating inefficiencies and weird taxations and customs duties not to mention the  earlier Government R&D policies.

The customer and the market

The market for this type of aircraft is said to be around 12,000 airframes. Currently this sector is served by watered down versions of 4^th generation aircraft and Advanced Trainers. It is tempting to think of the IAF as a launch customer but it may actually be fatal for any Private venture (PV) to even think of it because the “decision cycle” time of the IAF is financially unsustainable for any PV. It would be much better to keep the IAF in the loop, give it the full ego massage befitting a prospective customer but the main customer will be the Asian, American and African air Arms making do with over sophisticated equipment or with combat equipment whose spare parts have to be sourced from museums. The concept will be laughed to scorn but astute companies like Textron are investing. Cap in hand and shuffling my feet I would say that Textron got the balance wrong by being too much Cessna based. The Scorpion is a 1^st Generation airframe with fourth gen systems but the idea is right and it is awaiting the winners! It is inevitable that even the bigger air arms may see the sense of the Gnat F 125 but to be gulled into tailoring the aircraft too much to the point of exclusivity may result in the Bailiffs coming in before the Bank notes.

 A well strategized aircraft development programme is very predictable and within costs manageable as a PV. India has the resources both Human and legacy not to speak of the enterprise of our people to develop a remarkable range of combat aircraft of great potency. The then Government’s decision to anoint an yet to exist organization with no production facilities to develop a state of the art warplane defies logic even “Saturday night at the bar” type of logic. The psychological impact of such ill decisions must not be allowed to cloud our vision of the potential of the Indian Industry in supplying original combat equipment to the world

In sum

Modern airframes are unsuited for the most likely combat condition. This reduces their effectiveness and battle outcome.

Reprise of 2nd generation airframes matched with later gen systems will produce marketable combat equipment. This is the way to go for Indian private companies. India has considerable upgrade equipment and access to many excellent 2^nd generation airframes. This is an exciting potential.

The style of handling the project would like an “armed reccee”. Success at each low cost sub stage alone will guarantee farther exploration.

Past experience notwithstanding such aircraft couldbe in service within 6-7 years.

Table 1

Type	AR	W/L	T/W	ID/T	Target Volume
Folland Gnat	3.6	237	0.71	0.20	1	How good it was as a basis!
MiG 21 Bison	2.2	354	0.64/0.87/1.14	0.39/0.32	2.8
LCA Mk.1	1.8	242	0.52/0.84	0.35	3.1
Adour Gnat  (1987)	3.6	303	0.6/0.9	0.24	1.2	Low cost Demonstrator
Adour Gnat (1993)	3.6	286	0.63/0.94	0.23	1.2	Introduces Composites, made to order accessories
Gnat F125 2008	3.6	267	0.75/1.2	0.18	1.2	Current engines e.g. Honeywell F125
F 16 A	3.2	408	0.64/1.15	0.244	4.4	Target vol. shows the price paid for versatility.
JF 17	3.7	372	0.48/0.95	0.23	4.4

Notes:

AR      Aspect ratio

W/L    is wing loading in clean combat weight with full internal fuel kg/M²

T/W    Thrust to weight in kg/kg at above weight. Two ratios give full military and A/b thrust/weight ratios.

ID/T    Gives the percentage of power consumed at S/L ,3.5 G turn at clean combat weight.

Target Volume indicates the target the aircraft presents to AA defences. Span x length x height .Folland Gnat is taken as unit volume.

The simple table above accurately reflects why that the Folland Gnat gave the MiG 21 Bis a hard time in DACT sorties. The MiG 21 had to use its A/B to disengage. Such a move would be ill advised against the F 125  whose figures above are with two CCMs. These would find the large IR signatures of the F 16/ MiG 21/JF 17 very attractive.