The HJT 36 -   a 2024 review    Iss. 7                                      Professor Prodyut Das

It is interesting to be mentioned in Twitter X and more surprising when the reference is in positive terms. One can imagine my surprise when something I had said ten years ago was referred to. https://twitter.com/Indranil_Roy and  https://twitter.com/hvtiaf_bharat  referred to my blog posts in 2014 on the then subject of the HJT 36 spinning trials. The references are;

1.      for Indranil’s post of 2019 in Delhi Defence Review https://delhidefencereview.com/2019/04/23/hals-hjt-36-intermediate-jet-trainer-rises-from-the-ashes-a-detailed-look/

2.       2014  Prof. Prodyut Das 2:  Some Notes on the Forthcoming Spin Trials of the ... https://prodyut7.blogspot.com/2024/03/some-notes-on-forthcoming-spin-trials.html?spref=tw

3.       Prof. Prodyut Das 2:  Some Notes on the Forthcoming Spin Trials of the ... part 2 is at https://prodyut7.blogspot.com/2024/03/prof.html?spref=tw 2014

Despite the surprise at the long memories of the people I am still a bit curious about the trigger for the recent posts. To me in Calcutta, on the remote fringes of aeronautical civilization, the information is that the HJT 36 cleared the spinning trials with 6 turn spins being conducted for both left- and right-hand spins in 2022 and is ready for production. Has there been some further problem in spinning?? After reading Indranil piece of 2019 and seeing the photo 6 ( now removed) it seems to me that there is such a possibility in the “fix” . which is why I am posting this piece.

Apparently, HAL had (quite unnecessarily) brought in a foreign consultant and I was flattered that the consultant had after putting his hands on the aircraft (note 1) proceeded along the same lines as in my posts of 2014.

I have no inside information so what follows is pure conjecture but you might continue reading because it reflects what one person thinks about the subject of design for spin correction and its problems.

Indranil had a picture of the final solution (it is now missing since 19/03/24) and I thought it looks like a pig sty. Worse the solution given for the HJT 36 has a sting in the tail i.e. a part of the solution actually adds to the undoing of the solution. Indranil mentions a flat spin. When the aircraft is in a spin it should be designed so that the Cg/ C.P.  moment itself should be enough to push the nose down and unstall the aircraft even without the pilot pushing the stick forward to do so. This flat spin indicates too much of a rear CG position and inadequate stabilizer authority.

If  that is indeed the case this is what may have happened is simply this: All that metal work (bigger fin, strakes, rudder) whilst curing the problem by providing more keel area in one hand destabilized the aircraft by moving the CG back. May be the stability was marginal and what passed on one aircraft did not quite pass on the next.

What would I do?

If the IAF wants better spin/stall characteristics the following may help. 

Assuming what I have said so far is right this is what we need to do:

1.      Get the weight down and the CG forward and down, Must do all three. Excess Weight makes the aircraft badly behaved- especially at low airspeed. The airframe needs selective weight improvement so that both the weight is reduced and the CG is move forward. Try things like a composite fin, rear panels and strakes in composites etc so the Cg moves forward. How much weight can be reduced from the airframe cannot be fixed without knowing more but if you go by Horikoshi's method you would be surprised how much can be improved.  

2.      Reduce the coefficient of the side area. The cockpit of the HJT 36 is oversize – it was once heard- so that when one transitioned to the Su  30 MKI fighters one was not "traumatized". That was the genesis of the spinning problem though it may have been difficult to see at that time. Well now that we are here what do we do? We cannot easily recontour the fuselage lower line perhaps but we can recontour the canopy and the nose cone? It will help reduce destabilizing yaw. Getting a new trial canopy shape or a platypus nose cone should not take too long. It will help.

3.      Lower the vertical Cg. Again, it is a case of selective culling of the weight of the upper airframe.

4.      Dihedral increase: Can the dihedral be increased a little bit? About1 degree to begin with should help.

5.      Casual discussions raised the possibility of roll and yaw stabilities interfering with each other. It is an interesting possibility to work on but I need more info to delve into it..

6.      Increase the washout of the tips or give conical camber as in the Gnat L.E. This is why I had in 2014 argued about using a wooden L.E so that modifications could be quickly made. These provisions ideally should have been made ab initio during the preliminary design but given the low speeds at which the spin trials will take some “jugad” may still be possible. The Gnat’s conical camber could be analyzed and applied-after discussing with ex Ajeet jockeys- about its effectiveness. I remember they used to Dutch roll ever so slightly when they came in over the perimeter fence at Kalaikunda but that was probably due to its shoulder wing/45 deg. wing sweep layout.

7.   Manufacturing    The manufacturing “errors” between port and starboard wing should be quantified and rectified. 

      Platypus Nose Change the nice streamlined nose to a inverted  "Utkanos" or Platypus nose. Experts will tell you area ruling does not work at low speeds but then they have not heard of the work of the German auto aerodynamicists - Rumpler, Kamm et al of a hundred years ago. Area ruling works but the effect is not so visible at M=o.3 as it is in M=1 giving only a 4% effect at the speeds we are talking about but I believe every bit counts.. One needs to do CFD comparisons for this one but one will pick up some drag improvement also. 

Anhedral on the stabilizer. Significant anhedral on the stabilizer will add to the "fin" area and being anhedral it will be in relatively clear flow at the stall. It should work . By significant I mean something in the order of 25 degrees at least. It will help to get rid of those stupid little plates stuck under the stabilizer which will only attract AC2s to ride into them and damage both the "fix" and the "erk".A

Closing comments

The HJT 36 made a few mistakes but is a competently designed and engineered aircraft or else it could not have been sent off on its first flight within three years of go ahead and 14 prototypes built thereafter. The subsequent delays are worth studying because Technology difficulties was not an excuse. The project shows that by working in silos we are wasting much time and funds by making small elementary mistakes at the start and then spending years and crores trying to rectify those very mistakes when they are embedded and difficult to correct. Instead of acknowledging an error the present structure of the Industry conditioned the people to see an error or mistake as a sin and a loss of face rather than acknowledge there has been mistakes made and doing everything required to get it right in the shortest possible time. Interestingly when the F 35 turned out 2 tons overweight the team acknowledged the problem, formed SWAT ( Structural Weight Attack Team) and got the weight down. By telling the truth the first time they were free to take all resources to correct, By pretending to be infallible we try to do things secretly and delay though I have a feeling that if you speak Mandarin and ask them nicely the Chinese will tell you exactly what is wrong. So much for "secrecy".. 

     The HJT 36 is also an example of why we should change from a “science based” approach to a “culture” or “feel” based approach. Most of the problems of this kind could have been avoided if our designer’s developed a respect or empathy for the customer’s and his deep knowledge. It is an old wife’s tale that Tandem seating has too much side area ahead of the CLA . If you look at the HJT 36 there is too much side area forward of the CG and it is made worse by being absolutely flat and a lot of that fin is “going to waste” correcting that. There are very simple tricks to get this right- before doing the first layouts but now it is all water under the bridge. However, getting the aircraft back up to trim should be easily possible.

HAL tried may solutions- wing fences, vortex generators, wash out ,bigger tail, strakes  et al. Any combination of three should have worked  but it seems to me HAL was too timid and  too anxious about the result resulting in too hurried an application. One cannot make corrections unless one is personally confident of the fix. Otherwise feafr of failure, fear of criticism makes for timid application and the Babu in the Engineer  takes over.

ALL of them should have worked there was lack of confidence about the outcome. This mental anxiety does not give results in in development. There has to be BOTH an unshakeable conviction in a well thought correction giving results AND a humility that if it does not work well lets try something else. I will illustrate with a story. The French Epsilon basic trainer of 1980s failed in its spin recovery. The French original fin design had checked all the 'old wives tale " boxes and it still failed. They simply redesigned the entire empennage ,introducing an elegant swept fin which made the Epsilon- comme dit on? - Tres Chic! 

One last kick to conclude. Whilst HAL team showed commendable effort in trying various solutions they should not have taken 6 years to do so! We will have to do better, Lads!

Note 1.

We literally “see” with our hands just as we “fly by the seat of our pants”. Half the nerve endings-some 7000 of them in the human body end up in the hands. This is why we instinctively reach out when we say “show”. An incredible amount of information is conveyed to the brain by the hands and getting one’s hands on the subject is a rewarding experience.

Note 2

We seem to give up too easily after achieving a modicum of performance. The HT 2 was a rugged and useful trainer and though bad mouthed by the foreign Instructors of the Ghana Air Force (India had exported 12 in the ‘60s). The HT2 it was never improved. It had a fuel system that cut out when it half rolled and IAF pilots ( incl. TR  Patel 14 Sqdn. Kalaikunda 1970) who had flown both types said that the Chipmunk was nicer to handle. Possibly DHC Chipmunk Designer Zaikimuk had spent more time on getting the 1:2:4 ratio of control harmonization right and the fabric covered wings of the Chipmunk though less :modern: helped to reduce yaw and roll inertias?. The control harmonization could not have been difficult but was a bit of a fiddle which no one bothered to do once the aircraft was certified. Apart from the fuel system mentioned above one would have liked to see cockpit canopy with reduced framing and a prototype with a tri-cycle u/c with a YaK 18 style externally retracted undercarriage. We lose interest too easily perhaps?