Geiger HDP 10  The Geiger HDP 10 is a German electric motor for powering electric aircraft, designed and produced by Geiger Engineering of Seigendorf, Hirschaid, Oberfranken. The HDP 10 is a brushless 58 volt design producing at 1950 rpm, with an outrunner coil. It has a 93% efficiency. The low working rpm of the engine means that it can turn a propeller at efficient speeds without the need for a reduction drive.