Defining what the combat aircraft of tomorrow could be, having an aircraft that is both stealthy and maneuverable, is what ONERA is working on through the Superman project. Particularity: the latter has control surfaces hitherto unused in France, the Levcon. (This article appeared in issue 2825 of Airlines and Destinations)
Superman and supermaneuverability
Superman. This is one of the research programs unveiled to the public by Onera, during the visit of the DGA and the DGAC which took place in Modane, where part of the large wind tunnels are located, on March 17, 2023. This research program aims to study the behavior of a combat aircraft at the limits of its flight envelope, with the aim of preparing tools for the design and evaluation of future combat aircraft. “Superman refers to supermaneuverability. The survivability of a combat aircraft results from a compromise between manoeuvrability, stealth and velocity,” explains Eric Garnier, Deputy Director of the Aerodynamics, Aeroelasticity and Acoustics Department at Onera. “Combat aircraft developed in Europe in the 1980s were designed with limited integration of stealth constraints. The next generation of aircraft will have to achieve a more balanced compromise between the three components of survivability. The choice of the name of the project is perhaps a little badly chosen in the sense that it perhaps makes one think of extreme maneuverability, whereas it is rather about maneuverability under the constraint of stealth”.
Reconciling stealth and maneuverability
Until then, stealth devices were not very maneuverable or at least had lower maneuverability than other devices. But contrary to what one might think, it is possible to reconcile the two areas to a certain extent. “A machine like the F-22 is both maneuverable and stealthy. But at the risk of cringe, the aerodynamic formula close to ideal for maneuverability remains that of an aircraft that is not stealthy. I am thinking in particular of the Sukhoi 27 family. The versions derived from the Su-35 as well as those equipped with canard planes from the same aircraft manufacturer, with vector thrust on three axes is a fine example. By comparison, the F-22 only has two-axis vector thrust, complicated by the fact that it had to be made stealthy on the rear side. In general, there is an antinomy anyway, because the larger the surface of the drift, the higher the maneuverability of the device and the less stealthy it is. Everything is therefore a matter of compromise. Finally, the specification aspect of the need should not be neglected either. On the NGAD for example, the Americans seem to want to achieve what they call a ” command plane »a device that flies very high, intended to be very stealthy, which will direct other aircraft including remote carrier. We can therefore imagine this device as not needing to be very manoeuvrable”, comments Eric Garnier.
A beautiful baby of 60 cm wingspan for 7 kg
Superman, for his part, is in the logic of an aircraft that has not sacrificed too much maneuverability on the altar of stealth. “However, at Onera we are not making a combat aircraft but rather making a generic shape that is realistic enough to have the same problems as those encountered on a real aircraft. The aircraft chosen is based on a logic similar to the Sukhoi Su-57, that is to say that of a maneuvering aircraft that incorporates stealth constraints. The shapes are therefore in a logic of aerostealth compromise”, adds Eric Garnier. The generic model of Superman is on a scale of 1/25, it measures 60 cm wingspan for an equivalent length and weighs approximately 7 kg. It is made of an aluminum alloy and its control surfaces actuated by servomotors. The model was blown in a research wind tunnel at Onera’s Lille center, the L1 horizontal wind tunnel. Tests are underway in the SV4 vertical wind tunnel in the center of Lille.
Levcon or leading edge vortex controllers
The tilting planes located at the apex are Levcon (acronym of Leading Edge Vortex Controllers, literally leading edge vortex controllers). “These devices are of particular interest to us because they are supposed to control the vortices emitted at the leading edge of the wing. They appeared on both the Indian Navy’s Su-57 and Tejas, but remain little known. It’s actually the only control that we don’t know enough about,” comments Eric Garnier. The model is devoid of air intake and/or nozzles. Because for the moment, it is the wing alone and the interactions with the Levcon that are studied.
100 degrees per second
“We were not interested, in this project, in the possible positioning of the air inlets. However, interactions between the air inlets and the levcon must be expected for certain angle of attack ranges. The example of the Su-57, however, shows that this situation is manageable. It should also be noted that the model is a technical masterpiece: the control surfaces move at 100 degrees per second and their positioning is guaranteed to within 0.1 degrees. This is a technological feat to be credited to the Lille service of the Onera wind tunnel, engineering and mock-up department,” explains Eric Garnier. For the moment, the model has only been blown at “low” speeds. “With the mobile levcon we will not be able to go at high speed because they have not been dimensioned in terms of effort for this purpose. In fixed version, it is feasible. The possible extensions of the model would be around a motor integration, a more complete set of control surfaces, the increase in speed, since for the moment we are limited to around 200 km/h. We must nevertheless remember that very high maneuverability is not achieved at Mach 2,” comments Eric Garnier. If the model bears no resemblance to the silhouette of the Su-57, the differences are nevertheless notable.
A sail shape resembling
“While it’s not a real fighter, the shape is both generic and realistic, so both the aerodynamics and fluid mechanics issues are very similar to those of a fighter plane. This device is the result of a creation, since we work within a NATO group. The shape in plan had been decided by other members of the group, who had not worked on the profiles at all, which were, after several iterations, chosen by ONERA. The generic shape of the aircraft, representative, has the usual profiles found on combat aircraft, on which we have worked on the camber, the twist, the radius of the leading edge to have something sufficiently realistic. The shape of the airfoil, seen from above, is quite similar to that of the Su-57. The arrows are 45° at the root at the level of the Levcon, then 75° and again 45°”, explains Eric Garnier. In particular, work has been done in the field of twist laws, to limit the incidence at the wing tips and avoid stalling the wingtips first. Furthermore, “We particularly focused on the pitch balance. At its angle of attack in cruise, the pitching moment of the aircraft is zero. We have carried out work to be certain of being balanced in pitch at both low and high speeds, ie up to Mach 0.85”, adds Eric Garnier.
Levcon and eddy flows
The Levcon are only one aspect of the project, the characterization of vortex flows is another subject of Superman’s study. “The vortices are a zone of depression which is reflected on the wall by an aspiration, that is to say a free vortex lift. If the airliners do not use this lift, the combat planes use it enormously. There are issues of fluid mechanics which are crucial, among other things to understand the movement of these vortices, their intensity, their positioning. From these phenomena will result the aerodynamic forces and consequently the trajectory of the aircraft. Superman’s goal is to make the connection between these vortices and the efforts and then the efforts to the trajectories, all in the presence of Levcon. If the Russians called it “controller”, it is not without reason since it is a control surface that controls the leading edge vortices. This can be seen in the images, when the Levcon turns, the vortices move drastically”, explains Eric Garnier. Aerodynamically, the Levcon modify the pitching moment, or the balance of the plane. “They give nose-down capability, in addition to the elevators – which can therefore be smaller in size (see Airlines and Destinations n°2823 “Reduce the surface of the fin and increase its efficiency”) -, with a higher rate . Typically, during a very brutal maneuver, during which a lot of energy has been lost with an aircraft being positioned with the nose very high, the Levcon will allow you to go back into a nose-down attitude to regain speed. The Levcon therefore manipulate these vortex flows”, explains Eric Garnier.
Light up the future
Will Levcon equip future air combat aircraft? The answer cannot be affirmative for the moment, because the choice of their use or not will belong to the industrialists, namely the aircraft manufacturers, according to the choices made. Onera has, however, provided what appears to be extended documentation to those concerned. “We are in our role to enlighten the future by proposing new means of measurement, new actuation technologies as well as new modeling strategies, among others”, concludes Eric Garnier.