10-04-2017, 07:44 PM
DESIGN, ANALYSIS, FABRICATION AND TESTING OF MINI PROPELLER FOR MAVS
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INTRODUCTION
National Aerospace Laboratories (NAL) is
having major program to develop Micro Air Vehicles
(MAVs). Thrust requirements of MAV s are
provided using different propulsive systems, namely
high pressure micro-jets or propellers. Commercially
available propellers manufacturer does not share the
design, aerodynamic and structural details with the
end users. Design of propeller for low Reynolds
number applications with proper matching of
propeller characteristics with that of small, light
weight electric motors with appreciable increase in
efficiency is very challenging work. Worldwide
miniaturization and continues improvement in
efficiency of electrical brushless motors are
addressed vigorously. Airfoil designed for higher
Reynolds numbers are highly inefficient at low
Reynolds numbers. Lower flight velocity combined
with the lower altitude and massive flow separation
makes it more inefficient.
METHODOLGY
Overall geometry of a propeller is limited by
the thrust requirement or the power availability and
rotational speed by its centrifugal stresses as well as
tip Mach number. A two bladed 6 inch diameter,
variable speed, fixed pitch propeller was designed.
Eppler-193 was chosen for most part of the span of
the propeller and NACA 66-021 was chosen near the
hub section to provide sufficient strength. Two blades
are chosen in comparison to three blades on slightly
higher efficiency and weight consideration thou one
can get lower thrust. The twist and chord distribution
shown in were arrived using minimum induced loss
method to give required thrust at design condition.
PROCEDURE
The baseline design was carried out using
the thrust requirement at design condition and the
blade geometry using minimum induced loss method
[5, 6]. Figure 1 show the propeller blade geometry
and Reynolds number at various sections. NACA 66-
021 profile was chosen up to 30% of the blade height
and Eppler-193 was chosen for rest of the span.
RESULTS AND DISCUSSION
The thrust measured at lower air velocities
are slightly higher than the computed as well as
calculated from BEMT method. However thrust
measured at very close to operating conditions of
MAV are in good agreement. In the absence of
validated drag polar at lower air velocities and
Reynolds numbers could have lead to calculation of
lower thrust. However the experimental results have
conclusively shown that the requirements of thrust at
take-off and cruise conditions were met. The
propulsive efficiencies are greater than 76% at the
design condition.