On May 24, 2:59*am, wrote:
On May 23, 7:52 am, Bertie the Bunyip wrote:

wrote i
* * * So it's a process to cause the air to exert a force in a
direction perpendicular to the airflow. It fools the air, if you like,
which is why we call it an "air foil." A foil is a device to deceive.

Actually it's a synonym for blade.

Not quite. From the Merriam-Webster Dictionary:

* * Foil:
1 obsolete : trample
2 a: to prevent from attaining an end : defeat always able to foil
her enemies b: to bring to naught : thwart (foiled the plot)
synonyms: see frustrate.

* * * So my "deceive" is much less accurate than "frustrate."

* * * *Your definiton matches one of the the Cambridge Dictionary's
definitions:

If you look a bit further in the MW disctionary you will see both
blade and keaf. But the key is to use a proper dictionary like the
OED. Your definition of foil is a verb, not a noun and an aerofoil or
air foil is a noun.

Cheers