Linda Carlton's Perilous Summer

This is the fourth book in the now public domain Linda Carlton aviatrix series (and my personal favorite). Linda Carlton and her Aunt Emily are vacationing in Michigan when Linda and her friend Dot Crowley, while flying around in Linda's autogiro, witness an accident. They see a young girl struck by a hit-and-run automobile and immediately land the autogiro to investigate. The girl, who they discover has amnesia, goes back to Linda's home to stay with her and her aunt while they attempt to locate her family. A strange woman comes to claim Emily and before everything is straightened out at the end, Linda and Dot and the lost girl are in many adventures, all making this Linda's perilous summer.