*pel(e)-, *pleu

The original idea of this family was:

flowing, gliding

And it could be a liquid as well as a flowing motion in the air.
Here’s an (incomplete) list of English members of the family. You’ll see that most of them are about a “continuous motion” in or of liquid or in air.

• flow
• float
• flood
• fleet
• fly
• flee
• flight
• pulmonary (Originally: the body part that floats)
• pneumonia