Drone Types: Multi-Rotor vs Fixed-Wing vs Single Rotor vs Hybrid VTOL
Although multi-rotors get most of the attention in the drone world, they are not the only option. If you’re serious about aerial mapping then you need to consider a fixed-wing aircraft, and single-rotor helis are also a great solution for some niche applications too. Here’s a run-down of the four main types of aerial drone platforms, and the strengths and weaknesses of each.
If you want to get a small camera in the air for a short period of time, then it is hard to argue with a multi-rotor. They are the easiest and cheapest option for getting an ‘eye in the sky’, and because they give you such great control over position and framing they are perfect for aerial photography work.
The downside of multi-rotors is their limited endurance and speed, making them unsuitable for large scale aerial mapping, long endurance monitoring and long distance inspection such as pipelines, roads and power lines.
Although the technology is improving all the time, multi-rotors are fundamentally very inefficient and require a lot of energy just to fight gravity and keep them in the air. With current battery technology they are limited to around 20-30 minutes when carrying a lightweight camera payload. Heavy-lift multi-rotors are capable of carrying more weight, but in exchange for much shorter flight times. Due to the need for fast and high-precision throttle changes to keep them stabilised, it isn’t practical to use a gas engine to power multi-rotors, so they are restricted to electric motors. So until a new power source comes along, we can only expect very small gains in flight time.
Fixed-wing drones (as opposed to ‘rotary wing’, i.e. helicopters) use a wing like a normal aeroplane to provide the lift rather than vertical lift rotors. Because of this they only need to use energy to move forward, not hold themselves up in the air, so are much more efficient.
For this reason they are able to cover longer distances, map much larger areas, and loiter for long times monitoring their point of interest. In addition to the greater efficiency, it is also possible to use gas engines as their power source, and with the greater energy density of fuel many fixed-wing UAVs can stay aloft for 16 hours or more.
The main downside of a fixed-wing aircraft is obviously their inability to hover in one spot, which rules them out for any general aerial photography work. This also makes launching and landing them a lot trickier, as depending on their size you can need a runway or catapult launcher to get them into the air, and either a runway, parachute or net to recover them safely again at the end. Only the smallest fixed-wing drones are suitable for hand launch and ‘belly landing’ in an open field.
Other downsides are their higher cost, and that it is much more difficult to learn the ropes with fixed-wing drones. One reason why multi-rotors have become so widespread is that it is easy to get started: anyone can buy a cheap quad-copter and start hovering in their back yard, practicing the skills and gradually getting more and more confident before flying further, higher and faster. That isn’t the case with fixed-wing drones: the first time you launch one you need to be confident in your abilities to control it from launch, through the flight and then bring it back to a soft landing. You don’t get a chance to put it into a hover and think, putting the sticks in the middle won’t keep it in place: a fixed-wing drone is always moving forward and they move a lot quicker than a multi-rotor!
Another consideration of fixed-wing drone work is that it is much more about the data, not just taking pretty pictures. With a multi-rotor session you’re generally done with the job when the flight is over, you only need to hand over the imagery. With fixed-wing work the flight is just the beginning, you’ve captured the images but it isn’t yet the data the clients are looking for. The imagery is fed through the first stage processing to stitch the hundreds (or thousands) of separate images into one big tiled image, but there can be a lot more to be done after this in performing data analysis such as the stockpile volume calculations, tree counts, overlaying other data on to the maps, and so on.
Finally, a big challenge when operating fixed-wing drones in Australia is our Wedge-tailed Eagles. You won’t find them in the city, and they won’t often attack a multi-rotor flying low regardless, but for the average fixed-wing job further out of town and flying 100m high, these huge birds are a menace. Many people have had their new drones ripped from the sky by these amazing but incredibly aggressive birds of prey. Keep a close lookout, and be ready to make some quick evasive manoeuvres!
While a multi-rotor has many different rotors to hold it up, a single rotor has just one, plus a tail rotor to control its heading. Helicopters are very popular in manned aviation, but currently only fill a small niche in the drone world.
A single-rotor helicopter has the benefit of much greater efficiency over a multi-rotor, and also that they can be powered by a gas motor for even longer endurance. It is a general rule of aerodynamics that the larger the rotor blade is and the slower it spins, the more efficient it is. This is why a quad-copter is more efficient than an octo-copter, and special long-endurance quads have a large prop diameter. A single-rotor heli allows for very long blades which are more like a spinning wing than a propeller, giving great efficiency.
If you need to hover with a heavy payload (e.g. an aerial LIDAR laser scanner) or have a mixture of hovering with long endurance or fast forward flight, then a single-rotor heli is really your best bet.
The downsides are their complexity, cost, vibration, and also the danger of their large spinning blades. While a multi-rotor prop can certainly leave you with a bad scar, it is unlikely to do much more than that. The long sharp blades of a heli can cause more serious damage if you get in their way, and there have been a number of fatalities from RC hobby and drone helicopters.
In terms of difficulty, single-rotor heli drones lie somewhere between multi-rotors and fixed-wing aircraft. On one hand they can hover on the spot, so it is possible to start easy and work your way up, but on the other hand they aren’t as stable or forgiving in the event of a bad landing, and they also require a lot of maintenance and care due to their mechanical complexity.
Fixed-Wing Hybrid VTOL
Merging the benefits of fixed-wing UAVs with the ability to hover is a new category of hybrids which can also take off and land vertically.
There are various types under development, some of which are basically just existing fixed-wing designs with vertical lift motors bolted on. Others are ‘tail sitter’ aircraft which look like a regular plane but rest on their tails on the ground, pointing straight up for take off before pitching over to fly normally, or ’tilt rotor’ types where the rotors or even the whole wing with propellers attached can swivel from pointing upwards for takeoff to pointing horizontally for forward flight.
Many of these configurations were tried in the 1950s and 60s for manned aircraft, but they proved too complex and difficult to fly, with some disastrous results. With the arrival of modern autopilots, gyros and accelerometers, suddenly these whacky types are feasible because the autopilot can do all the hard work of keeping them stable, leaving the human pilot the easier task of guiding them around the sky.
There are only a handful of hybrid fixed-wing aircraft currently on the market, but you can expect this to be a much more popular option in the coming years as the technology is perfected. One example getting a lot of attention is Amazon’s Prime Air delivery drone, shown below.
Pros and Cons Table
|Pros||Cons||Typical Uses||Price ($AUD)|
||Aerial Photography and Video Aerial Inspection||$5k-$65k for pro drones|
||Aerial Mapping, Pipeline and Power line inspection||$25-$120k for pro drones|
||Aerial LIDAR laser scanning||$25-$300k for pro drones|
||Drone Delivery||TBD, in development|
About the Author
Andrew Chapman is the NSW Director of Operations for Australian UAV (www.auav.com.au), specialising in aerial mapping, survey and inspection work since 2013.
This article was originally written for the Australian DRONE magazine, issue 3 (June 2016).