HF-24 Problems From Its Design

By Group Captain Kapil Bhargava VM (Retd)


This is the third and final part of my personal story of Design & Development of the HF-24. My earlier caution declared before the text of Part 1 stands for this Part 3 as well, hopefully the last one on the subject. To remind, this is also from memory and should be taken as my personal and coloured views. Please do not treat any of these articles as authentic history of the HF-24 project.

 My Very Limited Participation

 I would like to caution everyone who thinks that I had a lot to do with the development of the HF-24. The truth is that I ended up with a grand total of 30 hours and a few minutes on all variants of the type, except the Mk IR. These included the Glider, Mk I, Mk IA, Mk IBX, and Mk IT. As you can guess, my contribution to it was only theoretical and partly during its pre-flying days as Chief Test Pilot of HAL. I have already written a brief account from the three flights on the Glider and one on the first prototype, which gave adequate warning or demonstrated the troubles to follow. You could read it now if you have not already done this.

 My main contributions as Officer Commanding A&ATU and later ASTE were to clear the Extended Chord Wing and the view from the rear cockpit of the Trainer version. Now let us look at some early flight testing, all of which was done by other test pilots. Most of this is actually hearsay and it should be treated with due care.

 Stalls on the Glider

 In the write up on the HF-24 Glider story, I  had mentioned that after several rejected options, the sawtooth wing gave stall warning of about 12 Knots, which everyone accepted with glee. Apparently, I was the sole dissenter. After I heard this figure from Winco Suri and Prof. Tank, back in my office I picked up my slide rule, we did not have calculators or computers those days. I calculated the expected stall warning on the actual aircraft, not even at intended maximum all up weight. This turned out to be 28 Knots. I pointed this out to Winco Suri but he was not interested in what I was saying. No one else took this up as we were following the British Air Publication AP-970. This had no caution against it but only required the fact to be reported. Obviously, some boffin sitting in a back room would have had to clear or reject it. On the other hand according to the American document for military aircraft requirements called Mil-Std (for Military Standards) it was clear that the stall warning was much too high and unacceptable. More than thirty years later the two standards were  integrated as Def-Stan. These are currently in use, perhaps even for the LCA.

 The Drag Curve

 After reading his comment about flying on the other (or wrong) side of the drag curve, I had requested Air Mshl Shashi Ramdas to elaborate on this. But he was on a short holiday, and now might not have the time for it. I am attempting to say something about it here.

 If you draw curves showing Parasite and Profile Drag against speed (TAS) during take off, they will start at a low figure and increase with speed. On the other hand, Induced Drag, the penalty for generating lift at low speed, decreases with increase in speed. If you add them together you will get another curve higher than all three, starting high and ending high as speed increases from low to high. Plotted against speed such a curve in part will look like the bottom surface of a Chinese soup bowl seen edgeways. Thus, in between at some point at the bottom of the curve the total would be the minimum. Getting airborne at any speed below the minimum drag point, would be flying on wrong side of the drag curve. The speed would definitely be higher than stalling speed, though perhaps not by much. Both during take off and landing, it would be dangerous to be at higher drag values with inadequate power. But the HF-24 had another major contributing factor, its stall behaviour.

 Consequences of Excessive Stall Warning

 I believe that the narrow escape of Air Mshl Jayal’s take-off, graphically described by SK, was at low speed but also subject to judder of the closeness of the stall. These would surely prevent getting the aircraft to accelerate. I was then serving abroad but I heard a rumour which may be untrue that it was an HF-24 Mk IA and that the reheat on one or both engines cut off during take off. With the jet pipe nozzles open this would result in considerable loss of thrust. I feel we need to appeal to Air Mshl Jayal to give us an authentic account of what happened. That would be the definitive story of it and would be a worthwhile lesson. I also think that Piloo Kacker’s crash could have been due to poor take off performance with four full drop tanks installed. I have always believed that if the drop tanks were jettisoned he could have got away with it. But then almost no pilot, including me, would actually act this way. It feels criminal to lose four tanks during take off. The reluctance to do it would be very natural. In any case, a Court of Enquiry would follow where the first question always is ‘who is to blame’. There is virtually no emphasis on prevention.

 The excessively high stall warning caused more than one problem on the fighter aircraft. The common one to encounter was a judder even at high speeds if high G was pulled. I am not sure if squadron pilots ever encountered this, or recall it. Comments on this will be very welcome to confirm my personal quick test.

 A Partial Solution to one Problem from Judder

 The second more awkward problem was the judder during bank reversals when flying scissors manoeuvres. Very often these were at relatively low speeds. Too often, strong judder used to shake the wings violently enough to bend the pitot tube mounted on the left wing. It often ended up pointing  backwards.. As has already been brought out by other contributors, practising this exercise was essential. Until the arrival of No. 29 Squadron at Jodhpur, Marut aircraft had to mount CAP. An aircraft with the bent pitot tube was left with no indication of speed and unreliable reading of height. Whenever possible, a shepherd was used to help it land. The pitot tube on the Marut was from Rosemount (Type 9G) common with the Gnat. But the Gnat never had a single such failure.

 After a spate of incidents, I decided to get the pitot tube moved to the nose where it had started with on the prototypes. As a general rule, a flying body affects the airflow ahead of it for up to twelve times its frontal dimension facing the airflow. I had known that the nose installation would have large errors in reading height. As far as I know, the Marut pitot tube was moved to the wing not due to errors but perhaps in the hope of fitting a radar in the nose. But I could be wrong in this assumption. I obtained the pressure error correction graph from HAL. Sure enough, the errors were large in one critical condition. The worst of these was with drop tanks at low altitude. The altimeter would read about 1200 to 1500 feet above the actual height. But I decided that the aircraft was very unlikely to be flown at low altitude at night. In daytime, safety could be ensured by visual cues. I think that the decision was right as I never heard of a crash due to this problem and pitot tubes stopped bending.

 The Single Hydraulic System

 Kurt Tank belonged to the old school, suspicious of new technology such as powered controls. His mix between old practices and new innovations was most interesting. He seriously tried to clear the HF-24 to high IAS and Mach No. with manual controls. The Glider had manual controls but three tiny winglets for each aileron with spans of about six centimetres. These were conventional cambered aerofoils  but of a very small chord of two centimetres or less. They were fitted just above the leading edge of each aileron. The lateral control over the aircraft throughout the cleared envelope was excellent. For longitudinal control Tank used a variable incidence tailplane. Rather reluctantly he decided to power the controls but only with a single hydraulic system, including the services such as wheel brakes, undercarriage, flaps and airbrakes. A single jack in the fuselage was used to drive both ailerons. All the same, he used to talk about clearing the aircraft to Mach 1.5.

 The control run from the cockpit to the single hydraulic jack in the fuselage and from it to the two ailerons was purely mechanical. It would have been virtually impossible to eliminate all free play and backlash in the control run. Perhaps the push-pull rods, bearings and hinges etc. between the jack and the two ailerons were more critical than elsewhere. On the first prototype Winco Das discovered soon enough in early flight tests that at medium altitude the aircraft developed a nasty rocking motion of banking side to side by about 10° or so. This was not quite a waggle of wings but was much faster with small oscillations around level wings. The feature was not dangerous but was quite unacceptable. Much effort had to be devoted to correcting this behaviour. The obvious answer would have been to provide separate jacks for each aileron, with minimised control runs. I believe that this may have been done on the Mk IR, the final ill-fated version of the HF-24.

 Tests by Wg Cdr RL Suri

 There is little doubt that Winco Suri displayed much courage and flying skill to test the HF-24 Glider for 83 flights before I got a turn and perhaps one or two more launches after it. Unfortunately, he did not bring his personal knowledge or opinions to flight testing. I had seen this in relation to the Krishak aircraft which he asked me to taxy for the first time and to do a little hop. During external checks, I discovered that even though it was a tail wheel aircraft, it had just 1° up elevator but 38° down elevator. I did the hop with great care rising by only about a foot. On closing throttle, it was impossible to prevent the aircraft from coming down with a thud. When I told him about it and the risk involved, his answer was that it was the designer’s problem, not his. I even pointed out that there was no designer since it was a tape-measure copy of the Aeronca Sedan. I said that he would be flying it and the risk was his. In the event, on the maiden flight while landing the aircraft came down so heavily that the undercarriage flayed out like a can can dancer doing a split. I  told him that I was not willing to be his No.2 for the HF-24. Fortunately I was to pulled out of HAL for the Avro748 project by the CAS himself. The eventual results of his attempted first flight in BR-462 were no surprise to me.

 When Winco Suri did taxy trials on the first HF-24 prototype, he ignored the heat generated due to braking. A fire resulted and did much damage. Sqn Ldr PDA Albuquerque (now AVM, retired) developed a thermocouple probe to measure temperatures of the wheel drum and brake pads etc. to ensure that the problem was not repeated. It did not. The portents for a successful maiden flight were not good but were ignored by everyone.

 For the maiden flight attempted in March 1961, I heard some horror stories while posted in the UK. Apparently Winco Suri got briefed by Prof. Tank that the aircraft would take-off by itself with nose oleo extended for the shortest ground roll. Perhaps a decision speed of 155 knots was agreed by which if the aircraft was not airborne, the take-off would be aborted. Some one even mentioned to me that a speed of 175 knots was discussed, though presumably rejected as too high to stop safely. And even Prof. Tank must have forgotten that friction between the wheels and runway could delay the automatic take-off by causing a nose down pitching moment.

 Obviously, the aircraft did not fly off by itself. Despite having talked about aborting take-off, no procedure was discussed or finalised  Winco Suri aborted the take-off in an unprofessional and unprepared manner. He closed throttles, but did not switch off engines. He did not use the brake parachute but simply retracted undercarriage. He must have had no faith in the crash barrier stopping the aircraft safely.

 The Air Advisor Air Cmde H Moolgavkar summoned me from AVRO’s Woodford airfield (Cheshire) south of Manchester to explain to him what could have happened. I immediately said that it was a simple case of incorrect trim setting and not pulling the stick back to get airborne. The Air Advisor shouted at me, ”Don’t be stupid. He is a test pilot”. I kept my mouth firmly shut, rather than express my different opinion on this. I have often wondered what conclusions he came to once he got all the facts. Eventually Winco Das did the maiden flight, deliberately using the same trim setting, nose oleo extended, but getting airborne in the normal manner.

 My Flight in First Prototype

 Just one flight by me in BR 462 followed a lot of confusion within IAF, Ministry of Defence and HAL. I wrote about collaboration with Egypt earlier. The relevant article is on Bharat-Rakshak at


 I was suddenly ordered to go to Bangalore, fly the first prototype and then leave for Cairo just two or three weeks later. It must have been thought that just the one flight would prepare me adequately to handle the joint HF-24-E300 project! Eventually the task was very competently handled by Wg Cdr IM Chopra.

 On arrival at HAL, I heard a lot about the various experiments to confirm the source of the rocking behaviour at mid-altitude and speeds above 320 or 340 knots. Tank always knew that the trouble was due to free play in aileron circuits, even when the ailerons were in power, controlled by the single jack in the fuselage. A dramatic test tried was when after spraying oil on the rear fuselage,white powder was dispensed from the top of the (right?) wing.  This confirmed the cause but the solution was not easy. Tank then said that instead of the dihedral setting of the tailplane, anhedral might get it the tailplanes out of the oscillating down wash.  Everyone was aghast fearing a huge amount of re-engineering work. But Tank explained that there was none involved. The tailplanes on either side were identical and symmetrical. All that needed to be done was to fit the left one to the right and vice versa. It was the master stroke of a design genius.

 The tailplanes were at anhedral when my flight became due, out of the blue. Many fears were expressed about my ability to cope with this. But Winco Das said not to worry, I would be just fine. He must not have explained that the changeover had no noticeable effect on handling the aircraft, and also did not cure the rocking. Eventually powered friction brakes were fitted which prevented the problem and kept out of the way on reversion to manual. I have vaguely heard that the Mk IR had dual hydraulic systems and separate aileron jacks. Perhaps the only surviving pilot, Wg Cdr MW (Chuchu) Tilak to have flown, it could explain this for our education.


 While Prof. KW Tank was a very good designer, he obviously did not know much about production technology to minimise manufacturing time, costs and time or to ensure maintainability. I have some ideas about this and a separate article could cover them. The article could also suggest why the Marut died prematurely. There is much confusion and speculation about this. I believe most causes attributed to its death are just guess work, I hope to add my own guesses to the bundle and let others come to decisions on their own.


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© Copyright Gp Capt Kapil Bhargava (Retd). All rights reserved. Reproduction or distribution of this article in any form without the express written permission of the author is prohibited.


One Response to HF-24 Problems From Its Design

  1. prodyut says:

    True the Marut had a lot of problems -just like the Hunter but it still has potential specially when one considers the advances in modern propulsion technology. The Saturn AI 55/Adour/ Honeywell 125 together with the Bison’s avionics, missiles and cockpit equipment could be the STARTING point for an alternate solution should the LCA Mk 2,just by mischance, repeat the LCA Mk1 experience. Such pessimism can be labelled “negativism” or simple pragmatism depending on what job you have to do.

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