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2017 Asia-Pacific International Symposium on Aerospace Technology, Seoul, Korea
* Presenting and Corresponding Author: Research Engineer, f.finger@fh-aachen.de, Student Member AIAA
1
A Review of Configuration Design for Distributed Propulsion
Transitioning VTOL Aircraft
D. Felix Finger 1*, Carsten Braun1, and Cees Bil 2
1 Department of Aerospace Engineering, FH Aachen UAS, Aachen, Germany
2 School of Engineering, RMIT University, Melbourne, Australia
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Abstract
One of the biggest challenges in aviation is the design of transitioning vertical take-off and landing (VTOL) aircraft. Thrust-borne
flight implies a higher mass fraction of the propulsion system, as well as much increased energy consumption in the take-off and landing
phases. A good VTOL design will offset this disadvantage by transitioning to conventional forward flight, thus travelling at much higher
efficiency than a comparable rotorcraft, for an overall improvement in mission performance.
This paper intents to support the configuration designer of VTOL aircraft by giving a review of some of the available configuration
possibilities, considering the latest advancements in technology. While VTOL aircraft can use the conventional wing-fuselage-stabilizer
configuration, much of new development efforts involve unconventional planforms. The advent of distributed propulsion and electric- or
hybrid-electric propulsion systems offers additional opportunities to optimize the vehicle layout and improve flight performance. This
review considers propeller driven designs, lift fans and ducted fans, as well as jet lift and hybrid configurations that use a mix of propulsion
methods.
Keywords: Aircraft Configuration Design, VTOL, Transition, Distributed Propulsion
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Nomenclature
b = back
C = cruise
CTOL = conventional take-off and landing
f = front
FOD = foreign object damage
g = gravitational acceleration
ISA = international standard atmosphere
l = left
L = lift
L/D = lift-to-drag ratio
M = figure of merit
M = Torque
MSL = mean sea-level
P = power
r = right
S = area
STOL = short take-off and landing
T = thrust
T/W = thrust-to-weight ratio
UAV = unmanned aerial vehicle
v = velocity
VTOL = vertical take-off and landing
w0 = design weight
ρ = density of air
ω = rotational speed
1. Introduction
VTOL transitioning aircraft combine a helicopter’s
ability to take-off and land almost anywhere, with the
speed, range, endurance and load carrying capability of a
fixed wing aircraft. Some sources call these vehicles
‘convertiplanes’ or ‘hybrid aircraft’ and many different
configurations are used [1].
Because a transitioning VTOL aircraft must perform
satisfactory in both flight regimes, its design consists of
trade-offs and compromises [2].
This paper presents a review of some of the possibilities
available to the configuration designer, for both manned
and unmanned aircraft. Concerning size or intended
mission, this study covers all classes of aircraft. However,
some concepts will be more suitable for small UAV
applications, while others may be preferable for large
transport applications. The focus is on aircraft capable of
transitioning to forward flight, and a broad discussion of
helicopter design will not be provided. This special field is
covered comprehensively in literature (e.g. [3], [4], [5]).
Nevertheless, the fundamentals on the layout of helicopters
and multicopters will be treated, to provide the reader with
a complete overview. Just recently, the fusion between a
fixed-wing aircraft and a multicopter system has become
very popular, especially for unmanned aircraft. Therefore,
a discussion of the design of multicopters, a topic rarely
covered in classical aircraft design literature, is included in
section 5. This way, the configuration designer will find the
most relevant information in one place.
The highly specialized topics of compound helicopters,
autogyros and rotodynes are not covered.
There are many missions where a VTOL aircraft is
superior to a CTOL solution. Without the need for a runway
or other infrastructure, they can attain a mobility that is
2017 Asia-Pacific International Symposium on Aerospace Technology, Seoul, Korea
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superior to conventional take-off and landing (CTOL)
vehicles.
Nevertheless, VTOL aircraft also have disadvantages:
They are mechanically complex and difficult to control in
hover and transition. Furthermore, combining VTOL with
conventional forward flight is weight critical and raises
costs significantly.
Technical progress in lightweight composite
construction, load and stress optimization methods,
computerized flight controls and high performance
propulsion systems, have introduced new VTOL concepts,
particularly for unmanned systems [2], and it is useful to
review or revisit some of these concepts.
This paper discusses a number of past, present and future
VTOL concepts with their specific features, focusing on
designs that can transition to conventional forward flight or
otherwise can achieve speeds comparable to CTOL aircraft.
2. Hover Efficiency and Downwash
Hover efficiency (w
0
/P in kg/kW) relates the aircraft
weight with the power required to lift it in vertical take-off
and hovering flight. This efficiency is linked to the rotor
disk loading (kg/m²), i.e. thrust force T (N) dived by both
the gravitational acceleration g and the rotor disk area S
Disk
.
The relationship is shown in Fig. 1, based on the
assumptions of the momentum theory [6].
As Eq. (1) shows, hover lift efficiency is inversely
proportional to the square root of the disk loading.
Therefore, the easiest way to increase hover efficiency is to
increase the disk area.
∙2∙∙
/
∙
(1)
Besides reduction of disk area, negative influences on the
hover efficiency are operation at high altitudes (decreasing
air density ρ) and M, the figure of merit that indicates how
efficient the rotor system is, compared to the theoretical
optimum.
Eq. (2) allows to calculate the flow velocity induced by
the rotor, assuming a uniform lift distribution across the
disk.
2∙∙
(2)
As the column of air is forced down below the rotor, it
constricts. For hover, the final slipstream velocity can be
shown to be 1.5 times the calculated amount (Eq. (3)) [4].
3
2∙
2∙∙
(3)
At high disk loadings, a small volume of air is moved
downwards very quickly, yielding a low lift efficiency. The
high downwash velocities necessary at high disk loadings
can also cause soil erosion and high noise levels. Jets can
still cause considerable damage, especially because of the
high temperature exhaust gases. If jet propulsion is used for
direct lift, (e.g. F-35) the disk loading typically exceeds
5000 kg/m² and the slipstream velocity approaches the
speed of sound.
In general, the designer should strive for low disk
loadings, to minimize power requirements. However, there
are several drawbacks: Rotor weight becomes prohibitively
high, as does overall size, and low disk loadings increase
the gust sensitivity. Therefore, the performance balance
between forward flight performance and hover efficiency
must be assessed very carefully.
Typically, for smaller VTOL vehicles a lower disc
loading can be achieved, as from a scaling perspective, it is
easier to manufacture a relatively large rotor for a smaller
rotorcraft.
Distributing the required thrust over a greater number of
disks allows to keep the rotors small, while maintaining a
low disk loading. Such a distributed propulsion design is
particularly efficient if electric propulsion is used, as
described in [7]. However, Reynolds number effects have
to be taken into account, to establish the smallest suitable
rotor size.
3. Thrust-to-weight ratio
For vertical flight, regardless of the configuration, the
thrust-to-weight ratio (T/W) is T/W > 1. According to [8],
a 5% margin is needed for acceptable heave control. This
margin is increased to 20 and 50% for thrust vectoring, the
suck-down effect and hot gas ingestion effects. The effects
are explained below:
0
40
80
120
160
200
240
280
320
0
2
4
6
8
10
12
14
16
10 100 1000 10000
Flow Velocity [m/s]
Hover Efficiency w0/P [kg/kW]
Disk Loading T/(g·Sdisk )[kg/m²]
Hover Efficiency M=1,00
Hover Efficiency M=0,75
Induced Velocity
Slipstream Velocity
Fig. 1. Hover efficiency
2017 Asia-Pacific International Symposium on Aerospace Technology, Seoul, Korea
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A reaction control system (RCS) must be installed, as the
lack of dynamic pressure makes aerodynamic controls
ineffective. The RCS can operate on any form of thrust
vectoring, can include additional propellers, rely on bleed
air for jet propulsion systems, or be of hybrid nature (e.g.
electric fans and thrust vectoring as described by the author
in [9]). Thrust vectoring can be achieved by placing
aerodynamic controls in the slipstream of a rotor, but the
corresponding drag has to be correctly accounted for.
For all VTOL vehicles, the suck-down effect, depicted in
Fig. 2, cannot be neglected. The freestream effect of a
propeller or a jet causes the air mass around the aircraft to
be accelerated due to viscosity. This creates a downward
flowfield both in, and out of ground effect. In ground effect,
however, the improved mixing of the jet exhaust increases
the effect greatly. Raymer gives the magnitude of this
“vertical drag” as 2-6 % of T/W in free air and up to 30 %
of T/W in ground effect [8].
If rotors are used to provide the necessary lift, their
placement has to be carefully considered. If one considers
the tilt rotor configuration as an example, the wings placed
below the rotors impinging wake produces a download,
reducing the net lift. The US Army Material Command’s
Helicopter Design Handbook [3] gives Eq. (4) to
approximate this effect.
1
2∗
∗
(4)
The typical, “average” effect is impossible to assess,
however a reduction up to 30% of the effective lift is
certainly possible.
VTOL vehicles using jets as lift engines face an
additional problem, illustrated in Fig. 3. Because the hot
exhaust gases always find a way back into the inlet, thrust
is considerably reduced as the compressor inlet temperature
rises. Raymer states: “It takes about 30 s in hover for the air
around the [Hawker] Harrier to heat up by 5°C. This 5°C
increase in air temperature entering the inlet reduces the
engine thrust by about 5%.” [10, p. 767]
For all the above reasons, the overall installed T/W for
VTOL aircraft – both jet and rotor propelled – ranges
between 1.2 and 1.5. Nicolai and Carichner [11] suggest to
use a T/W of 1.2 as a rule of thumb, while Raymer suggests
a T/W of 1.3. If the aircraft is required to hover at gross
weight in hot and high conditions at increased density
altitudes, the nominal T/W (usually specified for ISA MSL
conditions) will increase further.
If control of the vehicle in hover is supposed to be
maintained by only varying the rotor speed, like for a
multicopter, the thrust requirements increase further. For
heavy lift multicopters, a T/W of less than 1.6 is not
recommended and results in a less responsive aircraft. For
better control in gusting conditions, a T/W of 2.0 is more
reasonable for these configurations.
A fail-safe operation of any X-planform multicopter (e.g.
quad- or octocopter) will need a T/W greater than 2.0. In
general, VTOL aircraft face a great problem when it comes
to the probability of engine failure. The aircraft must not
only be capable of sustaining sufficient vertical thrust, but
also balance this thrust about the center of gravity. To
achieve redundancy, a massive surplus of power and/or a
complex driveshaft and gearbox system must be used.
For UAV operations this requirement is often dropped,
especially for small systems. If an engine fails in hover, the
UAV is lost. The benefit is a reduction in aircraft size and
cost, at the price of reduced reliability.
4. Transitioning VTOL Configurations
The simplest method to enable VTOL capability for any
aircraft is to add lift engines to the airframe (Fig. 4a). This
takes up internal volume, requires some effort to avoid
excess drag in cruise, and causes a considerable weight
increase. However, the big advantage lies in the possibility
to size the main propulsion system for efficient cruise or
loiter – thus reducing the fuel fraction for that part of the
flight. Consequently, also the lift engines can be designed
for a single high power operating point.
Another extreme method to enable VTOL capability for
aircraft is to use the same propulsion system for both cruise
Fig. 3. Recirculation and Hot-Gas Ingestion
(adapted from [34])
Fig. 2. Free air entrainment (left) and ground effect (right)
(adapted from [8])
2017 Asia-Pacific International Symposium on Aerospace Technology, Seoul, Korea
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and hover (Fig. 4b). This eliminates the need for an
additional propulsion system, which would only represent
dead weight during the cruise or loiter part of the flight.
Unfortunately, the powerful engines cause low efficiency
and high fuel consumption during forward flight.
Additionally, UAVs are rarely used on missions requiring
very high speeds or very high climb rates, so the excess
power cannot be coined into an advantage.
A middle ground is represented in Fig. 4c. The cruise
engines are used for both forward flight and hover, but
supplemented by some sort of dedicated powered lift
system. This is usually considered the best way to achieve
VTOL for any aircraft – unmanned or manned. This way,
the massive weight and volume increase for a dedicated
propulsion system is minimized, and the efficiency loss of
the cruise propulsion system is kept in reasonable bounds.
For best performance, the ratio of lift between the lift
engine(s) and the cruise engine(s) must be carefully
investigated and optimized. This optimization process
depends on the given design mission and is outside the
scope of this paper.
In the following paragraphs, the most relevant VTOL
configurations are briefly presented. Below each subsection,
a table lists example aircraft for the corresponding
configurations.
4.1 Lift + Cruise
All concepts in this subchapter are discussed under the
premise that exclusive propulsion systems are installed for
hover and flight.
4.1.1 Multicopter + Conventional Airframe
Fusing a multicopter (see section 5) with a conventionally
laid out aircraft is the easiest way to give a fixed-wing
aircraft VTOL capabilities. Each system is decoupled and
used in its most effective state. Using the multicopter
system the aircraft ascends to obstacle height and then uses
the regular propulsion system to accelerate and sustain
wing-borne flight. Of course, the landing procedure is
reversed.
Usually, the chosen multicopter layout is symmetric
about the aircraft’s longitudinal axis (e.g. a quad-, hexa- or
octo-configuration). The rotors can then be mounted inline
for minimal drag impact.
Fig. 5. Octocopter/conventional pusher
Hybrid propulsion options (meaning electric lift motors
and combustion engines for endurance flight) are used
frequently and minimize the weight impact [12]. Because
electric motors have a vastly higher power-to-weight ratio
than internal combustion engines (about 5 kW/kg vs.
1 kW/kg), they are highly suited for this application.
However, electric hover propulsion is only a viable option,
if the time spent in hover is kept to an absolute minimum.
Otherwise, the weight savings of the electric motors will be
completely negated by a heavy battery system. Another
benefit of using electric motors for short durations, is their
ability to operate in overload conditions for a short time.
While this heats up the motors significantly, this can be of
benefit in case of an engine failure. This is not possible for
a traditional combustion engine.
Multicopter systems can be used as an ‘add-on’, allowing
the airframe to be converted between the VTOL and CTOL
configuration with respect to mission requirements.
Another possibility is to design the multicopter system to
be an integral part of the airframe, to minimize the impact
on structural weight.
Fig. 4. Methods for VTOL
2017 Asia-Pacific International Symposium on Aerospace Technology, Seoul, Korea
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Table 1. Example Aircraft
Multicopter + Conventional Airframe
ALTI Transition (UAV)
Arcturus Jump 20 (UAV)
GerMap G170-V Quadplane (UAV)
Latitude HQ-Series (UAV)
SkyPro M6 VTOL QuadPlane (UAV)
4.1.2 Jets + Conventional Airframe
At least for small UAV applications, the modular ‘add-
on’ setup can be applied as well. The small form factor of
turbojets with radial compressors may allow the vertical
installation of such small jet engines. This uses very little
internal space, but requires careful inlet design. Since small
jets for UAV applications usually do not allow the offtake
of bleed-air for a reaction control system it can be necessary
to establish control authority by placing ducted electric fans
in suitable locations. If the VTOL capability is unnecessary
for a certain mission, the jet engines might be removed and
replaced with fuel tanks for extended range and endurance.
Specialized jet engines for hover can be used to improve
performance for a jet Lift + Cruise aircraft. For jet engines,
the average T/W ratio is about 6-8, depending on their size.
If the multitude of operation points necessary for “regular”
jet engines can be reduced to only vertical take-off and
landing conditions, it is possible to develop lightweight
engines of higher thrust. For example, the Russian designed
Kolesov RD-38 (first run 1964 as RD-36) lift jet used by
the Yakovlev Yak-38 (Lift + Lift/Cruise design, see section
4.3.3) and Rolls-Royce’s designated VTOL engine RB162
(first run 1962) both achieve a T/W of about 18. Kohlman
estimates that a T/W of up to 25 is possible for future lift
jets employing modern technology [13]. This will
necessitate the employment of composites in the
compressor section and the casing, as well as accepting a
marked deterioration of time between overhaul compared to
conventionally designed jet engines.
Fig. 6. Jet L+C
If hover engines are positioned about the center of gravity
like in Fig. 6, they can increase the required fuselage
volume in this area significantly. This can lead to a severe
increase in wave drag.
For manned aircraft, the Lift + Cruise approach was used
successfully for the supersonic French Mirage III V.
However, the eight lift engines imposed a severe range and
payload penalty on the aircraft and it was never able to take
off vertically and successfully attain supersonic flight
during the same flight. (This feat was only accomplished by
the X-35B in 2001 [14].)
Table 2. Example Aircraft - Jets + Conventional Airframe
Dassault Mirgae III-V
Short SC-1
4.2 Lift = Cruise
All concepts are discussed under the premise that the
propulsion system is sized by hover requirements and also
used for cruise flight.
4.2.1 Tiltrotor
A tiltrotor (and for the sake of simplicity this title shall
also cover tilt-nacelle and tilt-duct concepts) uses the same
motors for both vertical and horizontal flight. It can rotate
its propellers, rotors, ducts or nacelles in such a way, that
the thrust vector can support the aircraft’s weight in vertical
flight, and provides forward thrust during level flight. In
some cases, the entire nacelle with the engine is tilted, while
other tiltrotors use drive shafts and gearboxes to transfer
power to swiveling rotors. A mechanism that synchronizes
the movement is necessary to avoid loss of control. The
wings and fuselage stays level during the transition
maneuver. This allows any attitude sensitive payload (e.g.
optical sensors or humans) to operate in all parts of the
flight regime.
Fig. 7. Tiltrotor
Because wings placed below the rotors’ impinging wake
produce a download reducing the net lift (see also section 3,
Eq. (4)), the rotors are usually positioned at the wing tips,
to minimize exposure. The wings can be small and sized to
cruise requirements, because low speed flight is achieved in
the hovering configuration. To reduce the download to a
minimum, tiltrotors are usually equipped with full span
flaps, which will deflect to a 90° down position during
2017 Asia-Pacific International Symposium on Aerospace Technology, Seoul, Korea
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hover. This way, the flapped wing area is not producing any
vertical drag.
Conventional take-off is usually impossible for propeller
driven aircraft, because the size of the rotors prevents a
forward tilt on the ground. This, just like the hover
download, can be avoided if a jet propulsion system is
used– at the expense of reduced hover efficiency.
Ducted fans can be an interesting alternative to the pure
rotor, as a duct can increase propeller thrust and can provide
additional lift during forward flight. However, these
benefits can be offset by the additional weight and friction
drag.
Compared to a Lift + Cruise configuration, tiltrotors offer
the benefit of reduced weight and improved aerodynamic
efficiency, because a combined propulsion system can be
packed tighter. However, during transition the control loops
are coupled, increasing complexity, and powerful actuators are
necessary to overcome the gyroscopic moments of the large,
fast-spinning rotors.
Tiltrotors usually use wing-mounted propellers. When a
tandem-wing or lifting-canard layout is used, this allows
multicopter-like control in hover – provided the T/W is
high-, and the vehicle’s inertia is low enough.
Fig. 8. Tandem-wing tiltrotor/tiltduct
Table 3. Example Aircraft - Tilt Configurations
Tilt Rotor
AgustaWestland AW609
Bell XV-15
Bell XV-3
Bell-Boeing V22 Osprey
Bell Eagle Eye (UAV)
KATI Smart (UAV)
Tilt Prop
Curtiss Wright X-19
Curtiss-Wright X-100
Tilt Duct/Nacelle
Bell 65 ATV
Bell X-22
Doak 16 VZ-4
Nord 500 Cadet
American Dynamics AD-150 (UAV)
Compared to other concepts, a disadvantage of the
tiltrotor is the need to mount (at least) two large rotors side-
by-side at the tips of the wings. This dictates a minimum
wingspan and gives a larger footprint, especially when
compared to helicopters.
Because tiltrotor aircraft usually use the same propulsion
system for hover and cruise, the engines are oversized for
cruise and loiter conditions, leading to an increase in fuel
consumption.
4.2.2 Tiltwing
A tiltwing aircraft is quite similar to the tiltrotor, as it is
able to rotate the thrust vector of its main engines. However,
not only the nacelle and rotor are swiveled, but the entire
aircraft’s wing, with the engines fixed relative to the chord.
This has some advantages: A single rotation mechanism is
simpler, therefore more reliable and should be lighter [10].
There is also little blockage effect from the wing.
However, during transition from hover to forward flight,
the wing reaches very high angles of attack and will stall
[15]. This can be controlled by submerging the entire wing
in propwash and running the engine at a high power setting.
This is acceptable for take-off, but during approach, with
the intention to slow down, this is undesirable. Additionally,
the tilted wing offers its entire surface to the wind, causing
degraded controllability during hover in gusting conditions.
As explained for the tiltrotor, if at least four rotors are
used, control in hover might be accomplished similar to a
multicopter. However, most of the time, the wings’ control
surfaces are submerged in the propwash and thus can exert
control forces.
For propellers without swashplate actuation an additional
method to control the pitching motion during hover is
required. This can be either a reaction control system or an
additional rotor, typically mounted in the tail, as shown on
the LTV XC-142.
To cover the complete span with propwash, multiple
smaller propellers can be used instead of few large rotors as
for tiltrotors. This allows conventional take-off and landing
operations are possible due to the reduced propeller
diameter.
Fig. 9. Distributed propulsion tiltwing (with tilting
horizontal stabilizer)
2017 Asia-Pacific International Symposium on Aerospace Technology, Seoul, Korea
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The concept can be extended to the concept of distributed
propulsion. NASA’s GL-10 ‘Greased Lightning’ UAV
(now licensed by the ‘Advanced Aircraft Company’) is the
perfect example. A total of ten propellers provide
propulsion to the airframe. “The GL-10 concept takes the
well proven tilt-wing aircraft configuration and combines
distributed electric propulsion in a concept that provides the
new capability of extremely efficient aircraft operations
fully exploiting salient features of both.” [16, p. 2]
Aurora Flight Sciences’ XV-24A ‘LightningStrike’
carries this concept to the extreme. The canard
configuration employs a distributed embedded electric fan
system that consists of 24 separate motors. Power is
provided by a turboshaft engine driving triple redundant
generators [17]. The concept of the German startup Lilium
Aviation is similar: it uses 36 embedded electric fans that
are attached to the wing’s and canard’s high lift system.
Their aircraft is called the ‘Lilium Jet’ even though it is
propelled exclusively by electric motors and batteries.
A hybrid between the tiltrotor and tiltwing is possible.
Tilting nacelles are attached to a fixed central wing. An
outboard wing can be attached to the nacelle and tilt with it.
This design combines the structural benefits of the fixed
central wing, where the largest forces and moments are
introduced in the fuselage, with the aerodynamic advantage
of the reduced hover download of the tiltwing configuration.
This way, a higher aspect ratio wing can be used, with the
corresponding induced drag benefit. It must be said,
however, that the lift distribution will be far from the ideal
ellipse, due to the negative impact of the nacelle.
As described in [18], such a wing can be easily attached
or disassembled from the nacelle, allowing to optimize the
airframe depending on different mission requirements.
Fig. 10. Tiltrotor/tiltwing hybrid
Another hybrid concept is possible if a more
unconventional configuration is chosen: A fixed tandem
wing layout can be supplemented with a tilting stub-wing
to which propellers are attached [19]. If carried to the
extreme, this might be considered a three-surface layout.
Depending on the design philosophy, this layout also lends
itself to box-wing configurations.
Fig. 11. Boxwing- and three-surface tiltrotor
Table 4. Example Aircraft - Tiltwing and Hybrid
Canadair CL-84
Elytron 2S
Hiller X-18
Karem Aircraft AeroTrain/JHL
Ling-Temco-Vought XC-142
Vertol VZ-2
Acuity AT-10 (UAV)
Aurora Flight Sciences XV-24A (UAV)
NASA/Advanced Aircraft Company GL-10 (UAV)
4.2.3 Flow Deflection and Thrust Vectoring
Flow deflection and thrust vectoring are related.
Typically, thrust vectoring happens directly at the engines
exhaust nozzle, while a flow diversion involves a movable
blocker device that redirects the flow through ducting to a
different outlet. It is therefore applicable to aircraft
propelled by ducted fans or jets.
The Hawker Harrier’s Pegasus turbofan engine allows the
vectoring of fan- and core-airflow through swiveling
nozzles. However, this causes the hot air to be ejected along
the fuselage, where the Coandă effect, keeps it attached.
This causes thermal fatigue, which is aggravated by
acoustic loads. Additionally, it requires the engine to be
positioned very far forward, leading to a poor volume
distribution for transonic flight [8]. These issues can be
mitigated, if the concept is applied to multi-engine aircraft,
where the engines are placed in pods on the wings [20].
Then, a much simpler integration is possible because the
aircraft’s tail is not in the way of the exhaust gases.
Fig. 12. Fan and core vectoring
Lockheed’s “Reverse Installation Vectored Engine
Thrust” (RIVET) configuration is a very interesting ‘twist’
on this problem. To quote from the patent: “The main
2017 Asia-Pacific International Symposium on Aerospace Technology, Seoul, Korea
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propulsion engine which will provide both vertical and
horizontal propulsion forces is mounted in the airframe in a
reversed position aft of the center of gravity, with the
engine front face facing to the rear and the exhaust nozzle
assembly located at the center of gravity. The nozzle
assembly is comprised of a pair of exhaust nozzles, one
mounted on each side of the airframe, in the form of 90
degree "elbows", which can be rotated from a vertically
downward facing position to a horizontally rearward facing
direction.” [21]
Fig. 13. . RIVET Concept [22]
This concept allows significant weight savings over a
conventional engine installation, and allows the installation
of any standard turbofan engine. An afterburner can be
integrated in the rotating nozzle, but it must not be used in
hover, since the ground erosion is excessive. The weight
savings, coupled with the corresponding sizing benefits of
this configuration, result in a lighter aircraft.
The deflected slipstream concept was applied to propeller
aircraft as well. However, it was only of very limited
success. The US Army explored the concept with the Ryan
VZ-3 and Fairchild VZ-5 research aircraft. The flight-
testing revealed that the “deflected slipstream concept
proved to be better suited to STOL than VTOL operation.”
[23]
Table 5. Example Aircraft - Flow Division and Vectoring
Vectored Thrust
Boeing X-32
Hawker Harrier
Yakolev Yak-36
Deflected Slipstream
Fairchild VZ-5
Ryan VZ-3
4.2.4 Tail-Sitter
The tail-sitter concept also uses the same propulsion
system for take-off and landing, but differs from the two
concepts described before. The whole aircraft is tilted, not
only a part of it. As the name suggests it sits on its
empennage during take-off and landing, and transitions by
pitching the entire aircraft.
If propeller propulsion is used, either large flaps (which
can degrade performance) must be placed in the slipstream
to control propeller torque, or a coaxial rotor system must
be used. The motor and rotor/propeller design requires
many compromises, because they cannot be optimized for
either vertical or horizontal flight. Therefore, propeller
efficiency is degraded, especially for fixed pitch propellers.
An interesting concept is the combination of a tail-sitting
flying wing and the quad-/octocopter layout (see sections
4.3.2 and 5.5). The propulsion system doubles as a
relatively wide landing gear in this arrangement and thus
the aircraft is less prone to topple over in high winds.
Jet propulsion can also be used for tail-sitters.
Afterburners are not used at take-off and landing for such a
concept since the re-heated exhaust gases will melt any
landing pad. Because of the cooler exhaust stream and
larger static thrust, high- or ultra-high-bypass turbofans are
superior to low-bypass- or turbojet-engines for VTOL
performance. Unfortunately, this will restrict such aircraft
to subsonic speeds and shows how difficult it is to combine
supersonic flight and VTOL requirements.
If a jet engine is used, it must not touch the ground during
take-off or landing, since this will choke the engine. The
Ryan X-13 was designed with the tailpipe as the absolute
end of the aircraft, and took off (and landed) on a hook on
a wall, instead of sitting on its tail on the ground.
Ducted fans can be successfully applied to a tail-sitter
design, since the duct can increase the static thrust, thereby
improving performance in the vertical flight regime.
Generally, tailless (sometimes also tail first, see [24])
configurations lend themselves very well to the tail sitter
approach. Many concepts involve a delta (Ryan X-13,
Convair XFY-1) or flying wing planform. A conventional
tail-aft configuration (like the ducted fan concept developed
by RMIT [25] standing on its empennage offers too much
area to the wind, making it unstable when standing on the
ground.
Fig. 14. Prop and jet tail-sitters
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According to [26], tail-sitter concepts can be split in
‘Control Surface Transition Tail-sitters’ and ‘Differential
Thrust Transitioning Tail-sitters’. Both require
sophisticated control systems, with the former relying on
control surfaces submerged in the slipstream and the latter
using either a swash-plate actuated rotor or a 3-D thrust-
vector-nozzle. The thrust vectoring approach is more
maneuverable, but comes with a weight penalty.
For manned tail-sitters a large problem is pilot visibility
[27], but this is no drawback for an unmanned aircraft.
However, optical payloads might not be effective in both
flight regimes. Problems also arise from a tendency to tip
over (observed for several small tail-sitter UAVs) and the
heavy tail structure needed to take the landing loads.
Nevertheless, as the general approach needs less moving
parts, tail-sitters should offer a slight weight advantage
compared to tiltwings or tiltrotors.
Transition from flight to hover is very complex, because
the wing operates at large, post-stall angles of attack. The
so-called diving transition seeks to minimize that phase of
the transition, by diving down, rather than sustaining level
flight during the transition. By trading altitude for airspeed
level flight is resumed.
Just as the other Lift = Cruise concepts, in its pure form
the propulsion system is oversized for forward flight.
Table 6. Example Aircraft - Tailsitters
Convair XFY-1
Lockheed XFV-1
Ryan X-13
Aerovel Flexrotor (UAV)
AeroVironment SkyTote (UAV)
AVIC VD200 (UAV)
Martin UAV V-Bat (UAV)
University of Sydney T-Wing (UAV)
4.3 Lift + Lift/Cruise
The best overall system performance is obtainable by
combining the power of the cruise propulsor and a
dedicated separate system for hover. However, if the
systems are not properly laid-out, the system’s performance
can easily be degraded to the point where it performs worse
than a well thought-out Lift + Cruise configurations.
4.3.1 Tilting Multicopters
If a multicopter configuration is desired for hover flight,
additional actuators can be used to rotate some of the rotors
in a way they produce thrust during forward flight. This
approach can be combined with the tiltwing/tiltrotor
technology. Naturally, any Y- or X-multicopter
configuration (discussed in detail in section 5) is suitable
for such a concept.
The selection of the tricopter configuration (see section
5.5.1) has the distinct advantage of having the least amount
of motor/rotor pairs and is therefore the easiest to integrate.
This gives the possibility either to actuate a single rotor in
a way that offers 3-D vector control, or to actuate two of the
three rotors for 2-D motion. Since the rotor-torque does not
need to be evenly distributed, an uneven disk loading can
be applied. This enables the tilting rotors to be optimized
for forward flight and the dedicated hover rotors to be
adapted to their separate flight regime.
IAI has created a very adaptable unmanned airframe called
‘Panther’. It swivels two of its three motors to transition to
wing-borne flight. The Panther is available in two sizes and
with pure electric or a hybrid gas-electric propulsion system,
demonstrating the scalability of the concept.
Quantum Systems uses a tilting mechanism on all four
rotors of their SLT UAV. However, only the front engines
are used to provide forward thrust during cruise flight. The
aft engines are swiveled to allow the propeller blades to fold
into a low drag position, increasing aerodynamic
performance. A careful tradeoff is required, if the increase
in L/D is worth the weight of the additional swiveling
mechanism.
If a high speed flight requirement is a high priority, then
efficiency in hover can be exchanged for lower drag during
cruise by replacing one rotor (or even all rotors) of the
tricopter layout with a ducted fan. The RWTH Aachen
developed the ‘Parcelcopter 3.0’ together with logistics
provider DHL based on this concept. Two propellers are
mounted to tilting wings and an electric ducted fan is used
to supplement the system in hover.
Fig. 15. Tricopter with front engines tilt
Fig. 16. Tricopter with aft engine tilt
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Table 7. Example Aircraft - Tilting Multicopters
Tilt Prop
IAI Mini Panther (UAV)
IAI Panther (UAV)
Quantum Systems SLT (UAV)
RWTH Parcelcopter 3.0 (UAV)
Tilt Duct
XTI Trifan 600
4.3.2 Tail-sitters Using Engine Shutdown
Tail-sitter aircraft usually are grouped under the Lift =
Cruise label, however if they are designed to shut down one
or more powerplants in flight, they can be discussed under
the L+L/C banner.
For UAVs the concept of combining a quadcopter with a
flying wing is appealing. This setup eliminates the need for
a landing gear, since the aircraft lands tail first on the
multicopter frame. Additionally, if the topside motors are
shut down in cruise flight, the bottomside motors, generate
a nose-up pitching moment, which allows a reduction in
trailing edge up control surface deflection for trim. This
improves the aerodynamic performance of the airfoil and
wing.
Fig. 17. Flying-wing L+L/C tail-sitter
4.3.3 Jet L+L/C
Jet aircraft concepts, that are developed for the Lift +
Lift/Cruise configuration, usually use a vectoring nozzle on
the main engine in the back, to deflect its thrust vector
vertically during VTOL operation. This thrust is then
supplemented and balanced by an additional lift engine in
the forward part of the aircraft. This configuration is similar
in operation to jet Lift + Cruise concepts, but offers a
reduction in weight and volume, with the corresponding
sizing benefits.
Fig. 18. Jet L+L/C
Table 8. Example Aircraft - Jet L+L/C
Dornier Do 31
Entwicklungsring Süd VJ 101C
Lockheed XV-4B
VFW-Fokker VAK 191B
Yakolev Yak-38
Yakolev Yak-141
Aurora Excalibur (UAV)
4.3.4 Augmented Jet Powerplants
If the thrust level of a jet powerplant must be increased
beyond its normal range, an augmentor device must be used.
For level flight, this augmentor is usually an afterburner.
For hover, this is an impractical solution, because of the
high exhaust temperature. Instead, different auxiliary
devices have been developed to provide additional thrust
for VTOL operation.
One method uses the viscosity of the air to its advantage:
Ejector nozzles allow the exhaust gas stream to mix
efficiently with additional air. The free stream effect will
accelerate the ambient air around the core stream as well,
which is then exhausted together, resulting in a higher total
thrust. While controlled laboratory tests showed thrust
augmentation factors of 1.5 to 2.0, the additional weight of
the necessary ducts and problems with incomplete mixing
prevented practical application [28].
The second method is to extract shaft power from the
main engine and use this energy to drive a horizontally
oriented fan system buried in the aircraft’s fuselage (e.g.
Lockheed XV-4) or wing (e.g. Ryan XV-5). This method
was applied to the F-35B, the only supersonic VTOL
aircraft currently flying.
Fig. 19. Ejector augmented jets (fuselage configuration)
The shaft driven lift fan is an attractive concept, due to
the reduction in exhaust gas temperature of the primary jet,
since the work for the fan is extracted from the engine’s hot
section [29]. Driving a lift fan with exhaust gas instead of a
mechanical coupling mechanism is also possible. Typically
tip turbines are used, driven by either fan air, compressor
belled air or core flow. However, the gas coupling is less
“stiff” compared to a driveshaft, which results in a slight
deficit in control authority.
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Table 9. Example Aircraft - Augmented Jets
Lockheed Martin F-35B
Lockheed XV-4A
Rockwell XFV-12A
Ryan XV-5A
Vanguard Omniplane
4.4 Summary
An overview of the configurations suited for transitioning
VTOL aircraft, which were discussed in this chapter, is
provided below. Table 10 shows an evaluation matrix of the
strengths and weaknesses of each configuration.
While the performance evaluation of an aircraft cannot
merely be attributed to a certain propulsion system layout
but needs to take into account the entire configuration, this
matrix does provide a starting point for a detailed design
synthesis. Good performance is marked with a ‘+’, average
performance with a ‘○’ and below average performance is
marked with a ‘-’.
Table 10. Performance overview of VTOL configurations
Performance
Subsonic
Speed
Transonic
Speed
Supersonic
Speed
Hover
Simplicity
L=C
Tilt Rotor + ○ - + ○
Tilt Prop + ○ - ○ +
Tilt Duct + ○ - ○ +
Tilt Nacelle ○ + + - +
Tilt Wing + ○ - +
○
Tail Sitters (Prop) + ○ - ○ ○
Tail Sitters (Jet) ○ + ○ - ○
Vectored Thrust ○ + + - ○
Deflected Slipstream + - - - -
L+C
Jet Lift + Prop + - - - +
Prop Lift + Prop + - - ○ +
Jet Lift + Jet ○ + ○ - +
L+L/C
Jet Lift + Jet ○ + + - ○
Aug. Jet Lift + Jet ○ + + ○ -
Prop Lift + Prop + ○ - ○ +
5. Rotorcraft
To provide the reader with a complete overview of VTOL
configurations, a brief overview of rotorcraft design
concepts is presented in this section. For rotorcraft, no
exemplary vehicles will be listed.
5.1 Helicopters
The conventional helicopter is the best air vehicle
configuration if extended hover periods are required. This
is owed to its low disc loading. Helicopters achieve control
by a swashplate actuated main rotor and a counter-torque
rotor at the tail. To spin large rotors at low speed, large
gearboxes are needed. Both the rotor hub and the tail rotor
cause high drag in forward flight and a typical helicopter
reaches a maximum lift-to-drag ratio of only 4.5 [5]. Fixed
wing aircraft reach 2-4 times this L/D value. VTOL
concepts usually cause a slight degradation of aerodynamic
performance, but still requires vastly less power for
cruising flight. This is the main reason, why transitioning
VTOL systems are even considered.
During forward flight, the helicopter suffers from a
problem arising from the relative velocity of the blades to
the surrounding mass of air.
In hover, at zero forward speed, both blades encounter the
same flow conditions. Each blade-station, however,
experiences a different velocity. Therefore, the low velocity
area near the rotor hub contributes little to the total lift. This
lowers the overall efficiency of the complete system.
When moving, the retreating blade sees the helicopter’s
forward speed subtracted from its rotational velocity.
Because the blade speed linearly varies between zero and
the tip speed, a portion of the retreating blade is always
stalled in forward flight, causing asymmetric lift and
vibrations. The advancing blade’s relative airspeed is equal
to the rotational velocity at each blade station, plus the
helicopter’s forward speed. Because tip speeds are limited
by compressibility effects, helicopters are typically
confined to maximum speeds of less than 100 m/s.
A fixed wing operates at a constant inflow velocity at
each spanwise position, enabling greater efficiency of lift
generation. However, this comes at the expense of a certain
minimum forward speed.
5.2 Multicopters
“Just as the conventional twin boom pusher configuration
became the iconic arrangement for fixed wing UAS’ from
the 1980s to the 2000s, the quadcopter is becoming the
iconic small rotorcraft configuration today.” [30, p. 473]
Multicopter technology has matured enough that man-
carrying aircraft are employing this technology. However,
the consequences of choosing between a tri-, quad-, hexa-
or octocopter are difficult to grasp, especially in
conjunction with the impact on the overall aircraft
configuration.
In this section, the basics of multirotor design shall be
presented, so that the designer can use this information to
mate such a system successfully with his fixed-wing
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concept (see sections 4.1.1, 4.3.1 and 4.3.2).
Multicopters (sometimes called multirotors) have rapidly
taken a very large share of the small UAV market and
nowadays are used frequently for commercial and
recreational applications. Considering terminology,
multicopters really are a subtype of the helicopter.
Typically, rotorcraft with up to two rotors are called
helicopters, with the dual rotor type being called tandem-
or coaxial helicopter, depending on the layout. If three or
more rotors are used, then one speaks of it as a multicopter.
The typical multirotor has an even number of rotors,
where each rotor pair spinning clock- and counter-
clockwise to cancel out torque. Even though the general
flight mechanics are comparable to helicopters, multirotors
offer a very simple control system. No actuators or variable
pitch rotors need to be used and besides the motors, there
are no other moving parts, because the rotors usually are
fixed pitch. For simplicity, the most commonly used
propulsion system are ungeared electric motors, even
though there have been successful manned flights of
multicopters using direct drive gasoline engines. The
simplest multicopter arrangement that allows pure motor
control is the quadcopter [30].
The general concept is not new; in fact, it can be traced
back to the 1907 ‘Breguet-Richet Gyroplane’ [31], designed
by Louis Breguet, of fame for his range and endurance
equations. However, it was only with the miniaturization of
processors and accelerometers that this kind of aircraft
concept found widespread use. Multicopters are highly
unstable systems, and it is near to impossible for a pilot to
stay in control without an automatic flight controller.
Payloads are usually carried in a central body, with the
rotors distributed symmetrically around this fuselage. This
increases efficiency compared to the helicopter, because it
avoids the suck down effect (sometimes called ‘hover
download’) by minimizing the area below the rotor disk.
Unfortunately, efficiency and controllability do not go
together for multicopters. Considering an arbitrary
multicopter with a given weight, an increase of rotor disk
area will lower the disk loading, improve efficiency, and
decrease power consumption. However, the larger rotors
will spin slower than their smaller counterparts will.
Coupled with the weight increase of larger rotors, the rotor
inertia is increased, and, because vehicle control is based
on the acceleration of said rotors, the control
responsiveness is reduced.
Correspondingly, while the concept works for small
unmanned and large manned aircraft alike, growth potential
is not unlimited because the flight dynamics are based on
the moments generated by inertia and very large vehicles
exhibit sluggish reactions to control inputs. Typically,
growth is achieved by using additional motors and rotors
instead of increasing the size and power of each motor/rotor.
5.3 Multicopter Dynamics
To illustrate the basic control principles of multicopters,
only the quad rotor design will be discussed. Nevertheless,
this theory can be extended to vehicles with six or more
rotors.
Consider a copter system with four counter rotating
propellers aligned in an X-shape, as shown in Fig. 20. At a
given rotational speed (ω), propellers generate a thrust force
(T) and a torque (M). Due to Newton’s third law of motion,
a clockwise spinning rotor generates a counter-clockwise
torque on the vehicle. The four independent motors control
the attitude angles roll (Φ), pitch (Θ), and yaw (Ψ), as well
as the heave motion, for a total four degrees of freedom.
Roll rate is controlled by a left-right differential of thrust,
while the front-back motors control the rate of pitch. The
yaw rate is controlled by differentiating the speed of the
counter-spinning motors: Accelerating the clockwise and
slowing down the counter-clockwise rotors lets the vehicle
rotate counter-clockwise. Forward flight is controlled by
rotating the thrust vector opposite to the desired direction
of travel.
Fig. 20. Multicopter Definitions
5.4 Optimal Multicopter Layout
All multicopters use arm mounted propellers to generate
their lift. The exact configuration of the motor/rotor setup
influences the efficiency and hence the performance of the
multicopter. Theys et al. from the KU Leuven [32]
investigated several configurations experimentally for
small multicopters, and their most interesting findings shall
be presented here.
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5.4.1 Two Blade or Three Blade Propellers
A direct comparison between two- and three bladed
propellers of the same pitch and diameter was conducted,
with the two-bladed variants being about 4% more efficient.
However, if noise is a concern, three bladed propellers may
offer some advantage, as they produce the same thrust at a
lower rotational speed.
5.4.2 Pusher or Puller Propeller
A pusher propeller is 3% more effective than a pulling
propeller. The difference of these two propulsion concepts
is shown in Fig. 21.
If a pusher layout is chosen, this has the benefit that
unsealed motors are covered from rain, hail and snow.
Drawbacks are the reduced ground clearance of the
propellers, and possible interaction with the landing gear.
On an additional note, the width of the engine mount is
influencing the efficiency more than its shape, and should
be kept to a minimum.
Fig. 21 Pushing propeller (left) and pulling propeller
(right)
5.4.3 Coaxial propellers
Theoretically, a coaxial propeller setup should exhibit
superior efficiency over a single propeller, since swirl
losses can be minimized. To get this benefit, the
downstream propeller needs a higher blade pitch since it
operates in accelerated flow. In addition, the stream tube
contracts, hence a smaller diameter should be chosen as
well. The ideal design achieves its maximum benefit only
at a single design point. In practice, the coaxial propellers
are usually a pair of identical pitch and diameter.
Testing revealed that for disk loadings above 100N/m²
this simple coaxial setup may offer about 3% more
efficiency compared to a single propeller.
5.4.4 Optimal Arrangement for Yaw
The motor/rotor combination can be tilted several degrees
(2°…7°) away from the horizontal plane, to allow better
yaw control during hover. Since the yawing motion in the
basic multicopter layout is only influenced by the rotors’
inertia, this means that control might be marginal for
heavier systems. A small tilt can introduce an additional
force in the horizontal plane, which in turn gives a torque
about the center of gravity and thus allows a much quicker
response of the aircraft.
5.5 Multicopter Variants
5.5.1 Tricopters
Tricopters have three motors and rotors. Because the
number of motors is uneven, their torque cannot be canceled
out completely. Instead, at least one (sometimes two) of the
three rotors is actuated to pivot and generate a torque to
control yaw. Due to gyroscopic forces resulting from the
fast-spinning motors, the actuation forces are non-trivial.
There is no redundancy for motor failure in a tricopter
system and since the moving part count (three motors and
at least one actuator) is equal to the quadcopter the failure
rates are similar.
Because of the uneven number of motor arms, only
unaligned radial arms can be used. These arms must be
joined in the center of gravity, where the maximum bending
loads occur [30], resulting in a heavier structure of the
frame.
Fig. 22. Tricopter
5.5.2 Quadcopters
Quadcopters have four pairs of motors and rotors. As
stated before, they offer the simplest control arrangement,
and have risen to the most used multirotor configuration. In
fact, if anybody is speaking of a “drone” chances are high
that a quadcopter is described.
Quadcopters come in either a + (‘plus’) or an X-layout.
The former has the advantage to offer improved visual
attitude feedback to the ground observer. The latter has the
advantage to offer a free forward field of view to any optical
payload mounted in its center of gravity. Also, for equal
size motor arms, the X-layout offers an increase in
maneuverability, since all 4 motors are used to control the
pitching and rolling motion (control forces increase by a
factor of 1.41, while the inertia stays constant).
If the quadcopters advantages of simplicity and a low
moving parts count shall be retained, this system does not
offer full redundancy in case of motor failure. Nevertheless,
researchers at ETH Zürich have demonstrated the ability of
a quadcopter to stay airborne after motor failure [33].
However, yaw control is lost in this case and the copter
rotates about his center of gravity.
Because a pair of motor arms is always aligned for this
2017 Asia-Pacific International Symposium on Aerospace Technology, Seoul, Korea
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layout, a single tube can be used, which can carry the
bending loads efficiently, without a heavy connector in the
center of gravity.
Fig. 23. + Quadcopter (left) - X Quadcopter (right)
5.5.3 Pentacopters
Pentacopters have five pairs of motors and rotors. Like
for tricopters, at least one of the motors needs to be actuated
to allow yaw control. The pentacopter offers a distinct
advantage for hybrid propulsion options: A combustion
engine can be used to drive a low disk loading rotor in the
center of the vehicle as well as a generator, which delivers
energy to the four electric motors positioned in a typical
quad layout. Such a design might be suitable for missions
where high endurance and extended time in hover is
required.
Naturally, a disadvantage of the penta-layout is the
increased complexity over the quad. In addition, just as for
the tricopter, the motor arms must be joined at the position
of the maximum bending loads.
Fig. 24. Pentacopters
5.5.4 Hexacopters
Hexacopters have six pairs of motors and rotors. They
come in either a six-arm or a Y-layout. The latter is a
tricopter with coaxial rotors (pros and cons of coaxial rotors
were discussed in section 5.4.3). The advantage of
hexacopters, compared to the tricopters is in the even
number of rotors. Therefore, torque is cancelled out and
control is achieved using differential motor speeds without
having to rely on changing the thrust angle of one (or more)
rotors. Hexacopters can sustain flight in case of a motor
failure, giving improved reliability over systems with a
lower number of motors. However, since they have greater
inertia compared to a copter with lower rotor count, they
are less agile.
Fig. 25. Hexacopters
5.5.5 Octocopters
Octocopters have eight pairs of motors and rotors. They
come in either an eight-arm or a X-layout. The latter is a
quadcopter with coaxial rotors (pros and cons of coaxial
rotors were discussed in section 5.4.3). Octocopter with a
sufficiently large T/W can tolerate a failure of any two
engines and thereby offer high redundancy. At ETH Zürich
an overactuated octocopter was developed. This special
setup is not only extremely redundant but can also sustain
flight regardless of attitude. However, this feature is usually
not desirable for transitioning aircraft. Their higher inertia
makes octocopters more stable.
Fig. 26. Octocopters
Of course, configurations with more than eight rotors are
possible. They offer a very high degree of redundancy and
since many rotors with a low inertia are used, the agility of
the system is retained. As an example, the German E-Volo
Volocopter 2X uses a total of 18 rotors and is currently
undergoing testing as an air-taxi in Dubai.
6. Conclusion
This survey paper has presented the key concepts
available today to the VTOL configuration designer. The
relationship between efficiency in hover, the disk loading
and the downwash-velocity was explained. A guideline for
selecting an appropriate T/W was established, dependent on
the propulsion system. The most relevant options for the
configuration of transitioning VTOL aircraft were
thoroughly discussed. Finally, for completeness, an
overview about the design of rotorcraft, with major focus
on multicopters, was presented.
2017 Asia-Pacific International Symposium on Aerospace Technology, Seoul, Korea
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