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23 by the drag, both of which can be measured in pounds or kilos, so the units cancel each other out to leave a dimensionless figure, which coincides with the glide ratio. Because powered-lift vehicles by definition have to consume power to generate lift and overcome drag, it is hard to measure the drag, so the calculation is more complicated. “That’s why, in the case of a helicopter or a multi-rotor, we use effective lift-to- drag ratio,” Fredericks says. “Weight is the same as lift in the numerator, but drag is replaced with velocity divided by power.” This gives the drag, because the more power required to achieve a given velocity at a given weight, the more drag there must be. The actual eL/D calculation involves multiplying the vehicle’s weight by its velocity and dividing the result by the power required. Again, the units cancel out to leave a dimensionless ratio. “The aerodynamic efficiency of conventional multi-rotors is terrible, so we held an aerodynamic clean- up brainstorming session,” Fredericks says. From that, the team selected three solutions. Advanced Aircraft Company HAMR UAV | Dossier Unmanned Systems Technology | December/January 2018 The fourth HAMR prototype in an early test flight without its signature passively pivoting aerodynamic fairings (Courtesy of AAC) Artist’s rendering of the HAMR with the boom fairings in place. The balls on sticks attached to the leading edges are mass balance weights for stability (Courtesy of AAC)