Delivery Drones Use Bird-Inspired Legs to Jump Into the Air

Passerine's fixed-wing drones can take off (and land) using a pair of legs

Passerine drone Sparrow
Photo: Passerine
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Drones have a fundamental design problem. The kind of drone that can carry large payloads at high speeds over long distances is fundamentally different from the kind of drone that can take off and land from a small area. In very simple terms, for the former, you want fixed wings, and for the latter, you want rotors.

This problem has resulted in a bunch of weird drones that try to do both of these things at once, usually by combining desired features from fixed-wing drones and rotorcraft. We’ve seen tail-sitter drones that can transition from vertical take off to horizontal flight; we’ve seen drones with propeller systems that swivel; and we’ve seen a variety of airframes that are essentially quadrotors stapled to fixed-wing aircraft to give them vertical take-off and landing capability. These sorts of compromises do work, more or less, but being compromises, they’re inevitably adding weight, cost, and complexity in order to be able to do everything they need to do.

A South African startup called Passerine has a better idea, which is to do what birds do: Use wings to fly efficiently, while relying on legs and feet for takeoff and landing.

This is a rendering of Passerine’s drone, called Sparrow. The first thing to notice is of course those legs, which we’ll certainly get into. But the fixed-wing design of the airframe might be a better place to start. Those over-wing engines create what’s called a blown wing, where the engine exhaust passes over the top of the wing and over a portion of the wing flaps. The forced high-speed air passing over the wings and flaps generates a substantial amount of lift (two or three times the lift of a conventional wing), and since the air is coming directly from the engines, you get that lift even if the aircraft isn’t moving very much. This is in contrast to most conventional wings and flaps, the performance of which depends on the aircraft’s forward velocity. The upshot is that aircraft with blown wings or blown flaps can takeoff and land over a much shorter distance, and can fly much more slowly before they stall.

To be clear, blown wings aren’t Passerine’s idea, and they’ve been around for a while. Ukraine’s Antonov currently produces a freight aircraft with a similar over-wing engine arrangement, and NASA tested this Quiet Short-Haul Research Aircraft (QSRA) in the 1970s, showing that it could takeoff and land on an aircraft carrier without catapults or arresting gear, with room to spare.

There are several reasons why over-wing engines never really caught on. The first is that they’re more difficult and expensive to maintain, because you can’t easily reach them from the ground. They’re also potentially riskier to use—since the engine itself produces so much lift, losing an engine during takeoff or landing has much more immediate consequences than with a standard engine arrangement. But the biggest reason why blown wings aren’t used in more aircraft seems to be simply that they’re not really necessary—runways are long enough that the extra lift they offer just isn’t worth the downsides.

For drones, however, these downsides are minimal. At a smaller scale, the over-wing engines are actually easier to maintain. There’s still some risk with engine loss on takeoff or landing, but since you’re only hauling cargo, it’s not nearly as serious. And for many drone use cases, you have little or no infrastructure to rely on, making short takeoffs and landings far more important.

The blown wings that Passerine’s Sparrow drone uses definitely can’t lift the aircraft off of the ground by themselves, which is where the legs come in. You can think of the legs sort of like a self-contained and reusable catapult system that the drone carries along with it. They’re spring loaded, and provide the majority (80 percent) of the energy required for takeoff. 

After takeoff, the legs retract into fairings alongside the fuselage to make sure that they don’t cause undue amounts of drag, and Sparrow flies just like any other fixed-wing aircraft. Once it gets to its destination, the legs can be used in reverse: The drone slows down as much as possible (using the blown wing to maintain lift), extends it legs, and then uses them as shock absorbers. 

What this system means is that you can have all of the advantages of a fixed-wing drone (payload, speed, range, and efficiency) along with the pinpoint landing capabilities of a rotorcraft, without having to compromise with some kind of hybrid design. Sparrow can’t hover like a rotorcraft does, which will be a slightly limitation on the kinds of missions it’s able to perform. It might not be ideal for camera work, for example. But that’s okay, since Passerine has their eye on deliveries at the moment, where payload, range, and speed are all critical, and being able to takeoff and land without requiring infrastructure like runways opens up many more options.

For more details, we spoke with Passerine founder and CEO Matthew Whalley. But first, here’s a quick clip to illustrate that the combination of legs and a blown wing can in fact get Sparrow into the air:

IEEE Spectrum: Can you put the takeoff test video into context for us?

Matthew Whalley: The video is with our test airframe: It shows that on launch we got past our stall speed and had some control, although we didn’t have our full control system onboard. The transition from what you see in the video to actual flight is basically control: The drone retracts the legs and keeps accelerating, climbing out at about 30 degrees. The flaps get raised, and it goes into cruise configuration. The real takeoff won’t look too much more exciting than the part you see in the video.

Can you describe what happens when the aircraft launches?

When it launches, it essentially jumps into the air. The launch is very similar to a bird. What a lot of people don’t realize is that when a bird takes off from the ground, it’s not generating the lift with its wings. Most of that initial takeoff velocity comes from a jump. Many small birds will do about a 5g jump to get them up to speed before they start flapping their wings. Our aircraft does essentially the exact same thing. When it jumps, it’s not about gaining height, it’s about launching the drone forward to get it up past its minimum flight speed, and at that point it’s flying like a conventional aircraft. 

Where did this idea come from?

The idea of having a drone that could do long range and carry a fairly large payload originated back when I was at university, where it was this need for something in Africa to basically bridge the infrastructure gap that we have present in a lot of countries here. Interestingly enough, South Africa was the first place to do drone delivery—about 15 years ago we were trying to do medical deliveries. So that was the basis for the drone. I knew the capability it needed to have. 

Also, knowing that specifically in Africa, but also generally in the developing world, there is potential for this massive improvement by using drones, but there’s not a lot of infrastructure, not a lot of places where you could use a conventional fixed-wing airplane or drone. So you need something that can operate from very low infrastructure, and the legs came about as being a more efficient way of getting a long-range airplane into the air rather than trying to strap a quadcopter to it.

How closely are the legs modeled on the legs of birds?

It’s an interesting story. We started off with something that looked nothing like a bird’s leg. We knew that we needed a certain acceleration to get to flight speed, and every time we did an iteration that gave us more efficiency, it started to look more and more like a bird’s leg. The current design that we have now, which resembles a bird’s leg very closely, is actually the result of an iterative design process. It wasn’t our intention to make a bird’s leg, it just turned out that that was quite an efficient way of doing it.

What are the advantages of Passerine’s drones relative to hybrid drones, tail sitters, and other VTOL designs?

There are several different things. One is, hover is the least energy efficient point of flight by quite some margin. What we’ve essentially done is looked at most of the missions people are trying to do with drones and said, well actually, none of these require hover, they just need zero infrastructure takeoff and landing. So, by avoiding hover, we avoid the least efficient point of flight, which means we don’t need to carry as many batteries, we don’t need as powerful motors, and the result of that is that everything can be made a bit lighter, which means either you can fly farther, or you can carry additional payload. 

How much of an advantage is this for your design over other systems?

That depends on the mission, and on the specific system, obviously. But generally, we’re looking at 10 percent better performance than a hybrid drone. It depends on what the mission is: If you’re just doing a low-speed survey mission, it’s more in the 10 percent range. Where we really shine is on delivery or high speed missions, because we’re able to cruise at about 50 percent higher speed, and still have the same sort of energy usage as any of these other drones. We can do deliveries a lot faster, we can respond to emergencies a lot faster, and that’s where there’s a really big advantage over a hybrid drone.

Where does that higher speed come from?

Compared to most fixed-wing drones that are out there, our big advantage comes from our engine layout. We can design a drone that is optimal for high speed flight, but because we can use our blown wing to fly very slowly, we’re still able to bring it in at a low speed so that the autopilot can land it easily. That’s one of the major advantages that we have—our ability to be optimal for high speeds, but still able to perform low speed maneuvers, due to the wing and engine layout.

This system can also be used for landing, right?

If you can imagine how a bird lands, or how a hang glider lands, essentially you flare the aircraft very sharply, stalling the wings and using the entire aircraft as an air brake. You do an approach at a reasonably low speed and altitude, and then you do the flare maneuver which essentially stops the aircraft and it starts dropping towards the ground. One of the advantages of our engine layout and airframe is that in that flared, stalled configuration, we still generate a fair amount of lift.  Not enough to fly, but enough that you’re only getting a very small acceleration as you descend. So, even though we’re sinking, we sink quite slowly towards the ground. And then we deploy the legs below the aircraft to essentially act as shock absorbers for the last remaining energy that wasn’t dissipated by the flare maneuver.

How important is being able to land?

My personal belief is that yes, the landing is critical. Particularly when what you’re delivering is either sensitive or critical. In my opinion, things like blood really shouldn’t be being dropped by parachute. You want to be able to deliver it very precisely, and to the correct person, possibly into a secure environment.

Is this kind of landing really achievable with a useful payload?

Yes. Absolutely. We’re very confident on this approach to landing. 

Are there significant downsides to your design? For example, do the legs add a significant amount of weight, is the autopilot particularly complex, that sort of thing?

The one obvious disadvantage that our design does have is that it’s not able to hover. There are some missions, not many, but some missions where you want to be able to hover. We’re not designing for that. The other things are, yes, having legs on this airframe, they do weigh something. But the weight is not that significant—they’re more comparable to a retractable undercarriage that you’d have on a large remote controlled aircraft. You do need a reasonably complex flight controller, but nothing beyond what’s available off the shelf.

Can you give me a sense of what your targets are for speed, range, and payload?

For the first aircraft that we’re building, which we’re hoping will be the smallest of several versions, we’re looking at a cruise speed of 120 km/h, carrying 2 kg of payload, and a flight time of an hour. We plan to scale up to 100 kg payload with a 6-meter wingspan; that’s the largest size that we’re doing on-paper designs of, and that’s where we want to get to in the fairly near future.


Whalley tells us that Passerine will be starting pilot programs in the second quarter of 2019, deploying in several places across Africa doing real-world missions. They’re still working on a complete flight cycle (takeoff and landing), but Whalley expects that they’ll achieve that within the next month or two. 

Drones have the potential to be a valuable logistics tool in Africa in the near future, and from what we’ve seen, the ability to takeoff and land from areas without infrastructure will be critical to their effectiveness, especially when it comes to serving the areas that need them most. Passerine is certainly not the only drone company targeting this space, but they’re one of the most innovative, and we’re very much looking forward to seeing how Sparrow performs.

[ Passerine ]

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