Spiny Norman said:
This one? http://aiac17.com/
Yes, it does look most interesting.
Yep. That one.
> Australia needs blimps
On the Potential of Lighter-Than-Air and Hybrid Aircraft for the Australian Civil Sector
G. E. Dorrington 1, J. Silva1, C. Bil 1 and P. Marzocca 1
1 School of Engineering, Royal Melbourne Institute of Technology, PO Box 71, Bundoora, Victoria, 3083, Australia
Abstract
A brief comparison between traditional non-rigid airships and hybrid-lifting-body-type,
lighter-than-aircraft of similar size is presented. Mass breakdown is discussed. Brief
comments are made on ground handling, transport efficiency and life-cycle costs. A mission
example presented is intended to match Australian geographic parameters. Although further
study is needed to verify the findings presented, it is suggested that in some niche applications
hybrid aircraft could offer useful advantages/gains within the Australian civil sector.
Keywords: airship, dirigible, aerostatic, transport, efficiency, cargo, surveillance, patrol
Introduction
Lighter-Than-Air (LTA) aircraft are often overlooked as possible solutions to a gamut of
commercial aviation roles which are dominated almost entirely by Heavier-Than-Air (HTA)
aircraft. Australia is relatively unique geopolitically in terms of land area/perimeter, resources
and population distribution. Hence LTA aircraft might have advantages in the Australian
context. This question must include the so-called hybrid LTA aircraft which balance weight
with a combination of propulsive, aerodynamic and aerostatic lift. Hybrids challenge the
general prevailing perception that all LTA solutions are impractical and uncompetitive with
HTA aircraft. One such hybrid aircraft that has recently flown in the UK is the Airlander 10
, shown in Fig. 1, whose specification includes: a maximum cruising airspeed of ~148
km/h (80 knots); a payload carrying capability ~10 tonnes; operational ceiling (pressure
height) up to 4880 m (16,000 feet); and an endurance of 5 days.
Fig. 1, Airlander 10 (images courtesy of Hybrid Air Vehicles, Cardington, UK).
It has been claimed that the specific transport costs ($/tonne) of this hybrid are somewhat less
than any commercial cargo aircraft currently in operation . As well as cargo transport, the
diverse possible markets for such vehicle might include passenger sightseeing flights,
emergency services and surveillance, mining resource surveys, and coastal patrol, etc. This
paper presents a brief discussion on the performance of an Airlander-type hybrid craft, vis-avis
more traditional, near-neutrally-buoyant airships, such as the former Goodyear ZP3G ,
which had a similar load capacity. The numerical examples presented, are intended to match
Australian geographic parameters. It is concluded that further study is needed to verify the
preliminary findings, but that in some niche applications hybrid LTA aircraft such as
Airlander 10, or larger versions, could have some useful advantages/gains within the
Australian civil sector.
Aerodynamic Comparison
The fundamental aerodynamic difference between a lifting-body-type, or multi-hull, hybrid
LTA aircraft and the conventional airship lies in the parameters of drag polar – which for
illustrative purposes may be simply expressed as:
(equation here)
Fig. 2, Drag Polars of Conventional Hull Airship and Hybrid Airship.
Whereas the traditional airship is limited to maximum lift coefficients ~0.3, the hybrid can
achieve lift coefficients as high as ~0.8. This simple comparison suggests that the traditional
airship has lower cruise power at near neutral buoyancy conditions and/or offers high
maximum cruise speed, but the hybrid offers increased dynamic lift and/or maintains the same
dynamic lift at lower loiter airspeeds. However, caution is needed in this immediate
interpretation, since a comparison based on equal hull volumes is not necessarily fair.
Traditional airships are typically operated when static lift is ~85+% of the total lift ,
whereas the hybrid may be designed to operate at as low as 60% static lift, i.e., it might be
expected that for a given payload capability the hull volume of hybrid is lower than that of the
traditional design. In the following sections, it is shown that this is not actually the case.
Simplified Mass Breakdown
Various simplifications may be employed to predict airship mass breakdown with some
reasonably good comparisons with real data . For illustrative purposes, the useful
load of a pressurised non-rigid airship can be approximated by the following simplified
expression,
(equation)
Actual Mass Breakdown Comparison
Table 1 lists the parameters of the Airlander 10 with those of selected non-rigid airships. The
useful load of the Airlander 10 is comparable with the ZP3G which had a much lower hull
volume. The main reasons for the lower payload fraction appear to be the increased empty
weight fraction of the hybrid which is partly associated with the higher surface-area-tovolume
ratio hull and the lower value of specific power, peng.
The maximum cruise speed of the Airlander 10 is lower than that of the ZP3G and the ZPG-
3W, although this also reflects the lower installed engine power of the Airlander 10 which is
comparable to that of the much smaller ZS2G1. However, direct comparison of these
parameters should be tempered by that the fact the Airlander 10 has been designed for a much
higher ceiling. Note that at loiter conditions at 10 m/s (20 kts), the Airlander 10 can generate
much higher dynamic lift.
The flight test programme of Airlander 10 is still in the early stages. It is not yet clear if the
Airlander 10 can achieve the specified maximum cruise speed of Umax = 41 m/s (80 kts).
Aside from the uncertainty in the drag coefficient (in an essentially non-lifting flight mode), it
is also still not yet clear what the value of propulsive efficiency of the aft ducted propeller
units will be when operated in close conjunction with the aft hull, although the wind tunnel
studies performed by McLemore suggest relatively high propulsive efficiencies.
The ZP3G baseline adopted three Allison GM500 engines with an installed specific power
0.89 kW/kg, whereas the Airlander 10 employs four variants of the AVIC Continental CD-
300 turbo-diesel engines normally rated at 231 kW (310 hp), with a lower installed specific
power. The advantage of the turbo-diesel is that specific fuel consumption ~210 g/kWh is
about half that of the former Allison engines.
Table 1: Comparison of Airlander 10 with Past US Non-Rigid Airships
Parameter ZS2G1 ZP3G ZPG-3W Airlander 10
Hull volume /m3 18200 24777 42928 38000
Static buoyancy /kN ~156 232 369 ~292
Empty weight /kN ? 150 223 196
Gross weight /kN ? 250 416 ?
Useful load /kN ? 100 193 98
Max. cruise / (ms-1) 37 50 46 41?
Engine power /kW 1020 1790 1790 969
Endurance / h ? 101 170 120
Ballonet ceiling /ft ? 9700 5000 16000
Estimated CDV 0.048 0.028 0.025 0.03-0.05?
Max. CLV 0.3 0.3 0.3 0.8
Max. dyn. lift @ 10 ms-1/kN 12 15 22 54
In summary, after consideration of the factors presented above and inspection of Table 1, it
cannot be confidently predicted that Airlander 10 offers improved performance over previous
non-rigid airships of comparable size. However, it is important to stress that the Airlander 10
probably offers other unique advantages in terms of low speed controllability and
manoeuvrability that would undoubtedly impact on ground operations.
Ground Operations and Influence of Gusts
A major consideration in LTA operations is the number of ground staff needed to assist with
ground operations. The Airlander 10 has vectored thrust units both fore and aft. Low speed
yaw manoeuvring capability would also be enhanced by the intended bow thruster.
Consequently, Airlander 10 should be capable of controlled hovering in static conditions
when ~30 kN heavy. This suggests it would be possible to offload 20 kN without having to
pick up ballast, which would represent a major operational improvement over fixed-thrustvector
airships. The reduced-size docking mast required is set back from the nose on the belly
as has been done in the past. Wind speeds in many regions of Australia are relatively low
much of year, and would not present any operational difficulties. However, it is not yet clear
whether, or not, such docking scheme would cater for the extreme gust conditions associated
with cyclones which occasionally track across the northern regions of Australia. Gust speeds
in excess of 285 km/h were recorded during cyclone Yasi in 2011. Research is also still
needed on such possible gust loading effects.
Airships of the type considered here can accelerate and decelerate to and from maximum
flight speed within ~60 s, i.e. of the order of five hull lengths. Calculations of added (or
virtual) mass are needed to make predictions of such accelerations, but it is also essential
to consider the behaviour of the viscous boundary layer in such non-steady conditions and
actual flight data suggests dynamics are often affected by flow separations that are not
accounted for in potential flow simulations and analysis.
Transport Efficiency
The productivity of any cargo transporter may be defined as the useful load carried multiplied
by the transport speed. Since the transporters acquisition cost is often directly linked to its
empty mass, a useful figure of merit is specific productivity,
(equation)
It could exceed road freight which is limited to 100 km/h over much of Australia.
For example, assume 10 tonnes of cargo needed to be transported from Freemantle Port in
Perth to a mine (say) near Cloudbreak in the Mulga Hills (WA). The road trip over 1374 km
mainly along a highway would take ~ 16 hours (without break), i.e. the average transit speed
is ~88 km/h, whereas the direct flight would take ~8 hours at 148 km/h (80 knot) at maximum
speed. Such a hybrid aircraft cargo flight would probably not be of economically viable,
unless the cargo happens to be an oversize item and/or the final drop-off point cannot be
reached by road. If it is assumed the Airlander 10 is powered by all four turbo-diesels
operating at their maximum power rating, then the fuel consumption rate would be ~200 kg/h.
However, since Ysp varies as the inverse square of the cruise speed, travelling at half speed, 20
m/s (40 knot) results in the consumption rate dropping to ~50 kg/h and required fuel load
being halved for a given range. At these low speeds, both the ZP3G and Airlander 10 have
much higher Ysp than any HTA aircraft.
Life Cycle Costs and Mission Roles
A comprehensive life-cycle cost analysis was performed on the ZP non-rigid, maritime patrol
airships in 1980. The conclusion of this study was that these airships had lower life-cycle
costs than any other competitive HTA options. It is likely the detailed role requirements for
aerial maritime patrol have changed since 1980 with the introduction of satellite systems
(etc.), but it is still likely that a similar comparative result would be found today. The
Australian requirements for maritime patrol are also arguably more demanding that those of
the US, given the larger coastal perimeter, although current operating budgets are likely to
much more restrictive. Nevertheless, the budget for Australian Customs and Border Protection
for 2012-2013 was $1.3 Billion hence it is not inconceivable that potential civil airship
maritime operations are relevant. Other civil mission roles appear to be worthy of
consideration. For example, tourist flights over land marks such as Uluru might be
marketable. The ability to loiter for long periods is another particular advantage of airships.
For example, one could imagine Airlander 10 being used as an aerial command centre for
coordinating the suppression of a major bushfire over a 5-day period. Of course, these are
speculative examples, but serve to illustrate the need for detailed study.
Concluding Remarks
Airships (hybrid LTA or more conventional types) cannot simply be dismissed from
consideration for many civil operations within Australia. The opportunity for improved
modelling now exists not only to perform realistic design synthesis, but also to
predict flight behaviour with good accuracy, including behaviour under gusting conditions.
Life cycle modelling tools can also be employed to better predict operational costs. With such
models, the effects of scaling-up the Airlander 10 to the Airlander 50 could be addressed.
In summary, there is a good argument to develop comprehensive models to evaluate whether,
or not, conventional and unconventional airships have possible competitive roles within the
Australian civil sector. It seems likely that in certain niche operations, airships will have
certain eco-economic advantages that will deserve attention in the future.
A picture of the Airlander 10
