Aircraft engine with an open rotor: a missed chance for Russia
Examples of placement of a promising turbopropfan aircraft engine.
open rotor
Managers' dreams from aviation industry to keep within the bounds of decency is very difficult. Abroad, they seriously believe that by 2050, global aviation will become carbon-neutral, that is, aircraft will not produce excess carbon dioxide at all. More precisely, emissions will not disappear anywhere, but they will be compensated by new types of fuel, as well as by direct capture of carbon dioxide from the atmosphere.
History sounds a bit absurd - over the past five years, global aviation has increased COXNUMX emissions by a third2. Despite this, the International Civil Aviation Organization (better known as ICAO) adopted a resolution last September to zero carbon dioxide emissions.
Now it seems a little fantastic, especially in light of the Russian special operation and international sanctions. Simply put, what does this have to do with modern Russia?
The fact is that the notorious carbon-neutral aviation, in whatever form it appears, will sooner or later capture the world's airports. And it may turn out that the domestic MS-21 and SSJ-100 New in fifteen or twenty years will not even be allowed into the airports of China and Turkey. Engines will be too "dirty". All that remains is to buy aircraft equipment abroad, which is very unlikely, or to launch carriers from friendly countries.
ICAO plans to phase out carbon emissions more than once - the next tightening of the standards is planned in 2026 and 2037. Looking to the future is a thankless task, but air travel is growing by 3-5 percent every year, which by 2050 will increase carbon dioxide emissions from aircraft engines by 3-3,5 times compared to 2020.
Carbon-neutral hydrogen engines and electric motors with propellers should not be seriously considered - these are too revolutionary designs. The future is definitely for evolutionary solutions that will allow at least partially approaching the fantasies of ICAO. One of these, of course, is the turboprop-fan aircraft engine or propfan, which is a complex product of the evolution of the turboprop engine.
The key feature of CFM International's "open rotor" is the fixed second row of blades.
Historically, the emergence of propfan is associated with the oil crisis of the 70s, which prompted engine builders to search for economical schemes. An open propfan differs from a classic turboprop by a large number of blades and a special shape.
If it’s completely according to the textbook, then the screw can be considered a propfan if there are more than eight blades, and they look like sabers in outline. This allows you to reduce the speed, so as not to reach supersonic speeds, leading to the "locking effect", that is, to a sharp drop in efficiency. By increasing the number of blades, there is no need to increase the diameter of the propeller, which is beneficial for the layout of the aircraft.
On paper, an open fan allows you to save, all other things being equal, up to 20-30 percent of fuel. This is only achieved by a large noise level. For example, the turboprop D-27, developed in the late 80s at the Progress Design Bureau in Zaporozhye, which in 2005 barely received a Type Certificate for noise on the ground, is excellent. The engine was supposed to be mounted under the wing of the transport An-70.
Neither the plane nor the engine has been alive for a long time. Just as the NK-93 is no longer alive - a hooded turboprop-fan aircraft engine developed for the promising military transport Il-106 in the Samara Scientific and Technical Complex named after. Kuznetsova.
From the outside it may seem that the engine belongs to a turbojet, just with a high degree of bypass. But this is not so - the front fan has eight blades, and the second one has ten, which automatically translates the product into the category of turboprop fans.
NK-93 (top) and D-27
The giant hood surrounding the fan was supposed to reduce noise levels.
The main problem with an engine with a thrust of 18 tons is a three-meter diameter, which makes it necessary to seriously reconsider the design of the aircraft.
Be that as it may, Russia is currently not conducting development work on the NK-93 or its analogue, not to mention rethinking the design of the once domestic D-27. The theme of an open rotor, open fan or propfan does not go beyond theoretical research.
While abroad, it receives a lot of attention.
RISE
The requirement to reduce fuel consumption and, as a result, to reduce carbon dioxide emissions, forced abroad to recall the open rotor.
Since the summer of 2021, the French-American company CFM International has been working on the Revolutionary Innovation for Sustainable Engines (RISE) project. The ambitious goal is to reduce fuel consumption by 20 percent compared to the most economical counterparts in the class. Engineers propose to develop a modular product capable of operating on hydrogen, as well as with a fan driven by an electric motor.
The developers abandoned the birotational scheme that underlies all previously developed propfan and open fan. A pair of fans on the same axis of the motor rotated in different directions, which increased thrust, but significantly increased aerodynamic noise.
So it was with the D-27, and with the hooded NK-93, and with the experienced American GE36 and P&W Allison 578-DX.
A small digression.
CFM International was established in 1974 by two companies - the American GE Aviation and the French Snecma, which is a subsidiary of Safran. The French have always had a special craving for exotic solutions, and the "open fan" was no exception.
Safran's birotational "open fan" test in 2017. Did not go into the series, although it provided 15% fuel economy
In 2017, Safran completed work on the SAGE 2 project pusher turboprop. The design was recognized as successful, but market conditions did not promise market success - ICAO's carbon-neutral solutions were still far away. Although 15 percent of saved kerosene impressed many.
And now CFM International returns to the topic, but this time it was decided to abandon the pusher circuit and the second fan on the axis. In place of the second rotor, there are fixed guide vanes that can only change their pitch. As planned, this should significantly reduce the perturbations of the air flow behind the main rotor.
Bench tests of the aircraft engine of the RISE project are planned to begin as early as 2026.
An open fan is planned with a diameter of four meters. This, by the way, is hello to all the skeptics who buried the domestic NK-93 with its three-meter cross section. Say, the aircraft engine would not fit anywhere. The French and the Americans think differently. According to calculations, the RISE engine with a thrust of 13,6 tons should provide the carrier with the same Mach 0,8 as traditional high-bypass turbojet engines.
The giant A-380 should become a flying laboratory for RISE
The most unusual thing in history is the alleged carrier of a promising aircraft engine.
CFM International agreed with Airbus, and they sent a whole A-380 to the engine builders. This is a four-engine double-deck giant, bearing the title of the world's largest passenger aircraft. Although no longer mass-produced. Under its wing, they plan to hang the first experimental aircraft engine and take to the skies. Approximately the same NK-93 mastered the work in the air with the domestic flying laboratory IL-76LL in Zhukovsky.
For unknown reasons, the authors of the project abandoned the Boeing 747 flying stand. Although the "jumbojet" is also with four motors, which guarantees safety in case of failure of the prototype. It is premature to talk about the use of an "open fan" on wide-body aircraft - it simply does not have enough thrust for this. Typical consumers include regional Fokker and Bombardier.
Conceptually, the placement of huge motors is considered not under the wing, but in front a little above the edge of the wing. Or in the tail, which is especially beneficial in terms of reducing noise in the passenger compartment. Of course, the most preferred scheme is with a high wing, like in the An and Il-76 families.
As you can see, military transport aircraft may well become carriers of a promising aircraft engine.
Despite the initially civilian nature of an aircraft engine, RISE will clearly find its place under the wings of military transport aircraft. Especially if the developers manage to keep their promises about flight speeds. All other things being equal, even the military will not refuse a 20 percent fuel economy.
Russia missed the chance to be among the world leaders in the development of turboprop-fan aircraft engines. In the West, they are clearly seizing the initiative in a fairly narrow specialization, which in the future may capture the entire market.
But all is not lost - before the planned acceptance into the RISE series, there are still eight to ten years. During this time, there is every opportunity to catch up and achieve parity with the West. And the sanctions for Russian engineers only play into the hands here.
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