Aeronautical Bionics: How does a delivery UAV take off with a bird's leg?
Time: 2019-02-23 17:19  Click:100
How to design UAVs with short takeoffs but long distances? The common method used in the industry is to combine fixed-wing and rotors. However, this design is obviously too complicated. At present, we have seen tail-wing UAVs that can transition from vertical takeoff to horizontal flight, UAVs with rotatable propeller systems, and even fixed-wing UAVs with Four-rotor to achieve this goal. The scheme of vertical takeoff and landing capability. These designs play a role more or less, but inevitably increase the weight, cost and complexity of UAV, which means that it will inevitably lose some of its mission execution.
Passerine, a South African startup, has come up with a better way to mimic the bird's idea of flying: using wings for long flights while taking off and landing on legs/feet. The company has designed an unmanned aerial vehicle called Sparrow (as shown in the figure below). The engine is mounted on the upper wing of the aircraft. Its legs are loaded with springs, which provide 80% of the energy needed for takeoff. When it reaches its destination, the unmanned aerial vehicle slows down and its legs stretch out to serve as shock absorbers.
You must first pay attention to the leg design, which will be described in detail later. Let's start with the fixed wing design of the fuselage. These super-airfoil engines can produce the so-called "blow-wing effect". The exhaust of the engine will flow through the top of the wing and part of the wing flaps. In this way, the forced adjustment air that blows through the wing and flaps quickly will generate a lot of lift (twice or triple the lift of the traditional wing), and because the air comes directly from the engine. Aircraft, means that even if the speed of the aircraft is not fast, it can get considerable lift. Unlike most traditional wing and flap designs, the lift of the latter depends to a large extent on the forward speed of the aircraft, while the aircraft with blown wings can take off and land in a shorter distance, and the stall limit has been greatly improved.
It needs to be made clear here that the blown wing design is not originally created by Passerine, which has a long history. Antonov cargo planes in Ukraine use similar on-wing engines. NASA even tested this low-noise short-range research aircraft (QSRA) in the 1970s, which can take off and land easily on an aircraft carrier without ejectors or brakes.
 But there are reasons why wing engine design has not been really popular. First of all, the maintenance of such designs is more difficult and expensive, because the maintenance personnel can not operate directly on the ground. In addition, the risk of using this design is also great, because the engine itself will generate huge lift, compared with the traditional standard engine layout, such aircraft is likely to take off or landing when the body overturns and so on. Of course, in most cases, the reason why airplanes do not use blown wing design is entirely unnecessary. After all, the runway is usually long enough to withstand the above disadvantages in order to obtain additional lift.
But in the case of UAVs, the impact of these problems is much smaller. Because the airframe is lighter and smaller, the wing engines of UAVs are actually easier to maintain. Although there is still a certain risk of overturning during takeoff or landing, the problem is not particularly unacceptable because it is only used to transport goods. For most UAV use cases, we often have little or no ready-made infrastructure to borrow, which means that short-range take-off and landing capabilities will be the key to its feasibility.
The blown wing design used in Sparrow UAV can not lift the aircraft itself off the ground alone, so it is necessary to match the leg structure. This leg structure can be understood as a kind of portable elastic system which can be used repeatedly for UAV. Its power is provided by spring, and it can provide most of the momentum (80%) needed for take-off.
After takeoff, the legs will be retracted into the fairing next to the fuselage to ensure that no excessive drag is generated, and Sparrow will then be able to fly smoothly like other common fixed-wing aircraft. Once arrived at the destination, the legs worked backwards: no one had the chance to decelerate as much as possible (using a blown wing design to maintain lift), stretch the legs and use them as shock absorbers to achieve a smooth landing.
The existence of this system means that Sparrow UAV supports all the advantages of fixed-wing UAV (high load-carrying capacity, speed level, range and energy efficiency), plus precise landing capability of rotorcraft, without the need for compromise hybrid design. Of course, Sparrow can't hover in the air like a rotorcraft, so the type of tasks it can perform will also be affected. For example, it may not be suitable for shooting jobs. But that's OK, because Passerine is currently focusing on delivery UAVs, which have high requirements for carrying capacity, range and speed, and the ability to take off and land without runway will open up more practical application space for them.
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