Voting Result: 2.37881690581
Overview for Flugokura
We named our aircraft Flugokura, a compound word in Esperanto: flugo (flight) and kura (cure).
Having read the requirements, we immediately decided to work on a BWB concept for several reasons. Indeed, the BWB's main issue is around flight maneuverability and control, especially during TOL; yet, having our UAV a VTOL system, such difficulties would be less limiting. Other difficulties that BWB concepts face, like security requirements are overdone, having our aircraft an electric propulsion system. On the other hand, we get to have most of the advantages of the BWB concept, specially in terms of potential payload and cruise efficiency.
As for the selection of cruise motors for our UAV, we considered that, at the most, we would need to consider for a possibly underestimated value of the aerodynamic efficiency (E=10), a thrust of around one tenth of our aircraft's weight. We therefore thought to use just one motor for cruise to be necessarily positioned on the axis of symmetry of our aircraft. We preferred to position such an aircraft on the leading edge of our root chord.
We considered the wing reference area (1.05 m2) and span (2.22 m) to be constant throughout the optimization process. The layout has been sketched as the joining of 4 trapezoids, among whom the central trapezoid is rectangular, as we would want it to be straight at the trailing edge and sufficiently long, in order to position effective elevators. We also considered the root chord to be fixed (1.20 m).
The trapezoids have been defined by their heights referred to the wing span, their taper ratio and sweep angle. We had therefore a total of 8 design variables. For the sake of simplicity we considered the airfoils fixed, therefore having expected lift coefficients. We have neglected the positions of the VTOL motors as we thought to keep them as degrees of freedom for the balancing equation. We considered the positioning of the cruise motor at the leading edge of our center chord. As our UAV will be a BWB, the neutral point coincides with the aerodynamic center of the vehicle. Both the neutral point and the center of mass have been obtained through approximate moment equations. The balancing test verifies if by moving the weights of the aircrafts fixed equipment (ie: batteries) into certain ranges the layout obtained gives a possible positive static margin between 0.1 and 0.4. We have therefore obtained a Pareto front with 26 optimal solutions. We used as a refined decision criteria for the selection of our final layout, that is the one that best fitted our conceptual requirements: best geometry for the installation of control surfaces and for the construction of the additional structural supports for our VTOL motors (in order that they have no sweep angles.
We decided to position twin vertical fins at the tips of our aircraft wings. This solution not only is possibly better in terms of structural weights, it also gives the opportunity to have a less large trailing edge line in the central body, as so would be necessary for the installation of two elevators divided by a unique central vertical fin. Also, twin fins positioned at the tip of the wings give better aerodynamic efficiencies to our aircraft. We sized the fin as suggested in Raymer's “Aircraft design: a conceptual approach”.
Once our layout was sufficiently defined, we refined it using splines and smoothing out our geometry.
Our aircraft has quite different structural requirements if we were to compare it with conventional aircraft. The difference lays not much in the shape, indeed in it's weight. We therefore thought that a proper structure designed for our special requirements would necessarily have to be revisited.
We decided to use a topological method following Kovar and Liu's important work.
To begin with, we designed the structural supports for our VTOL motors. Then we extended this concept to the wing structure itself.