Abstract: This report indicates three possible adaptations to improve the capabilities of the Merlin HC3 helicopter. These adaptations look

to improve the efficiency of the system, the safety of the crew and the visibility of the

pilot. This report will look into energy harvesting and possible, practical solutions for brownout; a major cause of helicopter crashes

in military use.

Aerospace Challenge 2018 Team CAN

Team Captain info: Amaan Karim Ahmad, 27/01/2001 AhmadAmaan@stpaulsschool.org.uk

Team Members info: Cole Hunt, 11/02/2001, HuntA@stpaulsschool.org.uk

Navonil Neogi, 25/10/2000, NeogiN@stpaulsschool.org.uk St Paul’s School, Lonsdale Road Barnes London SW13

9JT, 02087489162 Dr Herceg, Engineering Teacher, TMH@stpaulsschool.org.uk

1. Thermoelectrics: The Seebeck effect is the name given to the creation of emf across a junction due to a temperature difference. The materials used are generally semiconductors – the Seebeck coefficient α of a material describes how large relatively the emf generated by the effect in the material is. The effect arises because when heated, the electron energy levels in the two metals shift differently, so a potential difference is created between the two metals (and so an emf). The Seebeck effect is described very simply by:

𝐸 = 𝜎∇𝑇 Where σ is the Seebeck coefficient, E is the induced emf and the gradient of T is the derivative of temperature. Thus, the Seebeck coefficient for any given material is defined as the thermoelectric voltage per unite temperature difference.

Localising the heat: Recent studies have shown that heat storage units (HSUs) can be used to keep a large temperature difference between the two reservoirs large by minimising the rate of heat transfer. More recently, phase change materials (PCMs) have shown to be useful in maximising efficiency of HSUs compared to the usual medium – water, due to their high specific heat capacities, allowing them to store heat energy when undergoing a phase change.

Device design / Circuit diagram: The device is a small cuboid seen in Figure 1. The casing is made out of an insulating material; acrylic will often be sufficient. Inside, the majority of the space is occupied by the phase change material (PCM) which is contained within the area known as the heat storage unit (HSU). Thermal bridges, thin in order to maximise heat transfer surface area to volume ratio, occupy the central space within the HSU. In the interface between the bridges and the aircraft surface lies a series of thermoelectric generators (TEGs), which utilise the Seebeck effect.

Figure 1 shows the CAD model for the energy harvester

Flight simulation: We may run a simulation of the behaviour of the PCM temperature under flight, using the following iterative model:

𝑇𝑖𝑛(𝑛 + 1) = 𝑇𝑖𝑛(𝑛) + (𝑇𝑜𝑢𝑡(𝑛) − 𝑇𝑖𝑛(𝑛))∆𝑡

𝑅𝐶

(when no phase change is occurring)

𝑇𝑖𝑛(𝑛 + 1) = 𝑇𝑖𝑛(𝑛) + (𝑇𝑜𝑢𝑡(𝑛) − 𝑇𝑖𝑛(𝑛))2

∆𝑡

𝑅2𝑘𝜌𝐿𝐴2

(where phase change is occurring) Where the constants are:

• Tout – the aircraft surface temperature

• Tin – the internal HSU temperature

• Δt – the timestep of the model (here 1 minute)

• R – the load resistance applied across the generator

• k – the thermal conductivity of the PCM

• L – the latent heat of the PCM

• A – the cross-sectional area of the HSU

• 𝜌 – the density of the PCM

Typical values for the PCM quoted in the source paper were used in Graph 1, with a generator of the approximate size of a small tumbler glass. (marketsandmarkets.com, 2017) (M.E.Kiziroglou, 2013)

Graph 1 an example plot of Tout vs Tin for a specimen plot of the exterior temperature: Tout is orange, Tin is blue

Correspondingly, the voltages produced are of the order of 0.2-1 V; the power output is around 10-20 mW. The open-circuit voltage is proportional to

𝑇𝑜𝑢𝑡(𝑛) − 𝑇𝑖𝑛(𝑛)

Application: Assuming a relatively wide-body aircraft of diameter 3 m, there was at least (by calculation of half of the curved surface area of a cylinder, and then halving again assuming aerodynamic space constraints – so that they are simply arranged in one strip on the underside of the hull) 100 m2 of available space for sensors to sit on the underside of the hull. Although each has cross-sectional area 0.008 m2, when assuming some spacing, at least 1000 sensors may optimistically be placed on the aircraft, leading to a total peak power output of up to 20 W. Therefore, the small sensors on the plane can easily be powered such as:

• Wind speed sensors

• Proximity sensors

• Doors and slides (and other electronic detection)

A ring circuit would be an effective implementation to connect these harvesters together, because

• These sensors are to be placed on the fuselage and so thinner wiring is beneficial: since in a ring circuit there

are two paths for each flow of current, the wires can be made thinner.

• Flexible positioning of the sensors also becomes possible.

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Example plot of T_out vs T_in (°C vs minutes)

2. Scanner Drone Brownout in aeronautics is an in-flight visibility restriction due to dust or sand in the air. This issue causes spatial disorientation and loss of situational awareness which could easily lead to an accident. As a result, the design of this ‘mapping craft’ is therefore to map out the landing surface of the aircraft, to give aid to the pilot through HELIVAS (this being AGI’s Helicopter Visual Approach System) or other visual aids. The craft itself, though interesting would not be integral to the design, as the sensor mounted to it could just be mounted to the helicopter. The only advantage in using the surveillance craft, is that the landing surface could be thoroughly inspected before the landing itself.

Figure 2. Mapping craft used to survey the landing area

The sensor itself would be a spinning disk of sensors, rotating at a suitable speed to be able to map a 360-degree plane to suitable accuracies. The pitch of the sensors would be alternated from a local maximum to a local minimum as shown below.

Figure 3. The mechanism of pitch alternation of the sensors

Figure 4.Graphs showing the effect of the max pitch angle of the scanner has on the field of view. (from left), theta = 3 rads, theta =2.5 rads,

theta = 2 rads.

This, combined with the rapid spinning of the sensor, would allow high definition scanning with an even larger vision radius. If the scanner were to be mounted on the craft itself, it would use the onboard gyroscopic pitch, yaw and roll values to calibrate each measurement in 3D space. If the scanner were to be mounted on a mapping craft, as previously hinted, the individual data values would have to be a built within the gyroscopic senseor to normalise the readings. The readings would be made using the spherical coordinate system, for ease, to allow forcraft angle changes. Data handling would be made to handle three pieces of information; angle theta, angle phi and the distance between the outer position and the craft. The angles would then be added to the angles of the craft itself, to make

sure that the results were plotted for the correct position. This data could then either be directly graphed onto the pilot’s heads up display or could be used to highlight places of danger on the helicopter’s windshield. (Wulf, 2003) The use of retinal displays to add information about the terrain calculated on the internal computers within the helicopter could also be done. This would mean that the helicopter itself could interpret the data and use complex algorithms to give the pilot a better understanding of the data without compromising the reaction time needed to react to the information. Though visual retinal displays are not yet a feasible technology, they will most likely be used in the next decade, and therefore they should be considered as a possible form of pilot interface. Currently, the drone could easily incorporate anemometers and temperature sensors and provide photographic images to indicate the amount of rubble in a specific area and so map out the safer paths of travel for the helicopter.

3. Ducted Propeller Although in the project the vehicle would use propellers instead of wings, it is easier to understand this concept if thought about as a wing since this can represent a single blade of a propeller. Thus, Bernoulli’s principle could be applied as with the wing of a plane (Figure 5) which creates a vortex of wasted energy.

Figure 5 Trailing tip vortex formed as air travels through the wing of a plane

If we now translate this to a propeller, as seen in Figure 6A, this would also form this vortex which mostly becomes noise pollution and causes the downwards deflection of sand and dust (downwash). Since missions in places such Afghanistan and Iraq, brownout has become a major issue with pilot visibility and the safety of the crew. And so, it has become a hot topic in international aviation community which this adaptation improves. Between the tip of the propeller blade, there is a smaller gap which reduced the size of the vortex. As Bernoulli’s principle, we know that above the blade there is an area of low pressure which is also shown in this figure. To further improve this, wing shaped ends are added to create an area of low pressure and bring in more air. In addition, this curved surface causes a greater lift generated which the output of the propeller without needing more power; it also means that less power will be required. (RCModelReviews, 2015)

Figure 6 Propeller and ducted propeller

The effect of this adaptation is seen in the simulations below. The data is represented as flow trajectories to highlight the change in size of the energy vortex.

a)

b)

Figure 7 showing the flow trajectories which highlight the size of downwash

The flow trajectories display the efficacy of a propeller duct is. Since this is a closed system, a duct with a 2mm gap from the propeller blades was compared to a duct with a 25mm gap from the propeller blades. Since the downwash of sand and dust are the causes of bad visibility, the simple addition of the duct (Figure 8) reduces the amount of downwash and therefore the amount of brownout.

Figure 8 shows the rendered image of the ducted propeller system which would be incorporated

Conclusions: To conclude, the three disruptive adaptations explained above use current, practical technologies to further improve the Merlin HC3. The energy harvester system suggested above employs available technology that is not currently used by the RAF. This system is available can be used to power some systems such as the connection to the small drone suggested above. The 3D laser scan incorporated is a novel form of technology which can be easily incorporated in as the only major setback is the retina display for the pilot to view or by the helmet mounted display which is currently being developed by contractors of the RAF. Currently, the drone can be used to provide data to the pilot in order to indicate a safer path which would reduce the risk of brownout. Another way in which the risk of brownout can be reduced is by incorporating propeller duct theory to reduce downwash. This is a simple and easily incorporable adaptation which will significantly reduce energy loss, increase the thrust generated and – most importantly - reduce the downward deflection of sand which allows for greater pilot visibility in sandy locations.

Bibliography M.E.Kiziroglou, 2013. Performance of phase change materials for heat storage thermoelectric harvesting. Applied Physics Lectures, 103(19). marketsandmarkets.com, 2017. MARKETSANDMARKETS. [Online] Available at: https://www.marketsandmarkets.com/Market-Reports/aircraft-sensors-market-53630527.html [Accessed May 2018]. RCModelReviews, 2015. YouTube. [Online] Available at: https://www.youtube.com/watch?v=Cew5JF8q6eY [Accessed 2017]. Roche, M. S. D., 2013. Peripheral control ejector. US, Patent No. US 8413932 B2. Wulf, O., 2003. Fast 3D Scanning methods for laser measurement systems. Fast 3D Scanning methods for laser measurement systems.