Noting that silent UAVs are more useful than noisy one, an Israeli firm has developed a hybrid motor for small (under a hundred pounds) and medium size (up to half a ton) UAVs. The gasoline powered portion of the engine can spin the propeller, while also charging the batteries. When over an area where the enemy might hear the UAV, the operator can switch to battery power. Especially at night, loud motors on UAVs often spoil the element of surprise, by letting the enemy below know that there's a UAV watching. Until now, the only battery powered UAVs were the micro (under ten pound) variety. The larger UAVs fly high enough (over 20,000 feet), and have powerful enough sensors to see clearly what's down there, to avoid this problem. But the smaller UAVs cannot carry those high powered sensors, and have to fly lower to get a good view. Alas, at a few thousand feet up, that internal combustion engine can be heard clearly down below.