The UAV fiasco – An analysis & corrections. ISS3.                                                      Prof. Prodyut Das

Iss2. 15/1/2024 Note 1 on stabilizer effectiveness added

Iss3.  11/02/24Syntaxes corrected.

DRDO is to be monitored by the PMO. It was overdue that the common-sense credo i.e. the Maalik (Hindustani: owner) must keep an eye on his enterprise be implemented but we hold our breath. DRDO failures (or more precisely the failures of elements like ADE and ADA) are far too valuable an aid for the Import Lobby for the existing cosy arrangement to be tampered with for the sake of mere reforms. I wait to see implementation of monitoring by PMO.

Most of DRDO failures have been due to poor quality of Program Management unless it is subtle sabotage caused by lack of owner’s supervision but before discussing that let us examine some of the History of this present round of “development” and some technical question that have been bandied around on Twitter (“X”).

For the record ADE has been “competence building” (Oh! what a lovely phrase!) on UAVs since the 1970s.  For the thousands of crores and 50 years barring the dubious Nishant no one has got a serviceable Boxkite out of it all.

 Let me begin from the beginning. I was there entirely by chance. One of the younger cadets of the 3^rd Bengal (Tech) Air Squadron had developed an UAV that looked rather like the Nishant. When IIT wanted more funds reportedly ADE took it over. I was privy to regular moans about how ADE hijacked the project. In my callow youth the angst of the Professor had my sympathy though now I think it was the loss of control of the funds that triggered the man’s ire and umbrage! That sounds cynical but a lot of silos are built and defended in this struggle to have unilateral control of what are de facto unmonitored funds. What fun such funds are! One rascal in another time and place actually got a “personal” four- wheeler with running cost allowed by his one- time Institute cronies. Crony socialism in the 1990s was even more comfortable than crony capitalism.

The scene then changed to the LCRA (the Light Canard research Aircraft). Circa 1990s Prof. Rustom Damania (one of the UAVs was named after him) had very commendably got hold of a set of Bert Rutan’s plans for the Vari Eze (available on mail Order for home building) and built one. It was pretty established design and NAL did not claim to make any significant changes in it. Why or who decided from the start to call it “LCRA” (Light canard research aircraft) with the accent on Research I really don’t know but to label an established homebuilt design as an LRCA is to “poodle-fakery”. Many believed that NAL was actually doing research. I mention this as the present set up the important “Maliks” at Raisina Hill who fund R&D lack elementary knowledge of Aviation so there is no check if they are buying into junk bonds.

Now we have the Tapas/Rustom/ Tapas2/Swift/Goosey Bumps etc and it is a case fit to sicken your heart because like all ADE and ADA projects these projects have absolutely no excuses for taking the amount of money (Rs.3600 Crores @2024 INR), time (12 years) and other resources and ultimately ending in imports. It is not that the Tapas was ready in all respects in 2016 and we have gone in for imports. Indeed the accusers of the import lobby can be asked if programs are delayed so that imports are needed. The carelessness with which the programmes are done raises the question: Fool or Knave?

Technical questions

The Twitter Punditry have suggested that reciprocating engines cannot deliver the power at 30,000’ and perhaps turboprops will be needed. They might be chagrined to know that Frank Barnwell who had a degree in Naval Architecture designed the Bristol Type 138 single-seater of went up to 16,636 mts (53,560) feet on the power of a supercharged 373 kW Bristol Pegasus with a manually cut in 2^nd stage blower. The aeroplane, the pressure suit (an adaptation of the Siebe Gorman underwater diving suit (!) and the oxygen system were designed from scratch within 17 months and a FAI record established. The top speed was 285 km.p.h.at 13715 mts and endurance was 3 hours. There will be a storm of fanboy protests about how complicated the Tapas etc is but what has failed is the airframe and a hundred years ago a B.Sc in Naval Architecture working in a small company with 1930’s technology could design and build a record breaking airframe in seventeen months. I am pointing out so many several failings and engineering design lacunae here that I cannot detail them.

Table 1 The table has certain parameters are not relevant to the discussion but is included for the purpose of record

Sl.no.	Parameter	Bristol 138	Tapas BH-201	Predator
1	Air density at operating altitude kg/cu.m	0.18	0.3782	0.5572
2	Span Loading     kg. /M	120	87.32	71.9
3	Wing Loading    kg. /Sq. Mts	45.67	n.a.	53.6
4	Power Loading   kg.  / kW.	6.51	19`	12.3
5	Span	20.12	20.6	14.8
6	Aspect ratio	7.67	n.a.	11.53
7	Length	13.41	9.5	8.23
8	Height	3.12	2.4	2.1
9	Wing area	52.77	not available	19
10	Installed Power	370 kw Bristol Pegasus Supercharged	2x73,5 (147 kW) Saturn Turbine	Rotax 83kW
11	MTO (kg)	2409	2800**	1020 *
12	Empty weight ( kg)	1982	2080	513
13	Warload ( kg)	n/a	350 kg	120
14	MTO/We	1.2154	1.346	1.98

 

·        *Incl 387 kg of fuel  ** Attempts are NOW being made to reduce wt by 400 kgs. Should have been done in 2016 before flight trials.       

·        **Designed to Predator standards the Tapas BH should have MTO weigh between 1700-1900 kgs at which it should meet or exceed all platform performance parameters comfortably. One gets the feeling that some labs are the dregs when it come to platform design failure in which area negates all the successful laboratories.

What went wrong?

Looking at the above figures these are the things that are making me uneasy:

i)                 The bottom line shows that the platform is far too heavy- by structure, systems or layout for the capability it offers.  I don’t know but I guess it is mainly the clumsy layout (see iv &vi below).  The design entered a weight spiral.

ii)                Is the airframe -particularly the fuselage-too large for the job? That adds weight and the drag which is the root problem.

iii)              Following i) above - was a packing fraction worked out before the fuselage design was worked out? How were the weights worked out? “Pi by the window squared” or using the rich source of the existing drones carefully studied. Obviously, something was missed out. The pictures of the wrecked UAV does not indicate intelligent packing.

iv)              The undercarriage looks bad! The Tapas is to be viewed as a powered glider despite its weights and its frequency of landings will be a tenth of a lightplane- is that beefy u/c really required? It looks like the work of some engineering “clerk” just applied the rules instead of his mind. One hears that CVRDE of Arjun Tank fame designed the u/c which may be the explanation.

v)                 iv) shows of the degree of silo-ization. Similarly the remote control systems was done at another DRDO laboratory but perhaps ISRO could have been brought into the loop.

vi)              The T tail stabilizer, the (over?) large fin and the slender rear fuselage is a VERY LARGE red flag.( pl. see Note1) Structurally it is a flagpole on another flagpole. Un-commanded control inputs may have been experienced due to sheer structural flex. 

vii)             The shoddy design of the nacelles is irritating. I am not convinced that further reduction in weight, drag and lube oil cooler size is possible.  

viii)            Almost certainly no one has bothered to tinker and fiddle with the propeller dias and speeds  to wring the best possible performance.

The above is on the basis of my Tapasya (Searching). I am interested in other views but counter me with engineering-based reasons figures and examples not whose maternal uncle told what to whom.

The wrong remedies

VK Thakur writing in the Eurasian Times listed the remedies carried out by ADE 

   The weight spiral mandated the following measures to cope with it.

1.      Use of higher power engine

2.      Re-design of Engine Interface Module & Nacelle

3.      Local reinforcement/stiffening of the span of the wing

4.      Re-sized VT (Vertical tail to make the rudder more effective)

5.      Aerodynamic re-design (Twin Element Airfoil) for meeting QR

6.      Increased take off & landing distance

7.      Increase in minimum loiter velocity

8.      Reduced endurance

9.      Reduced ceiling

10.  Wing twist to mitigate the tip stall at a higher roll rate during approach for landing

Suggested Weight and Aerodynamic corrections.

If the reported corrections done by ADE are true they are all (barring 2,4 &10.)  retrograde  exchanges of problems because they do not address the root cause which is overweight. The overweight is close to an incredible 100% of the designed structural weight and is cited variously as 2800 kgs! The overweight indicates criminal neglect of detail design and absence of prototype management. Who issued those drawing for fabrication. Had the overweight- been controlled NONE the modifications ADE did would be unnecessary.

No2. (nacelle design) is still unsatisfactory and needs further improvement as I have already discussed. It is possible that with the reduced pitch and yaw inertias due to better weight control may also mitigate the need for 4 but one would have to see the hard data to suggest anything. What does come to my mind is the present horizontal stabilizer mounted directly under the fuselage with two endplate underslung fins and rudder immersed in the prop wash. The arrangement would be lighter and given crisper controls. The other is Reaper like ventral “Bonanza” fins?  Fix No 10 is “washout” which any schoolboy aeromodeller will remember from his free flight days with the APS “Tomboy” or Mercury “Magna”. It should have been put in from the start instead waiting for it to discovered during trials.

ADE’s desperate measures to mitigate the weight spiral included

I have kept the same headings used by Mr.Thakur. It says ADE did the following.

1.      Integrating avionics package in a single LRU (70-80 kg weight reduction)

2.      Airframe weight optimization (180-kg reduction) by using lighter material.

3.      Reducing the safety factor from 1.5 (the standard for manned aircraft) to 1.25 in a gradual manner.

At best, DRDO aimed to reduce the weight by 280 kg! Going by Table 1 there is no reason that the Tapas cannot achieve its design weight.

1.      Is excellent. That is what Heinmann did on the A4! I only hope it was from general knowledge of History and Heinmann not an original invention because it is such a waste to re-invent the wheel.

2.       Composites as weight savings particularly ab initio enthuse only the amateur. In airframes of this size perhaps 60 kilos would be saved if a 100 percent composite airframe is made- which is not possible. A well- engineered metal frame is much cheaper and often lighter in the early stage of the programme.

3.      The ‘g’ factor for this class is I think +4/-2. Reducing that to 0.9 is going to trouble with a begging bowl. It was criminal culpability specially since the problem is overweight. 0.9 you will get in windshear and gust

Corrections.

Given that the problems are known and the solution, I am convinced are known a calibrated programme of modifications, starting with the rear fuselage, nacelle and tailplane and some relocation of accessories should give very satisfactory performance and better handling.  

Note 1. More detailed explanation of the structural problem. The "Bump" to house the dish antennae in the upper forward fuselage sheds vortices at high AoA which disturbs the flow. To maintain stabilizer effectiveness ADA has mounted the stab. at the top of the fin and out of the wake. The wake increases at higher altitudes and near the stall. The fin in the Rustom has to be about 2.5 mts more than necessary just to clear the wake completely. Structurally this is the first "flagpole" which have whips equal to cube of the length. The load of the stabilizer acts on the  fin and the fin load then acts on the tail boom .

If the stabilizer is brought down to the top/bottom of the fuselage and end plate ventral fins added the following are likely to happen:

1. The stabilizer effectiveness will increase due to end plate effects of the fins resulting in smaller size and less weight. 

2. The fin size will decrease because with under fins it will be sized only for directional requirements and no to clear the wake. leading to further weight improvement

3. being immersed in the propwash the fin and stabilizer will give crisper handling at all altitudes.  Just redesigning the Tailfin and stabilizer location will start a weight saving spiral.  

The end plate fins will eb smaller and design will have a scarfed bottom for obvious reasons.