The HF-24 Glider

After the First World War (1914-1918), the victors imposed many limitations on the Germans. In terms of aviation research, they were not permitted to design, develop or manufacture any powered aircraft, especially fighters. German designers switched over their work to gliders on which considerable research was still possible. Kurt Wolfgang Tank who in World War I was a cavalry officer and a qualified electrical engineer also dabbled in designing gliders. His first design, naturally a glider, had an all flying tail and fin. In time the Germans ignored the ban (or it lapsed) and work on fighter aircraft began in earnest. By 1939 before World War II broke out, Willy Messerschmitt had manufactured more than 35,000 Me-109s at the rate of more than one each day including all holidays or Sundays. Tank was working at Focke-Wulf as a designer-cum-test pilot. I had the opportunity to work with both of them, with Tank between 1957 and 1960 and Messerschmitt between 1963 and 1969.

After World War II, severe limitations were re-imposed on German designers. Messerschmitt moved to Spain to avoid them and Tank to Argentina. When India wanted to develop a fighter aircraft, both of them offered their help. We chose Prof. Tank as the better designer while without a doubt Messerschmitt was the better aircraft constructor.

Professor Kurt Tank arrived in India in 1955 with a team of sixteen other Germans to begin work on the HF-24. It was not very surprising when their first job was to design and manufacture a plywood glider version of the HF-24. This was a full 1:1 scale aircraft with the airframe geometry and even the thickness chord ratio of the wing (at 6%) exactly equal to the projected fighter aircraft. The glider body was almost entirely made of locally available plywood. Adhesives for joining parts were imported from Germany. However, it was a rather well equipped glider. It was designed for two pilots in tandem seating.

The HF-24 Glider was fitted with a pneumatic air bottle to retract undercarriage and to operate brakes, flaps and airbrakes. The bottle when fully charged could work all these services and retract or lower the undercarriage several times before running out of pressure. For all except two flights, the rear cockpit was unoccupied. Just above where the head of the pilot in the rear seat would have been, a camera was installed. This took pictures at short intervals of the instrument panel to record the readings for subsequent analysis. No trace recorders were installed. An anti-spin parachute was provided but it was not possible to accommodate an ejection seat. The cockpit layout was as close as possible to the intended fighter aircraft.

The towing for the first and all later flights was done with a Dakota Mk IV (BJ 449). This aircraft had Pratt & Whitney R-2000 engines and its rudder had a balance tab to cope with an engine failure.

Click to Enlarge Dakota BJ449 towing the HF-24 Glider prototype

With Flt Lt OM Kunhiraman as the pilot in command and me in the co-pilot's seat, we practised towing both on the ground and in the air, Our one fear was that if the glider pulled up higher than the Dakota, both aircraft could crash into the ground. Normal and emergency releases of the glider were simulated.

Wing Commander Roshan Lal Suri, the senior-most test pilot in India had been deputed to HAL to work on the HF-24 project. With him in the glider, it was first launched on 3rd April 1959. HF-24 is perhaps the last fighter aircraft to have been tested in the shape of a 1:1 scale glider. We air-towed it to 17,500-ft amsl. Roshan Suri then released the tow cable and came whistling down at 3,800 ft per minute. The rate of descent increased even further with flaps, undercarriage and air brakes out. He carried out a flawless landing and followed it with 82 more. This was his finest hour.

Wg Cdr Roshan Lal Suri, entering the Glider cockpit for one of the gliding flights. The lack of an ejection seat meant that the pilots had to wear the normal Seat Parachute. Click to Enlarge

Apart from confirming Reynolds numbers, the glider was mainly used for stability and low speed trials. With the German preoccupation with very stable aircraft, Prof. Tank planned and Roshan Suri executed 64 of the 83 flights for phugoid oscillation tests. For these, the aircraft was trimmed accurately and then a sudden longitudinal input of about 2G was given. The resulting oscillations were recorded to see how quickly they damped out. Unfortunately they did not, hence the huge effort spent on it. Prof. Tank did not consider that the problem could have been due to the flexibility of the plywood glider.

The final aircraft made of metal was very unlikely to behave in the same manner. I was unable to convince Roshan Suri to take up issue with Prof Tank for wasting a large number of launches doing phugoids to the exclusion of low speed handling and spins. The glider was extensively used for checking stalls. When the stall warning was found to be too low (at around 2 or 3 knots) many modifications were tried to increase the margin. These included aerodynamic fences and methods for boundary layer activation. The final choice was the saw-tooth wing which yielded a warning of 12 knots. As the final fighter aircraft showed, this was excessive and became the main shortcoming of the aircraft. The HF-24 was prone to judder even at low altitude and very high speeds when pulling G. Also many aircraft had their pitot tubes bend due to the severe judder during bank reversal when pulling G during simulated combat manoeuvres.

A camera was installed on top of the fin to photograph wool tufts on the left wing. These showed that the aircraft was aerodynamically smooth. The aircraft flew very well and turned out to be easier to control and land than had been anticipated.

I did the last three flights of the glider. By default, I became the only pilot to have flown both the glider and the fighter aircraft. When I flew the HF-24 fighter three years after the glider, I found its stall behaviour identical to the glider except for the rather large stall warning (around 28 knots). On my third flight (86th of the glider) with Gp Capt S Chenna Keshu in the rear seat, the glider was damaged during landing with the nose wheel retracted which had failed to come down. It never flew again.

The anti-spin chute of the glider was never used as no spin trials were ever done on it. This was just as well. Wind tunnel test were carried out abroad. These conclusively showed that the aircraft was quite difficult to put into a spin but impossible to recover from it The full scale HF-24 Glider was perhaps the last one to be made in the world. It was a magnificent effort even though it may not have contributed too much to design and development of its fighter version.

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