It measures air speed—not ground speed, but the speed of the airplane at altitude.
He crafted an outline of an airplane and filled it with water so it darkened like a shadow.
There is a large area to be scoured and the amount of time the airplane can spend over its assigned zone is critical.
1907, from air (n.1) + plane (n.1); though the original references are British, the word caught on in American English, where it largely superseded earlier aeroplane (1873 in this sense and still common in British English). Aircraft "airplane" also is from 1907. Lord Byron, speculatively, used air-vessel (1822).
A tweezerlike clip for holding a marijuana cigarette stub; roach clip