It's not clear what the terms in the equation denote.

Am I correct to establish from the Calculations spreadsheet and Burgers & Chiappe (1999) that:

Cl is the lift coefficient (taken as 1) f the flapping frequency Amp the amplitude of a flap U the ground speed of the running animal (?) S the wing area

It's not clear whether p corresponds to the "average factor that considers lift being generated only during the downstroke (0.5)", or whether it represents air density (ρ); presumably this is given as the constant 1.23 in the spreadsheet.