Available performance data of selected open, contra-rotating and
ducted propulsors operating in a uniform onset field are assembled to display
the maximum efficiency as a function of thrust-loading coefficient,
C
_T
=
T/(½ρV
^2
R
_{R^2}
). Satisfactory
correlation is presented between this observed best efficiency and estimates
from a numerical method based on an inviscid lifting-line representation of the
rotor, or other blade row, plus, when appropriate, an axi-symmetric
panel-method analysis of the duct and hub surfaces. Viscous drag is added
empirically as a local blade-section friction coefficient. For the
open-propeller series and various sets of conventional accelerating ducted
propulsors, the maximum efficiency of the ducted propulsors is greater by
several percent at a C
_T
= 1 and by about 5 percent
(straight difference in values) at a C
_T
approaching 10. The
contra-rotating configuration displays a greater increase in the observed
maximum efficiency at reduced C
_T
values compared to the
accelerating ducted propulsor but at C
_T
> 2 the
accelerating ducted units develop greater efficiency. Only at reduced thrust
levels is there an indicated beneficial interaction between the blade rows of a
contra-rotating propulsor compared to a simple thrust division without
interactions.
