Anybody remember what a "prayer wheel" was?

I didn't recognise this phrase, at first, but soon worked it out from
context. In the 80s in the UK we knew it as a John Willy - after a well
known coach, John Williamson, who marketed a range of such devices,
calibrated for all sorts of performance of glider.

I think I still have one in my Ventus, but haven't used it in decades as
(ironically, considering today's electronic wizardry) I can monitor the
situation mentally quite well enough.

(I don't know whether "John Willy" has the same alternative connotation
in the US as in the UK?)

James.