Blade Area Ratio:
Blade area in a Propeller Series is expressed by one of two
ratios: Expanded Area Ratio (EAR) or Developed Area Ratio (DAR). The
difference between EAR and DAR is usually very small, and is due to the way
blade area is measured in a propeller drawing. Both EAR and DAR are ratios
of total blade area divided by the propeller disc area. Since the
difference between EAR and DAR is usually small, a generic term called Blade
Area Ratio (BAR) is used in PSModel to refer to these area ratios.
The following example shows how to calculate the BAR for a 3
bladed, 12 inch diameter propeller with each blade face having a face area of 15
square inches:
Total blade area = 3 × 15.0 = 45.0 square inches
Propeller disc area = 3.142 × (R^{2}) = 3.142 × (6^{2})
= 113.11 square inches
EAR = 45.0/113.11 = 0.398
DAR is related to EAR via the following equation:
EAR = 0.34 × (2.75 + DAR/Z) × DAR
or
EAR = 0.935 × DAR + 0.34 × DAR^{2}/Z
where Z is the number of blades
An estimate of the total blade area for a small propeller can
be obtained as follows:
1) Place a sheet of graph paper on the side of a table
(the graph paper should have a minimum of 2 grid lines per inch horizontally
and vertically).
2) Place the leading edge of a propeller blade on the
graph paper with the propeller hub pressed against the side of the table.
3) Roll the blade from the leading edge to the trailing
edge, and as you roll, have a friend trace the blade outline with a pencil.
Alternatively, place the propeller on the table and lay the graph paper on
top of a blade and then trace the blade outline.
4) Count how many squares on the graph paper equal one
square inch, then count the number of squares within the blade outline just
traced.
5) blade area = (number of squares within the blade
outline) / ( number of squares that make one square inch on the graph
paper).
6) Total blade area = (blade area) × (number of blades)
