Voting Result: 3.16985550969
Overview for The Cascadia Colibri
**Final Update** Check out the new Airframe Walkthrough Video with timestamps in the description to specific features. I've also added my first-ever 3D model, and eCalc performance datasheets, plus a whole new bit at the bottom detailing the propulsion system technical specifications - for some nice, light reading. Also better photos with clearer organization. Enjoy.
So a DC-3 and a Leatherman have a baby…
I present the Cascadia Colibri Cargo Drone Concept and Prototype.
That’s right, I designed mine in meatspace!
First off, let's discuss what a cargo drone needs to be. It's a utility vehicle. Designed to carry a wide variety of packages and payloads through the air quickly and efficiently (read: autonomously), it needs to easily withstand harsh environmental conditions and rough handing. It needs to be minimally complex, cheap to maintain, and easy to transport. Like any great tool, it should have all these qualities: Simple. This applies doubly to aircraft. Simplicity means a low parts count, lighter weight, less points of failure, cheaper maintenance regime, and cheaper to manufacture. Reliable and Safe, and just as importantly, perceived to be so. Many people have still not interacted with a drone so early impressions can literally influence policy and law. I want my drone to be trusted and admired not legislated against. Robust. A cargo drone will be exposed to harsh and sometimes extreme environments, used heavily, and handled by people who are not pilots nor technical experts. Doesn’t matter. It just needs to work. Versatile. The more tasks it's good at, the more useful it is. Simple. For a cargo drone versatility means it's more likely to be widely adopted and adapted to new roles. A ubiquitous cargo drone will end up with an entire ecosystem of parts, mods, improvements and adaptations built around it. Which leads me to Easy to Maintain, as well as modify and upgrade. Technology moves fast, so a drone that isn't simple to maintain and easily upgradeable won't be around for long. I also specifically wanted to design an aircraft that’s a pleasure to work on. I think it's easy to forget the operators and maintenance personnel when designing a high-performance aircraft, but they’re precisely the ones sticking their hand in this thing like it’s their job. I designed the Colibri with the maintenance and repair personnel in mind. It may just be the friendliest aircraft to work on of all time.
When unmanned cargo delivery scales - as it will, and our world is going to be changed by it - things like parts, workflows, package/pallet size, delivery and deployment methods, etc. will start to coalesce around standards. I am not familiar with the cargo industry’s specific needs yet, but I hope that by releasing this design I'll gain valuable insight on what some of those standards should be. It's an incredible opportunity to anticipate the needs of a budding industry and design a vehicle with smart standards and organization right from the beginning. This is why open source design is so great!
My goal here is to develop a great tool. That would be my ultimate success.
A vehicle capable in a huge range of mission roles, using a minimum amount of standardized components, which are robust, easy to maintain, and upgradeable. If I can do all that, the big economical to operate piece of the puzzle falls into place.
- A newish lightweight, strong, and inexpensive monocoque construction method for SUAS: Single-structure chassis (printed/molded, or flat-pack assembly) that includes all framing and attachment points, tightly nested in an EPO/EPP foam sandwich which performs with the frame as an integrated structural body and skin. Entire enterprise-quality airframe can be formed out of 2 relatively inexpensive materials.
- Removable jump jets that attach securely in less than a minute with zero tools.
- Updated flight profiles to maximized energy efficiency and take advantage of the latest flight control stability advancements.
- Ultra-modular, ultra-flexible cargo capabilities.
Key Design Considerations:
- Simplicity! In aircraft components, manufacturing processes, operations and maintenance, and cargo design.
- Capable in a huge range of mission roles by being simultaneously highly modular and minimally complex. Like a good cargo aircraft.
- Friendly to maintain.
- Disassembles into 6 neatly packable pieces for transport using zero tools.
- 100% serviceable with one special hex-style wrench. Like, everything.
- All pieces, modules and hatches secure with a single latch each. All latches come in just two flavors, big and small, and are conspicuous when open an satisfying to close.
- Whole VTOL aircraft minus the electronics is manufactured in just 10 pieces (all simple assemblies).
I'm so excited for this project not only because it fits so well with my interests, but also because when I heard of it I knew immediately where I wanted to start:
The XUAV Talon has for a while been one of my favorite UAS designs. It has a proven high-endurance wing design, pleasant handling characteristics and a forgiving CG, plus it's fairly robust and already starts with a large payload bay.
In other words, the perfect thing to destroy!
To build better, faster, stronger.
Also, I’ve wanted to add jump jets to a V-tail or Y-tail configuration like the SULSA UAV or Talon because I know that by "stacking" the tail surfaces over the rear props I can create a sort of bullpup version of a more traditional jump jet design like the excellent Arcturus Jump 20. So turns out this competition was the perfect catalyst. Thanks for emptying my wallet again guys. ;)
Let’s get into it.
The chassis is key. I wanted it to incorporate as many required structures as possible into a single, contiguous piece.
The chassis essentially is the cargo and electronics bays, and it incorporates the wing and tail attachment mechanisms, cargo attaching system harness, electronics system harness, nose payload hardpoint, wiring harnesses and guides, plus all attachment points for hatches, the catapult hook, and the landing gear. Basically all required connections are incorporated directly onto the unibody chassis. This helps makes the design compact while minimizing weight.
During initial assembly, pretty much all the electronics can be easily installed onto the bare chassis before attaching the EPP foam body halves.
It is also not difficult to detach the foam halves later to gain the same level of access to all components and wiring as the day it was assembled - but this isn't necessary for nearly all maintenance.
Placing the main spar directly above (or above and very slightly forward) of the optimal CG location provides the user with a convenient handle/hook for easily checking CG balance as well as handling the aircraft.
- Front and rear electronics bays provide good separation of RX from TX components.
- The smaller main wing “sparlet” in the back of the wing is a permanent part of the wing assembly and seats in a small reinforced sleeve on each side of the frame, thus it does not cross above the cargo bay like the main spar does.
Foam Structural Body and Skin:
The body is molded in two halves out of expanded polypropylene foam (EPP) and fits to the chassis like a glove. This adds a significant amount of stiffness and rigidity to the chassis and helps distribute stresses through the body; enabling the chassis to be built light and remain exceptionally strong.
The foam also forms the aircraft’s external shape and all aerodynamic surfaces. It’s like encasing the cargo and chassis in a perfectly molded shipping block that also conveniently happens to be an airplane.
Also good for insurance and public perception (see safety section).
- EPP foam is not durable enough to withstand repeated high-abrasion areas or frequent high impacts (basically don't use it for landing gear). But those areas are easy to identify and protect by adding a hardened contact point like a small protective skid. On the Colibri these areas are the chin, forward wingtips, and the 3 Y-tail tips.
Removable Jump Jets:
The jump jets consist of 2 identical symmetrical bars (one for each wing), that attach securely in less than a minute with no tools.
Each jump jet bar has a vertical lift motor on each end and the corresponding electronic speed controllers (ESCs) housed internally.
Added weight and complexity is minimal to the point of being negligible.
- User keeps all the benefits and simplicity of a fixed-wing design when that’s the best configuration for the mission.
- User also gains access to all the enhanced capabilities of a jump jet design when needed.
- In addition, an enhanced quadcopter configuration is unlocked.
- Weight penalty is very small. For the standard jump jet config it is no heavier than it would be otherwise. For the fixed-wing config, there's the extra 21 cm (8.3 in) of wire and plugs that remain permanently installed in the inboard wing sections. That’s it.
- Minimal added complexity. The jump jets install in the same way as the wing sections with no tools in less than a minute (see video). Since the flight controller code already knows how to handle conventional fixed-wing and VTOL flight modes, the only added software complexity is for the flight controller to understand when the jump jets are attached so it can switch to VTOL mode.
Attachment system: A slim 80 mm wing section is an integral part of each jump jet bar. This section slides onto main spar in-between the 2 wing sections. A standard plug bus located just behind main spar automatically connects power and control when sections are slid together.
- Plugs are housed inside the wing just behind the main spar and are therefore protected and weatherproof.
- ESCs need to know the correct way to spin motors. Either L and R motor pod are not interchangeable (mechanically simplest), or jump jet bars have a small switch in them next to the plugs, and a notch or ridge on the adjoining surface that forces the switch to the proper position as it’s being slid on.
- Jump jet booms are angled for min drag in cruise flight at normal loaded weight.
- As you can see in the 3-way view the jump jets have a tiny frontal area and streamlined fairings to reduce drag.
- Props stop and align with boom during forward flight for lowest drag.
- In jump jet configuration, starting with a catapult launch increases overall flight endurance because the aircraft doesn’t use the most energy-costly phase of VTOL flight: fully loaded takeoff (see efficiency section).
- *Must make sure system clicks all the way in. User should not be able to secure wings loosely to main spar.*
The empennage is a Y-tail configuration.
The upper Y-tail control surfaces are stacked above the rear jump jet rotors for a compact footprint but are sufficiently clear of jump jet rotors so the two don’t aerodynamically interfere (but only just - one of the testing goals with the Colibri prototype is to find min distance between these aerodynamic surfaces).
The upper Y-tail surfaces fold down to meet the lower vertical stab and click in with a small snap for transport.
- I had a lot of trouble with the tail design but I think this is my favorite configuration because it has the advantage of reducing assembly/breakdown to 4 easy steps (see transportability section), plus by folding the tail instead of removing it, you can house the rudder servos and even the GPS modules in the tail (ideal reception location) without having to plug/unplug anything for transport.
- Y-tail is superior to the V-tail for pusher prop designs because it can be used for landing gear and prevents prop strikes.
It all starts and ends with the cargo. A good SUAS system should be completely designed around the payload it’s intended to carry, and in this case we have the most interesting payload of all: cargo! I think that to design a great cargo hauler you are by definition designing an aircraft which can carry a wide variety of payloads, and carry them well.
For the standard variant the cargo bay dimensions are 1000x350x250 (see variant section) and offers unrestricted through-hole cargo access from both the top and bottom except for the main spar located near the center and above. This means that even a cargo module taking up the entire volume of the bay - all 77000 cubic centimeters (4700 cu. in.) - can still be cleanly loaded from the bottom.
A weatherproof top cover and rigid floor both attach easily to protect sensitive cargo from the elements as well as minimize form drag. But they are not required and the aircraft operates fine with the cargo bay wide open.
The hard floor comes with general purpose securing points or straps, thus allows for nonstandard cargo to be carried without a container or bracket. In actual use the bottom may only need to be removed when airdrop/auto-unload is desired.
Cargo/payload modules must conform to a standard width and simple hole standard for the attaching system. But other than that they can be any size, shape, or function - container, bracket, or shaving cream can filled with dino-dna.
This means cargo containers are very simple and can be manufactured out of almost any rigid material.
This cargo concept supports almost any type of cargo/payload imaginable - powered or unpowered, in a container or without one - as long as it doesn't put the aircraft over it's maximum weight and fits within the 1000 x 350 x 250mm bay dimensions… or even if it doesn’t the cargo can protrude from the top or bottom. Easy loading/unloading from the top or bottom, either manually or by automated package handling system, airdrop, auto drop-off, winch ops, it can be operated with the bay closed or wide open, it allows for quick turnarounds (open latch, swap battery and/or cargo, close latch).
Cargo module types:
- Basic, unpowered cargo container.
- Powered container or payload.
- Bracket for stuff that doesn't fit in a container or can't be in a container for whatever reason.
- Releasable container.
- Container with door on bottom designed to spill contents.
- Anything you can think of.
Easy to lock in a known payload configuration or any individual module that the operator doesn’t want the end user messing with by simply screwing down the module's locking lever with either the maintenance tool or with a special tamper-resistant screw for extra security.
*A locked configuration will probably be used on most routes. When using a locked configuration, this saves the end user from having to know a thing about positioning the cargo or adjusting for CG. They simply load/unload payload from the only spot it fits and go.*
Batteries and Power Handling Concept:
- The full length of cargo bay and the wing hardpoints all have easy access to a common regulated and protected power rail.
- Everything on the power rail is connected in parallel.
- Normal batteries are fitted as cargo modules and can be carried anywhere in the bay same as cargo.
- Allows batteries to be stored and swapped as simply as anything else.
- Batteries hook into the same cargo attachment system concept (see section below) and energize the rail.
- Optional externally mounted battery “fuel tanks” can be mounted to wing in the same location or in addition to the vtol jump jet modules. This is not a standard configuration but can be a useful option for tricky cargo load outs or to carry extra range.
- For operations in subzero temperatures a cold weather battery module with integrated self-powered heating coil would be available.
- This way of handling the batteries maximizes payload configurability because it allows for easy and hugely adjustable component placement while still maintaining proper CG. Need to carry a heavy sensor all the way in the nose? Carry the battery far back in the cargo bay. Multiple packs can also be placed in different locations to account for a tricky loadout or to simply increase capacity.
- Disclaimer: I’m sure I don’t fully understand all the issues of such a wiring system, so I’d love to hear from an electrical technician or engineer on this.
- Parachute launcher is also a special cargo module that is optionally mounted in the rear of the cargo bay.
- Makes adding or replacing the optional parachute as simple as swapping a cargo container.
- The special parachute module has ridge that only allows it to slide into one location at the rear of the cargo bay. This is because that is the optimum position for deploying a parachute, and the chassis/cargo attachment system in that location is reinforced to handle the extra stress of parachute deployment.
Cargo Attachment System Concept:
*I want the cargo attaching system to perform several functions, but I haven’t quite figured out how to make it all work yet. Feel free to skip this section as I haven't "solved" this idea yet.*
Cargo modules slide in from either the top or the bottom and lightly click into place along a horizontal groove or rail that runs the full length of the cargo bay. This allows the modules to “soft lock” into place while still sliding forward or backwards for easy CG adjustment and/or the loading of more cargo.
Once everything is positioned, a conspicuous red lever is closed, extending pins from aircraft into holes in cargo container to “hard lock” the cargo in place. Cargo bay is now ready for flight. System will display alert and won’t arm unless this lever is closed.
Locking pins have shielded female bullet connectors inside, so if cargo/payload module requires power, it simply needs male bullet connectors inside the receiving sockets and it is automatically connected to power when the cargo is hard locked. Cargo that requires no power simply has no male bullet connectors.
Problem is, without placing the hard lock system on the individual containers instead of the airframe (which makes all cargo containers unacceptably complex in my opinion), I don’t know how to make the modules individually removable/droppable. Which is important. You don’t want the battery or other cargo/payload modules coming out of hard lock when a different module is released.
Deceptively simple landing gear: chin and tail skids and protective scuff ridges near wingtips. Really doesn't need more.
Alternative: If this concept proves to be too hard on the airframe, I'll use low-profile carbon fiber landing skids like a helicopter.
Aircraft is fully capable of self-powered vertical or catapult-assist takeoffs... and conventional, vertical, or deep-stall-blip landings.
Deep stall blips: More precisely, a stabilized deep stall approach followed by a brief flare and gentle hover landing all in one smooth motion - but I like deep stall blips. In this maneuver the aircraft descends sharply in a deep stall, stabilized by the jump jets operating at very low power (near idle like betaflight Air Mode). This makes the maneuver safe and reliable even in gusty conditions. Then by automatically “blipping” the throttle 1-3 meters above the surface and flaring slightly, forward and vertical speed is arrested by the jump jets and the aircraft lands gently under hover power. The idea here is this type of landing can remain precise while using less power than a vertical hovering descent.
Here is an example of a conventional mission profile involving a roundtrip flight from home base (fixed station or mobile GCS) to an unprepared delivery location:
- Catapult launch from base. *Not required, but this conserves a lot of energy during the most energy-costly phase of flight: fully loaded ascent to cruise altitude.
- Efficient forward flight to the delivery point.
- Aircraft determines the wind direction over delivery point.
- If the area is somewhat open, a DSB is used. If area is not suitable for a deep stall blip landing, the aircraft descends vertically using the jump jets.
- Once the package has been delivered, the recipient only needs to ensure “clear props, clear sky” before the aircraft can take off again. This time the aircraft ascends vertically to minimum operating altitude using the jump jets (but now there's no cargo unless there's a return package).
- Efficient forward flight back to base.
- Aircraft determines the wind direction over landing zone.
- The flight ends with any type of landing the operator prefers.
Perfect for medical or urgent delivery drones to hospitals or disaster areas or for getting packages to remote or underdeveloped parts of the world.
3 configurations: Basic fixed wing, standard jump jet configuration, and enhanced quadcopter configuration.
Basic fixed wing config is the most efficient in forward flight. Used when maximum range or endurance is needed, or when the jump jets simply aren't needed for the mission. But the aircraft can't hover in this config and it has less flexibility in takeoff and landing options.
Jump jet config is standard. It has nearly the forward flight efficiency of the fixed wing, plus all the sweet operational flexibility of a multirotor.
The enhanced quad config is like the the Hind heavy-lift helicopter of drones. The wing sections outboard of the jump jets are removed leaving stubby quadcopter wings. Best for operations where the capabilities of a multirotor are needed more than a fixed-wing. This configuration is also superior to a conventional multirotor in forward flight speed and efficiency due to the extra lift provided by the stubby wings and a more streamlined body angle. The jump jets (and body) remain horizontal the entire flight and operate at low hover efficiency (hover efficiency minus lift generated by the wing). All pitch/roll/yaw is controlled like a multirotor, but all forward thrust is provided by the pusher motor.
- Broad wing helps slow deep stall descent rate.
Reliability and Safety:
Component Failure Mitigation:
- Enterprise-level critical component redundancy like triple gps, dual imu, robust failsafe modes, etc.
- During forward flight if the aircraft exceeds a critical pitch/bank angle or vertical descent rate outside the normal operating range, this indicates a control surface malfunction and the flight controller immediately uses the jump jets to return the aircraft to a level attitude.
- During hovering flight if the aircraft exceeds the critical angle or descent rate, this indicates a jump jet motor malfunction and the parachute FTS is triggered. A complete power system failure during flight triggers the parachute via a capacitor carried in the parachute module specifically for this purpose.
- Aircraft does not arm before running a simple diagnostics check. If a problem is detected it alerts operator to the issue in mission planner (such as wing or jump jets not detected, wing not locked in place, payload not secured, bad GPS lock, etc). Careful not to add too many extra failure points or nanny features, but just enough. Something like what the 3DR Solo has is good.
- The flight controller reads the capacity of the battery/batteries from the battery management system. If total capacity is insufficient for the mission plus a safety margin, the system alerts the operator.
- On throttle-up, the flight controller quickly self-right the aircraft from any angle up to 30 deg (or more) without significant drift. Aircraft is always recommended to be launched from a level surface, but having this capability baked-in increases safety and truly reduces the requirements of what can potentially be used as a launch area to "clear props, clear sky, clear to fly".
- If on takeoff the flight controller detects too large a difference in rpm between the jump-jet motors - indicating a hazardously uncentered cg, the aircraft will settle back down, disarm, and warn the operator to check cg.
Selectable failsafe behavior in case of lost link or problem not requiring parachute deployment. Default is climb to 200 ft and return to home, but it can be set to loiter, or continue to destination, and the safe altitude can be adjusted.
*Also able to turn on/off certain nanny features in advanced mode.
Aircraft disarms immediately once on the ground.
Aircraft disarms immediately if jostled after arming but before takeoff.
Active ESC motor braking is used to stop propellers quickly.
A foam aircraft is good for safety and the public image of safety. Also insurance rates. Yes, this aircraft still has the kinetic energy to hurt things. That is why we make it safe in many other ways. But I also know if I had to choose a type of aircraft to be hit by, I'm going with the foamie every time.
- Operating hypothesis is 4 reliable motors + a parachute is an overall lighter and more robust safety system than 8 motors. But an 8 motor "H8" configuration is easily possible.
All bay hatches have a squishy compressible closed-cell foam gasket on the underside that forms a nice moisture barrier when closed.
Silicone conformal coating on the electronics is what provides true protection even if moisture seeps in. This would make the electronics IPX58 or better. I can run a multirotor underwater using this method, so it should be good for inside a protected bay.
One of the biggest challenges of the competition is efficiency. It’s stated in terms of carry a relatively large weight a relatively long distance while keeping everything under a certain max weight, but what that really boils down to is efficiency.
The Colibri tackles efficiency from several angles.
Central to the design is a medium aspect ratio, low reynolds number, high-lift wing and minimal form drag body shape. When creating the 3D model I designed the body profile as a lifting body, but this needs testing to optimize or determine what exact shape would be most efficient.
Just as important, the flight phases are improved to most efficiently use this vehicle using rotor-stabilized deep stall descents and catapult-assisted launches when available as they use much less battery power than pure VTOL ascents/descents.
I believe with a single catapult launch from the base station I can get the total round-trip jump jet use-time down to 30 seconds or less. 5-10 seconds each for 2 deep stall blip landings, and a 20 sec or less vertical punch out to minimum operating altitude (or at whatever the most efficient vertical ascent rate is). That means a round trip profile of 2 hours forward flight time plus less than 2 minutes of jump jet use time is possible or very nearly possible right now with hobby parts and I'm going to prove it.
Ease of Maintenance:
All electronics mount to simple vertical trays that slide straight in and out of the airframe. This provides compact stowage and easy access to ALL electrical components with nothing buried or hidden from view.
This method is lightweight and simple, easy to vibration dampen when needed, and will easily accommodate both individual electronic components or fully integrated modules. I imagine carriers operating a fleet of drones will want to have one standard electronics module installed on all their aircraft. Thank you William for that idea.
Zero hard-to-access maintenance areas.
For the most serious modifications the foam halves can detach exposing every nook and cranny.
EPP is a friendly material to work with, modify, and repair.
The entire aircraft is completely serviceable with a single hex wrench style tool, and a compact version of this tool can even be carried onboard the vehicle itself.
All electronics are located in one of two electronics bays under full-length access hatches. The only exceptions to this are control servos, GPS modules (located in the tail and also easily accessible), and possibly the IMU if it actually needs to be located closer to the CG location rather than where it is now in the aft electronics bay: centered on the longitudinal axis and 48 cm (18.9 in) behind the lateral axis.
4 aerospace-quality servos rated at IPX67 move the aileron and empennage control surfaces and are easily accessible in the bottom of the wings for maintenance.
Aircraft breaks down into 6 pieces in under 2 min using zero tools.
Packed Up Dimensions just 2000 x 600 x 650mm.
Assembly/Disassembly is a super easy 4 steps: Remove wings, remove jump jets if installed, slide out wing spar, fold tail down.
No plugs to forget. Other than a single plug in each wing just behind the main spar that automatically connect when the wings are slid on, there are no electrical connections to be made. If it’s got all the pieces, everything’s connected.
The pieces are: Body, L wing, R wing, wing spar, and 2 jump jets. All pieces are 2m or less in length.
Ease of Manufacture:
Foam parts: 2 body halves, 4 wing sections, 2 tail surfaces, 4 bay lids, and possibly the 5mm wing accessory extensions.
Chassis: Bonded 2-piece molded or printed frame, or flat pack assembly consisting of two main structural runners plus 4 molded, printed, or glued-up flat pack bulkheads and 8-12 sections of carbon fiber tube.
Jump jet structure: an 80mm accessory extension from above, 1 carbon fiber tube, 2 motor mounts, plus tube clamps or another way of bonding the boom to the accessory extension.
That's the entire aircraft minus electronics and fasteners.
- "Skycrane" variant with an open cargo carry concept. Same body shape minus a 1000mm belly cutout. Cargo bay is replaced by strong internal spine structure and completely cut-out bottom and sides for carrying outsized or slung loads like the Sikorsky Skycrane.
- "Noble" variant made at minimal cost. Probably flat pack ply (or fully molded frame halves) and cheaper foam than EPP (if necessary), comes with a low-cost electronics package that prioritizes reliability.
- "Icarus" variant with photovoltaics in wing.
- "Spruce Goose" biodegradable variant made to be minimally environmentally impactful. Useful for a one-time use scenario where the craft is expected or likely to be lost carrying out its role, or in sensitive enviroments. Made from "green" ply and a tough biofoam. Battery and electronics are rendered as inert/minimally environmentally damaging as possible.
*All pieces are 2m or less in length for transportability.*
- Body length: 2m (plus removable nose sensor).
- Wingspan: 4m.
- Wing area: 2.16 m^2.
- Wing loading very good. On half-scale prototype @ 5kg AUW it's 11kg/m^2.
- Rotor spacing: 1600mm front to back, 1400mm laterally.
- Total disc area 1.6 m^2.
- Disc loading very good. On half-scale prototype @ 5kg AUW it's 12.5kg/m^2.
- Front electronics bay dimensions: 250 x 200 x 200mm.
- Rear electronics bay dimensions: 350 x 200 x 200mm.
- Cargo bay dimensions:
- Standard variant: 1000 x 350 x 250mm.
- Skycrane variant: 1000 x 450(+) x unlimited.
Key things to test with the new prototype:
- Work with an engineer to perform a loads analysis to optimize the chassis structure and select the best material and manufacturing method.
- Test how the jump jets and fixed-wing surfaces interact. How close can you move the props to other aerodynamic surfaces before they interfere?
- Cargo bay attachment system needs refinement.
- Develop the common power rail. Need an electrical expert to look at the feasibility of a common power rail. Is it best to have all power regulation done in-payload? Or have a main rail plus an auxiliary (5 or 12v) rail?
- How stable can I tune the prop-stabilized deep stall descents? It should be easy to avoid a vortex ring state as long as some forward motion is maintained.
- With testing I am hoping to move the props even closer to the body and wing because until hover instability or aerodynamic interference become an issue, moving the props inward has the triple advantages of shaving weight due to decreasing material used, increasing rigidity by reducing the arm, and allowing for lighter prop guard solutions to be implemented.
- Develop lightweight prop guards. The closer the rotors can come to the wing and body, the better the options become for guarding the props.
- Winch can be easily installed onto main spar sleeve which is located conveniently right above the CG location and nearly centered above the cargo bay. Especially useful for the Skycrane variant.
- All fasteners should have a quality finish and a satisfying “snap”. The car door effect. Good action, secure shut, and easy release works wonders for customer confidence and satisfaction.
- All hatches and attaching points use a slot and tab plus an aluminum latch with a conspicuous lever that sticks up when open and sits flush/aerodynamic when secured.
- Hatches use soft compressible foam gasket on the bottom that acts as a moisture barrier when hatch is closed.
- Can jump jets in stabilize mode be used to "fix" hazardously out-of-range CG? Not efficiently, but in an emergency or for short range?
- Are flaps worth it?
- How about tubercles? - which could be easily molded.
- Wide chin hook for catapult launching that also acts as forward skid. No complicated attachments just big gentle secure hook designed to catch rope or the rubber tubing used for the catapult.
- Catapult is simple to manufacture, or easy to construct out of globally common materials.
- Min clearance (of maybe 1-2cm) is needed above and below the cargo bay and below the electronics bays for wiring or irregularities jutting out of components.
- With jump jets, are aerodynamic control surfaces still needed? It seems like jump jets can handle yaw needs pretty easily, what about pitch and roll?
Propulsion System Technical Specifications:
Here we get into the real nitty-gritty... I hope you like numbers!
- Vertical Lift System:
4x T-Motor U11 120kv motor, 26x8.5 2-blade propellers, and Flame 80A ESCs.
Calculations show that when producing 8kg thrust (x4 motors = 32kg total thrust), the T-motor U11 120kv motors spinning 26x8.5 props are only ~13% less efficient than the U11 90kv motors spinning 28x9.2 props (which is the combo I originally selected). I elected to go with the higher kv/smaller diameter props because as mentioned in the jump jet design I’d like for the props as close as possible to the body/wing, plus a smaller dia. permits the prop guards to be lighter/stronger, the 120kv 26in combo will provide a *much* higher max thrust without adding weight (49kg total vs 37kg) if it’s ever needed for high altitude ops or in an emergency, plus the higher kv motor will nominally operate at a lower % of max with is good for motor lifespan, plus shorter props = less inertia therefore faster control response. The minuscule efficiency hit is a price I gladly pay.
The U10Plus 100kv motors on 10 or 12S is also an appealing option because it saves 230g per motor (or a significant 920g in total weight), but when lifting 25kg the U10 motors will regularly need to operate at greater than 85% of max throttle so I worry about reliability and longevity running them that hard all the time. Ideally I’d like a motor in the middle, only slightly more powerful and heavy than the U10Plus, instead of 40% heavier, but no middle option exists.
- Pusher System:
1x T-Motor U-11 120kv motor, 23x15 or 24x16 2-blade propeller, and T80A ESCs.
I spent hours combing through dozens and dozens of combinations and recommendations on eCalc and surprisingly, the U11 120kv motor again proved to be the best option to use for the pusher motor as well. This time spinning a 23x15 or 24x16 prop.
The main problem with the recommendations given in the ignition kit was the weight. Even though it’d probably be a more efficient setup, there is no way a 2.6 kg motor and accompanying large ESC can squeeze onto this airframe and still remain under all the weight requirements (my opinion).
Amicell High Density LiPo
60km battery: 12S2P, 44.4V, 27Ah, 5kg.
100km battery: 12S3P, 44.4V, 40.5Ah, 7.5kg.
Since weight is such a restricting factor in this competition we need the highest energy-density batteries possible that are capable of doing the job. With this in mind there are three battery cells worth considering. The Panasonic NCR8650B, the GEB8043125 (both LiIon), and the Amicell Extreme High Density LiPo cells or other very high quality LiPos possibly with special chemistry.
Both of the LiIon cells have proven energy densities of about 250Wh/kg (see here and here), plus LiIon is a safer battery chemistry than LiPo. BUT there’s a big problem with using LiIon cells. They have a maximum discharge rate of just 2C continuous, 3C burst - something I have not seen addressed in many of the entries that use them. So even with the very large capacity batteries such as the ones the Colibri will use (27-40Ah+), LiIon cells can’t handle the load generated during VTOL flight. In forward flight they're ideal - and since this is 80%-95% of the total energy expenditure it’s very appealing to use them - still, my concept's VTOL phase requires a minimum discharge rate of about 5C or 2.5x what LiIon cells are capable of. The system could carry a separate 4Ah LiPo battery specifically for the VTOL phase, and I calculate that would shave up to 1.7 kg off the total battery weight compared to LiPos, but then swapping and especially charging batteries becomes more complicated, the operator must always track two different types of batteries or unique battery packs incorporating 2 different cell chemistries are needed, plus there’s the major operational disadvantage with this method of then only being able to use the jump jets for the 2-minute hover phase and never for extended hovering if the mission calls for it. So complication is added and versatility lost. For these reasons I decided not use LiIon batteries for now until a higher discharge rate is possible.
Amicell High Density LiPos claim an energy density of up to 250Wh/kg. Since this is higher than other LiPos I wanted some more information before going with them but I emailed the company and haven't heard back yet. But from what I’ve heard they are a reputable UAS battery supplier so I went with them anyway. Specifically I used the ALBP1158150HG 13.5Ah cells in a 12S2P config for the 60km mission and the same cells in a 12S3P configuration for the 100km mission. There is a number of different cell sizes so different battery sizes could easily be made to match mission requirements.
Notes: For my calculations I also provide the figures for a 200Wh/kg energy density battery to account for a margin of error and also to show the performance figures regular high-quality LiPos would give.
- Hover Flight Power Requirements:
I calculate that at the maximum weight of 25kg, during hover each lift motor will use ~0.5Ah per minute, or 4Ah total to satisfy the 2 minute hover requirement.
*This seems like the perfect time to note that when using my more efficient flight profile concept the aircraft never needs to hover at MTOW and only hovers for ~30 seconds total per round-trip flight so in that case the total energy required for hover is less than 1Ah. Knowing these figures helps simplify battery size calculations because it means we can simply add 4Ah to whatever capacity is required for the forward flight phase and know that we’re covered for the whole mission. 4Ah = 178Wh @ 44.4V, which equals 0.7 kg of battery @ 250Wh/kg or 0.9kg at 200Wh/kg. We can simplify this by saying 1kg of battery is needed to handle the hover phase of flight. Notice I have rounded up at every step as a safety margin. 30 seconds of hover flight would really only require 0.2 kg of battery weight.
- Total Flight Power Requirements:
By my eCalc calculations I need ~780Wh or 18Ah to complete a 60km flight with a 5kg payload, and ~1230Wh or 27Ah to complete a 100km flight with a 3kg payload.
Add these figures the 4Ah for hover and you get 22Ah or 978Wh for 60km, and 31Ah or 1376Wh for 100km.
However the frame sheet calculates these values to be 1046Wh and 1575Wh, so we’ll use these higher figures to calculate weight - again rounding up. *When I have to make estimates on top of estimates I start to question everything. :)
At 250Wh/kg, 1046Wh = 4.18 kg worth of battery, and at 200Wh/kg, 1046Wh = 5.23kg worth of battery.
At 250Wh/kg, 1575Wh = 6.3 kg worth of battery, and at 200Wh/kg, 1575Wh = 7.7kg worth of battery.
In conclusion, I calculate that for a 60km mission at maximum weight the Colibri will need a battery close to 5kg (and probably lighter if anything since we always rounded up), and for a 100km mission at max weight it will need a battery near 7kg. These numbers are highly dependent on the precise efficiencies of the power system and airframe so it’s kinda impractical for me to calculate battery weight down to grams.
I just graduated MCL from ERAU with a BS in UAS and am looking to return to the beautiful PNW for a JOB.
I learned about this awesome challenge in early May, directly before instrument checkride and finals week. Then the family visited, then I graduated. I was so excited about this challenge but had no time! So after all that craziness I worked on this idea sunup to midnight – as much as I could and still keep my sanity to bring you this design. In the end the clock still ran out on me.
Independent of the challenge outcome, look forward to seeing this little bird fly in the near future.
Thanks for checking out my submission.