Your picture shows a wing rib.
It creates the wing 'section' that determine the ratio of lift and drag generated as it travels through the air.
The classic Clark Y wing section does have a flat underside over most (2/3) of the underside and has a thickness to chord ratio of 11.7 so is
much thinner than the section you as showing.
View attachment 119670
A true flat bottom section is a bit less efficient and can have odd characteristics at the stall than one with a proper rounded leading edge.
In general at model sizes a thicker the section does give more lift but the drag it creates rises faster than the increase in lift so you need more power.
I hope this helps.