Article
An Empirical Study of Overlapping Rotor Interference
for a Small Unmanned Aircraft Propulsion System
Mantas Brazinskas *
,†
, Stephen D. Prior
†
and James P. Scanlan
Boldrewood Innovation Campus, University of Southampton, Southampton SO16 7QF, UK;
S.D.Prior@soton.ac.uk (S.D.P.); J.P.Scanlan@soton.ac.uk (J.P.S.)
* Correspondence: M.Brazinskas@soton.ac.uk; Tel.: +44-023-8059-1204
† These authors contributed equally to this work.
Academic Editor: David Anderson
Received: 25 July 2016; Accepted: 28 September 2016; Published: 10 October 2016
Abstract:
The majority of research into full-sized helicopter overlapping propulsion systems involves
co-axial setups (fully overlapped). Partially overlapping rotor setups (tandem, multirotor) have
received less attention, and empirical data produced over the years is limited. The increase in demand
for compact small unmanned aircraft has exposed the need for empirical investigations of overlapping
propulsion systems at a small scale (Reynolds Number < 250,000). Rotor-to-rotor interference at
the static state in various overlapping propulsion system configurations was empirically measured
using off the shelf T-Motor 16 inch
×
5.4 inch rotors. A purpose-built test rig was manufactured
allowing various overlapping rotor configurations to be tested. First, single rotor data was gathered,
then performance measurements were taken at different thrust and tip speeds on a range of overlap
configurations. The studies were conducted in a system torque balance mode. Overlapping rotor
performance was compared to an isolated dual rotor propulsion system revealing interference factors
which were compared to the momentum theory. Tests revealed that in the co-axial torque-balanced
propulsion system the upper rotor outperforms the lower rotor at axial separation ratios between 0.05
and 0.85. Additionally, in the same region, thrust sharing between the two rotors changed by 21%;
the upper rotor produced more thrust than the lower rotor at all times. Peak performance
was recorded as a 22% efficiency loss when the axial separation ratio was greater than 0.25.
The performance of a co-axial torque-balanced system reached a 27% efficiency loss when the axial
separation ratio was equal to 0.05. The co-axial system swirl recovery effect was recorded to have
a 4% efficiency gain in the axial separation ratio region between 0.05 and 0.85. The smallest efficiency
loss (3%) was recorded when the rotor separation ratio was between 0.95 and 1 (axial separation ratio
was kept at 0.05). Tests conducted at a rotor separation ratio of 0.85 showed that the efficiency loss
decreased when the axial separation ratio was greater than 0.25. The lower rotor outperformed the
upper rotor in the rotor separation ratio region from 0.95 to 1 (axial separation ratio was kept at 0.05)
at an overall system thrust of 8 N, and matched the upper rotor performance at the tested overall
thrust of 15 N.
Keywords:
SUAV; propulsion system; empirical; overlapping rotor; tandem; inter-rotor spacing;
axial separation; low Reynolds
1. Introduction
The interest in small multi-rotor unmanned aerial vehicles (SUAVs) is increasing. These platforms
are used in both the civilian and military sectors. Civilian applications are related to fire control, search
and rescue, agriculture, maintenance of structures, and media. The military uses these platforms for
ground control operations, border surveillance, crowd control and, in some cases, attack operations.
The Teal Group’s study estimates that: “the drone market will increase from the current worldwide
Aerospace 2016, 3, 32; doi:10.3390/aerospace3040032 www.mdpi.com/journal/aerospace
