May | 2015 | Old Machine Press

By William Pearce

In early 1939, Soviet authorities sought the design and development of a new aircraft engine rated in excess of 2,000 hp (1,491 kW). Soviet aircraft engine technology was falling behind that of the western powers at the time, and this new engine was intended to close the gap. Gleb S. Skubachevskiy at the Moskovskiy Aviatsionniy Institut (Moscow Aviation Institute or MAI) completed the preliminary design of the new 2,000+ hp (1,490+ kW) engine, and development of a prototype was approved in July 1939. The new engine was given the designation M-250. Vladimir A. Dobrynin was brought in to assist Skubachevskiy on the M-250.

The six bank, 24-cylinder, 3,111 cu in (51.0 L) M-250 aircraft engine with contra-rotating propeller shafts.

The M-250 was a 24-cylinder, water-cooled engine with six cylinder banks, each with four cylinders. The inline cylinder banks were spaced radially around the crankcase at 60 degree intervals, giving the engine an inline radial configuration. One cylinder bank extended horizontally from the crankcase on each side of the engine. A hexagon was formed by connecting the outer points of the six cylinder banks, making the M-250 part of the hexagonal engine family. Other hexagonal engines include the Curtiss H-1640 Chieftain, the Wright H-2120, the SNCM 137, and the Junkers Jumo 222. The M-250 employed a master/articulating connecting rod arrangement as used in a typical radial engine. The engine had a single-stage, three-speed supercharger mounted at its rear. A carbureted version of the engine was built along with a direct fuel injected version. The engine had a compression ratio of 6.2 to 1.

Each cylinder bank had a single overhead camshaft that was driven by a vertical shaft at the front of the bank. Intake and exhaust manifolding occupied the space between alternating cylinder banks, and the spark plugs were located in the intake Vee. At the front of the engine, the crankshaft drove contra-rotating propeller shafts via a reduction gearing. The M-250 had a 5.5 in (140 mm) bore and a 5.4 in (138 mm) stroke. The total displacement from the 24-cylinder engine was 3,111 cu in (51.0 L), and the engine weighed 2,822 lb (1,280 kg). The M-250 produced 2,200 to 2,500 hp (1,640 to 1,864 kW).

The M-250 was developed into the 3,628 cu in (59.5 L), 3.500 hp (2,610 kW) Dobrynin VD-3TK.

Dobrynin was sent to Voronezh, Russia to assist with the M-250’s construction and testing while Skubachevskiy remained at the MAI. The M-250 was first run on 22 June 1941. However, the M-250 development team was evacuated from Voronezh in October 1941 because of advancing German troops. Skubachevskiy was also evacuated from the MAI in Moscow and was no longer involved with the M-250 as a result. After the evacuation from Voronezh, the M-250 design team and the manufacturing team were split, which caused long delays in further engine testing and the completion of additional prototypes.

M-250 development and testing was continued at what later became OKB-36 (Opytno-Konstruktorskoye Byuro-36 or Experimental Design Bureau-36) in Rybinsk, Russia. However, the M-250 engine program was never able to fully recover after the evacuation, and the project was cancelled on 25 June 1946. A total of 10 M-250 prototype engines were built. The M-250 engine was proposed for use in several projects: a version of the Ilyushin Il-2 Sturmovik attack aircraft, an undesignated Yakovlev heavy fighter, the Alekseyev I-218 attack aircraft, and an undesignated Alekseyev fighter. However, none of these projects progressed beyond the drawing board, and the M-250 was never installed in any aircraft.

A Tupolev Tu-4LL testbed with a contra-rotating Dobrynin VD-3TK engine installed in each outer position. The LL in the aircraft’s designation stood for “letayushchaya laboratoriya,” which means flying laboratory.

While at OKB-36 and under Dobrynin’s supervision, A. L. Dynkin developed the M-251TK from the M-250. Compared to the M-250, the M-251TK had a larger bore and stroke, a higher compression ratio of 6.6 to 1, and strengthened internal components. In addition, the engine was fitted with fuel injection, a single-speed supercharger, and two turbosuperchargers. Two versions of the M-251TK were developed—one with a standard propeller shaft and one with contra-rotating propeller shafts.

After the M-250 was cancelled, the M-251TK was approved for prototype manufacture in late 1946 and was first run in August 1947. The M-251TK passed various certification tests throughout 1948, including 50 and 100 hour tests. The engine was approved for manufacture in January 1949 as the VD-3TK. The VD-3TK had a 5.8 in (148 mm) bore and a 5.7 in (144 mm) stroke. The engine’s total displacement was 3,628 cu in (59.5 L), and it weighed 3,351 lb (1,520 kg). The VD-3TK had a takeoff rating of 3,500 hp (2,610 kW) and a continuous rating of 2,500 hp (1,864 kW).

The restored Dobrynin VD-4K engine preserved at the CPO Saturn facility in Rybinsk, Russia. The power recovery turbines are mounted in the exhaust Vees of the engine. The red plates cover inlets through which air flowed to cool the units. The 4,300 hp (3,207 kW) VD-4K represented the pinnacle of piston-engine development in the Soviet Union. (www.missiles.ru image)

In the first half of 1950, VD-3TK engines were test-flown in the outboard positions on a Tupolev Tu-4 bomber. The engine was also proposed for the Alekseyev Sh-218 attack aircraft, which was never built. The VD-3TK did not enter series production, and only 34 engines were made.

In 1949, Dobrynin’s team at OKB-36 had begun further engine development, this time based on the M-251TK. The intent was to create an engine with improved fuel economy to be used for a new long range, strategic bomber. The new engine was known as the M-253K, and its development proceeded under chief designer P. A. Kolesov. Along with other modifications, the engine’s compression ratio was raised to 7.0 to 1, and three power recovery turbines were installed in the exhaust Vees. These turbines would recover energy from the exhaust gases and feed that power back to the engine’s crankshaft. The two turbosuperchargers used with the M-251TK engine were replaced by a single, large unit that incorporated an adjustable jet outlet to harness thrust from the exhaust gases.

The Tupolev Tu-85 strategic bomber was the only aircraft powered by VD-4K engines. The engines and aircraft preformed well, but the future lay with turboprop and jet engines. Note the turbosupercharger housing above each engine nacelle.

The first M-253K was completed in January 1950. Prototype engines were tested and developed throughout 1950. During this time, test engines passed 50 and 100 hour tests and were flown as the No. 3 engine on a Tu-4. Twenty-three engines were built and given the designation VD-4K. While the VD-4K had the same bore and stroke as the VD-3TK, the VD-4K produced a lot more power. The engine had a takeoff rating of 4,300 hp (3,207 kW) at 2,900 rpm and a continuous rating of 3,800 hp (2,834 kW) at 2,700 rpm. The VD-4K was fuel injected and achieved a specific fuel consumption of .408 lb/hp/hr (284 g/kW/hr) at cruse power. The engine was 63 in (1.6 m) in diameter, 119 in (3.0 m) long, and weighed 4,552 lb (2,065 kg). The turbosupercharger weighed an additional 485 lb (220 kg).

VD-4K engines were installed in Tupolev’s new strategic bomber, the Tu-85. The Tu-85 was ordered in 1949 and made its first flight on 9 January 1951—Aleksei Perelyot was at the controls. The Tu-85 had a 183.5 ft (55.9 m) wingspan and was 130.9 ft (39.9 m) long. The aircraft had a maximum speed of 396 mph (638 km/h) at 32,810 ft (10,000 m). Designed to counter the long-range Convair B-36 Peacemaker, the Tu-85 could deliver 11,000 lb (1,000 kg) of bombs 7,580 mi (12,300 km) or carry 44,000 lb (20,000 kg) of bombs.

A diagram showing the VD-4K’s installation in the Tu-85 and its intake and exhaust paths. Note the cooling fan and how air is diverted from the turbosupercharger inlet to flow through an aftercooler.

In the Tu-85, an annular radiator was installed around the front of the VD-4K engine. An axillary fan was added behind the spinner to increase the flow of cooling air, but it appears no other major improvements were needed. The turbosupercharger for the VD-4K engine was positioned on top of the nacelle, and the engine exhaust flowed back over the wing. Incoming air to the engine was compressed by the turbosupercharger, flowed through an aftercooler, and was then delivered to the engine.

While the Tu-85 and its VD-4K engines achieved excellent test results, the Tupolev Tu-95 “Bear” strategic turboprop bomber was under development and showed greater promise than the Tu-85. As a result, development of the Tu-85 and the VD-4K engine was stopped. Both Tu-85 prototypes were later scrapped.

The VD-4K was the last piston engine developed by Dobrynin and OKB-36; their efforts shifted to designing and building turbojets engines. A VD-4K engine is preserved at the NPO Saturn (former OKB-36) facility in Rybinsk.

With its impressive range and payload, the Tu-85 was one of the most capable piston-engine bombers ever built. Because of the transition to turbine engines, the Tu-85 was outclassed and never went into production.

Sources:
– Russian Piston Aero Engines by Vladimir Kotelnikov (2005)
– Unflown Wings by Yefim Gordon and Sergey Komissarov (2013)
– Soviet and Russian Testbed Aircraft by Yefim Gordon and Dmitriy Komissarov (2011)
– Tupolev Aircraft since 1922 by Bill Gunston (1995)
– http://www.redov.ru/transport_i_aviacija/aviacija_i_kosmonavtika_1997_07/p3.php

By William Pearce

Romanian Henri Marie Coandă is perhaps best known for observing the way a stream of fluid (such as air) is attracted to and will flow over a nearby surface. This component of fluid dynamics became known as the Coandă Effect. Coandă recognized this phenomenon while testing his first aircraft, built in 1910. This aircraft had a unique propulsion system that Coandă called a turbo-propulseur, and it is recognized as the first “jet” aircraft. A four-cylinder, 50 hp Clerget engine was used to power a rotary compressor that provided thrust. While there is some debate about the validity of the aircraft’s first and only flight and its subsequent destruction, the aircraft was certainly built to be propelled by a jet of fast-flowing air.

Henri Coandă’s 1911 monoplane at the Concours Militaire in Reims, France in October 1911. Note the tandem main gear wheels.

Coandă’s second aircraft was built in France and completed in 1911. It utilized some salvaged and spare parts from the 1910 aircraft. The 1911 aircraft was originally designed to use a turbo-propulseur, but it was finished with a conventional propeller. The aircraft’s engine arrangement, however, was not conventional.

The 1911 aircraft was a rather large monoplane with a parasol wing mounted above the cockpit. A small lifting surface with a nickel steel spar joined the two main landing gear which were each comprised of two tandem wheels. Each main gear wheel set was encased in a large fairing. A single vertical strut made of nickel steel extended above each gear fairing and supported the wing. The wings had a nickel steel spar and were covered by fabric. The aircraft’s roll control was achieved by wing warping. Coandă’s 1911 aircraft had a cruciform tail similar to that used on the 1910 aircraft. The fins of the tail formed an X, and each fin had a trailing control surface that acted as both a rudder and an elevator.

This photo shows a detailed view of the Gnome installation on Coandă’s 1911 aircraft. Note the various struts and braces used on the aircraft. The aluminum-covered front fuselage is easy distinguished from the plywood-covered cockpit section. The aircraft’s control wheel can just be seen at right.

A rectangular support structure was formed by the upper and lower spar and the vertical struts above the wheels. The fuselage was suspended in this support structure by a series of brace wires and small struts. Additional wire bracing and struts supported the rest of the aircraft’s structure. Except for where the engines were mounted, the fuselage had a circular cross section that narrowed to a point at the tail. The front of the fuselage was covered by aluminum sheeting, the cockpit section was covered by plywood sheeting, and the rear of the aircraft was fabric-covered.

Perhaps the most unusual feature of Coandă’s 1911 monoplane was its engine installation and propeller drive. At the front of the aircraft were two Gnome 7 Gamma rotary engines. The seven-cylinder engines had a 5.1 in (130 mm) bore, a 4.7 in (120 mm) stroke, and a total displacement of 680 cu in (11.1 L). The 7 Gamma produced 70 hp (52 kW) at 1,200 rpm and weighed 194 lb (88 kg).

This photo shows an engine and gearbox arrangement similar to that used on Coandă’s 1911 monoplane. It is not clear when this photo was taken, but it may have been at the Salon de l’Aeronautique in Paris, France held mid-December 1911 through early January 1912. (Harry Stine image via New Fluid Technology)

The engines were installed front-to-front with their crankshafts perpendicular to the aircraft’s fuselage. While the engines’ cylinders were exposed to the slipstream for cooling, the front of the engines were enclosed within the fuselage. Mounted between the engines was a gearbox that drove a propeller shaft. The propeller shaft extended to the front of the aircraft where it drove a four-blade propeller. The engines and gearbox were mounted to a steel frame. Coandă claimed that the aircraft could fly with just one engine operating.

Most likely, the engines turned in opposite directions relative to each other. While this arrangement would cancel out the gyroscopic effects of the rotary engines along the pitch axis, it would induce some tendency to roll, even if just slightly. Some sources indicate the engines were “handed” —they rotated the same direction relative to each other. In addition to the complications in making a rotary engine run “backward,” the “handed” engine configuration would create a noticeable pitch moment on the aircraft as the engines were throttled (blipped), but it would also alleviate any tendency for the aircraft to roll. However, an early sketch of the engine arrangement indicates “handed” engines were not installed, and that a simple beveled gear arrangement was used to transfer power from the engines to the propeller shaft. Additionally, the transfer gearbox did not appear to be of sufficient size to accommodate the differential gearing needed for a “handed” engine arrangement.

Note the cruciform tail and its control surfaces in this photo of the Coandă 1911 monoplane. Also, the plywood-covered cockpit section can be easily distinguished from the fabric-covered rear fuselage.

The 1911 Coandă monoplane had a wingspan of 53 ft 6 in (16.3 m) and a length of 41 ft (12.5 m). The aircraft had an empty weight of 1,036 lb (470 kg) and a maximum weight of 2,756 lb (1,250 kg). Two fuel tanks of around 30 gallons (115 L) each were housed in the center section of the wing. Reportedly, the aircraft could accommodate a pilot and two passengers. The estimated speed of the 1911 monoplane was 81 mph (130 km/h).

Coandă’s 1911 monoplane was tested in the Concours Militaire (Military Competition), held in Reims, France in late October 1911. Georges de Boutiny flew the aircraft, but it reportedly did not meet performance expectations. Later, wing extensions were added to the wheel fairings, turning the aircraft into a sesquiplane. Along with additional wire bracing, a vertical strut connected the end of the wing extension to the upper wing.

Mechanic George Bonneuil checks a Gnome engine as pilot George de Boutiny looks on from the cockpit. (Harry Stine image via New Fluid Technology)

A Coandă aircraft catalog from 1911 offered both the monoplane and sesquiplane versions of the aircraft with either 50 hp (37 kW) Omega or 70 hp (52 kW) Gamma Gnome rotary engines. It appears that only the single prototype of the Coandă 1911 aircraft was built, and exactly what happened to it is not known. The 1911 aircraft faded into history, and Henri Coandă went on to build other aircraft and further explore fluid dynamics.

Note: Some claim that Coandă’s 1911 aircraft was the first twin-engine aircraft. However, at least four other twin-engine aircraft preceded it in flight: the Daimler Lutskoy No. 1 (flew 10 March 1910, or possibly earlier), Edward Andrew’s twin (flew early 1910), Roger Sommer’s twin (flew 27 September 1910), and the Queen Speed Monoplane (flew 10 July 1911).

This photo shows Coandă’s 1911 aircraft with its wing extensions. The extensions effectively made the aircraft a sesquiplane. Additional struts and braces for the extensions can be seen. Note the three people in the cockpit and also the warp of the wing tip.

Sources:
– Henri Coandă and His Technical Work During 1906-1918 by Dan Antoniu, et al (2010)
– French Aeroplanes before the Great War by Leonard E. Opdycke (1999)
– Romanian Aeronautical Constructors 1905-1974 by Gudju, Iacibescu, and Ionescu (1974)
– Henri Coanda: The Facts by New Fluid Technology (4.3 MB pdf)
– http://flyingmachines.ru/Site2/Crafts/Craft28597.htm
– http://en.wikipedia.org/wiki/Coand%C4%83-1910
– http://en.wikipedia.org/wiki/Henri_Coand%C4%83
– http://www.secretprojects.co.uk/forum/index.php/topic,18780.15.html