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24 Dossier | Advanced Aircraft Company HAMR UAV Spreading the load The first was to spread the rotors out along thin cylindrical booms over a wider span to reduce the interference between them. “Every multi-rotor these days puts all the rotors equidistant from the centre of the frame, and that is the lightest structural weight solution,” Fredericks says. “So, if you have a mission where you are mostly just hovering, that’s the right answer. “If however you have a mission where you are flying with forward airspeed then the optimal system-level answer is not minimum structural weight. You want a bias towards better aerodynamics, even at a small expense of higher structural weight. That would be a more efficient vehicle overall. “In a typical quadcopter, the props at the back are mounted directly behind those at the front, so they are always effectively operating with a rate of climb because the front props are grabbing air and directing the flow downward. So, by separating the props laterally, we are reducing the interference between the rotors. That saves power when you are operating with forward airspeed. “We’re spreading out the weight of the aircraft on rotors that create a larger span, so we are working a much wider streamtube of air less hard, rather than working a narrow streamtube of air harder,” he says. A change of attitude The second solution was to mount the fuselage in a nose-up attitude so that when the aircraft pitches its nose down to build airspeed the fuselage levels out to reduce the base drag area. “In a typical multi-rotor at high speed you are dragging the fuselage through the air in a nose-low attitude, so all of the belly of the fuselage creates separated airflow, leading to high drag,” Fredericks says. Separated airflow creates two orders of magnitude more drag than skin friction from attached airflow, he explains. “With the HAMR, when it pitches forward to go forward, the fuselage becomes level and the airflow is streamlined along its entire length, apart from the very back, the base drag area. At least the whole belly of the helicopter is no longer separated though.” The tubular booms on which the electric motors are mounted are circular in cross-section, making them cylinders from the point of the airflow, and a cylinder in a cross-flow creates a lot of separated airflow behind it. That is why AAC decided to add the pivoting aerodynamic fairings to the booms, which constitute the third solution. These fairings are not control surfaces, they are only there to reduce drag and generate lift, as all the control forces come from varying the propeller rpm. Running the span of the booms, the fairings have a chord of about 7 in. They are free to rotate and do not require servos to articulate; the aerodynamic moments articulate them to the proper angle. Instead of relying on standard NACA aerodynamic profiles, the company developed custom aerofoils for the fairings. “This required the maximum thickness location of the airfoil to be moved forward to enclose the booms,” Fredericks says. “It also required a reflexed trailing edge to trim to a positive angle of attack to generate lift passively, as well as the aerodynamic centre to be located aft of the axis of rotation to ensure its static stability.” AAC has applied for patents for all three of these solutions. What all this means is that the HAMR is comparable with manned helicopters in terms of aerodynamic efficiency, with an eL/D of 4. That figure may not seem particularly impressive, but it is about double that of conventional multi-rotors, which it also beats in terms of hover efficiency thanks to its lower disc loading, according to Fredericks. “The power consumed per pound of thrust is lower for the HAMR because the props are loaded more lightly,” he says. The HAMR’s maximum speed and optimum cruise speed are the same, 40 knots, to which it is limited by the autopilot and which emerges as a result of the power available and the drag forces acting on the aircraft. “In December/January 2018 | Unmanned Systems Technology The front and rear propellers are offset to minimise aerodynamic interference in forward flight. The fuselage sits nose-up on the ground and in hover, but levels out in forward flight to reduce drag (Courtesy of AAC)