Citation: Giljarhus, K.E.T.;
Porcarelli, A.; Apeland, J.
Investigation of Rotor Efficiency with
Varying Rotor Pitch Angle
for a Coaxial Drone. Drones 2022, 6,
91. https://doi.org/10.3390/
drones6040091
Academic Editors: Andrzej
Łukaszewicz, Wojciech Giernacki,
Zbigniew Kulesza, Jaroslaw Pytka
and Andriy Holovatyy
Received: 28 February 2022
Accepted: 30 March 2022
Published: 4 April 2022
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Article
Investigation of Rotor Efficiency with Varying Rotor Pitch
Angle for a Coaxial Drone
Knut Erik Teigen Giljarhus
1,
* , Alessandro Porcarelli
2
and Jørgen Apeland
3
1
Department of Mechanical and Structural Engineering and Materials Science, University of Stavanger,
PB 8600, 4036 Stavanger, Norway
2
Neptech AB, Enskededalen, 121 34 Stockholm, Sweden; alessandro@neptech.se
3
Nordic Unmanned AS, Rådhusgata 3, 4306 Sandnes, Norway; ja@nordicunmanned.com
* Correspondence: knut.e.giljarhus@uis.no
Abstract:
Coaxial rotor systems are appealing for multirotor drones, as they increase thrust without
increasing the vehicle’s footprint. However, the thrust of a coaxial rotor system is reduced compared
to having the rotors in line. It is of interest to increase the efficiency of coaxial systems, both to extend
mission time and to enable new mission capabilities. While some parameters of a coaxial system
have been explored, such as the rotor-to-rotor distance, the influence of rotor pitch is less understood.
This work investigates how adjusting the pitch of the lower rotor relative to that of the upper one
impacts the overall efficiency of the system. A methodology based on blade element momentum
theory is extended to coaxial rotor systems, and in addition blade-resolved simulations using compu-
tational fluid dynamics are performed. A coaxial rotor system for a medium-sized drone with a rotor
diameter of 71.12 cm is used for the study. Experiments are performed using a thrust stand to validate
the methods. The results show that there exists a peak in total rotor efficiency (thrust-to-power
ratio), and that the efficiency can be increased by 2% to 5% by increasing the pitch of the lower rotor.
The work contributes to furthering our understanding of coaxial rotor systems, and the results can
potentially lead to more efficient drones with increased mission time.
Keywords:
computational fluid dynamics; blade element momentum theory; coaxial rotor;
aerodynamics
1. Introduction
Several designs for unmanned aerial vehicles exist today, with varying compromises
in terms of size, fuel system and speed [
1
]. Multirotor drones have the advantage of small
footprint, non-restrictive take-off/landing area requirements, high maneuverability and
high hovering capability. Multirotor drone designs can be found in sizes ranging from light
vehicles below 50 g [2,3] to larger vehicles designed for human transport [4,5].
The power unit on a multirotor drone is important, as it determines the type of op-
erations a drone can perform. The most common power source on multirotor drones is
batteries, although some designs exist that employ, e.g., hydrogen [
6
,
7
]. Increasing the effi-
ciency of the drones is important as it can extend the flight time, thereby extending mission
time or even enabling new types of mission capabilities.
The projected area of a drone can be a critical factor, as it determines how the drone
can be transported and the type of areas it can fly in. In a coaxial rotor system, one rotor
is put above the other instead of placing them side by side. Compared to larger aircrafts,
coaxial multirotor drones typically use fixed-pitch rotors to avoid complex mechanical
design [
8
]. The coaxial layout increases the thrust without increasing the projected area,
making this an attractive option for multirotor drones.
However, a disadvantage of a coaxial rotor system is that there is a loss of efficiency,
since the lower rotors operate in the wake of the upper rotors. Some attempts have
Drones 2022, 6, 91. https://doi.org/10.3390/drones6040091 https://www.mdpi.com/journal/drones
