Over the last twenty years, since 1996, when I first started the study for the design of electric propulsion aircrafts, I was oriented to a kind of parallel pathway in which two main propulsion formulas where the basic of such projects. They were and still are basically two, a twin-rotor and a quad-rotor aircraft typology. The first two more important projects designed in 1995-96 were the G150 CellCraft and the DDRH. Both aircraft were equipped with electro-rotors, although the first one was a quad-rotor type with a tilting system working according to the flight mode which the aircraft was capable to perform (see CellCraft chapter) while the second project, the DDRH, was also an electric propulsion aircraft, twin-rotor but having a limited tilting range. A revised version of the DDRH, named DDVL, was equipped with a new rotor system developed in 2013, reviewing the original 1996's design with a new version designed only for aerial work or as a flying crane. The twin-rotor formula, however, remains an interesting concept although it requires a more complex control mechanism that reminds the control device used by Kaman helicopters on their rotor system.
In fact, in lateral transverse flight it is necessary not only to vary the rotor's differential thrust with respect to the other, but also control the resulting torque that would force the machine to rotate on itself along the Z axis during the lateral translation. That's why lateral translation is obtained by a difference in thrust of a rotor with respect to the other, but also by the control of the tilt angle of the two rotors in the opposite direction in a such way to keep a perfect alignment of the aicraft along its longitudinal axis, counter-acting the torque that is generated by the differential trust.
Since the development phase of the DDRH, it was already assumed that a machine with a higher load capacity of at least four seats would be designed for passenger transport or other civilian applications provvided of a similiar rotor system. Several designs and project were produced in the following twenty years since 1996 in order to develop a four seat aircraft, and the best solution seemed to be to use the first conceptual project of an existing fuselage belonged to a project known as the EJ440, to which I made some essential modifications both in the structure and in the development of two new electro-rotors mounted on variable sweep wings. Here is from where the concept comes out, the EJ420 represented in these renderings is the result of that project.
body is entirely made of aluminum and carbon frame with thermoplastic
panels mounted on it, there are no rivets or screws of any kind, it
is a monocoque structure assembled in one body. The cabin is large and
contains four comfortable seats.
The EJ420 is a twin-rotor aircraft with side-by side rotors and it's subjected to the same identical problems as occurs on Helicopters in-fact the shifting of the Center of Gravity represent one of the main problem, unlike a four-rotor system which on the other hand, can support a wide CG's range position.
For this reason I designed a variable sweep wing system which contains the two rotors, capable to move either forward or backward the rotor system within a maximum range of 15° to compensate for any displacement of the longitudinal Center of Gravity, which will always be set in the right position.
The Stability Control System works on algorithm computation moving the rotor system according with the CG position at any time. The balancing process occurs already since the loading stage of the machine, when it is still grounded through sensors placed in the skids providing the data needed to calculate the right displacement of the CG, which allows the stabilization system adjust the position of the Variable Sweep Wing automatically.
This system is designed to operate autonomously, however it can also be manually controlled by the pilot, he/she can check the situation on the control panel about the status of the Gravity Center in real time, or even getting a CG forecast position due to fuel consumption along the flight which will alter the CG position.
No helicopter can shift the entire rotor-system forward or backward, this would require an expensive and rather heavy mechanism to correct the position of the CG, indeed weight and balance calculations on helicopter require a special attention. To find out more, please visit the chapter on Weight and Balance in Helicopter Aerodynamics on this web site (Italian version only).
are both oriented downwards to let the thrust flow be accellerated efficient
downward so that no obstacles are encountered along its path, eliminating
the problem of parasite drag produced
by the supports which in this case are characterized by the two variable
sweep wings. The efficiency of the rotor reaches the maximum level and
the fuel consumption is reduced. The rotor is equipped with five blades
which the pitch is collectively variable as well any RPM
variations. In other words the aircraft can use both possibilities which
may be mixed or controlled individually, whatever it is, the mixing
mode is always selected by the control system automatically according
to the flight mode, the mass and speed of the aircraft.
flight, however the system uses the control of the RPMs
mixed with the pitch angle of the blades which is always collective,
as cyclic control in this aircraft is absent.
Electro-rotors aircraft are equipped with electric motors, installed in a pair on each rotor. These are super-flat induction motors digitally controlled, capable of operating in any condition and cooled by air. Both the motors and the blades are easily replaceable in just a few minutes. A safety device keeps the rotors always locked when the machine is on the ground, or as soon as the doors are unlocked. In fact it is not possible to board or disembarking passenger unless the rotors are completely stopped. The pilot can only activate rotor spinning when the machine is completely safe, therefore the rotor stops after 30 seconds from landing or immediately afterwards for manual activation.
In this project it is noted that both rotor are very close to the fuselage and the doors, the reason is strictly related to some aerodynamic questions that in this project represent just an experimental concept. In fact, this project will not provide for further development because of some functional controversy that would not be acceptable for a commercial aircraft. In other words it is not a deliberate design errors, but rather a configuration dictated by aerodynamic requirements that are already underway for a reasonable resolution on a new project. The EJ420 is the basis for experimentation for a new project that is much more suitable for civilian use, which include also a military version.
The EJ420 can reach - theoretically speaking -the maximum speed of 210 knots. This occurs thanks to the forward tilting of both rotors forward for a maximum angle of 12° or backwards for a maximum angle of 9°, in case of rapid stop or backward flight. In forward flight the Control System acts also on the pitch moment of the tail through a dynamic stabilizer which is automatically controlled by the Dynamic Stabilization System, which assumes a negative angle according to the speed that the aircraft is reaching at any due instant along the flight path.
is equipped with a hovering and automatic pre-landing device, the pilot
will only have to act on the small power joystick by moving it slightly
backwards until the machine is lifted by itself by one meter from the
ground, maintaining its position in space thanks to a GPS
device. The EJ420 is able to keep its position hovering
even with a transversal wind of 30 knots without any
effort required by the pilot.
control does not provide a precise or progressive position
of the joystick as it happens with the collective control of a conventional
helicopter. In our case instead, the command always maintains a central
position that guarantees a stable flight to the altitude to which the
aircraft is brought. In other words pilot will just pulling it backwards
to go up or pushing forward to go down. Leaving the power Joystick will
stop any aircraft vertical movement, because it returns in the center
position freezing any further ascending or descending, so the machine
will keep flying at that desired height automatically.
The EJ420 as well as all Verticraft designs are provided of a Vortex Ring alarm which suggest the best maneuvering to adopt according with the circumstances at that moment in which this phenomenon occurs, furthermore it is also not possible to approach to a vertical speed above 300 feet per minute or forward less than 30 knots to avoid any situation which would cause Setting with Power to which even multi-rotors are unfortunately subject exactly as conventional helicopters.
One of the most important problems of multi-rotor is that it is not possible to perform any type of auto-rotation maneuver in the event of engine failure otherwise quite possible with helicopters. The surface of the rotors of a multi-rotor aircraft is smaller if compared to that of a helicopter rotor and thus its inertia is as well lower. However as these are electrical systems, all aircraft are equipped with a special lithium battery capable of providing a flight-time of at least five to seven minutes to guarantee an emergency landing or a safe descent allowing the restart of at least one of the two Power Unit.
The power system consists of two 240kW turbo-generators which supply the four electro-rotors and the main aircraft devices, a service group of batteries guarantees the required power to all vital functions in flight as well in case of failure of both turbo-generators. In addition, the EJ420 is capable to perform a continuous fly-time at 100% power with only one turbo-generator with the only pilot on board. The pilot can handle the power required along the flight or activating the power plan recommended by the Aircraft Managing System before take-off, which would be based on information relate to the instantaneous mass and the flight plan parameters, like speed, altitude, temperature etc in order to optimize fuel consumption.
Further updates on this project will be published during the experimentation of this aircraft. The EJ420 represent just a conceptual project that could undoubtedly open up new paths in the development of multi-rotor propulsion systems in the near future.
|©Gino D'Ignazio Gizio|