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October 16, 2019

New Home

By Matthew Payor

Recently the hard work of some incredible staff at UNSW has come to fruition with a huge expansion of the UNSW Makerspace Network, where essentially 3 new workshops have opened up around the campus for everyone to work on projects, research and share resources. As a part of that, FLU has been moved to a new spot: we have an alcove in the Tyree (Renewables) Makerspace.

This spot came with some shelves but we wanted to incorporate the BAC cabinet we have into this alcove, and have a mixture of large shelves for crates and small drawers and shelves for components and compartment boxes. So Daniel Mann, Sarah Stormont and I embarked on some furniture making escapades.

Since we like to spend our money on stuff for drones, we had to use material that was at hand. The original shelving unit provided with the alcove had 18mm wood shelves, and a lot of the tables in the Makerspace had their reinforcing struts removed, so between those and some munched up form and structural ply leftover from a building site, I designed something to occupy our alcove.

Designed to be CNC’d on one of the Design Futures Lab router tables, joints were made to be self aligning and require little post work after coming off them machine besides glue, screws and paint. From the renovations that birthed our new alcove, there was some leftover wall paint in grey, so we made use of it.

One challenge that presented itself was that I wanted a full width table area at the height of the top of the BAC cabinet. This meant that the top shelf above needed to be significantly cantilevered to allow for an uninterrupted torso height working area. We bent some of the steel bar and routed channels in the underside of the top shelf for them to press into to arrest any worries of a wobbly top shelf. The tolerances were loose enough that we could bend the bar by hand with blocks and clamps.

So yeah, welcome to our new home as of 2019.

October 16, 2019

How does Wren get off the ground?

By Marco Alberto

Just when you think you’ve figured it all out, trust the software to mess up! I’ve been working on the programming and electronics integration with the Wren, primarily using Mission Planner to set all the relevant parameters correctly and plan any autonomous missions as required.

The “brain” of the WREN is a Cube Autopilot (from Cubepilot) running Arduplane, which allows us to have Quad-plane support and features for the Wren. This gives us access to hundreds of different parameters, the ability to tune both quadcopter and regular plane flight, and accurately collect log data from each flight. This has allowed us to diagnose all issues with aircraft centre of gravity and controllability concerns in particular, during the development process.

In addition to these core features, we have begun experimenting with the use of UAVCAN for servo control, which worked wonderfully at the AUVSI SUAS competition in June 2019. Future versions of Wren will continue to use the UAVCAN architecture, while also including battery monitoring and ESC (electronic speed controller) control over the CAN lines as well.

Following some issues we had at the Land of Lakes challenge in 2017, the digital ESC protocol DShot was successfully implemented for use with the ESCs on Wren at the AUVSI SUAS competition. Using APD ESCs, DShot was found to be an extremely reliable and easy to implement protocol with Ardupilot. With no ESC calibrations required, like with a PWM ESC, setup time was reduced significantly with one less calibration process required.

Overall, we are extremely happy with the performance and future capabilities of the systems that were integrated onto WREN v1. As we move towards the end of the year, and progress towards competing in the AUVSI SUAS 2020 and Medical Rescue 2020, we will be testing these changes on WREN v1, streamlining the programming and parameter updating processes, as well as developing a lighter, faster WREN.

Please do not hesitate to contact us with any further questions about the WREN, its systems, or the team! We have some exciting things coming, so be sure to keep an eye on social media.

October 15, 2019

Build Positive

By Matthew Payor

This article is about the manufacturing processes we’ve developed with Wren V1, with focus on foam core positive layups.

Motivation

Coming into the Wren build, Daniel Mann and I had a long list of nagging points from previous work that we wanted to try and nip in the bud. We wanted to build a composite skin airframe where the skin truly composed the majority of the structure, with embedded electronics and compartments for different objects.

Whilst the production of optimal and attractive composite skin sections is relatively easy by layup on negative moulds, this leaves a lot of work to follow. The production of an internal structure, the joining of the skin to this structure in a way that is both strong and light, and embedding hardware ends up being a slog. It also introduces many possible weak points that require additional reinforcing.

The way I see it, every joint is either wasted weight or a weak point. It’s work that relies heavily on having multiple independently produced elements having close tolerances, using epoxies, filling and hiding joints. All these aspects increase the difficulty of the construction, move the final product further away from the CAD / design, and add weight that is hard to predict and budget accurately.

I’ve previously created laser cut plywood skeletal internal structures with accomodation for mounting harware and wire routing. These projects came across issues locating everything accurately, producing satisfactory sliding fits, etc. – nothing insurmountable with some time and care, but seemingly suboptimal for the applications we’re exploring. It is still certainly true that this method is exceptional for lightweight model aircraft with solar film skin.

All these aspects increase the difficulty of the construction, move the final product further away from the CAD / design, and add weight that is hard to predict and budget accurately. I’ve previously created laser cut plywood skeletal internal structures with accomodation for mounting harware and wire routing. These projects came across issues locating everything accurately, producing satisfactory sliding fits, etc. – nothing insurmountable with some time and care, but seemingly suboptimal for the applications we’re exploring.

It is still certainly true that this method is exceptional for lightweight model aircraft with solar film skin.

So whilst laser cut internals provide a lot of control over balancing weight and strength at different areas, the job of adhering the internals to two or more composite skin sections is something we wanted to avoid.

With our growing CNC experience, the solution that first presented was to incorporate both the mould and internal structure as one – a positive foam mould with embedded electronics, spars, mounting points and other features.

We posited that a single positive layup of the entire skin that grips tightly to a core with adhesion along the full surface area would make best use of the benefits of a composite skin – although bound to produce an uglier surface. Furthermore, since there was to be a cargo bay in Wren, it seemed appropriate to lay onto the inside walls of this cargo bay to create strong vertical sections in the centre of the airframe for rigidity (important for the quadcopter aspect of the vehicle).

Initial experiments

We began by doing some small scale rough tests of positive layups over a core with some scraps of carbon just to practice the process and discover the difficulties in the process. Luke Jackson, Arfin Trisakti and I did a quick and dirty lay on a 3D printed scale Wren model, and a hastily hacked up piece of foam that included the characteristic challenging geometries we’d be seeing on Wren – a cutout cargo bay and a sharp trailing edge.

These tests were very promising, bringing up both the benefits and difficulties of positive layups over a core.

The strength of the test pieces was impressive, and I believe that’s because the compressive strength of foam is amazing for its weight, leaving the skin to handle tension and torsion. Being tightly wrapped by the vacuum, loads should transfer well through the structure.

One difficulty is in achieving a decent surface finish without a negative mould, which is not just about looks but also strength, since wrinkles will effect performance under tensile load. Also, doing one layup instead of multiple to produce a shape means that naturally the geometry will be more complicated to deal with, since you’ve taken on more at once. This means that having a good plan and the right tools for the layup is tantamount. Especially when gravity is working against you on the downfacing side of the core.

The next step was to come up with methods to embed electronics, connectors, spars, and other internal components. By splitting the positive core into two halves about the approximate wing chord, I was able to machine cavities and aligning features for internals, and then flip the foam using aligning pins to machine the outside profile. For housing connectors and other things that needed to protrude from the skin, we glue all these into the foam and then plug them up for the layup. After release, the entirety of the post work is digging out these parts, filleting the boundaries, and doing finishing work on the carbon fibre.

Here’s some vids and pics from machining the foam core for the prototype.

Simultaneously… 3D Printing

Whilst I was working on the foam experiments, Daniel Mann was investigating the promising features 3D printing could bring to a core material. The notion of an object being comprised of a skin and a minimally infilled core that is optimised to support the skin seems like it could outperform foam as a core, since building in strength only where it is required should result in a lighter and stronger item than having a constant substrate. The additional things 3d printing could bring to the table would be endless possibilities in terms of inbuilt complex connector housings, wiring channels and other things you would be unable to machine without great difficulty. So we decided to have a foam centre section and 3d printed wing stubs for our first prototype of Wren, to put these thoughts to test. Things wing stubs would include strong mounting points for the quadcopter booms, which we wanted to be bulletproof.

Here’s an example of a gyroid style infill with nylon filament.

Dan looked into the strength and weight of 3d prints of different materials, skin layers, infill style and percentage. Nylon proved to be very strong, but an impractical core material due to its compressibility – it was hard to break but easy to deform. It is also rather difficult to print due to its sensitivity to humidity and temperature, and its high shrinkage. PLA and its many variants were weaker but more rigid and much easier to print. All we really required was something that would survive the vacuum during layup, so this seemed to be the way to go. Dan ended up settling on a single skin print and a low infill that seemed to make the best compromise between strength and weight. Unfortunately, this still was about twice to three times the weight of an equivalent foam part. The main issue is that with a fixed nozzle size and extrusion characteristics, lower infill means scaling up of the infill pattern and thus larger areas of unsupported skin. Ideally, there would be some way to achieve a lower infill weight without greater spacing of the infill, by thinning the walls. Perhaps this can be achieved with a constant nozzle size by fiddling with extrusion parameters, but with typical slicing software we could not find a good solution.

We did our second positive test layup on a 3d printed wing stub section, this time using fresh fibreglass instead of offcuts and trying to achieve a good surface finish. This wing stub section had hollowed areas for batteries, spars, and connectors. The battery hollow crushed under the vacuum bag, and the trailing edge softened and smushed due to the heat produced by the resin cure and the low melting point of the 3d printed core. This taught us to plug up large cavities for future layups, and to keep temperatures down during the lay. The surface also had dimples between infill areas. Despite these issues the surface finish of the fibreglass was great, although the geometry was very simple to work with.

Some scientific stress testing…

So we concluded that plastic FDM 3D printed cores were an inferior alternative to foam cores for our specific application. Where they still excelled for us was in situations where we needed small components stronger than foam in the core, with complicated features across multiple axes. Mounting points were a prime example. The D-sub connectors we used for the wings screwed into small 3D printed housings that were glued to the foam. Where we wanted to bolt through the wing we inserted strong 3D printed sections to handle the compressive load of a bolt, whilst still maching the 3D contours of the wings. It is also worth noting that a 3D printed wing, without being reinforced with a composite skin, could outshine foam wings in terms of easily incorporating internal features and reinforcements, and providing a more durable skin than foam. Wings typically dont see compressive loads so there’s strictly nothing wrong with the weaknesses of 3D prints besides the weight (which could probably be dealt with by custom infill and smaller nozzle width) for making something that flies.

Prototype Wren

So all these elements came together in a first prototype / proof of concept that incorporated a CF tube skeleton, foam centre section, 3d printed PLA wing stubs and a 3d printed nylon forward flight motor firewall. The internal CF skeleton was designed by Rowan Whiteman and Arfin Trisakti with the intention of being bulletproof for the inevitable crashes we’d face during flight testing, and the completed core prior to reinforcement was already really strong. We were unsure at the time of how much weight and strength the positive layup would contribute so we were playing it safe there.

The gluing of all the parts together proved difficult. A significant problem was that with the compound angles of the skeleton tubes, shrinkage in the 3Dprints took us out of tolerance and we needed to really force some things to fit. Despite attempting to account for shrinkage in the modelling of the 3D prints, the nylon motor mount section fit especially poorly due to the high shrinkage of that particular filament.

We started by prepping the carbon tubes by drilling some small holes and heavily scratching around the joints to other tubes, and scuffing up the other surfaces that would be contacting 3D prints and foam. We then applied Scotch-Weld to the tube joints, and then a PU woodworking adhesive to the foam and inside the 3D prints, and then fitted everything together.

We weren’t prepared and had to improvise with the clamping. I sprayed some canola cooking oil on carboard and wood scraps and rigged up a monstrous concoction of clamps.

Then it was time to layup. In being frugal, and not knowing how things would turn out, we decided to use standard weave fibreglass for the prototype, two layers of roughly 65gsm glass. This also meant that we were diving in the more difficult end with the standard weave, and using twill in future lays would be easier to hit the geometry.

The layup proceeded pretty much perfectly, and simultaneously Daniel Wong and Arfin Trisakti whipped up some solar film & laser cut plywood outboard wings. There was one issue that occured after the prototype centre section went under vacuum – three of the four 3D printed wing stubs were out of a slightly different PLA (coloured black) to the last piece (coloured grey), we had printed all four in this filament and one of the prints failed, so we used a different PLA filament for the last piece. This one grey piece, which was printed with the exact same settings as the others, crushed under the vacuum. This wasn’t an insurmountable issue, but it was annoying because the CF VTOL booms were going to sit in adapting plates along the bottom of the wing stubs, and since one section was crushed, it was both weaker and also would seat the adapting plate at a different angle. What was a purely cosmetic issue was that the nylon 3D print squished under the vacuum. It did not break, but the nylon filament is quite flexible and it just compressed. It became super clear after this point that all foam was the way to go next time.

The post work on the centre section after the layup was to cut off the excess fibreglass, and make the necessary repairs to the crushed 3D printed section. I didn’t want to add too much weight in the repair, so I built up an area to roughly the original wing profile on either side of the stub using PU woodworking glue and various ratios of water as a kind of rigid casting foam, and masking tape as a form. I also built up a small section in the middle of the stub on the underside to seat the boom adapter correctly. Then I solar filmed over the section, gluing to the built up edges to create a fairing that hid the crushed section (but just had air underneath it). As an afterthought we decided to spray paint the section lightly with some leftover cans – it did not look great close up, but did the job.

Not really the topic matter of this post, but here’s some pictures of the prototype flight testing.

So what did we learn from this initial prototype in terms of manufacturing? We decided to primarily use foam for the core and keep 3D prints to small, high infill, low volume parts selectively reinforcing or providing mounting points at specific areas. We validated our assumptions about producing a structural skin over a core via a positive layup – the centre section was incredibly strong, to the point where Daniel Mann and I tried jumping and stomping on it and couldn’t do any damage without point loads of impact like a heel. This meant that we were happy to strip down the internal structure, which frees up more room and weight. Something we hadn’t explored was testing embedding electronics and plugs in the positive and laying over these things, but we definitely wanted to do this for the next build. So with all these things in mind, it was time to get started on a carbon fibre airframe.

Wren V1

The first parts we made were the wings, since there was less time and cost invested in making them and we could test out the last few little things on these without huge consequence if mistakes were made. I machined the foam cores with wiring channels, cutouts for connector housings, and electronics like servos. Daniel Mann tried using a 3D printer filament consisting of nylon with chopped carbon fibre reinforcement to make connector housings and other fixings for the wings, and they turned out amazing – the strength of the nylon paired with the rigidity of the fibre reinforcment to create things that could almost mimick an injection moulded part in terms of properties, we were very impressed with the filament. It was extremely temperamental to moisture and other environmental factors during printing though. I also machined end plates for the sections out of 2mm carbon fibre plate. The layup of these was super simple because purely wing geometries require almost no stretch in the weave when forming to the core. Vineeth Rao and I were able to assemble the wings including electronics and lay them up across two afternoons. The resulting wings were equal in weight to within 2%, and well within tolerance.

Next was the centre section. After machining the foam, we potted all the electronics and connectors into the bottom half of the fuselage. Then we glued the foam sections together using thin aluminium sheet and whatever else we had on hand to spread the load of clamps and weight. Then we glued on the end plates, plugged up the large voids like the battery cutouts, and laid over the core.

The finishing work required was cutting off the excess fibre from the layup, adding in the externally mounted elements such as the wing latches, and painting.

The wing latches that Daniel Mann designed connected both structurally and electronically the wings to the centre section. They were a major win from this build. Incredibly satisfying to use. The only problem was that they were quite heavy, and we’ll be looking to do something equally cool at less cost to our weight budget in future.

Here’s a few pictures of the completed Wren V1. The wingtips were routed carbon plate with some 3D printed housings for the radios, and the nose cone was a machined HDPE (with calcium carbonate filler) mounting plate for the hardware with a vacuum formed PET fairing.

ADD FUSION AND RHINO SCREENSHOTS + timelapse

October 15, 2019

Preliminary Research On The Optimisation of Flying Wings In Forward Flight

By Read Liston

Longitudinal Stability and Trim

The differentiation of stability and trim is an important factor in the design of an aircraft, specifically a tailless one. In terms of static longitudinal stability, the definition remains the same for a flying wing and a traditional aircraft; a centre of gravity location ahead of the neutral point of the aircraft. For a flying wing, this neutral point is analogous to the aerodynamic centre of the wing/airfoil and is generally located along the 20% to 25% line of the chord along the span of the wing. A statically stable aircraft, however, is insufficient for steady flight, the aircraft must also be trimmed. A trimmed aircraft has all the moments about the centre of gravity balanced. With no horizontal stabiliser, the wing design of a tailless aircraft is significantly more involved, requiring both lifting and trimming capabilities. In the statically stable case, a typical main wing will produce a pitching down moment due to the fore centre of gravity location. From [1]; there are two ways in which to attain trim:

  1. Use a combination of wing sweep and wash out. 
  2. Use an airfoil that produces a positive pitching moment.

A traditional cambered airfoil produces a large negative pitching moment, exacerbating the pitching down moment. As such, an airfoil with a positive pitching moment can be used to instead balance the main wing moment. This solution is generally only applicable for a wing with no sweep. A much more feasible method is the combination of wing sweep and washout. Typical washout, or wing twist, is implemented to create a negative angle of incidence on the outboard section of the wing. Its combination with a sweptback wing creates a sufficient pitch-up moment arm to balance that of the main wing, trimming the aircraft. It follows that the larger the wing sweep angle, the less washout required (for a constant CG position). Both excessive sweepback and washout, however, have adverse effects on lift and flight performance. In practice, the most optimal solution is to combine wing sweep and washout with an airfoil that produces a minimally negative to zero pitching moment. 

A symmetric airfoil would produce a zero pitching moment, however, the minimum drag for a symmetric airfoil occurs at zero lift, therefore during level flight, an angle of incidence is required and excessive drag will be created. To obtain the desired pitching moment a reflexed airfoil is the most appropriate choice as they have minimal negative pitching moments along with a positive coefficient of lift at zero angle of attack.


Figure 1: Demonstration of the physical approach behind a reflex airfoil [1]

Cg Location

The center of gravity (CG) is one of the most important factors in the design of a flying wing. Acting as the pivot of the aircraft, its location is often influenced by many factors, particularly payload. In the case of Wren, to avoid sudden shifts in CG, its optimal location is coincident with the center of the cargo bay. This follows, that upon releasing the UGV, the flight characteristics remain unchanged. Specific internal configurations will be required in order to obtain this. 

Additionally, while a forward CG location improves longitudinal stability, it is not without its drawbacks. When the center of gravity of the aircraft is displaced from the center of lift, wing twist is necessary to provide the moment balance, as previously discussed. This twist, resulting in a down lift at the wingtips, adversely affects the maximum lift of the aircraft. The figure below demonstrates the percentage loss in a maximum lift for various CG locations.

Figure 2: Effect of CG location on the maximum lift [1]

More significantly, however, is the impact on the induced drag whose value is quadratically proportional to the distance of the CG to the elliptical center of lift. Figure 3 below, combines the losses of lift as well as the increase in induced drag, to demonstrate the optimal CG locations for a given taper ratio. Early Horten sailplanes are plotted as a reference.

Figure 3: Optimal CG locations [1]

Tests by [1] showed that the critical cg location ahead of the centre of lift before controls became unflyable was around 6-12% of the mean chord length.

Aspect Ratio

A large aspect ratio is a trade-off between reduced drag and reduced maneuverability, while the opposite is true for a smaller ratio. For a given wing area, as the aspect ratio increases, the wingspan follows suit. The adverse effect of tip vortices then occupies a smaller percentage of the entire wing. To optimise forward flight a larger aspect ratio seems to be the obvious choice however depending on the flight requirements and mission profile of the particular competition, an appropriate aspect ratio can be selected in conjunction with the average operating speed.

Figure 4: Display of the affect aspect ratio plays on the coefficient of lift [2]

Taper Ratio and Stall

Due to the delicate balance of the wing, local stall can disturb the flight significantly. For example, stall at a wing root when ahead of the CG will cause a nose dive while local stall further outboard, behind the CG will cause a pitch up. The ideal location for flow separation can, therefore, be inferred to be in line with the CG A crude approximation to achieve this given an idealised lift distribution, a minimum taper ratio of 0.6 should be selected with an increase to 0.7 perhaps being more desirable, as proved by [1]. Moreover, it was discovered that for a sweptback wing with washout, the optimal taper ratio to minimise parasitic drag lies between 0.7 and 1. Conversely, a strong taper ratio will produce generally all positive results apart from the increased induced drag above and center of lift implications displayed in figure 3. 

While it seems that the Horten design neglect the previous fact, by having strong tapers, authors Nickel and Wohlfahrt, stress that a flying wing with a taper ratio less than (greater taper) 0.7 has either:

  • No optimal Lift Distribution.
  • Unfavourable stall Characteristics.

Sweep

As mentioned, sweep is necessary to balance the locations of the centre of gravity and aerodynamic centre, below is a brief summary of the effects different amount of sweep has on the aircraft. 

For larger sweep:

  • Creates a larger moment arm for control surfaces located toward the wingtips.
  • Creates a larger moment arm for wingtip washout, increasing its effectiveness.
  • Creates less lift when compared to a straight wing by approximately the cos of the sweep angle. 

The opposite is true for smaller sweep angles. 

Washout

The amount of washout required can be either a small amount, needing elevon deflection at low speed, a large amount, needing elevon deflection at high speed or somewhere in between. The selection should depend on the operating lift coefficient. For Wren, this would be cruising conditions given the influence of the quad-motors at low speeds speed, take-off, and landing.

The washout amount can be determined using the moment balance equation:

The value of C_mw can be expressed in the form below, and experimentally found from the graph.

Figure 5: Washout function in terms of aspect ratio and sweep angle [1]

Lift Distribution

Recent studies from [3] express the benefits of a bell-shaped lift distribution in its ability to create superior control and proverse yaw. Additionally, it is known that the Horten brothers implemented this lift distribution similarly in their design. The effect of proverse yaw was not attained by the Horten’s however, due to the inboard nature of their elevons explained by [3]. The bell-shaped lift distribution developed by Prandtl and implemented by Horten and NASA [3], however, produces more drag for a given wing when compared to an elliptical lift distribution as mathematically explained in detail by [1]. Horten and NASA increased the aspect ratio (Span) to reduce this loss in drag to attempt to justify the benefits of proverse yaw. A firm expression from [1] suggests that, particularly for models (small scale aircraft), other techniques are more effective in reducing adverse yaw and that a bell-shaped lift distribution is inappropriate. 

It would worth building models from the theory of both [1] and [3] to compare the results first hand. 

Figure 6: Horten Bell-shaped lift distribution [1]

Horten Design Philosophy [2]

  • Followed the above, used large washout at the wingtips 
  • Wingtip twist created a bell-shaped lift distribution which was theorised to reduce adverse yaw.
  • They used a larger aspect ratio to make up for the loss in lift near the tips. 
Figure 7: Photograph of the Horten IX/ HO 229 

Northrop Design Philosophy [2]

  • Early designs pre B-2, Chose to avoid excess twist and used pusher propellers relying on vertical stabilisers.
  • Pusher props along the trailing edge of the wing created a stabilising effect
B2
  • Due to the fly-by-wire relaxed stability excess wingtip twist was not needed and instead, the flight computer handled the numerous control surfaces to stabilise the aircraft. 
  • The B-2 uses wingtip mounted control surfaces that crack open to create drag and thus a yawing moment. This could potentially be implemented into WREN however the response is known to be very non-linear, resulting in difficulty in control. When the B-2 is cruising the yaw control surfaces are cracked open to the point where they are only just catching the air. The flight computer then can fine-tune the aircraft’s dynamic response.
Figure 8: Northrop Grumman B2 with yaw control surfaces cracked open

References 

[1] K. NIckel and M. W. Wohlfahrt ‘Tailless Aircraft in Theory and Practise’  AIAA educational series 

[2] D. Raymer ‘Aircraft Design A conceptual Approach’ AIAA educational series. 

[3] A.H Bowers ‘On The Wings of the Minimum Induced Drag’ 2016.