| Yakovlev Yak-24 2001 |  |
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| Yakovlev Yak-24 2001 | |
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Soviet rotorcraft development was suspended during World War 2, and it was not until late summer 1952 that the USSR made its first major effort to close the design gap between itself and the USA in regard to large transport helicopters. Two basic projects were selected, the first, for a 12-passenger machine of single main rotor configuration, being assigned to the Mil design bureau. The second, entrusted to the bureau headed by Aleksandir S. Yakovlev, was for a twin-engined, tandem-rotor machine capable of seating 24 passengers. Prototype flights of both types were required to take place within one year. Two flying prototypes of the Yakovlev design were completed, the first making its maiden flight in the hands of S.Brovtsev and Y.Milyutichev on 3 July 1953; two others were built for static and dynamic testing. Two 1700hp Shvetsov ASh-82V radial engines drove rotor and transmission systems basically identical to those already proven in the single-engined Mi-4, with each engine geared to drive one or both rotors. Unfortunately this arrangement, although intended as a precaution against failure of either engine, created the problem of 'sympathetic' vibration. From the outset, vibration hampered the Yak-24's development, aggravated no doubt by insufficient rigidity in the fabric-covered rotor blades and middle section of the fuselage. After only 178 hours of operation, the rear engine frame attachments of the static test prototype collapsed and, in the words of the designer, the rear rotor 'lunged forwards and the rotor blades began to chop the machine to pieces'. A satisfactory cure was eventually attained by cropping 0.50m off each rotor blade, and state trials (during which another prototype was lost) began late in 1953. Production probably began about a year later, and the Yak-24 made its public debut in the Aviation Day display at Tushino in July 1955, when four machines were exhibited. On 17 December 1955 a Yak-24 set two new world-class payload-to-altitude records. The early Yak-24's featured a Vee tailplane, but later production examples had rectangular endplate fins on a horizontal tailplane, and both have been seen with and without a narrow auxiliary rudder. Initial Yak-24 production was undertaken on behalf of the Aviatsya Vozdushno-Desantnich Voisk (Aviation of the Airborne Troops), in which configuration the aircraft could accommodate up to 40 fully-equipped troops according to range. Other typical loads of the "Letayuchiy Vagon" (Flying Wagon), as it was quickly dubbed, include 18 casualty litters, 2 anti-tank guns, 2 GAZ-69 command vehicles or 3 M-20 staff cars. In 1958 the Yak-24U became the standard military model, with all-metal rotor blades and fuselage skin, the revised tail configuration already mentioned, and the rotors restored to the original 21.00m diameter. Civil counterpart to the Yak-24U was the Yak-24A, with standard seating for 30 passengers. The Yak-24A can also be operated as a freighter or flying crane, being able to lift an external sling load of 5000kg. In 1960 the short fuselage Yak-24K, a 9-seat executive version, was announced, and in the following year details were released of the 39-seat Yak-24P with 2700shp Ivchenko shaft turbine engines. Neither of these is thought to have been built in any quantity, and overall production of the earlier variants was probably not high. K.Munson "Helicopters And Other Rotorcraft Since 1907", 1968
After having designed an experimental helicopter with coaxial rotors and the Yak-100 between 1944 and 1947, the Yakovlev design bureau embarked at the beginning of the fifties on the development of a heavy twin-engine helicopter with tandem rotors. The first prototype flew on 3 July 1952, with the same rotor blades, transmission and rotor hub as the Mil Mi-4. The four-blade rotor of the prototypes was made of light alloy and covered with fabric, but this was replaced by a steel rotor with a metal skin on the production models. The fuselage was of tubular structure, originally fabric-covered, then with a light alloy skin. There were two big end-plate fins on either side of the rear fuselage. The quadricycle landing gear had fully castoring wheels. The cabin of the Yak-24 could accommodate 20 troops or four staff cars or a similar payload. Final development work on the aircraft was extremely long and complex and full-scale production for the armed forces began in 1955, about 30 months behind schedule. In December of that year, a Yak-24 established a world helicopter record by lifting a 2000kg load to 5082m and 4000kg to 2902m. In 1957, the Yak 24U appeared, with modified rotors, a reinforced structure, more cargo room and provision for carrying slung loads. This version was capable of lifting 3.5 tonnes vertically. Aeroflot (the Russian state airline) evaluated the Yak-24A commercial version for 30 passengers, but turned it down. It also rejected the Yak-24K deluxe version for 8-9 passengers, while the Yak-24P for 39 passengers, with two 1500shp Isotov turbines mounted above the cabin was never built. About 100 of the Yak-24 (NATO reporting name Horse) were built, although some sources put the total at only 40, and it was used exclusively by the Russian Air Force. Its service life seems to have been beset by numerous accidents. G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984  When development of the large Yakovlev Yak-24 transport helicopter began at the end of 1951, the design team had little experience of rotary-wing aircraft, such experience being limited to the co-axial EG of 1947 and Yak-100 of 1949, neither of which progressed beyond the experimental stage. Considerable difficulties were encountered by the large design team, but after extensive ground testing, tethered flights were achieved by the fourth prototype from 3 July 1952. With official tests completed on later prototypes, production began in April 1955, and only four months later evaluation aircraft were demonstrated at Tushino airport during the Soviet Aviation Day display. At least 100 of these twin-rotor helicopters were built, the large fuselage accommodating a crew of three and up to 30 armed troops or 18 stretchers cases, or 3000kg of freight/vehicles with access by rear ramp. The structure included horizontal braced tail surfaces with endplate fins, and the fuselage was supported on the ground by fixed quadricycle landing gear. The Yak-24UB, flown in December 1957, included many design improvements and was placed in production from 1959, about 50 being delivered; this version could carry 40 fully equipped troops or up to 3500kg of cargo. The civil Yak-24A of 1960 accommodated 30 passengers, and the Yak-24 was a VIP transport with a shorter fuselage and provision for nine passengers. D.Donald "The Complete Encyclopedia of World Aircraft", 1997
This large and powerful transport helicopter was created in response to order of Stalin at Kremlin meeting autumn 1951. Instruction was for Mil to build single-engine machine with 1200kg military load, Yak a machine with twice this load, prototypes to be ready in a year. Mil had already prepared suitable design, and Yak gained permission to use essentially same main rotor and drive from similar engine, merely doubling up to use two engine rotor systems at ends of boxcar fuselage. Yak awed at size of task and short timescale, assembled large team including Erlikh, veteran helicopter man N.Skrzhinskii, P.D.Samsonov (famed flying-boat designer who had long managed Yak prototype dept), L.Shekhter, L.S.Vil'dgrub and many other well-known engineers. Plan was to build four four Yak-24, already called LV (Letayushchii Vagon, flying wagon), two for static and resonance test and two for flight. Promised "unlimited support" in crash programme. Basic engine rotor design described under Mi-4. Fuselage functional container beased on welded KhGSA truss with minimal secondary stringer/fairing formers of D1 or wood. Unstressed D1 sheet covering over front and rear engine bays and large fin, fabric elsewhere. Aluminium plank cargo floor with full-section access via rear ramp/door; passenger door forward on left side. Rear rotor mounted on top of vertical fin (TE curved to right to give side-thrust to left in flight) with drive from engine installed in normal horizontal attitude at base of fin, with open cooling-air inlets each side of fin and clearance under engine for vehicles and other cargo on ramp. High-speed connecting shaft to front rotor, mirror-image with rotation anti-clockwise seen from above, driven by engine at 60° angle between cockpit and cabin. Nose cockpit for two pilots, radio-operator and engineer, entirely glazed with aft-sliding door each side and sliding door(s) at rear giving restricted access past engine to main compartment. Latter measured 2m x 2m x 10m with intended accomodation for up to 40 troops on canvas wall seats or light vehicles or 4t cargo, with crane operation using central hook on underside of fuselage. Four similar levered-suspension landing gears, each normally castoring +/-30°, on rigid welded steel-tube outriggers. While numerous establishments tested complete engine/rotor rigs, blade fatigue and truss structure of fuselage, first flight article readied spring 1952 and began 300 hr endurance test with wheels tied down. Vibration in evidence from start, and usually severe. With greater experience OKB might have recognised a fundamental N1 main-rotor mode and altered critical dimension. As it was, at 178th hour, rear engine tore free from fatigued mounts, machine being destroyed by fire. Second flying article, ie, 4th airframe, finally began tethered flight piloted by Sergei Brovtsev and Yegor Milyutchyev 3 July 1952. Hops at partial power were followed by full-power flights, when vibration reared its head dangerously. Five months by every available expert found no cure; then Yak claims he personally ordered 0.5m cut off each main-rotor blade. This effected immediate great improvement. No.4 aircraft delivered for NII test Oct 1953, but destroyed when tethers snapped during ground running. OKB delivered improved aircraft with numerous mods including modified tail with no fins but braced tailplanes with dihedral 45°. This finally passed NII April 1955 and production began at GAZ in Leningrad. First four pre-series Yak-24 (visibly not all identical) flew at Tushino, Aug 1955. Series version had strengthened floor with tracks for vehicles, tie-down rings, attachments for pillars carrying 18 stretchers, full radio and night equipment and facilities for field servicing. Normal max load 20 armed troops or 3t. Only 36 built, most with tailplane dihedral only 20° and with fixed endplate fins canted 3°30' to give side-thrust to left. On Dec 17 1955 Milyutchev took payload 2t to 5082m and Tinyakov lifted 4t to 2902m. OKB produced improved versions but none built in series. Yak-24U (Uluchshennyi, improved) flew Dec 1957 with numerous mods resulting from prolonged research. Rotor blade length unchanged but diameter restored by adding long tubular tie at root. Side-thrust at tail reduced by canting axes of rotors 2°30' (front to right,, rear to left), so curved rear of fin removed. Fuselage frame strengthened, metal skinned throughout and cabin increased in width 0.4m. Flight-control system fitted with two-axis autostab and autopilot of limited authority, developed within OKB. External slung load attached to winch in roof of cabin with large door in floor. Rear landing gear oleos changed in rate to eliminate last vestiges of ground resonance, and other minor changes including revised fuel system. In production GAZ-33 early 1959, though halted at No 40. This variant could at last lift 40 troops or 3.5t and at least some production machines had tailplane dihedral 0°. One example built by 1960 of Yak-24A (designation from Aeroliniya, airline) similar to late Yak-24U with horizontal tailplane and latest avionics but with comfortable civil interior for 30 passenger seated 2+1. Continious glazing down sides of fuselage, compartment for 300kg baggage and fold-down steps at door; rear freight door eliminated. Appeared in Aeroflot markings though never in service. A further example built by 1960 of VIP model, Yak-24K. Fuselage shorter, electrically extended airstairs and luxurious accomodation for (usually) nine passengers with four large windows each side. The 30-pax Yak-24P was never built. Yak-24 posed immense problems, and though it took much longer than Stalin's year, development was eventually completed to point at which this Flying Wagon could be put into military service. It was used for various purposes including crane role and for special photo missions, but remained a slow and rather unpopular machine. Had OKB persevered it might have produced more satisfactory machines with turbine engines, but it was glad to leave helicopter field to others. Bill Gunston "The Osprey Encyclopedia of Russian Aircraft", 2000 A S Yakovlev has described how, in autumn 1951, he and other designers were called to the Kremlin and told by Stalin to create two helicopters, one to carry a useful load of 1,200kg or twelve armed infantry and the other just twice as much, prototypes to be ready in one year. It was to be a 'crash programme', with 'unlimited support' from the national research institutes. Nobody was eager, but eventually Mikhail L Mil' agreed to tackle the smaller machine and Yakovlev the larger, Yakovlev having the idea of simply using tandem rotors based on those of the Mil' design. This was agreed, and design of the Yak-24 started in December 1951. Though the first prototype was built extremely quickly, this programme was to prove more protracted than any previous endeavour by the OKB. Including later versions the chief engineers comprised I A Erlikh (the original leader) and P P Brylin, Yu I Orlov, V P Lashkov, G I Rumyantsev and G I Ogarkov. The concept could not have been more straightforward. Mil', with CIAM, CAHI and other organizations, including Shvetsov's engine KB, developed the rotor and its drive system. The engine was the ASh-82V, a special helicopter version of the four-teen-cylinder radial used in some Yak fighter prototypes. Rated at l,430hp, and with 1,700hp available for takeoff, it was developed with a cooling fan and centrifugal clutch and cleared to operate in any attitude. It was decided to install the front engine between the cockpit and cabin at an angle of 60deg to drive the gearbox under the front rotor. The rear engine was installed in the normal attitude in the base of an enormous rear fin which formed the pylon for the rear rotor, driving through a 90deg bevel gearbox. The rotors had fully articulated hubs made of D16 and steel, with drag and flapping hinges and friction dampers. In fact, the rotor was not identical to that of the Mi-4, and indeed later Mil' enlarged his rotor by using Yakovlev's longer blades. The four blades were tapered, with NACA-230 profile, based on a 30KhGSA spar with ply ribs and skin covered in varnished fabric, with tracking adjusted by a tab on the trailing edge near the tip. The rotors turned at 178 rpm in opposite directions, the rear rotor being a mirror image of the front rotor which it over-sailed. The gearboxes were linked by a torque shaft so that flight could just be maintained on one engine. The boxcar fuselage was based on a truss of welded KhGS A tube, skinned with Dl panels covering the engine bays, rotor pylons and fin, and by fabric elsewhere. Each engine was housed in a fire-resistant bay with large apartures for cooling air, those for the rear engine being forward-facing open inlets beside the fin leading edge. Each engine had its own fuel tank. At the front was the fully glazed cockpit for two pilots and a radio operator/engineer, with a sliding door on each side and a rear door to the engine compart ment, through which a narrow passage led to the main cabin. This measured 10m long, the cross section being 2m square. There were six windows on each side, one being in a door, and at the back was a full-width ramp door through which shallow loads such as a GAZ-69 'Jeep' could be moved under the rear engine on to the floor of aluminium planks. The undercarriage comprised four triangulated steel-tube outriggers carrying levered suspen-sion wheels able to castor ±30deg. The track was 5m. The OKB very quickly built four prototypes, two for structural and static testing and two for flight testing in the hands of S G Brovtsev and Ye F Milyutichev. The first liftoff was as early as 3 July 1952, but it was soon evident that resonance was a serious problem. The static-test machine began a 300-hour tied-down test programme, but vibration proved frightening and difficult to predict, and even with CAHI help progress was slow. At the 178th hour the rear engine tore free from its fatigued mountings and the machine was burned out. The free-flight article (fourth airframe) fared little better until (according to Yakovlev) he personally ordered 0.5m cut off each main-rotor blade, reducing diameter from 21m to 20m. This immediately effected a great improvement, but after the No. 4 machine had been delivered to NII-VVS in October 1953 its tethers snapped during ground test. By this time the tail had been developed to have a trailing edge curved to the right, with a ground-adjustable ‘rudder’ downstream, to give a side-thrust in cruising flight, and two braced tailplanes tilted up at 45deg. In this configuration the Yak-24 passed NII-VVS testing in April 1955, though stability and control were regarded as marginal. Four pre-series helicopters, all slightly different, flew at the Tushino Aviation Day in August 1955, hailed as the LV (Letayushchii Vagon, flying wagon). Production was ordered at a Leningrad factory, where thirty-five were built for the VVS in 1956-58. These were painted in dark green camouflage, and except for the first few had larger tailplanes with dihedral reduced to 20deg carrying large endplate fins set at an angle of 3deg 30' to give the required thrust to the left in cruising flight, the tail end no longer being curved to the right. They had full equipment for loading and securing vehicles and other cargo up to a maximum of 3,000kg. Canvas wall seats were provided for twenty troops, with racks for weapons and equipment, with pillar sockets for eighteen stretchers accompanied by an attendant. A three-tonne load could also be slung from a central hook, but on 17 December 1955 Milyutichev carried an overload of four tonnes to 2,092m. On the same day G A Tinyakov set a second world record in the same prototype by taking 2,000kg to 5,032m. In January 1958 a complete three-axis autostabilization system was cleared for service and retrofitted to each helicopter. This dramatically improved stability and control, making 'hands'off hovering possible, and greatly improving the tactical value of these now very capable helicopters. More would have been built had Mil' not developed the far more powerful Mi-6. The USAF called this helicopter 'Type 38', later replaced by the ASCC name 'Horse'. Yak-24U This Uluchskennyi (improved) helicopter was completed in December 1957, and tested from January 1958. The rotor blade spars were connected to the hub by oval-section steel tie rods at the root, restoring rotor diameter to the original design value. The axes of the rotors were canted 2deg 30', the front hub tilted to the right and the rear to the left, so that the entire tail could be redesigned for minimum drag without the need to generate side thrust. Avionics included a two-axis autostabilization system and limited-authority autopilot developed mainly within the OKB. The fuselage truss was strengthened and increased in width by 0.4m and made slightly higher, and metal-skinned throughout. The external slung load rating was increased to 3,500kg, and the cable passed through a large floor hatch to a winch in the roof of the cabin. The rear landing oleos were modified to eliminate any tendency to resonance (now a better understood phenomenon), and later the fuel system was improved and the capacity significantly increased. This prototype could carry thirty-seven armed troops, but its main use was as a crane, putting roof trusses on the Pushkin (Ekaterinskii) palace and carrying gas pipes from Serpukhov to Leningrad over impassable marsh. Yak-24A In 1960 a single prototype was built of this civil version equipped for a crew of two and thirty to forty-passengers, seated 2+1. The rear cargo door was eliminated, the passenger door on the left was fitted with fold-down steps, and the cabin was fitted with larger windows. With thirty passengers 300kg of baggage could be stowed in a separate compartment. Yak-24K Another 1960 prototype was this VIP version. The fuselage was shortened, fitted with even bigger windows, improved soundproofing and heating and an electrically-operated airstairs, and luxuriously furnished for nine passengers. Yak-24P Though a beautiful model was exhibited in 1961, this twin-turboshaft development was never built. The two 2,700hp AI-24V engines were mounted above the stretched cabin giving comfortable accomodation for thirty-nine passengers, with baggage space and toilet. Cruising speed would have been 210km/h. With hindsight, the Yak-24 may be seen to have been a very difficult challenge which finally led to complete success. Unfortunately, by the time it was entering production rival Mil' had produced the far more powerful Mi-6, so only a limited number of Yakovlev's 'flying wagon' were built. Later, when Yakovlev scaled up the Yak-24 into the huge Yak-60, it was again unable to displace Mil' helicopters from their position of dominance. Bill Gunston & Yefim Gordon "Yakovlev Aircraft since 1924", 1997 * * *
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