HOBBYWING XRotor 3115 900KV

The HOBBYWING XRotor 3115 900KV is the larger version of the 31 mm series, for 9 to 10-inch FPV builds and light cinelifters on 4 to 6S. Hobbywing is a Chinese maker of drives and speed controllers. The case measures 37.2 mm in diameter and 32 mm in height, and the mass is 125 g. With 9 to 10-inch propellers the motor sits at the upper limit of what still counts as an FPV drive.

The KV value of 900 corresponds to roughly 20,000 rpm unloaded on 6S (22.2 V nominal). Recommended are HQ9x4x3 up to HQ10x5x3, three-blade propellers of 9 to 10 inches diameter. As peak values Hobbywing quotes 1,940 W and 81 A for 9 s, and the maximum thrust is 5,177 g. The type code 3115 stands for 31 mm diameter and 15 mm height; a taller stator at the same diameter brings torque and thermal reserve.

The 9 s time basis means that the motor tolerates this peak briefly, for instance when climbing with a full payload. Thrust only means something with a propeller and a voltage; the figure of 5,177 g is to be read as an order of magnitude for the maximum thrust, not as a comparison figure. In cinelifter builds the battery counts: a 6S pack of 8 Ah holds 177.6 Wh and so exceeds the 160 Wh limit that must not be exceeded on a passenger aircraft; between 100 and 160 Wh the airline's approval is needed. The EU class of the drone depends on its take-off mass, not on the motor.

When buying second-hand, the load history counts: a motor that has carried heavy cameras has run its bearings and winding under high load for longer. The bell should turn evenly and quietly by hand, the shaft without wobble, the magnets firmly seated. Discoloured winding varnish or a smell of heat indicates overload. The controller needs headroom above 81 A.