Issue 41 Unmanned Systems Technology December/January 2022 PteroDynamics X-P4 l Sense & avoid l 4Front Robotics Cricket l Autonomous transport l NWFC-1500 fuel cell l DroneX report l OceanScout I Composites I DSEI 2021 report

26 drive servo) sits further back, near the tail, and when powered moves a clip with two external pushrods that act on the inner- rear sections of either wing. The wings thus pivot about the hinges, to fold against the fuselage or unfold into their flight angles. “The hinges need to withstand the loads imparted during transition, and meet key stiffness requirements to be able to manage during both hover and flight,” Whitehand says. “The wings are passively locked into place after take-off by way of a secondary wing interlock, so that the actuator isn’t needed for holding the wings in place. “The worm drive was chosen to be as lightweight and reliable as possible, with an efficient and accurate encoder to allow us to tightly control its position. “As Val’s tests have proven, we can safely fly the X-P4 in all modes of transition, because when the wings fold in it just behaves like a quadcopter. So although we’re relying on a single electromechanical device, it doesn’t constitute a point of outright failure in the traditional sense.” The disposition of the hinge (and thus the angle of rotation between the two extremes of wing positioning) can vary between Transwing aircraft designs. While the X-P4 has been designed with high robustness and controllability of transitioning for the US Navy (with ongoing tests validating these), future r&d will examine how the aerodynamics change during transition, to see if there is an optimal angle from that perspective. “While we’ve had hundreds of real- world tests, developing robust aero and performance estimation is a key ingredient to maturing the technology,” Whitehand says. “And while most of our analysis currently relies on proven first-order methods and empirical data validated through testing, selective CFD and wind tunnel tests will play an important role in taking advantage of the differentiating aspects of the technology.” As mentioned, the Transwing system has some key advantages over other, better established VTOL- transition systems. The most visible is compactness: as the wings fold tightly along the fuselage, the craft’s footprint is greatly narrowed when landing (from 4 m wide to about 1.2 m). X-P4s can thus be stowed in cases or compartments without having to be disassembled, just as Grumman’s Sto-Wing maximised the number of F4F-4s that could fit on an aircraft carrier. Furthermore, the folding of the wing makes for a much more stable VTOL. As mentioned, a larger aerodynamic surface means a greater impact from crosswinds and gusts whenever fixed-wing aircraft try to land or take off. That could pose severe issues for tail- sitters, or for tilt-wing UAVs such as NASA’s Greased Lightning, as large landing pads could be needed to avoid endangering technicians during recovery. Dr Petrov notes here that the X-P2 by contrast has been flight-tested amid 55 mph gusts, and performed VTOL autonomously to well within safe levels of stability. Whitehand says, “The Transwing design has a very broad transition envelope. A lot of tiltrotors and tail-sitters, like helicopters, have to climb to a certain altitude before switching to forward flight, creating numerous constraints on speed, power, angle of attack and so on. “By contrast, we’re very tolerant of multiple outbound and inbound transition profiles, speeds, weights and external disturbances. We can start transitioning into cruise almost immediately after launching, December/January 2022 | Unmanned Systems Technology Each pushrod connects to the inner-rear section of its closest wing to achieve the transverse folding and unfolding movements The Quattro autopilot and ground interface solutions from Applied Navigation are used in X-P4s for defence users