Multirotor drones have been all the rage for quite a while now. Thanks to the rising popularity of the DJI Phantom range
and others, they are now in the public consciousness. The commercial applications for them are so widespread too that businesses are using them for a vast range of jobs.
But take a step back a few years and most drones used for things like mapping and surveying were fixed wing - more sophisticated versions of traditional, radio-controlled models. They've continued to develop since but they've been overshadowed, to a large extent, by the rise of quad, hexa and octocopters.
So which is better? Multirotor or fixed wing?
As always with this sort of topic, the answer is full of ifs and buts. Each type has things it's good at and there are some jobs that either is suited to. Here we talk about the main features and decide a winner.
Endurance: Fixed Wing
One of the first things that fans of fixed wing aircraft tell you about is range or endurance. It's reckoned that fixed wing drones can stay in the air for up to ten times longer. The main reason for that is they get all of their lift from their wings and just use their motor for thrust. Multirotors, on the other hand, need a heck of a lot of power just to stay in the air. Fixed wings can also take advantage of internal combustion engines if they need to. Not only can the fixed-wing cover a larger area (hundreds of acres) it can also achieve it in just one flight. Recently there've been successful experiments with alternative fuel sources for multirotors and some of them show considerable promise for the future.
A Quest UAV fixed wing
Take-off Area: Multirotor
The flip side of a conventional wing is that it needs a larger area for take-off and landing. It's obviously not as much as a light aircraft but it will still need to be hand or catapult launched before climbing out gradually to its operating altitude. When it lands it will need space for its approach so you'll have to make allowances for clearing obstacles too. Some fixed wings land by parachute and most have one in case of power failure.
This is where the multirotor starts to come into its own. Vertical take off and landing (VTOL) coupled with the ability to hover reduces the size of your operating area and, provided you comply with CAA rules about distance from structures, you could fly your mission in a very tight space. In fact in congested areas vertical flights can be important for safety reasons. If anything does go wrong when your multirotor is filming above tall buildings it will return to home by descending to its take-off point rather than trying to whizz off over the rooftops.
DJI S1000+ octocopter
Speed: Fixed Wing
Traditionally fixed wings have been used for aerial surveying work but their speed of flight can work against them. Mapping software relies on photographs being tagged with positional data so that a large series of overlapping images can be stitched together accurately. That requires precise recording of the time and the place that the image was taken, but at speed there can be a delay between the picture being captured and the geotagging. That problem is reduced if the fixed wing is flying at a higher altitude. In fact the optimum height for many fixed wings is 400 ft (120 metres) which is the normal, maximum permitted height for drones. On the other hand a multirotor can fly at any speed from zero so not only is it more capable of accurate geotagging it can also capture data more easily at a lower altitude if more detail is required.
Manoeuvrability : Multirotor
Multirotors win all the time when it comes to manoeuvrability . Because a fixed wing is faster and can't hover, it's unable to turn on a sixpence or stay put. It can loiter at a location but that involves circling. A multirotor can fly on any axis and at any speed (up to its maximum obviously). This makes it ideal for filming or inspection work where space is tight like between buildings or under bridges. It is also capable of flying indoors. From the pilot's point of view multirotors are much more forgiving to fly.
In the past fixed wings have had the edge when it comes to the size and amount of kit they can carry but now, thanks to miniaturisation of technology and improved batteries and motors, multirotors are catching up. Standard electro-optical cameras and thermal ones can now be flown on the same rig. Heliguy has even developed a thermal set-up
for the relatively small quadcopter, the DJI Inspire 1
. Multirotors can also handle LiDAR, which is used for three dimensional surveying.
A LiDAR sensor for a UAV
Fixed wing aircraft for surveying tend to be large. Often the central pods used to house batteries, motors and cameras can be about the same size as a small to medium multirotor. The wings, although very light, mean you're dealing with a big aircraft. The advantage with many is that the wings will detach for transporting and storage. Even quite small quadcopters can be used for surveying work. The DJI Phantom 2 Vision and Vision+ can capture data for popular software package Pix4D mapper. The DJI Phantom 3
, Inspire 1
and the soon to be released Matrice 100
will all be able to supply images for the same software very soon. As an ideal starting point for multirotor work a Phantom 3, with a diagonal dimension of only 59 cms, is a much tidier size.
Fixed wing aircraft are, more often than not, used by professional organisations and academic institutions so they do have a tendency to be more expensive than multirotors. Sometimes just the business part housing the electronics can be as dear as a complete multirotor. Fixed wings tend to use pre-programmed flightplans more than multirotors so the bill can start to mount up. As an example, American manufacturer Falcon offers both types of aircraft. Their multirotor sells for $8,000 while the fixed wing retails for $12,000. On top of that are two different sensor payloads - a mapping camera set-up for $3,000 or a day/night infrared twin sensor unit for $8,000.
Falcon fixed wing
Here at Heliguy we are bound to be biased. We love our multirotors and they're obviously wonderfully versatile aircraft. Fixed-wings can out perform them with endurance and speed but, as we've discovered, that's not everything.
Do remember that if you are flying fixed-wing or multirotor drones commercially you will need CAA approval in the form of a Permission for Aerial Work (PFAW)