A critique of the HJT 36 present situation                                   Prof. Prodyut Das

This is based on a part of Indranil Roy/Jha’s presentation at  https://www.youtube.com/live/1r-O7QGwScM?si=-xFPRUfwviitfMtn on “Appreciation of Indigenization” podcast

I was listening to the pod cast. Indranil could have explained the spin in more detail. At the spin the aircraft is at very low airspeeds and ceases to fly (< 90kts) i.e. marginally below the stall and control effectiveness speeds; the aircraft begins to roll yaw and have a steep nose down all at the same time. It is a frightening situation and I have no wish to be in one. I think Indranil was not wholly right when he said recovery is by pushing the stick down. That works if you have unlimited height; The standard drill was to push the stick down, wait for the rudder to become effective again and then apply anti spin rudder. Why the spinning aircraft, yaws and rolls is simple but even in the IITs, when I taught “Aircraft Stability and Controls” as an adjunct Professor, in the 1980s, they did not seem to know or perhaps bother about the details. And I am not talking about the students.

My purpose to cite Indranil is not to belittle but to emphasise that far too many people- and I don’t include Indranil in this- who should know the spin often don’t know it half of well enough (Note 0). This is the question that the above podcast once again re-awakened i.e. How did we get into this mess and how did we increase the “viscosity” of that mess by doing what we did to recover from it. Eighteen years living with an uncured spin problem in a light jet trainer is a joke made worse by hiring a foreign consultant to “cure” it.

Unsatisfactory spin recovery is a very simple trap which can be avoided at the project studies stage by taking simple appropriate steps and is why most designs don’t face trouble in spinning and recovering but about once every thirty years along comes a design which stubbornly refuses to spin (Wackett Winjeel) or equally stubbornly- but perhaps more alarmingly, refuses to recover quick smart (SOCATA Epsilon, Hindustan HT- 2). (Note 1.) When that happened in the HT-2, V.M. Ghatge cured it by removing the “ineffective” rudder area above the tailplane where it was in the airflow disturbed by the tailplane and adding that area to the rudder area below the tailplane. This is why the HT-2 rudder is that odd triangular-ish shape.

Sometimes such a simple “fix” does not work and SOCATA redesigned the entire rear fuselage and tailplane to get the desired characteristics. There is no shame in that. I would recommend- unless the Customer is absolutely delighted with the present “messy” fix – that HAL swallows its pride and go for a full-scale redesign of the tail end and the intake position- even just as a one- off learning exercise. About that later, because there are more questions raised by the podcast. Not in any order these questions have a bearing in creating the mess.

1.      Who approves the layout selected for development? It is necessary to find out- for avoiding future such self- injury- why the original configuration was changed from the S342 layout- low wing, low tail to the pod and boom or “tadpole” as @dprasenjit call it. The original layout -reminiscent of the Macchi B 339- would have worked without any trouble. The tail plane is low set in the fuselage and if you draw the lines of “wake” at the stall mentally on the picture you will see that most of the fin and rudder is clear of the wake. add to that the deep intakes which would also throw up less turbulence  to the at the stall. The original configuration would not have any of the problems faced in spinning by the configuration that was finally adapted. There is a repeated pattern about how we seem to start with a sensible configuration only to switch to the worst possible a configuration with built in traps in them which guarantee prolonged development schedules. This results in we developing some truly remarkable systems but the platform being unsuccessful they don’t find application here but are leached out abroad.

2.      Stupid specifications: It is perhaps- my apologies in case of mistake (Dr. Bhadoria @HTTPS40 who rather proudly says that one of the reasons for selecting the later “tadpole” configuration was that the rear cockpit is 8” higher than the front seat- the most there is in the world- and the view from the rear is like a seat in the sky or something like that.

 I don’t agree with Dr, Bhadoria on that. My counter question is why stop at 8” Go for 10’? Go for 12?  More is not better. About the max. separation all one needs is the ability to see through the pupils HUD during gunnery exercises. In fact, this “biggest in the world” vertical separation has contributed to the present mess as the consequent taller canopy has blocked more of the fin flow ineffective fin and rudder area caused precisely by this separation, delays spin recovery. The second objection is that should the customer insist so, the same vertical separation could have been incorporated in the original S342 configuration.

No. Sir! Figures cannot be given out of the sky on the one hand and they cannot be accepted in sullen silence by the other. There must be dogged arguments and counter arguments before any figure is mutually agreed. It is simply not worth getting into troublesome configurations with one’s eyes open or shut. It- very flawed configurations- is THE major cause for our delays in Tejas, HJT 36. Saras and Tapas.

3.      TV screen? If outre forward view is indeed so important some experiments should be done using CCTV.

4.      Damming the Customer: It is to be noted that the customer, the IAF, is reportedly agreed to “order” a small batch for further development. If true that is about all the customer can do. You can’t have an untested product and expect- like some X-ers twit about– massive orders. Testing by CEMILAC is both of doubtful quality and inadequate comprehensiveness. What is the total hours the HJT 36 has flown? In fact the IAF has been most supportive if it has asked for a further development batch.

5.      Spin Tunnel: The best Spin Tunnels, like any test facilities, are the one’s one makes by oneself  over a period of time. It is not something one orders from Amazon. It is not a particularly expensive item comprising of a “well” with something which will blow up air at about 14 mts./sec. Most of the expensive components needed to set it up-generators, motors, control gear could be collected as junk from any ship breaking yard and the rest is civil construction and some instrumentation. Try it. I don’t know about the latest but in old spin tunnels the trick lay in how one chucked the model into the well. Evidently the chap who did for the Phantom got it wrong. Mind you the F4s was a particularly difficult case because ramp/deck strike requirements meant the tail had to be set high, Mc Donnel did their best by putting a anhedral to lower the stabilizer datum but again the F 4 was a beast- though a gorgeous one. I have written on the urgency of having test rigs but I must also caution that these rigs are ‘sharpening tools” and not the tool itself which is knowledge and imagination. There must also have sufficient workload. Used for developing one design in forty years and your facilities will “reduce to produce” like a pair of shoes you have not worn for a long time. Hence the concurrent need/ responsibility for a cheap set up and quick developments. Finally, no one will “give” you a facility. You have to realize the need and persist in asking for it. It is not the Babus at MoD’s fault if you want a tunnel and have not got it.

6.      Low costs. One point worth emphasising is that the HJT 36 has some truly advanced avionics and displays and yet costs 40% that of the Hawk. Its development costs was 137 crores @ inr 2003 i.e. about 400 crores in INR 2024. When we note that ADA took 500 crores in 2009 for one of its projects -possibly the AMCA if not the Mk2- it is clear that ADA is using funds most ineffectively.

The cost figures for developing the HJT 36 give hope that a 4^th. Generation platform can be developed by the Private Sector Industry for about 5 to 600 crores in the same time assuming the technical skill levels of HAL and the tighter organization of the private sector.  It also implies that the IAF’s final size of 50 + Combat squadrons will not break the bank if basic management precepts are followed.

7.      Consultant competence. I was bemused at the mention of using 150 kgs of ballast to increase the inertia. I may have misunderstood but in this business, no one increases the inertia anywhere. If it is anti- flutter it would be a very unusual case.

Suggestions

1.      We seem to have started with a promising trouble- free original layout and somehow exchanged it for layout that has built in flaws. There is a pattern in this. HAL should for its own safeguard revisit the historiography about how S346 layout got changed to the later stupid layout. The 8” cockpit vertical separation if at all required would have been just as easily incorporated in the first layout as in the tadpole layout. Why and who changed the decision?

2.      The wing drop indicates that rigging and manufacturing of the wings needs to be checked over the entire “fleet”.

3.      Unless the IAF is fully delighted by the solution that HVTIAFs team has found and perhaps even otherwise- even just as a low- cost learning exercise – one HJT 36 is to be modified with the intakes lowered to the fuselage bottom line and some small changes in the contours of the lower part of the “pod” aft of the cockpit. It is not difficult and give surprising improvement not justin handling but also performance.  

Note 0- they know the maths but not how the Maths got there.

 Note1 There was also the Erco Ercoupe which was designed – by limiting control movements- not to spin, It was marketed (trust them Yanks to do it- as- “Novices can’t spin”

Acknowledgments; The pictures of S342 were taken frm the HAL Museum website and the yellow modified HJT 36 were taken possibly from Bharat Rakshak. Pl. let me know so that I can make any corrections.

Recipe for a blunder.