A Polymorph for the Future? a review of inherent upgradability and the pontoon “chassis”

A Polymorph for the Future?    a review of inherent upgradability and the pontoon  “chassis”    Prof.Prodyut Das.

 

 

Poly morph –literally “many forms” here is taken to mean an airframe that is inherently suitable for “infinite” modifications and up gradations. Should design for upgradability be a conscious part of the design requirements for combat aircraft?  Given the technological factors that are shaping the conceptual design of combat aircraft  and India’s own lack of infrastructure which keeps the Industry at the mercy of foreign vendors, the answer is definitely a “yes”. Strategically this overdependence on vendors for key aggregates is an unacceptable situation that must be compensated by technological innovation as represented by the “Ponton” (pontoon).

 

The question of inherent upgradability to alternative engines and systems arose after Prof. KK Sarma of Guwahati ( to whom thanks) wrote to me about considering the possibility of using the AL 41 F with thrust vectoring for the LCA which, like all pretty young things, has with time, transformed from a maidenly LCA to now a more matronly MWF.  

 

As a knee jerk reaction we both agreed that the engine was “impossible” but further prodding by Prof. Sarma triggered the search. What emerged is a discussion on upgradability with some conjectures as to meeting the MWF ( Prof. Sarma’s original query) and AMCA requirements with a Ponton/AL41F based solution a solution whose aim is to be inherently upgradable..

 

The story is told in the backrooms of Aeronautical bars about the chap who designed the perfect aeroplane. He had to be sacked because he had violated a basic rule of aircraft design- his aeroplane could not be improved! Perhaps there is wisdom in old wives tales and bar room ditties that one does not pick up elsewhere because even a casual look around will show that the commercially successful warplanes were always “developable” or “upgradable”.

 

A short resume of upgradability

 

Until the 1930s upgradability was not to be considered. Wood and fabric aircraft structures would decay in normal service within a few years and in any case the pace of aeronautical development was so furious that the airframes’ aerodynamics was obsolete within months. The Albatros D III was formidable when it entered service with Herr Rittmeister Manfred von Richthofen’s Jasta 11 in 1917 but by June 1918 the Fokker D VII’s amazing-   what is now called high AoA ability- enabled the Fokker, even at high altitude,  to hang on to its prop and pour fire into its opponent from behind ( Note 1). The Fokker was such a sensation that it was required in the terms of the Armistice for Germany to surrender “alle apparte DVII”. The Albatross was obsolete within a year.

 

The advent of all metal monoplanes of the mid 1930s was the first time that upgradability tip toed onto the scene. Final versions of the fighters like the ME 109 had three times the power and perhaps 12 times the firepower of the first prototypes and pointed the way to the future.

 

After a point aircraft design subsumes Aeronautical Engineering and Science and gives precedence to the global scenario. One cannot design an useful aircraft by focusing just on the engineering. The early German Jet aircraft designers were probably conscious of the need to make the aircraft upgradable. The early German jets- the Arado Ar 232 Blitz, the ME 262 Schwalbe, the Heinkel He 280 and even the projected Messerschmitt Me P1101 all had under slung jet position which, given the state of flux the German jet engines development was at that time, was a very sensible engineer like decision despite the drag penalty. The under slung engine layout made re-engineering for another type of engine relatively less disturbing. Boeing’s under slung engine was a more modern avatar. Imagine what would have happened if the B 737 was designed to have the elegant De Havilland Comet’s “buried- in the wings” type of engine location. The Boeings could remain in production for long because the pylon mount for the engine could handle engines of more than twice the power and size engines than of the prototypes.

 

 

It was with the coming of the transonic and supersonic fighters that upgradability should have been considered as a design requirement.  Transonic flight required enormously strong airframes at Mach 1.2 low level- which as fast as anyone would wish to go at low level ,the fuel consumption being 300kgs per minute or roughly 20 litres per kilometer- puts on dynamic pressures in the region of 3.5  atmospheres. Designing for such dynamic air loads meant that the strength of the airframes were close to what would any way be required for having a life where the airframe fatigue life and the operational life-the life at which the airframe would be written off due to accident, wear and tear and in- service degradations. (Note3) This  bonus could not be reaped because at that time, the SAM was not yet perfected for the all weather point defence tasks the airframe performance demands were still creeping up for the Mach 2 regime so the aerodynamic design of the airframe was “unstable- going upward”. Therefore even in the sixties the “inherently upgradable airframe was not easy to practice. (Note 4) What actually happened was as below:

 

The service life of an aircraft is not entirely decided by airframe fatigue. The “other” factors determining the retirement of aircraft are:

 

i)                    Difficulty in obtaining “running” spares- tyres, brake pads rubber seals etc.

ii)                  Difficulties in replacing major aggregates such as engines and such type specific items as the Hobson unit etc which are particularly difficult to get in the open market.

iii)                 Operational attrition which reduces fleet size which makes maintaining the fleet difficult or uneconomical.

 

 

The succeeding generation of engines, avionics and accessories were smaller and lighter yet upgrading often faced problems. The item not only had to match the performance but also the “shape” for a successful fit without too much engineering. For example upgrading the engine of the Hunter/ Lightning with the suitable versions of the RB 199 was possible in theory and would give a significant all round improvement in range but unfortunately the aircraft had  ”tubular” semi monocoque fuselages which is a “hard” skin making changes difficult. If the outer skin did not take the entire load it would be easier to modify the aircraft significantly. It was economically impossible to “rebore” the fuselage to take the new engine and its ducting.  

 

Engineers beat this constraint by designing the replacement as a”drop fit” or specific to type item. A handy example would be the MiG 21. Given the enormous production run of about a thousand for the plain vanilla Mig 21 F for example the Russians developed the rather natty GSh 23 mm  for the MIg 21 M which gave the aircraft the roughly the same weight of fire as the Vulcan six barreled but of course at the cost of thermal stability. For the engine they had the R 13 and the R25 to replace the R11. This put constraints on the extent of upgradability.  One remarkable example of an “inherently upgradable” design of the era  was the Douglas A4 Skyhawk where of course the one piece low wing acted as a” Ponton” (pontoon) foundation which is almost a halfway house for the concept of the polymorph.

 

The Ponton.

The Ponton (“pontoon”) is a concept known well enough in Automobile design and indeed the Mercedes Benz 220 was nicknamed “Der Ponton” though for my money the little Citroen 2CV was a more brilliant example of the true “pontoon”. I had the opportunity to tape and micrometer an old 2CV for one of my projects (what one had to do to make a living as an engineer!ah, me!) and came away very impressed. The chassis, which used rolled sections and did not require heavy presses to make, was brilliant. It permitted the little Citroen’s coach work to be made from steel tube and corrugated sheet- again without needing heavy presswork. Though the design is over eighty years old, as an example of frugal engineering, its study was a humbling experience. If the idea of the Ponton were to be translated into aeronautical engineering terms what we will have is the use of the wing and stabilizer and the connecting structure to form a ponton which will carry the cockpit, fuel tanks, equipment on the top and the engines underneath all faired by unstressed composite panels. This would allow infinite upgradability because much of the fuselage volume would be accessible..

 

A current example of the “Ponton” approach is the Russians. A look at the photos of MiG 29 under production shows how easy it is to upgrade the aircraft because the centre section acts a “ponton”. Perhaps the Russians, who examined several conventional “tubular” semi monocoque layouts, realized that requirements were uncertain and were changing sufficiently to make sense to have future upgradability in mind ab initio. In the MiG 29 for example the changes to the systems or to the engines or a more “stealthy” configuration at a future date will be easy to engineer when compared to equivalent western types. The FGFA is but built on on a Su 30 Ponton.

 

The “Ponton” has its draw backs. Greater cross sectional area  which will limits its supersonic acceleration and the greater wetted area which will increase drag but current engines say the RD 33 in comparison to the Avon have about two and a half times more power per unit cross section. This additional power enabled Northrop for example to do away with area ruling in the F20 or for that matter the “conformal” packs on the F 16 are examples of how advanced engine capabilities can allow “abuse” of aerodynamics. The structure will be heavier but as the Russians have shown, this is within manageable limits.

 

India and the “Ponton”

For India the Ponton concept is an imperative for our future combat aircraft and I am including the AMCA and the MWF as “future “fighter projects. The caution that these two projects are “Future” is because:

i)                    the dates of the first flight are being given by people who will have retired before those dates. The present concept may require major redesign amounting to “new” design by the time the prototype flies.

ii)                  The aircraft is very likely to fail to meet the basic performance parameters- I am just going by the way the development cards are being played. Too many new technologies in being tested on a new unknown test vehicle – the AMCA. Even the US did not go for the moon ( F 117) without an intermediate tes vehicle- the “Have Blue”.

iii)                 The major vendors may increase prices to unacceptable levels.

 

 

We have been up these streets before. We must have inherently upgradable fighter designs for several compelling reasons.

 

Resistance to sanctions and arm twisting- is that we have had experience of a vendor upping the price of a product forcing us to choose an alternative at the last moment. In the case of the HJT 36 for example the Larzac had to be replaced by the NPO Saturn AI 55 fairly late in the day for reasons which included commercial unviability. Earlier with the HJT 16 the Viper had to be replaced by the de-rated Orpheus. Who will guarantee that it may not happen a third time? I have just heard that the planned re-engining of the Jaguar has run into “commercial difficulties” and may have to be dropped. Upgrading the Jaguars would have been an economical but effective solution. The problem of a ”tubular” semi monocoque is that outer load bearing skin is wrapped around the engine and ducting. The accessories are then fitted into various nooks and crannies and upgrading them with better is not always possible. Those who can make afresh design after twenty years but what about those who are trying to keep a leash on budgets?

 

Whilst vendors are offering airframes with 6000 hrs life this feature is meaningless without inherent upgradability as offered by the “pontoon”. Statistics show that about two thirds of the fleet will be written off- depending on the role – between 2300 hours and 3000 hrs which is about 15-20 years.

 

The MBT and the AMCA

The second reason is that Combat aircraft concepts and design is at a point that Tank design was in the early 1940s. Four unrelated changes- radio telephony, track metallurgy, the high speed Diesel engine and dual purpose main armament converted the “infantry” tank to the MBT. In fighter aircraft design we are also at such a point. The concept of the fighter being marketed may have changed by 2025 to that of a ”carrier” relying more on GW, AI and systems to do the job-that is if that job is not already been taken over by UAVs and SAMS, when we fly the prototype AMCA- if on time.

 

The potential for a significant change in the concept and specifications of the fighter has been long overdue but this has been always stymied by a lobby which wants to push the most complex and expensive specifications that the market will bear. The impact of the following uncertainties  and new technologies has to be noted.

 

i)                    Is “stealth” i.e. the F 117 style total stealth achievable for frontline fighters o[operating say from Ambala or Jorhat or it will be a specialist “hush hush” weapon operating from special bases and requiring may be only two percent of all combat sorties flown ? The rest 98% can be flown by vanilla warplanes. This question has to be settled because “stealth – as marketed- will make the aeroplane both inefficient and technologically an insuperable challenge to master. Things are made worse by the fact that stealth itself may also become obsolete/ obsolescent by 2025. Old stagers will say that that many of “must have “features of today are just very good marketing.

 

ii)                  To what extent will thrust vectoring and HMDS and the R 73 style “cueable” CCMs render traditional “aerodynamics based maneuverability” obsolete? Yes the technology exists to have the aerodynamics based maneuverability but it is not the least expensive “solution”. Meeting India’s defence needs by conventional i.e. Western-’ thinking is unaffordable. New thinking, starting from fundamentals is imperative.

 

iii)    People are tiptoeing around the “Vartamaan “combat. Was the result of that combat an “outlier” or was it the proverbial “you can tell from a single grain if the pot of rice is done”. Let me put it more bluntly. Could an aircraft even simpler, smaller, cheaper and lighter than the MiG 21- i.e. inherently less detectable by eyeball, radar or IR with only ,say Mach 1.3 aerodynamics but capable of cueing the R 73 CCM and replicating the aerodynamics performance of the MIG Bison to the extent as actually used on that day- have done the job? The answer is obviously “Yes” and a (relatively) low performance but smart platform with lower signatures is actually be a better solution given the fearsome proven reliability of the CCM though there will always be more expensive and “safer” ways of fighting a war. I concede there can be no perfect decision in procurement. We can play “safe” and order the same as everyone else until we skid to the borderline of dangerously low fleet sizes which without a doubt will be unsustainable in war.

iv)    What is the impact of sensor fusion and artificial Intelligence on Combat aircraft design? For example designers have always returned to the all round vision canopy after going on for “flush” canopies e.g. Mc Donnell F 4 Phantom to Mc Donnell Douglas F 15 but today but can rearward facing cameras and “facial recognition” techniques with promptings from ADGES do the job?

 

v)                  Finally there is the undeniable truth that Mach 2 plus capability was an old Western requirement to supplement the failings of the early SAMs and plays havoc with “sensible” airfame design. The English Electric Lightning design sacrificed range dramatically to boost climb rate and speed so that it could back up the Bristol Blood Hound for the point Defence tasks. It also had some undesirable servicing and safety features. (Note 4).Today the point defence tasks for VPs and VA should mainly be shouldered by the SAMs . We should trade supersonic aerodynamics for “fuselage volumes” which augurs well both for range – always a most useful feature for warplanes- and upgradability. As an example, the useful fuselage volume of the MiG 21 , that is the fuselage volume theoretically available after deducting the engine and duct volumes is for example only is 49 percent. The pontoon is around 63 percent and can be expanded with penalties. Even if the figure is approximate the fact that the “Ponton” is both practicable and “infinitely”( within limits of logic) modifiable is undeniable.

 

vi)                These questions cannot be ignored anymore by India which has limited design capabilities and limited budgets. Again let me repeat – there can be no perfect decision. The Japanese sacrificed “recklessly” by Western standards-battle damage resistance to get range. It was to the IJN an arguably  sensible decision and if Japan lost it was not because of its design philosophy but because its industrial potential was limited. To cut myself short- if we do not examine the possibilities of the polymorph-  what will happen is that, even if the AMCA and MWF programmes are on time, we will have produced aircraft that are not modifiable or upgradeable to the extent required. Nor can they stand up to commercial or political arm twisting. The Designer cannot estimate the likelihood of such an event but he can provide in his design features to reduce the impact of such moves.

 

 

The danse Macbre

Aircraft design is not just a three legged race of the junior schooldays but probably a five legged race where the lengths of the legs- i.e. the development cycles of all the various systema are all very different. In the case of this category of combat aircraft the “legs” comprise of:

i)                    Airframe aerodynamics

ii)                  Propulsion Technology. People are working now on variable cycle engines ( we should not even think of such things!)

iii)                Airframe materials

iv)                 RAM coatings ( applicability, maintainability and disposal)

v)                  Radar reflecting geometry i.e. robust stealth.

vi)                Control technology- FBW, FBL, “Puff Pipe” e.g Harrier,

vii)               Artificial Intelligence

viii)             Radar and avionics

ix)                 Power and power control systems- hydraulics, FBW, FBL, electrics.

 

Only the first- aerodynamics- is now relatively “stable”. Indeed it is regressive- top speeds are coming down and high AOA is less in demand than two decades ago but the rest have changes which can make significant impact on future configurations.

 

Aircraft Engineering is Technology but Aircraft Design is Humanities and is strongly linked to the technological environment .Indian designers must keep “sanctionability” or vulnerability to sanctions very much more in their minds than others. The LWF is coming in a big way once the last F 35 has been “flogged” and suppose then there was a genuine shortage of the F 404/ F 414 engines in the market. Where will we be then with the AMCA? As mentioned earlier It has happened to us before and who can take the responsibility if it happens again once we have say perfected the AMCA? The present people would have long retired.

 

A different way of doing things

We will also have to change the way we develop aircraft. The idea that if we tinker around long enough we will somehow come out with a trouble free aircraft does not work. (Note4), The successful method is to cobble together something promising and get it flying within 3 or 4 years of “go ahead” and then patiently squash all the inevitable worms that crawl out of the woodwork.  At present there is a great reluctance to do so and first flights are usually planned to coincide with the project Chief’s retirement.

 

The “pontoon” for the AMCA/MWF?

With all that as a part of what is going on in my mind I will propose the design.

 

We begin by accepting that we do not have the time to do ab initio design and the aircraft must be cobbled together using known and proven sub assemblies and come to a firm base which is flexible enough to develop further. This is the historical process. We have the following options:

Wings:

i)                    MiG 21 wing and stabilizer.

ii)                  LCA wings and since I dislike tailless deltas awe pickup the tailplane of the MiG 27

iii)                MIG 29 wing and empennage. I prefer a “tailed” layout as they are less draggy and we do not need STOL as much as the Swedish AF plans for.

iv)             The Su 30 wing and empennage to be scaled down to around 35 M² wing area.

v)                  The AMCA wings and empennage presuming that the documents are prototype ready.

It will be noticed that only aerodynamics that are “available” to us is being considered

 

In the engine we have a choice of four contenders.

i) ) The RD 33

ii) The AL 41 F / AI31F with thrust vectoring

III)  The Eurojet 2000

iv)                SNECMA M 88

In avionics the electronic suites to be considered are those of the Mig Bison, , the MiG 29 the Sukhoi 30 and the LCA.

 

Thus  we have the set of 5 wings X 4 engines X 4 avionics suites X 4 other systems and i.e. 320  options. Many of these 320 options will automatically discarded after cursory examination and tabulation e.g. the MiG 21 wing with RD 33 and the Su 30 systems This option will require a “new” centre section to be developed and give a wing area a shade too small whilst the potential for further development will be marginal. Nevertheless these have to be rejected only after due diligence. Record of the discards and the reason thereof is important. We will end up with about 12 or so near feasible solutions. It will not be possible to discuss all the twelve or so possibilities. Two of the possibilities stand out by “gut feel” and in no way dismissing the others will discuss these two in some more detail.

 

The MWF ponton

The first possibility is to use the wing panels of the LCA as available at the moment along with its systems and avionics assuming these are acceptable as is  and creating  a “pontoon” to pick up the two panels and the stabilizers. Since I dislike tailless layouts for their “finnickyness in terms of developability-’ a suitable stabilizer  is mandated. The stabilizer of the MiG 27 is in terms of area more or less “right” and probably has sufficiently low aspect ratio not to stall before the wing! These will be attached to a pontoon which will carry the SU 30 nacelle for the AI 31/41 F underneath and the LCA MK1A cockpit and systems are attached to the pontoon.       ( Fig1) The thing will look something like shown in the sketch. I expect that the disadvantages of the 1.76 AR wing will be compensated in combat by thrust vectoring and HMDS/R 73 systems so we have a possible solution- even with the present very low aspect ratio of the wing and its consequent high induced drag in a turn is negated by TV.

 

The MiG 29 Ponton

Another probable solution is based on the MiG 29’s with a single AL 41 F TV. (Figs 2, 3.) The basis of the aircraft will be the MiG 29 ponton which consists of the wing outer panels the tailplanes and the centre section which also carries the cockpit. There will be temptation to redesign the MIg 29s wing by replacing the R 177 aerofoil with something “more suitable” for the Mach 1.3 range- the promise of having 20 percent fuel volume in the wings is alluring but this goes against the philosophy of “make do and mend” type of design and will hamper the target of getting something into the air as fast as possible.

 

The centre section whilst based on the existing MiG 29’s centre section (CS) with such “irrelevant- for the flight test aircraft- features as the louvered intakes etc deleted will have to be remanufactured to replace the two tunnels that are used by the RD 33s and replaced with a single larger central tunnel for the AL41F- as per Prof. Sarma’s original surmise. The centre section will also carry hard mounts so that the engines position relative to the CG can be shifted longitudinally for the the flight trials. The entire engine nacelle of the SU 30 is the basis of the propulsion “egg” and is slung under the CS as mentioned. ALL systems and aggregates are identical except the change in loom length as dictated by the change. The MiG 29’s undercarriage will not reach down sufficiently to cater for the 200 mm bigger diameter of the AL31/41 engines and so the solution will be re-examine the MiG 27’s undercarriage and “appliqué” them, to use a term in dressmaking, on to the sides of the nacelle or shift the datum of the tunnel upwards for the AL41 by the same amount.. The nose wheel and the brakes of the MiG 27 will be used and the nacelle will have to be modified accordingly or – at least for the prototypes -a 4 wheel undercarriage where the nose wheel is also appliquéd onto the front of the nacelle can be considered. The existing twin fins , though probably slightly excess in area , can be retained for the prototypes.

Each of the solutions have their problems. The MIG 29 based solution is good because the development and detail design work will be the least but the problem is that the Russians may  need to be placated with license. The LCA based solution is free from such hazards but will mean more engineering work. It is always thus- no prefect solutions. The test aircraft will not be built to any operational requirements or comply to any standards being in the “X”-experimental- category. The aim is to clear the aircraft for about 500 hours of testing to see if the idea has promise.

 

The DASA/Rockwell X31

Of course no one builds an aircraft as described above- like a patch work quilt- in India! Elsewhere “patchwork quilting” method is standard. Fighter designers always minimize risks ( because of the pressing need to be commercially successful, by using proven ,yawningly familiar systems and aggregates to build a trial quick build “canter horse” e.g. YF 17, EFA etc usually without a very formal “specs”. This is what Rockwell DASA did for the X (note “X”-experimental) 31 and we will see a marketing shift to hyper-manovreabilty some time soon..

i)                    A new cranked Delta wing and long moment canard by MBB who had developed it for the EFA- note again- the precursor for the Eurofighter.

ii)                  Thrust Vectoring system –with paddles that had been developed for the F 14 Tomcat’s spin testing. To be noted was that it was “good enough” TV – certainly not as good what could have been done later or the AI 41F- but it worked!.

iii)                Landing gear adopted from the F 16 with wheels from the Cessna Citation and Vought A 7 tyres. Note the extent of “scrounging around” for ready- made task reducing parts!

iv)                Ejection seat, instrument panel ,stick and throttle taken directly from the FA 18

v)                  GE 404 GE 400 engine taken from the F 18

The entire aircraft as made with aluminum except for the Wing skins but as already noted the entire wing was a “bought off the shelf” item from MBB.

 

The aircraft had no specifications but explored what was possible in terms of supermanouvreability  -AOA of 70⁰ and low speed handling at 28kts and a turn radius of 100mts. The technology developed has been salted away and will reappear as tehnew “wonder formula”  for marketing at a future date. The flight test programme was limited to 500 sorties- pl. also note. It was destroyed in the last flight under circumstances which to me at least sounds so careless as to be fishy. Except for the last flight we can faithfully follow the American approach.

 

Summary

Aircraft design is different from aircraft engineering and the Chief Designer’s ability to “read the situation” makes all te difference between an effective warplane and a lemon. It is an admittedly difficult task. The West has employed – particularly in America- thousands of talented people over the years to predict what kind of wars will be fought and therefore the kind of equipment that should be procured. By a process that is not clearly understood the Americans ,in particular, has always chosen solutions that involved manufacturing the most expensive ( as opposed to useless) equipment which could at apinch do the job. I am not being uncharitable. The Americans, in the ‘fifties developed the F 105 for nuclear strikes against the CCCP whereas they should have developed low cost equipment to knock out NV truck parks, power stations and bridges ,preferably at Night and in bad weather which in Vietnam was really bad. That they have not lost the proclivity is seen by the fact that they are extremely cautious about where and how they use the F 22 and the F 35 for reasons that do not make common sense to the Taxpayer. They developed theB!,B1A and the B2 but its  affair bet that the B 52 will see the younger aircraft off-( especially if the Americans can think of refurbishing theB52s in India!) . Why they do such things is a topic in itself but I will caution that in following American procedures beyond a certain point means that we are going for a very expensive way to meet our defence needs. The Americans have always survived by their unfettered “thinking –on the- feet” ability and we must do so too. Aircraft Marketing is very incoherent. Take the case of airframe fatigue life where 6000 hours is being now flouted and talks are going on for 10,000 hours- anything you can do I can do better is the game!. My studies show that 50% of a fleet will have been written off by about 1500 hours to 2300 hours depending on the operational profile flown.

The pontoon chassis concept allows for easy upgradability of engines and systems over the years For India, with no base in terms of engines and upgradability the study of the pontoon concept deserves particular emphasis.

The pontoon also is relevant because as discussed above it can be an “experimental” aircraft to serve as basis for our AMCA and MWF aircraft.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Sl.no	Parameter	LCAmk2	MiG  29	MiG 29 Polymorph	AMCA	MWF	LCA Polymorph	Boeing X32-JSF
1	Length	14.2	17.61	17.61	17.6	14.55	14.8	13.72
2	Span	8.2	11.41	11.41	11.13	8.2	8.2	9.14/10.97
3	Ht.	4.4	4.357	4.357	4.8	4.4	4.357	4.02
4	Wing area	38.4	38	38	55	40.4	38.4	54.8
5	Empty weight	7700	10,800	10,200	12,000	7700	8100	10,000
6	Clean TOW	11,400	15,800	13,780	12,000	11,000	11,400	18,144
7	MTOW	16,500	18,500	23,900	20000/,25000*	17,500	20,600	22,680
8	T/W	0.86	1.05	0.97	1.02/0.81	0.89	0.93
9	SFF	0.366	0.255	0.218	0.255	0.336	0.269
10	Engine thrust	1xGE 414 – 98kN	2x RD 33 2x83kn	1xAL41F(TV)142.2/ 86kN	2x98kN	1x98kN	122.4/76.2AL31F	P&W F119 -156kN
11	Aspect Ratio	1.75	3.4	3.4	2.25	1.8	1.9
12	Internal Fuel	3300	3630	3630	5000	3300	3300	8618
13	Payload	5500	3600	10,000	1500/5000	6500

 

 

Note 1

The Fokker D VII sturdiness and tractability and sturdiness came from Goettingen 298 aerofoil of 14.5  percent  thickness not only give immense structural strength – spar bending strengths go up as the cube of the depth and torsional rigidity is proportional to the enclosed area - but also the thick rounded aerofoil with its large nose radius worked better at the Reynolds Numbers involved than the earlier thin “clutching hand”  aerofoils- based on birds wings of the earlier aircraft. There is some dispute about the nomenclature of the aerofoil. Some say the nomenclature is Go 298 and others say that the aerofoil upper surface is Go387 and the lower surface is Go 418 with 14.5 % at the root tapering to 9% at the tips. The confusion is because the aerofoil actually came “out of the head” of Rheinhold Platz and was later formalized by Goettingen. Rheinhold Platz, Fokker’s talented Chief Designer, was a welder by trade, had like his contemporary Sydney Camm of Hawker’s , a flair for aerodynamics. Platz went on to design many of Fokker’s thick wing monoplane airliners after the war. It so happens that Einstein (yes! HIM!) also designed an aerofoil “ Die Katzenbuchenflache ” ( Cat’s back) which was so weird that it is perhaps better that he stuck to Relativity!

One interesting side show to this is why the German’s did so considerably in advance to the British in terms of theoretical aerodynamics even though it was Stokes who formulated and first proposed the circulation theory ( the Navier’s Stokes equation) . I would cautiously suggest 400 pages 280 odd differential equations (easy ones) that if you get a chance to read Prof. Bloor’s “The Enigma of the Aerofoil” don’t miss it. The politics and personality clashes that affect  scientific development has relevance to our situation and brings to mind the old adage that Science progresses by Deaths!

 

Note 2

When the Lightning was a gleam in Petter’s eyes he was looking for an aircraft that would be a supplement to the Bloodhound SAM i.e. a point defence interceptor. The Light5ning was of the manned missile genre of its contemporaries eg F 104, F 106 etc.  After examining several dozen possible configurations, Petter chose the vertically and longitudinally staggered engine layout. The two Avons and their afterburners were thus behind the Pilot’s “shadow” and thus the cross section – critical in supersonic flight -was at a minimum. On the flip side it was a headache to maintain and any leakage of fluids from the upper engine or hydraulics would result in an inflight fires – a great plague in Lightnings.

 

Note 3

One is reminded of an old cartoon strip where a scientists is seen busily scribbling  data – humidity, density, hardness and many complex equatiosn on the side of a large block of marble in his laboratory. He then after much further scribbling marks appoint “x” on the side of the marble with a plumb line and with achisel gives atap. A rubble of marble falls away and there is revealed a magnificent horse, rampant, on apedestal that Cellini would have been proud to have produced . However the Scientist is distraught and is seen wailing “No!No! there was supposed to have been a rider on the Horse!”. In aircraft design also it is proven fact that there is a limit to what can be done by theoretical studies. Getting the first prototype up in the air is the job to do.