Mil Mi 24

During the early 1960s, it became apparent to Soviet designer Mikhail Leont'yevich Mil that the trend towards ever-increasing battlefield mobility would result in the creation of flying infantry fighting vehicles, which could be used to perform both fire support and infantry transport missions. The first expression of this concept was a mock-up unveiled in 1966 in the experimental shop of the Ministry of Aircraft's factory number 329 where Mil was head designer. The mock-up designated V-24 was based on another project, the V-22 utility helicopter, which itself never flew. The V-24 had a central infantry compartment that could hold eight troops sitting back to back, and a set of small wings positioned to the top rear of the passenger cabin, capable of holding up to six missiles or rockets and a twin-barreled GSh-23L cannon fixed to the landing skid.

Mil proposed the design to the heads of the Soviet armed forces, and while he had the support of a number of strategists, he was opposed by several more senior members of the armed forces who believed that conventional weapons were a better use of resources. Despite the opposition, Mil managed to persuade the defence minister's first deputy, Marshal Andrey A. Grechko, to convene an expert panel to look into the matter. While the panel's opinions were mixed, supporters of the project eventually held sway and a request for design proposals for a battlefield support helicopter was issued. The development of gunships and attack helicopters by the US Army during the Vietnam War convinced the Soviets of the advantages of armed helicopter ground support doctrine, which had a positive influence on moving forward with the development of the Mi-24.

Mil engineers prepared two basic designs: a 7-ton single-engine design and a 10.5-ton twin-engine design, both based on the 1,700 hp Izotov TV3-177A turboshaft. Later, three complete mock-ups were produced, along with five cockpit mock-ups to allow the pilot and weapon station operator positions to be fine-tuned.

The Kamov design bureau suggested an army version of their Ka-25 Hormone ASW helicopter as a low-cost option. This was considered but later dropped in favor of the new Mil twin-engine design. A number of changes were made at the insistence of the military, including the replacement of the 23 mm cannon with a rapid-fire heavy machine gun mounted in a chin turret, and the use of the 9K114 Shturm (AT-6 Spiral) anti-tank missile.

A directive was issued on 6 May 1968 to proceed with development of the twin-engine design. Work proceeded under Mil until his death in 1970. Detailed design work began in August 1968 under the codename Yellow 24. A full scale mock-up of the design was reviewed and approved in February 1969. Flight tests with a prototype began on 15 September 1969 with a tethered hover, and four days later the first free flight was conducted. A second prototype was built, followed by a test batch of ten helicopters.

Acceptance testing for the design began in June 1970, continuing for 18 months. Changes made in the design addressed structural strength, fatigue problems and reduced vibration levels. Also, a 12-degree anhedral was introduced to the wings to address the aircraft's tendency to Dutch roll at speeds in excess of 200 km/h, and the Falanga missile pylons were moved from the fuselage to the wingtips. The tail rotor was moved from the right to the left side of the tail, and the rotation direction reversed. The tail rotor now rotated up on the side towards the front of the aircraft, into the downwash of the rotor, which increased the efficiency of the tail rotor. A number of other design changes were made until the production version Mi-24A (izdeliye 245) entered production in 1970, obtaining its initial operating capability in 1971 and was officially accepted into the state arsenal in 1972.

In 1972, following completion of the Mi-24, development began on a unique attack helicopter with transport capability. The new design had a reduced transport capability (3 troops instead of 8) and was called the Mi-28, and that of the Ka-50 attack helicopter, which is smaller and more maneuverable and does not have the large cabin for carrying troops. In October 2007, the Russian Air Force announced it would replace its Mi-24 fleet with Mi-28s and Ka-50s by 2015. However due to economics the Russian Army will upgrade some Mi-24s and keep them in service past 2015. The Russian navy will keep its Hinds in service for years to come as well.

Design

The core of the aircraft was derived from the Mil Mi-8 (NATO reporting name "Hip"): two top-mounted turboshaft engines driving a mid-mounted 17.3 m five-blade main rotor and a three-blade tail rotor. The engine configuration gave the aircraft its distinctive double air intake. Original versions have an angular greenhouse-style cockpit; Model D and later have a characteristic tandem cockpit with a "double bubble" canopy. Other airframe components came from the Mi-14 "Haze". Two mid-mounted stub wings provide weapon hardpoints, each offering three stations, in addition to providing lift. The load-out mix is mission dependent; Mi-24s can be tasked with close air support, anti-tank operations, or aerial combat.

The body is heavily armored and can resist impacts from .50 caliber (12.7 mm) rounds from all angles, including the titanium rotor blades. The cockpit is protected by ballistic-resistant windscreens and a titanium-armored tub. The cockpit and crew compartment are overpressurized to protect the crew in NBC conditions.

Considerable attention was given to making the Mi-24 fast. The airframe was streamlined, and fitted with retractable tricycle undercarriage landing gear to reduce drag. The wings provide considerable lift at high speed, up to a quarter of total lift. The main rotor was tilted 2.5° to the right from the fuselage to counteract dissymmetry of lift at high speed and provide a more stable firing platform. The landing gear was also tilted to the left so the rotor would still be level when the aircraft was on the ground, making the rest of the airframe tilt to the left. The tail was also asymmetrical to give a side force at speed, thus unloading the tail rotor.

A modified Mi-24B, named A-10, was used in several speed and time to climb world record attempts. The helicopter had been modified to reduce weight as much as possible, and among the measures used was to remove the stub wings. The speed record over a closed 1000 km course set on 13 August 1975 of 332.65 km/h still stands, as does many of the female specific records set by the all female crew of Galina Rastorgoueva and Ludmila Polyanskaia. On 21 September 1978 the A-10 set the absolute speed record for helicopters with 368.4 km/h over a 15/25 km course. The record stood until 1986 when it was broken by the current record holder, a modified Westland Lynx

Comparison to western helicopters

As a combination gunship and troop transport, the Mi-24 has no direct NATO counterpart. While the UH-1 ("Huey") helicopters were used in the Vietnam War either to ferry troops, or as gunships, they were not able to do both at the same time. Converting a UH-1 into a gunship meant stripping the entire passenger area to accommodate extra fuel and ammunition, and removing its troop transport capability. The Mi-24 was designed to do both, and this was greatly exploited by airborne units of the Soviet Army during the 1980–89 Soviet war in Afghanistan. The closest Western equivalent was the Sikorsky S-67 Blackhawk, which used many of the same design principles and was also built as a high-speed, high-agility attack helicopter with limited troop transport capability. It was also designed using many components from the existing Sikorsky S-61. The S-67, however, was never adopted for service. Another relatively close western equivalent is the MH-60 Direct Action Penetrator, a special purpose variant of the Sikorsky UH-60 Black Hawk, which is capable of mounting a variety of weapons on its stub wings, including AGM-114 Hellfire missiles and Hydra 70 rockets.

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