Some Notes on the Forthcoming Spin Trials of the HJT 36 - Part 2

In the first part of this series we had published an analysis as to what is the possible areas of problems with the HJT 36 which has been withering on the vine since a spectacular launch in 2003. There have been some reader’s responses and since they raise important points or suggestions I thought I would put in part 2 as a conclusion.

Before I go on to the Technical portion I have a bone to pick with Indranil Roy because he has touched upon a very important point. He said, whilst agreeing with my prognosis, that (and I am quoting from memory) “what Professor Das has said is actually what  is taught in Second and Third year Aerodynamics” How I wish this were true! No! Indranil, very unfortunately this is not what the Second and Third years learn.

What they learn in Second and a Third year is this:

-T = Iθ^"́+ KAθ^́́

They will learn why there is a negative sign on the LHS and how to solve the equation by “substitute tan θ by R” and whilst the teacher will do several blackboards of derivation to obtain the equation (which is in the book in any case!) he will be unable to explain that what the equation means is “this” - this being what I have written in the earlier part. Harsh? Those of you who have done IC engines will remember the painstaking derivation of the Thermal efficiency formula of IC engine but no teacher discusses the physical phenomena and the co-relation behind Thermal efficiency and compression ratio. Any wonder we then have overweight engines?

The portmanteau explanation is “lack of time and vast course” but someone who wastes time will never have enough of it. In trying to teach everything our Institutions finally teach nothing. Unlike the Germans- who introduced the Sciences based Engineering in Prussia in the 1880s with great emphasis on the Physics of things -“Technische Mechanik” - we have replaced the Physics with Mathematics and preen ourselves on “Science based Engineering! Back in Berlin or was it Potsdam – one had to have “real” Industrial experience to be considered a Professor. Kurt Waldemar Tank , an electrical Engineer by the way, was Professor Tank.

Let us revert:

In the earlier part we put forward the hypothesis that there are three problems:

1. Engine flame out at the spin

2. Possible difficulty/delay in spin recovery due to suboptimal location of the tailplane.

3. Wing drop at the stall.

If the above hypothesis is true we now have to test the hypothesis and recommend a possible course of correction. Let us start with the easiest one first.

Wing Drop Correction

First we have to test the hypothesis that lack of “washout” and not say the engine gyroscopic moment due to sharp pull up at the stall is indeed the cause of the wing drop. This means “wool tufting” the prototype and exploring the airflow separation behavior at the tip as stall approaches. CFD and wind tunnel testing can follow if it is possible to organize quickly. The appropriate “washout” has to be introduced.

The simplest is to introduce some kind of a turbulator or a vortex generator. You must have seen them on the Boeing 737. The Brazilian AMX also has prominent vortex generator on the outer span. Did they have a similar problem?

Here I must mention a very interesting solution proposed by Mr. Mukut Pathak who suggested a “dog tooth”. If we overlook the terminological inexactitude” (A dog tooth is a local extension of the leading edge used in swept wings to break up the span wise flows) the suggestion is very good and I have improved on it. Instead of the local slat as MR has suggested I would use a “banana wing” i.e. a wing where the local L.E. sweep near the outer semi span is reduced. That way you increase the aerodynamic “washout” as also it is a good way to introduce the conical camber to solve the problem. The sketch at the end of this note clarifies the idea. Instead of plunging into massive redesign and tooling it may be just possible to prepare a new leading edge in wood (teak,  if spruce is not available) as on the lines shown hack off the existing leading edge at the appropriate location and attach the wooden L.E. segment to the existing wing with suitable brackets and bolts. The expected loss in performance will be negligible. The loss in the lift will be insignificant as towards the tips the wing will not be producing so much lift (elliptical lift distribution) and the drag will be not too much. An excellent solution possibility.

Wooden bits on a jet aircraft?! Now! Who has heard of that? Well the Canberra-larger and faster than the Sitara had a wooden fin and rudder! The story goes that to get the aerodynamic balance of the rudder right W.E.W Peter’s people would shave away at the profile until the control Harmonization felt good. Prototype aircraft are like musical instrument - they love being “tuned” and then what music they make!

Tail Plane Location

Many have suggested the cruciform tail-I presume like the MiG 15/17. This is indeed the simplest engineering solution though one would have to put in an “acorn” fairing a la Hawker Seahawk because at the junction the airstream separates into four parts and that did cause severe vibrations until “fixed”. Also the fin will require stiffening (thicker gauge fin skins will probably do). At the end of it all because the HJT 36 is short coupled like the MiG 15 and the cruciform tail will blank out a fair portion of the fin the spin recovery will still be a “white knuckles “affair. I have seen films of MiG 15 (at a presentation by the legendary Rostislav A Belyakov no less!) spinning and recovering but the problem was known to exist. No, I think I would like to go for the full treatment as I have in mind of lowering the datum of the Tail plane and pushing it back as far as possible - and if that does not work then we may be looking at anhedral on the stabilizer to get it out of the wake. That can be done in six months if we get a move on. If we don’t, it will take forever. 

Inlet Location and Engine Flame Out

The more one thinks about it the more one suspects the inlet position. I can imagine that in a fully developed spin the inlet will be “behind” the wake being shed by the thick (NACA 23015?) wing. One hopes we have data as to at what point of the spin the engine flamed out? This would be crucial to the reconstruction of the problem and rectification. Crashes during the development of an aircraft are quite acceptable but it is criminal treason if the relevant data is not available because “someone” did not “think”. I mention this because I have found we sometimes do tests without sufficient planning and foresight of what we may want.

Assuming that the prognosis IS right then we have to increase the length of the inlet until it is out into the “clear” air. Whilst CFD/wool tuft studies will no doubt be made perhaps the quickest solution would be to “jury rig” a new extended lip inserts over the existing one and do confirmatory tests. This is not alarming as it sounds because most of the tests will be done at speed of around 100-120 kts and at 2000-3000 mts the dynamic pressure will be still lower. The point is not to waste time fiddling around trying to get the best pressure recovery or whatever. It is sufficient that the rig up shows that the extended intake works and the engine does not flame out. IF that works then the solution is to refine the design and standardize the extended lip for the first batch because the best solution, the intake lowered to the level of the wing L.E will be a bit of a job and delay things too much. What HAL can promise is that if the IAF will accept the first lot with the extended lip- it will curtail the downward view somewhat for the rear cockpit- then HAL will remanufacture the fuselage to the final configuration when it comes back for its 1000 hour overhaul or whatever is the set period.

Reader Gupta has demanded as to why this happened in the first place? This is a very relevant question. One reason, though obviously not the only one, is the element of passion. It is my view that vehicle design involves more than the basic knowledge. It requires passion. Companies such as Ferrari or Porsche or AVL are not large companies nor are they financially powerful. Yet the Giants of the Automobile Industry speak of them in whispers. As a Nation our Aeronautical men know less about Aeroplanes than our Footballers know about football. If you look at the AMCA in its last public Avatar you might agree with me.

The second is a lack of conviction of one’s own decisions. Redesign is onerous but if one is sure it will work it becomes a mission. In the Government this spirit is significantly lacking because everyone is on drip feed. Why do anything non routine? You will only be blamed if things go wrong. So don’t move away from the Glucose bottle. In the private sector –where people are arguably less qualified- the very question of survival- spurs people on to try alternatives. Assuming they are not morons they then very quickly arrive at some “effective” solution.

These are my views based so far away in Kolkata.

16 August 2014