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Airships represent an attractive and promising solution to missions that can involve transportation, surveillance, recognition and monitoring, and so on. The many potential benefits that they can provide compared with UAV’s (Unmanned Aerial Vehicles), helicopters or drones in many practical applications have been proved, in terms of their high endurance capability with a lower investment, as they require much less power due to the lift generated through buoyancy force. Most of the traditional lighter than air airships are at the mercy of the surrounding air as the majority are used with advertising purposes (Haque et al. (2014)) Nevertheless, it must be taken into account for transportation missions heavy dusty winds will cause a great difficulty in handling, especially in the lateral stability of the vehicle. Taking this in mind, in the recent years there has been experienced an increased interest on the design of hybrid airships, able to combine both the lift generated by buoyancy forces as well as the produced by added wings, solving by this way the stability issues of traditional airships.
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This document will cover the complete design, manufacturing and testing of a mini airship for surveillance and terrain mapping, including the integration of the airship envelope, propellers, controllers, micro pimp and sensors. The aim of this project will be to compare the manoeuvrability and performance of the final prototype with a conventional UAV of similar weight under the same conditions. It must be stated that this is a continuation project of the designs carried on from previous projects, so using this together with the limited literature available related to different areas of research (Haque et al. (2014)), this project will help to complete and explore these research areas that require the completion of the details of the aerodynamic and stability behaviour at different flight conditions.
The history of aviation and all the challenges that it implied, suppose one of the greatest exponents of how human dedication and tenacity served to achieve the common objective of conquering the skies.
Lighter than air vehicles began their development in 1783 with the French brothers Joshep Michael Montgolfier and Jacques Etienne, that invented the hot air balloon and were able to send it up to an altitude near to 6,000 ft (1800 m) (Miller et al. (2013)). This concept was improved the same year by Jean Pilatre de Rozier, a French physicist that was achieved to make the first manned balloon, crossing the English Channel using flapping wings to propel the balloon and a birdlike tail for steering manoeuvres (Figure 1-1).
It was in 1900, when the Count Ferdinand von Zeppelin from Germany, invented the first rigid airship, leading to one of the most successful airships of all time. The term rigid comes from the metal framework composed by triangular girders covered with fabric that contained hydrogen-gas-filled rubber bags to generate the lift. The first one built used tail fin and rudders to stabilize the airship during flight, being powered by an internal combustion engine.
Several models where manufactured (Zeppelins) with military and civil purposes in the early 1900s, as well as for transatlantic travel (Frankfurt-Recife, Brazil). During this period, although many accidents were produced in different countries as the UK and Italy, having hundreds of dead’s, the investigation and development never stopped until 1937. Was this year when the largest and most iconic airship was built, the LZ129 Zeppelin. Designed to perform transatlantic flights, and after successfully carry out a travel between New Jersey and Germany, it crashed while landing in Lakehurst where 36 passengers died (Figure 1-2).
This accident supposed the end of the golden era of airships. Nevertheless, the interest on lighter than air vehicles technology did not disappear, using helium as the main advantage due to its non-flammability. US Army continued to develop them until 1962.
In recent days they are being used for advertising, research purposes, tourism…, however, the investigation of the new possibilities that they can offer is still in process.
At basic levels, an airship is a balloon that can be manoeuvred and propelled, but to the able to perform this it must accomplish two basic principles. On the one hand, in order to have a reliable and secure flight, it must sustain all the aerodynamic loads as well as the ones produced by the propulsive installations. On the other hand, the primary function of the envelope is that it should have a streamlined in other to reduce the drag coming from the air resistance (Khoury et al. 1999).
Traditionally, especially in the first half of the 19th century, there have dominated to main structural airship categories. Pressure (Blimps), and rigid airships (See Figure 2-1).
A blimp is an airship whose shape is maintained by the pressure generated by the lifting gas contained on its interior. Regardless in many pressure airships, there is no need to install a hard compartment to harbour the power plant or additional equipment, sometimes appears the need to incorporate a structure capable of sustaining the bending loads; becoming by this way in a semi-rigid airship.
Otherwise, rigid airships carry all the external loads through a surrounding framework composed of a set of cells, traditionally made of fabric skin wire-braced girders. This also helps to maintain the envelope’s shape independent of the interior pressure as well as reducing the material strength needed.
To this project, the semi-rigid configuration is considered as the most adequate due to its less complex design and manufacturing, combining the advantages of both previous configurations it will allow to sustaining bending loads without creating additional stresses on the envelope.
Starting from the base of the previous designs, many configurations had been proposed and investigated in order to adapt the models to specific missions and improve their general performance. The airships involved into this current are classified as ‘Hybrid Airships’, that at the same time can be divided into two categories.
The first one combines the characteristics of an aeroplane with a classical airship, primarily adding wings. With this premise, is tried to enhance the lift capacity aerodynamically by carrying a significant percentage of the carried payload through the dynamic lift generated by the wings. Figure 2-2 shows the Megalifter, a project developed during the 1970s using this ‘winged’ configuration.
This design would have a vast scientific interest to this project due to the capabilities that it would offer during take-off and landing, having much less runway needed compared to an aircraft of similar weight, as well as a safer ground handling than a conventional airship.
The second group would involve the combination of the helicopter technology using vectored propellers to generate thrust and help to the airship buoyancy (See example in Figure ). However, many studies have gone further, developing designs where the total weight of the airship was sustained by the aerostatic lift, leaving all the power capability of the propellers to sustain the payload.
This configuration, together with the ‘winged’ previously explained, will suppose one of the bases of this document as the scope of this document pretends to mix this both technologies in the most efficient manner. With this, it will be intended to establish which is the best arrangement and the main advantages that they can offer, either separately or combined.
In order to face and understand the incoming literature, there must be a basic aerostatic and aerodynamic airship knowledge regarding design purposes (Khoury et al. 1999).
The static buoyancy refers to the effect of a body immersed in a fluid (upward force), named ‘aerostatics’ when this fluid is the air. This force is represented in Equation 1 below.
B is the buoyancy force;
V is the internal volume of the body
a is the mean density of the air
Then, subtracting the total weight (W) of the body to the buoyancy force generated as shown in Equation 2 it can be obtained the net lift produced upwards (L).
In a balloon/airship case, the total amount of weight will be defined by the sum of the weight of the envelope (W0) and the weight of the gas stored inside, which is quantified multiplying the internal volume of the envelope (V) by the density of the contained lifting gas (
g), which is usually hydrogen or helium (Equation 3).
The total dispensable lift (Ld) by the internal gas is represented in Equation 4 (from Equations 1, 2, and 3) as a result of the subtraction of the empty airship weight (W0) from the total lift generated from the gasbag (Lg).
The volume of the gasbag/airship determines the lift that it can generate regardless of its shape. Nevertheless, spherical balloons offer the smallest surface area, favouring a minimum weight design and the lowest skin tension.
In the atmosphere, as the gasbag ascends the gas densities decrease with the pressure, increasing by this manner the volume of the gasbag. Despite this, at the same time the temperature decreases rising the gas densities. Nevertheless, the effects regarding the fall of pressure with the altitude are more pronounced than the ones produced by the temperature. This leads that at a certain point, after a continuous expansion the gas will occupy the entire volume of the envelope and no further expansion will be allowed. Then, from this point called ‘pressure height’, the lift will begin to decrease.
This pressure height will be dependent on the amount of gas inside the gasbag, the quantity defined as the ‘inflation fraction’ as shown in Equation 5 where V0 is the actual volume of the contained gas and V is the maximum volume of the gasbag. Then, at sea level, it will be assigned a particular I0.
Taking into account that pressure and temperature equally affect to the air and gas densities, it can be obtained the relation in Equation 6.
Following the International Standard Atmosphere (ISA) as a reference to establish the parameters variation with altitude, the value of I0 can be expressed. So from Equation 6, it can be obtained that the pressure height will occur when I=1; I0=pa/pa0. This means that once I0 reaches 100% of its value, the gas will be producing its maximum gross lift. However, this will only occur at sea level impeding the ascend without losing lift, meaning that at sea level I0 must be always less than 100%. Equation 7 represents the maximum dispensable lift theoretically.
Lighter than air vehicles, use gases with lower density than the surrounding air. As shown in Figure 4-1, there are compared the different lift capacities of a variety of gases at sea level. As it can be appreciated, hydrogen (0.0711 lb/ft3), is the gas with the lowest density among the rest, while the helium is the second in the list with a 7.3% smaller lifting capacity (0.0659 lb/ft3). (Pasternak et al. 2009).
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As it was previously explained, the use of hydrogen was abandoned after the Hindenburg disaster due to its flammability and the high risk that it supposes. Due to this, although many other alternatives have been considered such as ammonia (corrosive), natural gas and methane (flammable), it is considered that the most adequate lifting gas to carry out at these days is the helium.
The investigation of a new generation of hybrid airships is considered a relatively new topic that it is still under development and continuous research in numerous investigations. The majority of the scientific documents related with this engineering field cover the design phase as well as numeric simulations, so starting from them, it will be analysed the different design aspects carried out in order to lead the project in the most efficient manner.
The main base and background that will sustain this dissertation will be the investigation of the design and development of a hybrid airship carried out by Adamczyk G., (2017). The aim of this study was to develop a hybrid airship capable of generating lift using an aerofoil envelope shape. There were analysed different airship designs and concepts as well as CFD analysis of different aerofoils to obtain the most appropriate to perform the work, leaving a high and detailed background for the design phase.
The second section was focused on the manufacturing of the full-scale model, that although it was unable to fly, it served as a guideline of how to manage a manufacturing process and avoid the repetition of errors on future work.
Another similar recent study, but this time looking for the design never seen before to study its performance was carried out by Liu et al. (2011). This paper provided the design of a V-shaped airship with an aerofoil design that, in addition to the buoyant lifting capacity, two propellers were installed providing yawing, rolling and pitching moments (Figure 5-1).
The properties of this V-shaped aerofoil, composed by the combination of two hulls with a thick aerofoil section, are compared with one teardrop-shaped airship of the same weight class.
On this study it was discovered that the aerodynamic efficiency of the V-shaped airship increased rapidly with the velocity compared with the conventional teardrop model, reaching the conclusion that the effectiveness of this design is achieved at high speeds. At the same time, it was also discovered how two engines installed can help to provide a wide variety of control strategies.
On the study Stability and Take-off Ground Issues by Haque et al. 2014, it is analysed the effect of the gondola position on rotation angle during take-off, landing and ground roll on the IWHA-14 hybrid winged airship. This paper puts special emphasis on the possible configurations that can be met in order to operate at the minimum roll angle, being of great utility for the design and manufacturing phases.
This can be demonstrated as it can be observed in Figure 5-2, where the gondola position has a dominant effect during take-off on the rotation angle of attack. Moving forward the position of the gondola the rotation angle is 9.5o while moving it 2.5 m back the minimum rotation angle increases up to 12o.
As stated in Adamczyk G., (2017), the selection of the envelope aerofoil profile is crucial in order to obtain the maximum lift to drag ratio as well as having the sufficient internal volume to fit the required gas to sustain the airship weight including the payload. Aerofoils from the NACA 4-digit series with almost flat bottom surfaces and great thickness were considered as the most suitable, where the NACA 6322 profile was finally chosen as one with the greatest characteristics.
As this inflated shape aerofoil would constitute the airship envelope, the NACA profile was cut at 70% of is chord as depicted in Figure 5-3, allowing to work with a more likely airship structure making easier the stages of the manufacturing process
The main advantages and aerodynamic differences of a winged aircraft in comparison with a wingless one have been studied in detail in Adan et al. (2012). As it was previously explained in the Unconventional Designs section, part of the total lift of hybrid winged airship comes from the aerostatic buoyancy and the results from the dynamic lift of the wings. Taking this into account, the intention was to integrate wings to a conventional airship (Figure 5-4) and, using a commercial CFD package, analyse both models as a preliminary aerodynamic analysis for a winged airship.
The experiment was carried out at a Re = 2.16×106 at a free stream velocity of 40 m/s at different angles of attack (See Figure 5-5 for winged airship velocity contours).
This investigation outcome presented a promising reliability and performance for future winged airships flight tests. The lift generated was on average at positive angles of attack three times more compared to the wingless airship, where the highest increment is found at α = 5°. On the other hand, drag increases exponentially on the winged, increasing from 19% up to 58% in comparison. In terms of motion, both airships present a good longitudinal stability, being where the major contribution of the wings to the total stability. In addition, changing the wings position and orientation can contribute to a better static rolling moment stability.
Another study (Static Longitudinal Stability of a Hybrid Airship) carried by Haque et al. (2014), investigates how different empennage configurations can affect to the longitudinal stability of a winged airship. As it can be observed in Figure 5-6, two models where analysed, an inverted ‘Y’ type empennage (left) and a ‘+’ type empennage.
The comparative investigation showed that the ‘+’ configuration tends to present a higher static stability with the noticeable ability to develop a restoring pitching moment.
In work done by Adamczyk G., (2017), there was reproduced a preliminary full-scale paper manufacturing prototype of the design in order to have an initial approach of the steps to take on before build the real model, which could not be finished due to lack of time.
The initial plan was to divide the airship into two main components, the gondola and the envelope. Two gondola configurations were considered; the first one composed of plastic rods forming four equilateral triangles, two at the bottom and two at the top. The other configuration was simpler as it was based on a cuboid-shaped gondola manufactured with carbon fibre tubes.
Both designs would serve to carry all the electronic systems proper to a surveillance aircraft; including a camera, batteries, altitude control equipment, mapping lecture systems, GPS and remote control systems. The propulsion unit was initially composed by two vectoring propellers capable of generating either thrust or lift, although despite this, a 4 engine configuration was considered as a possible alternative bringing a better control and net thrust.
For the design of the envelope, the chord of the aerofoil chosen was cut at its 80% instead of 70% to increase the volume and keep the aerodynamic characteristics (Figure 6-1).
Finally, after finishing the paper model,  establish as the next step the creation of the definitive envelope using Tuftane Thermoplastic Polyurethane (TPU) film due to its less rigidity compared with the paper, putting emphasis on the importance of creating a preliminary full scale model with paper before start using the manufacturing materials, ensuring that all the measures fit and for the prevention of errors.
During the envelope manufacturing, there will be primordial take special attention to the pressurization, avoiding leaks that can jeopardize the airship structural integrity during a test flight.
To avoid this, Motiwala et al. (2013) establish a series of requirements and considerations for the material involved in this process:
-It must be least permeable than Helium.
-High tear strength (damage tolerance).
-Resistant to environmental corrosion due to temperature changes and humidity.
-Resistant to fatigue to be able to be inflated and deflated for a long life cycle
-Use of reliable joining techniques.
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