Early Design & Development of the HF-24: Gp Capt Kapil Bhargava (Retd)

Caution: This history of sorts, covering some aspects of Design and Development of the HF-24, is entirely from my memory and contains my personal views. No other person or organisation is responsible for the contents of this article in any way. Accordingly, this personal history is to be treated as made up of biased, even coloured, opinions and is highly selective. This is  not the truth, the whole truth and nothing but the truth. Today, I am not sure if some facts as I recall them are totally correct. Please take this account with the proverbial pinch of salt, or better still a teaspoon full – Kapil Bhargava.

 

 

Early Design & Development of the HF-24

By Group Captain Kapil Bhargava (Retd)

The Company

Aircraft design and development began in India with the appearance of the private company, Hindustan Aircraft Limited (HAL) in December 1940. The first aircraft to be designed and flown in India was the G-1 Glider which did not see any success. But during World War II (WW II), HAL did good work assembling and overhauling aircraft for the Allies. After India became independent, the first indigenous powered aircraft design was the HT-2 trainer first flown in August 1951. Despite derogatory opinions of it being just a copy of the De Havilland Chipmunk, it was an original work, quite different from it in many aspects. The aircraft served with the IAF for many decades. Surprisingly, the next aircraft to be designed within HAL was the HF-24 twin fighter,  http://en.wikipedia.org/wiki/Kurt_Tank/wiki/Kurt_Tank for which work began just four years after the HT-2 had first flown. The design study number 24 in its name is highly misleading.

The Designer & His Team

After World War I (1918), Germany was prohibited from designing any military aircraft. The designers mostly took up work on gliders and got to be very good at it. The list included Kurt Waldemar Tank (See: http://en.wikipedia.org/wiki/Kurt_Tank). As a teenager, Tank had joined the Army in the cavalry. When WW I ended, he was a captain with several awards for bravery. He then graduated as an electrical engineer but soon gravitated to designing aircraft and test flying them. His first design as seen on the model in his house in Bangalore was a small glider but with all flying tailplane and fin-cum-rudder, meaning that these two controls were one piece only. Later, he joined Focke Wulf, where his very first commercial design turned around its fortunes. He soon became its Chief Designer and eventually designed a fighter to match  those of the Allies..

Fw 190

(See its history here http://www.aviation-history.com/focke-wulf/fw190.html). Tank recited to me how he obtained the very hard to get permission to construct the new aircraft. Due to all round shortage of material, he was not being allowed to make even one prototype of what eventually became the Fw 190. Resources were earmarked only for Me-109 and other aircraft perceived as essential. In desperation, Tank requested an interview with Hitler. As he recounted, he did not know whether he would get a nod or a bullet from the Führer. He managed to convince Hitler and came out of his bunker with a big smile on his face. No one had believed that his aircraft would even match the Me-109. But everyone including the Allies were wrong. The Fw 190, test flown and cleared by Tank himself, was superior to all other fighter aircraft of WW II,including the Spitfire. It was no surprise that HAL requested his help in the design and development of a fighter aircraft in Bangalore.

By 1955 Tank’s work in Argentina was over. His fighter aircraft Pulqui had entered Air Force service. His team soon dispersed, most members moving to the USA. He was free and accepted the contract from HAL. Tank landed in Bangalore with around sixteen members in his team. I met all except one when I joined HAL as its Chief Test Pilot (CTP) in August 1957. The one missing person was the qualified Production Engineer. He took a look at  HAL and decided that it was not a place to produce aircraft in numbers. He resigned and went away. It is not clear if this caused the problem but the production rate of the HF-24 forever remained very low. The Indian Air Force issued an Air Staff Requirement to match what Prof. Tank was offering by his design.

Design Philosophy of the HF-24

The main concept of the aircraft was finalised by Tank perhaps even before his arrival in India. He had wanted to make the aircraft capable of considerable operation including supersonic flight to 1.5 Mach No. with manual controls. His other radical idea was to make the wing so strong that if it hit a tree, the tree would lose the battle. The structure separating the two engines was firm enough to prevent damage to the running engine if the other engine broke up due to any reason. The aircraft was pre-designed for a rear cockpit which could take a fuel tank or a multi-rocket launcher if no second pilot was to fly in it.

Structural test in progress. The entire structure was tested to very high loads, at least 50% higher than cleared for service.

 

The major complaint I had against Tank’s design philosophy was to provide only one hydraulic system for powered controls. His idea was that up to Mach.0.95 it would be possible to fly in manual. To my question about what would happen if hydraulics failed at supersonic speed, he responded by saying that he had catered for it. He said that the pilot should use airbrakes to slow down and then once it was subsonic comfortably fly in manual. I pointed out that when an aircraft goes supersonic, its centre of pressure of lift moves back. This movement invariably causes a pitch down. Secondly, using airbrakes could also give a pitch change. Would the pilot be able to manage these after losing his powered controls. He remarked quite scathingly that I had been flying old technology aircraft.

Tank claimed that his design had wings with a low thickness/chord ratio (6%?). Thus going supersonic or reverting to subsonic would produce little or no pitch changes. I did not tell him that I had already flown aircraft with that ratio and they all behaved as I was apprehending. About the airbrakes, he said that he had placed them at right angle to the fuselage below, either at or very close to the centre of gravity. They would not cause any pitch change at all. The truth came home to roost later. One day (then) Winco Suranjan Das (Dasu from now onwards) flying at high speed at low altitude had failure of hydraulic up-lock of airbrakes and they dropped out due to gravity. The result was dramatic. The aircraft pitched up sharply. Dasu blacked out until he recovered his sight up side down, much higher at a low speed. The aircraft had recorded +10.2G. My other concern did not occur as we never had the engines to power the aircraft  to sustain any supersonic level flight. Dives are easy to manage.

Tank’s rather unusual and awkward feature was nose wheel oleo extension. When I asked him why he had added this strange operation, he wrote down two equations. He said that if no aid of lift was given to the aircraft, its ground roll for take off would be increased due to friction. If the nose was raised too much, the induced drag would be too high. The nose wheel extension set the aircraft at an angle which was optimum for the shortest take off run. To me the reasoning seemed unjustified as most aircraft were operated differently, starting level, raising the nose at the right moment and pulling it off for take off at the un-stick speed. He did not agree. This feature later became one of the reasons why the first attempted maiden flight ended in disaster.

 

HF-24 model undergoing wind tunnel tests at NAL, Bangalore.

 

The Mock-Up

My earliest task as a test pilot came up when a wooden mock-up of the cockpit along with all intended instruments and controls was made. My test pilot colleague was Flt Lt OM Kunhiraman, also my course mate (53 PIC). He was a transport pilot but had been doing production testing of  Vampires. He had been encouraged and ‘converted’ to Vampires by Fg Off Sunandan Roy. Physically, Kunhi was rather tall and Sunandan (Shu) was overweight. The senior-most doctor of IAF, Gp Capt (later AVM) MM Shrinagesh picked me out to be the guinea pig for assessing the mock-up. According to him I was so close to average that the difference, if any, did not count. It was another matter that no one else was available anyway.

On a fateful day, I donned overalls, carried my helmet, knee pad etc and got into the cockpit. Tank, several member of his team and some HAL personnel surrounded the cockpit. I strapped in and to everyone’s utter shock was unable even to touch the front panel, or operate any controls on either coaming. It was not possible to perform any of the essential tasks eg set G-4 compass or altimeter setting, operate flaps or undercarriage levers, rudder pedals, etc.. Everything was very far from me and the only way to reach them was to release the catch to permit leaning forward  or loosen the straps. The truth soon emerged. Tank had himself done all the installation planning without using seat straps. Leaning forward, combined with his rather large bottom, he had no trouble accessing anything. No one suggested it, but I would have refused to enlarge my butt to suit the aircraft. The team went back to the drawing board

The Cool Overalls

After the cockpit had shrunk a lot, I was invited again to examine it. The assessment was routine. If you did not like the positioning or operation of anything in it, you know now who to blame. But surprises are never far away and I was about to get one.

I was asked to strip and put on an Air Ventilated Suit (AVS), designed and manufactured in England. By the time I got it on and strapped in, I was sweating profusely. The AVS acted like an insulated cover. A flexible tube from the AVS was connected to a cylinder of air under considerable pressure. This was to provide the cool air to the AVS which in flight would have come from the cabin conditioning system.

When the ground crew saw that I looked ready, they decided to let in the air. The next moment I had very sharp high speed jets of chilled air hitting me all over the body. It was as if a few hundred nails were being hammered in. I screamed and thankfully someone shut it off. I did not have to use any words to announce the result of this test. The supplier of the AVS and some designers asked me about it. They needed to know it it could be used at all at any time. I told them to forget it as no pilot would be able to get to the stage of using it till well after he was full of sweat. The result each time would have been sheer agony.

I had some other disagreements with Tank. But by then HAL’s Managing Director AVM AM Engineer (later CAS) had taken charge of the company from the Indian Civil Service officer. As the project began to need the services of a test pilot, the MD got Wing Commander Roshan Lal Suri, our senior-most test pilot, to take over as CTP from me, a mere Flt Lt, qualified full seven years after him. While he must have been a good operational pilot and had earned a VrC during the Kashmir War, in my personal opinion his understanding of test flying was quite inadequate. Surely it is not in good form to make such a statement about my senior professional – but is inescapable to explain the accident of his attempted  maiden flight of the first prototype. Unfortunately, I saw too many examples of this failure to conclude otherwise. When he asked me to be his official number two for the HF-24 project I refused his kind offer. He wanted to know why not and I said that it was because no one in HAL listened to the test pilot. He said they would listen if I were the No. Two. I pointed out that he was No, 1 yet no one listened to him, without clarifying that he never even attempted to correct even the most glaring errors of design which I saw during his handling of the Krishak project.

 

The first Marut Prototype with its HAL number. Later became BR 462.

 

 

Construction

Tank’s well advertised boast was that his aircraft would be so strong that if the wing hit a tree, the tree would get sliced off with the aircraft capable of flying back home. To achieve this a tapering steel frame was essentially the wing with everything else built around it. Indian Aluminium Co. agreed to take a contract for it. It delivered only one pair of plates for the first aircraft after which it said that due to extremely low rate of production planned by HAL, making the plates was a badly losing business. The entire construction of the aircraft was cumbersome with virtually no attention paid to maintainability.

An interesting feature was the separation between the engines. The partition between them was so strong that the failure of one engine did not damage the other even once. Later the first Jaguar was lost due to a bird hit on one engine parts of which went through the partition and broke the other engine as well. The pilot had to eject.

A couple of examples might indicate the reasons why the aircraft was very difficult to produce. Each 30 mm Aden gun needed a mount at the front end to fix to the fuselage which could be made only after 40 hours of machining. The saddle tank on top did not quite match the fuselage. A  strip of teak wood 1.2m long but only just over  a centimetre wide and less thickness had to be machined to fill the gap.  I do not know if methods were found later to avoid the filler piece.

The time and cost of making jigs and tools to raise the production rate were so high that HAL had to give up. This was the main reason for the early demise of the Marut with no later designs evolving from it.

The next part of the early work on the HF-24 would cover the flying of the Glider and the first prototype. The entry for the Glider is already on this blog here but I intend to describe some of the tests in a little more detail along with the problems resulting from their poor conception, planning and execution.

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