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The modern gas turbine (turbofan) engine that drives the liners is, of course, not a two-stroke rattler for gardening tools, but a highly efficient and very reliable machine. However, according to aircraft manufacturers, it is close to exhausting reserves for further improvement.
Why are there engines - all the airliners under construction are so similar to each other that only an aviation expert will immediately distinguish Boeing or Airbus from Bombardier or MS-21. And although there is not the slightest doubt that modern-type airliners with two gas turbine engines under the wings will roll us across the sky for decades, high hopes for a new layout and new aerodynamics of aircraft are associated with electric propulsion.
Fast, but not for long
Until recently, the term "electric aircraft" was understood as a "more electric aircraft" - an aircraft with a fixed wing, in which the mechanical and hydraulic transmissions were replaced to the maximum by the electric ones.
No more pipes and cables - all mechanical work, such as driving the rudders and mechanizing the wing, is performed by small electric motors-actuators, which are supplied with power and a channel for the control signal. Now the term has been filled with a new meaning: a true electric plane must itself move on electric traction.
Of course, the prospects for electric aviation depend not only (and not even so much) on aircraft designers as on progress in the field of electrical engineering. After all, airplanes, as they say, "on batteries" exist. Auxiliary electric motors were installed on gliders several decades ago.
The Extra 330LE, which first flew in 2016, already carries gliders and sets speed records. But its block of 14 powerful lithium-ion batteries and an electric motor from Siemens allow this baby to take on board only two people, including the pilot, and stay in the air for no longer than 20 minutes.
Of course, there are projects with much more impressive indicators. In September last year, the British low-cost airline EasyJet announced that in ten years it will launch an all-electric regional liner (range of 540 km, which is quite a lot for intra-European flights) with a capacity of 180 passengers.
The American startup Wright Electric, which has already built a two-seat flying demonstrator, has become a partner in the project. However, today the energy density of the best lithium-ion batteries is more than an order of magnitude inferior to hydrocarbon fuels. It is assumed that by 2030 batteries will improve their performance by a maximum of two times.
The situation with fuel cells looks much more advantageous, in which the chemical energy of the fuel is converted into electrical energy directly, bypassing the combustion process.
Hydrogen is considered to be the most promising fuel for such a power source. Experiments with fuel cells as a power source for an electric plane are carried out in different countries of the world (in Russia, CIAM is primarily working on projects to create such aircraft, and fuel cells for them are created at the IPCP RAS under the leadership of Professor Yuri Dobrovolsky).
From the flying and manned concepts, one can recall the European demonstrator ENFICA-FC Rapid 200FC - it used both electric batteries and fuel cells at the same time. But this technology also needs significant improvement and additional research.
The most realistic prospects for today seem to be the prospects for electric aircraft built according to the hybrid scheme. This means that the propeller of the aircraft (propeller or propfan) will be driven by an electric motor, but it will receive electricity from a generator rotated … by a gas turbine engine (or other internal combustion engine). At first glance, such a scheme seems strange: they want to abandon the GTE in favor of the electric motor, but they are not going to do this.
There are already quite a few hybrid projects in the world, but we are primarily interested in Russia.Work on an electric plane, in particular with a hybrid scheme, was carried out in various scientific institutes of the aviation profile, such as TsAGI or TsIAM.
Today, these and some other institutions have been united (since 2014) under the auspices of the Research Center "Institute named after N. Ye. Zhukovsky", designed to become a single powerful "brain trust" of the industry. The task of integrating all work on electric aviation within the center is assigned to Sergei Galperin, whom we already quoted at the beginning of the article.
Battery powered takeoff
“The transition to electric motors in aviation opens up a lot of interesting prospects,” says Sergei Halperin, “but there is no need to count on the creation of a commercial electric aircraft with a decent range for Russian conditions on purely chemical energy sources (batteries or fuel cells) in the near future: the energy potential is too different a kilogram of kerosene and a kilogram of batteries. A hybrid design could be a reasonable compromise. It must be understood that a gas turbine engine that directly creates thrust and a gas turbine engine that will set the generator shaft in motion are not at all the same thing.
The fact is that the energy requirements of the aircraft change significantly during the flight. On takeoff, the aircraft engine develops power close to its maximum, and during cruising (that is, for most of the flight) the power consumption of the aircraft is reduced by 5-6 times.
Thus, a traditional power plant must be able to operate in a wide range of modes (not always optimal from the economic point of view) and quickly switch from one to another. Nothing of the kind is required from a gas turbine engine in a hybrid installation. It will be similar to gas turbines of power plants, which always operate in the same, most economically beneficial mode. They have been working for years without stopping."
With the help of a generator, the GTE will be able to generate energy for direct power supply of electric motors, as well as for creating a reserve in batteries. Battery help will be needed just on takeoff.
But since the operation of electric motors in takeoff mode will last only a few minutes, the energy reserve should not be very large and the batteries on board can be quite acceptable in size and weight. At the same time, the gas turbine engine will not have any take-off regime - its business is to quietly generate electricity.
Thus, unlike an aircraft engine, a gas turbine engine in a hybrid electric aircraft will be less powerful, more reliable and environmentally friendly, simpler in design, which means cheaper and, finally, will have a greater resource.
Blowing on the wing
At the same time, the transition to electric motors opens up prospects for fundamental innovations in the design of civil aircraft of the future. One of the most discussed topics is the creation of distributed power plants.
Today, the classic liner layout assumes two points of thrust application, that is, two, rarely four, powerful engines hanging on pylons under the wing. In electric airplanes, the layout of a large number of electric motors along the wing, as well as at its ends, is considered. Why is this needed?
The point is again in the difference between takeoff and cruise modes. On takeoff at a low speed of the incident flow, an aircraft needs a large wing area to create lift. At cruising speed, the wide wing gets in the way, creating excess lift.
The problem is solved due to complex mechanization - retractable flaps and slats. Smaller aircraft, taking off from small airfields and having large wings for this, are forced to cruise with a sub-optimal angle of attack, which leads to additional fuel consumption.
But, if on takeoff many electric motors connected to the propellers will additionally blow the wing, it will not have to be made too wide.The plane will take off with a short takeoff, and on the cruising section, a narrow wing will not create problems. The car will be pulled forward by propellers driven by the propulsion motors, and the propellers along the wing at this stage will be folded or retracted before landing.
An example is NASA's X-57 Maxwell project. The concept demonstrator is equipped with 14 electric motors positioned along the wing and on the wingtips. All of them work only during takeoff and landing. On the cruising section, only wing tip motors are involved.
Such placement of motors allows to reduce the negative influence of vortices that arise in these places. On the other hand, the power plant turns out to be complex, which means that it is more expensive to maintain and the likelihood of failures is also higher. In general, scientists and designers have something to think about.
Will help out liquid nitrogen
“An electric plane provides many opportunities for optimization,” says Sergei Halperin. - You can experiment, for example, with a combination of pull and push screws. Electric motors are much more advantageous compared to gas turbine engines in convertiplanes, since the safe rotation of the electric motor to a horizontal position does not present such a complex engineering problem as in the case of traditional engines.
In an electric plane, you can ensure the full integration of all systems, create a new control system. Even hybrid cars will produce less noise and emissions.”
Like batteries, electric motors increase in mass, volume and heat dissipation as power increases. New technologies are required to make them more powerful and lighter.
For domestic developers of hybrid propulsion systems, a real breakthrough was cooperation with the Russian company SuperOx, one of the five largest suppliers of materials with high-temperature superconductivity (HTSC) properties in the world. Now SuperOx is developing electric motors with a stator made of superconducting materials (cooled with liquid nitrogen).
These engines with good aviation characteristics will form the basis of a hybrid power plant for a regional aircraft, which may take to the skies in the middle of the next decade. This year, at the MAKS air show, CIAM specialists presented a demonstrator of such an installation with a capacity of 10 kW. The planned aircraft will be equipped with a hybrid power plant with two 500 kW engines each.
“Before talking about a hybrid electric aircraft,” says Halperin, “it is necessary to test our installation on the ground and then in a flying laboratory. We hope it will be the Yak-40. Instead of a radar, we can put a 500-kilowatt HTSC electric motor in the nose of the car.
We will install a turbo generator in the tail instead of the central engine. The two remaining Yak engines will be enough to test our brainchild in a wide range of heights (up to 8000 m) and speeds (up to 500 km / h). And even if the hybrid installation fails, the plane can safely complete the flight and land. " The demonstration laboratory will be equipped according to the plan in 2019. The test cycle is tentatively scheduled for 2020.
Electric and hybrid propulsion occupies a significant place in the plans of the world's largest aircraft manufacturers. This is how the main features of passenger aviation of the middle of this century look like according to the Smarter Skies program of the AIRBUS company.
The aircraft of the future will be designed to minimize the hydrocarbon footprint in the atmosphere. Hydrogen gas turbine engines, hybrids and battery-powered all-electric aircraft will gain popularity.
It is assumed that the batteries will be recharged from environmentally friendly sources of electricity. The appearance of large wind farms or solar power plants in the area of airfields is possible.
Freedom in the sky
Intelligent liners will independently plot routes based on environmental friendliness and fuel efficiency parameters based on the analysis of weather and atmospheric data. They will also be able to gather in formations like flocks of birds, which will reduce drag for individual aircraft in the formation and reduce energy consumption for flight.
Rather from the ground
New propulsion systems and aircraft aerodynamics will allow them to take off along the steepest possible trajectory in order to reduce noise in the airport area and reach the cruising level as soon as possible, where the aircraft demonstrates optimal economic characteristics.
Landing without engine
The planes of the future will be able to land in a gliding mode. This will save fuel and reduce noise levels in the airport area. The landing speed will also decrease. This will shorten the length of the runways.
Airports of the future will completely eliminate the use of internal combustion engines that burn fuel. For taxiing, the liners will be equipped with electric motor wheels. As an alternative - high-speed unmanned electric tractors, which will be able to quickly deliver aircraft from the apron to the runway and vice versa.