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Advances in VTOL Drone Technology By Bob Young With small quad-rotor drones now well established, it is time to examine the advantages and disadvantages of this configuration. What does the future offer in regards to vertical take-off and landing (VTOL) aircraft? Image Source: https://unsplash.com/photos/e3hH6_pSk1g W ith years of extensive and valuable practical experience now behind quad-rotor drones, the little and not-so-little quad- and multi-rotor drones are here to stay. Drones with four, six, eight or even more rotors are in everyday use (see Figs.1 & 2). We reviewed the Parrot AR Drone 2 quadcopter in the August 2012 issue (siliconchip.com.au/Article/566). We also had a look at six- and eight-rotor drones in the same issue (siliconchip. com.au/Article/567) and more unusual designs in August 2016 (siliconchip. com.au/Article/10035). Despite their many advantages and versatility, these drones still fall short in some areas. By far, the biggest shortcoming is the lack of endurance that any vehicle powered by batteries is faced with. The energy density of batteries is sadly lacking compared to chemical fuel (liquid or gas), as shown in Figs.3 & 4. Combine this with the length of time required for recharging, and the shortcomings of electric-powered aircraft are serious indeed! On a brighter note, electric power wins hands down in terms of simplicity, reliability in starting and running siliconchip.com.au and, most importantly for drones, starting and stopping in flight. Combine this with the huge reduction in the number of parts that make up electric motors (and thus cost of manufacture), and we can see why there are incentives to push on with the search for a suitable electric power source. To demonstrate how difficult this problem is to solve, even liquid hydrogen (H2) ranks very poorly against fuels like gasoline (petrol) and diesel. Note that the LiPo batteries used by most drones are marginally better than the standard Li-ion cells shown in Fig.3, but not by much. While LiPo batteries have one of the best energy densities of lithium based Fig.1: a conventional small quad-rotor drone. Source: https://pixabay.com/ photos/quadrocopter-drone-modelling-1033642/ Australia's electronics magazine March 2022  79 Fig.2: this small human-carrying quadcopter basically follows the conventional quad-rotor layout. Source: www.flickr.com/photos/apbutterfield/23632731924/ batteries, they have also been responsible for starting many fires. Some resulted from poor charging procedures, while others are just due to the volatile nature of the chemical composition of the LiPo battery. Essentially, once the LiPo battery decides to fail, it often does so spectacularly. I once was asked to service a radio control transmitter fitted with a LiPo battery which had been left switched on in its aluminium carrying case. For reasons unknown, the battery caught fire; luckily for my customer, the fire consumed all of the oxygen in the case and it fizzled out, but not before it had done irreparable damage to the transmitter. However, the story of the battle for the best fuel for drones does not end with energy density. We have the fuel weight to take into account as well. Fig.4 tells that story. So as you can see, there is a definite requirement for a better way to power multi-rotor drones than batteries. Quad-rotor drones Fig 3: a chart comparing the energy density of a variety of fuels, including batteries. Note that the density (shown in megajoules per litre) relates only to the volumetric efficiency and ignores the weight; weight is considered in the following figure. Original source: US Department of Energy Efficiency and Renewable Energy Fig.4: a comparison of the energy content per unit weight and volume for common fuels against gasoline. Original source: US Energy Information Administration 80 Silicon Chip Australia's electronics magazine To understand how drones can be improved, let’s briefly look at how a typical small quad-rotor drone works. They are built using components like those shown in Fig.5. From left to right, they are a battery, a power distribution module, a flight control module, a receiver, four identical electronic speed controllers (ESC) and four identical motors with two clockwise-pitch propellers and two anti-clockwise pitch propellers. A video camera and associated components may be added to provide what is commonly known as first-person view (FPV). Fig.6 shows the main thrust vectors involved in controlling a quadcopter. Being basically stable, horizontal flight is the main task of the flight controller. Stationary flight (hover) is achieved when Fz = Zworld (Gravity) and Fy = 0. To achieve this, all motors should be delivering equal thrust with two motors rotating clockwise and two motors rotating anti-clockwise. Strictly speaking, there is no front, back, left or right side as the quadcopter can be flown in any direction. However, the flight controller needs to be mounted so that the transmitter sticks are coordinated with the flight controller to give the pilot a sense of control. The quad can move in any direction simply by reducing the RPM on siliconchip.com.au Beoavia Beoavia (https://beoavia.org/) is a non-profit student team within the Association of Aviation Students. The team was founded in April 2018 by students from the Faculty of Mechanical Engineering at the University of Belgrade, Serbia, and deals with the calculation, design, and production of aircraft to participate in various European and international competitions. By their respective area of interest and education, team members are divided into sub-teams: aerodynamics; structure; manufacturing; propulsion; electronics and programming; and marketing. By participating in aerospace engineering competitions, the Beoavia team represents the University of Belgrade, and enables its members to exchange knowledge and experiences with students from other European countries. Fig.5: an example of how a quadcopter is typically built from separate modules. Commercial modules might integrate some of these, but they use essentially the same configuration. the two motors in the direction of travel and increasing the RPM on the two motors on the opposite side. This introduces the thrust vector (Fy) into the equation, and thus the quad moves in that direction. To increase altitude, all four motors increase in RPM, and likewise, a common decrease in RPM will result in a loss of altitude. To achieve rotation in the yaw axis is a little more complicated; it requires the use of yaw torque. There are two sources of yaw torque in a quad-rotor or multi-rotor, but both are pretty weak relative to the other control factors. This will become significant later when we discuss quadplanes. The first is the imbalance between the torque generated by the clockwise spinning rotors and the anti-clockwise spinning rotors. This is entirely a function of friction in the motor bearings and aerodynamic drag. The second is torque arising from the conservation of angular momentum when the rotor speeds are changed, similar to how a reaction wheel works. This effect is present in a vacuum, so it does not rely on aerodynamic forces. The first effect causes angular acceleration of the vehicle proportional to the difference in rotor speeds between the sets of rotors. The second effect causes angular acceleration of the vehicle proportional to the difference in the derivative of the rotor speeds (ie, their rotational accelerations) between the sets of rotors. It is when dealing with rotation that we encounter the concept of props-in and props-out (see Figs.7 & 8). This refers to the relative direction of rotation on all four rotors. Fig.7 shows the direction of rotation for the ‘props-in’ configuration. This is the default for all flight controllers and most multi-copters with a boom span over 7.5cm. The props-out configuration is used by most pros for 7.5cm quadcopters Fig.7: the ‘props-in’ configuration. Fig.8: the ‘props-out’ configuration. Essentially, the clockwise/anticlockwise layout is reversed compared to Fig.7. Fig.6: the vectors involved in quadcopter control and motion. siliconchip.com.au Australia's electronics magazine March 2022  81 Fig.9: a typical quadplane combines a standard aircraft layout and a quad-rotor layout. Note the motor on the front of the centre fuselage to provide forward thrust. For horizontal flight folding props are fitted to the four electric motors. Quadrocopter designed and built by the author. Fig.10: a very neat quadplane featuring a rear-mounted motor to provide forward thrust. Fig.11: the problems confronting a quadplane in the hover position without a motor to provide forward thrust. Original source: MicroPilot (www.micropilot.com) – used with permission. and smaller, at least when they are focusing on notable flight characteristics; a fact that becomes quite obvious when making a sharp turn. A sharp turn will cause a sudden dip and lift when using props-in rotation, just like in a dull 90° turn due to the turbulence during the yaw rotation. Some earlier whoop crafts had this problem until a solution was found, which turned out to be using the reverse (props-out) rotation. There are other factors involved with the props-in/out argument, but 82 Silicon Chip they fall outside the scope of this article. However, one aspect worth mentioning is that props-in helps keep dust and dirt off the camera in the event of a flip-over during landing. So, to summarise the pros & cons of quad-rotor and multi-rotor drones. Advantages: • Multi-rotor drones are easy to control and manoeuvre • They can take off and land vertically • They can hover • They are very stable Australia's electronics magazine Disadvantages: • Multi-rotors have a limited flying time (usually 15-30 minutes) • They only have small payload capabilities • Most of the drone’s energy is spent on fighting gravity and stabilising themselves. Quad-planes It is the last point that has driven the next stage in the quest for better outcomes. That is the addition of wings to the ‘copter to improve the payload and range capability. Such an aircraft is called a quadplane, and typical examples are shown in Figs.9 & 10. Adding two booms to a conventional aircraft makes it possible to mount the quad motors in the correct arrangement. However, just adding the quad motors without a motor to provide thrust for forward flight is not good enough. In this case, we need to tilt the aircraft forward to achieve a thrust vector to provide forward thrust for level flight. This arrangement is far from ideal, as shown in Fig.11. Figs.11 & 12 are originally from the Micropilot web page. Micropilot is a long-established and well-respected autopilot manufacturer in Canada. In Fig.11, we show the quadplane (without motor) in the hover position with a headwind. To hold a position relative to the ground, we must tilt the aircraft forward to provide a thrust vector from the four rotors for forward motion, to cancel the drift. This places the wing at a negative angle-of-attack (AoA) relative to the wind, which is now flowing over the wing and thus producing negative lift, which in turn calls for more power from the motors to hold the required altitude. That means more current from the battery; as is the way of the world, you don’t get anything for nothing! So we must look beyond our simple quadplane concept and go to the next step. This is to provide forward thrust with a propeller mounted either in the nose (tractor) or at the rear (pusher). This propeller can be powered either by an electric motor or an internal combustion motor. Take your pick. For a whisper-quiet surveillance drone, the obvious choice is an electric motor up front. For long-endurance drones, though, the obvious choice is an internal combustion engine (ICE). Fine examples of such aircraft are siliconchip.com.au shown in Figs.9 and 10. So we now have a long-endurance quadplane that can take off and land vertically, capable of holding position in a hover in a strong wind. As we are no longer required to tilt the aircraft to hold the hover due to the thrust provided by the IC engine running at a low throttle setting, this reduces the lift required from the four rotors when in hover, thus saving electrical power. An additional benefit from this style of quadplane is that we can now completely shut down the four electric motors in forward flight, providing an even greater saving in battery power. Thus, rather than being of prime concern, the batteries are needed only to provide power during take-off, hover and landing. However, have we reached the peak of aerodynamic efficiency? We still have two large booms to carry and various protrusions, such as motors and props out in the breeze, which all provide drag. There are many gifted people in this world, and some of them have come up with what I consider to be one of the most ingenious and elegant quadplane layouts I have yet to come across. That is the Beoavia Wasp, a Quadplane designed by a group of European students (see panel). The ability to take off and land vertically is of paramount importance in many applications. It eliminates the need for runways or large clearings for landings or take-offs. But the price to be paid is the expenditure of a considerable amount of energy lifting and lowering the quadplane to and from what is known as transition altitude. This is the altitude at which it is deemed safe to put the quadplane into forward flight. There is another rather complex requirement for quadplanes: a control system that can handle the transition from vector stabilisation and control to aerodynamic control surfaces as in traditional aircraft when in forward flight (controlling the throttle, ailerons, elevator and rudder). Consider the Wasp quadplane shown in Figs.13 & 14. Once it has transitioned to forward flight, the rotors are tucked away inside the fuselage and can no longer play any part in the control of the aircraft. During take-off, landing and hover, the receiver feeds directly through a flight controller into electronic speed siliconchip.com.au Fig.12: problems for a quadplane in a crosswind hover. Original source: MicroPilot (www.micropilot.com) – used with permission. Fig.13: a most elegant and ingenious VTOL quadplane, the Beoavia Wasp. Source: screen grab from Beoavia YouTube video (https://youtu.be/ T8xTAOuBwKc) Fig.14: the Wasp with the undercarriage, motors and props retracted into the fuselage. Now we are talking real aerodynamic efficiency. Source: screen grab from same video as Fig.13. controllers (ESC) and finally, to the motors. However, in forward flight, we must revert to a standard radio control system where the receiver bypasses the flight controller and feeds servos instead. We might need both systems to be Australia's electronics magazine fully functional during the transition, depending upon a host of variables. All of this has been taken care of in the Wasp. It should be evident by now that the future for quadplanes is very bright, and this is only the beginning! SC March 2022  83