G-1 UAV Mk.2 under construction
- Andrea Gatti
- 9 gen 2019
- Tempo di lettura: 3 min
Aggiornamento: 2 giu 2020
After the promising failure of the Mk.1 version, the Mk.2 version will show the following improvements:
- Wing dihedral, for increased latero-directional stability;
- Twist of the wing sections for enhanced low-speed control;
- NACA 4412 instead of the NACA 23015 previously used in Mk.1 for the main wing;
- New wing structure;
-Many CNC-cut parts, and extensive fuselage redesign to save weight.
Some shots of the process...
Let's explain why I'm implementing those modifications in my design.
Wing dihedral and linear twist of the wing sections:
Following the accident that occurred to the Mk.1 version due to the loss of control in the initial phase of flight, it is necessary to significantly improve the authority of the ailerons and the behavior at low speed. In fact, the Mk.1 version had two critical points underestimated during the design and construction phase:
The quality of the airflow both on the top and on the bottom of the wing due to the balsa spar caps applied on the surface of the polystyrene core, forcing the boundary layer transition to turbulent in a too rough way at the very beginning of the chord and reducing the authority of the ailerons for small deflections;
No airfoil twist between the root and the tip of the semi-wings, with the risk that a possible stall may spread violently over the whole span of the wing, also affecting the ailerons making them ineffective.
Early flight tests had shown little aileron effectiveness despite their size, but by combining the roll control with the yaw command (rudder) it was possible to get a good and safe control. For this reason I decided to design the new wing with 4° dihedral angle to more importantly couple the roll control with the rudder deflection, in any flight condition.
Previously, in fact, this coupling was still present because the wings showed a considerable in-flight bending (about 15 cm at the extremities) due to the aerodynamic loading and their relatively low bending stiffness, ensuring the stabilizing contribution on the roll of the dihedral angle.
But in the case of zero load (such as the slip that brought to the incident the first version, with a bank angle at impact close to 80 °) this "apparent" dihedral does not manifest itself, and so is the stabilizing rolling moment.
New wing structure:
This new structure is made with the concept of placing material only where needed.
Wingbox starts from the leading edge until 1/3 chord and is responsible to resist against wing torsion and bending. It's made by two balsa panels, 1mm thick and a 4mm thick webb closing the section. Bending moment is kept under control also by means of the two stringers located at 1/3 of the chord. The leading edge stringer is used to glue together the upper and lower panels of the wingbox.
Closing the wingbox is very important to ensure the torsional stiffness of the wing since open sections are not supposed to.
As before, the entire wing can be divided in four sections for a simpler transport. Each section is joined by means of an aluminum tube linking two ribs per wing, aligned with a reference pin and kept in place by a set of very strong magnets.
The linear twist is enforced by two special brackets per rib. Each rib has two brackets located in its lower half to allow an easy construction of the wing section with precise local incidence. Those brackets will be removed once the wingbox is closed.
CNC-cut parts and extensive fuselage redesign:
Machining every piece with the appropriate tools ensures better quality of the final product. In this case, airfoils are repeated in each wing section with absolute precision (less than 0.1mm) ensuring fast and precise building of the airframe. I'm helping myself in placing the components in the right place by sticking them with a bit of hot glue to a sheet where I've printed the entire design in 1:1 scale.
It's a very simple yet effective technique.
The entire re-design should provide a 30% improvement on the empty weight (that's huge!) and aerodynamic benefits that only flight testing will reveal!
