Analysis of a Propeller
Your virtual propeller design can be analyzed at off-design conditions, i.e.
at a different speed or a different velocity of rotation. The analysis is a so
called "Blade Element Method" and uses the same airfoil polars as the design
procedure. The influence of blade number and tip loss are take into account by
the "Prandtl Tip-Loss Factor". More details can be found in references 
and . It should be
noted, that the analysis does not work accurately for high thrust loadings as
they occur under static conditions.
JavaProp offers two ways to use the analysis procedure:
- First, it is possible to analyze the propeller for its full operating
range, from static to the beginning of the windmilling range. The results
of this "Multi Analysis" are presented a a table and a graph showing the
thrust and power coefficient depending on the advance ratio v/(nD).
The only detail about the flow conditions on the blade is the number of blade
sections where the airfoil has stalled, which is given as a "percentage
stalled" number. When 100% of the blade are stalled, all airfoils operate
beyond their maximum lift, which is usually the case al low speed. Due to the
definition of the
efficiency the efficiency at larger advance ratios may be higher than at
the design point; this does not mean the efficiency in the design point is not
maximized. The results also show the maximum possible efficiency for each
advance ratio, labeled eta*. This efficiency could only be reached
with a propeller without any frictional losses.
- A second way to use the analysis method is to perform an analysis for
one advance ratio only. This "Single Analysis" gives you more details for
the aerodynamic conditions along the radius. A plot shows the distribution of
lift and drag coefficient and a table lists all data of interest. These
include the additional local flow velocity induced by the propeller wake in
terms of the so called "interference factors". These factors are labeled a
and a', where a is the ratio of the additional axial
velocity to the onset flow velocity. A value of 0.05 for a means that
an aircraft propeller adds 5% of the flight velocity v to its jet.
The total axial flow velocity in the propeller plane will be (1+a)*v.
In a similar way, the factor a' defines the ratio of the additional
swirl component at the propeller in relation to the local circumferential
velocity Omega*r. The total circumferential velocity seen by the
airfoil section is (1-a')*Omega*r. Also, it is possible to calculate
the local flow direction immediately behind the propeller in terms of a swirl
As the previously given total velocities, this angle is again valid in the
propeller plane only or immediately behind the propeller. The
slipstream extends downstream from this point, which makes a point far behind
the propeller feel twice the induced velocity components (here the slipstream
extends downstream as well as upstream). So the additional axial velocity far
behind the propeller is 2*a*v and the additional rotational velocity
in the slipstream is 2*a'*Omega*r. I have named the resulting swirl
angle delta_ff (far field).
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© 1996-2006 Martin Hepperle
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