The wingspan of a drone denotes the distance from blade tip to blade tip along one of its sides and is most often measured in millimetres. More recently though drones are measured from the centre of one motor to the other across the front of the quadcopter and this measurement gives a drone its Class. For example, a 250 Class racing drone is around 250 mm across the front motor to motor. Distance between front and rear motors can vary considerably, but does not affect a drones Class.
That said, a Nano drone is usually around 50mm or smaller across, micro drones can be 50 - 100mm and mid-sized drones 150 – 300mm. The largest toy drones around can be up to 600mm across.
At the consumer drone level most are around 300 – 500 mm across. The DJI Phantom, Cheerson CX-20 and the Mariner all fall within this range as does the most newer drones such as the Ehang Ghost, 3D Robotics IRIS+ and Solo, Walkera QR X350 and Tali H500 as well as amazing DJI Inspire 1.
I like to use wingspan as a metric because it gives you an idea as to how large the quadcopter is which has a lot of bearing on what it is capable of doing and where you are able to fly it. A drone flight stability is greatly affected by its wingspan. Smaller, lighter, quads with a shorter wingspan are highly manoeuvrable and zippy which is great indoors where wind is not a factor, but they can be somewhat unstable outdoors in the elements. Larger drones are more geared toward smoother outdoor flight and can generally handle a light breeze, but are harder to navigate through tight spaces.
With flight time we're looking at how long a drone can stay in the air after a single battery charge. This is affected by battery size, weight, and flying manoeuvres.
For the most part quads in general don't have impressive flight times. Usually in the order of 5 - 10 minutes for toy drones and 15 – 30 minutes for the hobbyist or consumer level drones. A flight time greater the 30 minutes is the current Holy Grail as there is a trade-off manufacturers have to make between adding a large battery with more juice and the added weight that comes with it.
Manoeuvres like constant altitude changes or flips and loops can drain a battery a lot faster than just a static hold pattern. These cause more demand on the motors and the power to drive them maintain lift and make stability corrections.
Recharge times can vary from 45 - 90 minutes whether you’re using an AC outlet or USB charger so buying a spare battery to switch out is always good idea to extend your flight time.
Many quadcopters are now outfitted with onboard cameras or the ability to carry a small cameras and gimbal as their payload. This provides a great way to record flights from the drone’s perspective and get a bird’s eye view of your surroundings.
One thing you'll have to consider is the quality of the camera and subsequently the images and footage it captures. To save on space and weight lower resolution cameras are often utilised on toy drones with data being stored on an onboard MicroSD card or live streamed back to the controller over W-Fi or 5.8 GHz. These cameras at present are mostly VGA (640 x 480 pixels) or 0.3 MP. A new crop of toy drones are carrying 720p HD or 5.0 MP cameras and it’s clear that next years models will probably be carrying Full HD (1080p) cameras at the same price point.
If high quality video and imagery is your goal a drone capable of carrying a GoPro or Mobius camera and a gimbal is required and that means you’ll need at least a consumer level quadcopter such as the Ehang Ghost. A gimbal is a necessity for smooth vibration free video as a rubber grommet mount will not reduce motor vibration enough to provide quality video.
You can read more about aerial photography and cinematography is this post.
You should always take into account what type of controller is provided with a drone as this is what you'll be using to fly it around. For the safety of the drone and all its surroundings an easy to use and capable controller is a priority.
Most RC controllers work in the 2.4GHz band and provide remote communications with the drone telling it how to control its motors. A basic controller will have analog sticks for adjusting the altitude, roll, pitch and yaw of the drone. Control for the camera if fitted should be available as well as any special functions such as a ‘one button flips'. Some controllers have an additional built in LCD screen that can provide real time flight diagnostics when the quadcopter is airborne such as battery level. Most controllers supplied with toy drones or low end consumer drones are quite basic and can only be paired with that particular quadcopter.
However, there are numerous controllers available on the market as standalone products varying in price from a few hundred dollars to well over the thousand mark. If you plan to get into quadcopters big time or are thinking about racing, it would definitely pay to purchase a good quality controller that you can become familiar with. Controllers at this level can store the settings for many different craft in memory and will provide you, the pilot, with a consistent and measurable flight experience.
Alternatively a quadcopter can be controlled using a local Wi-Fi network. Parrot’s AR Drone was the first to introduce Wi-Fi control of a quadcopter and more and more Wi-Fi enabled consumer drones are now hitting the market. The main advantage of Wi-Fi control is that they are using an app on a mobile device such as a smartphone or tablet to communicate with the quadcopter. Control is thus simplified to mealy pushing buttons and using sliders to control the movement of the drone or selecting waypoints to set up a flight path for your drone to navigate. The advantages are obvious, it’s easy for first timers to master and the complicated control required for some maneuvers are taken out of the pilots hands and are automated by software.
If you wish to be a FPV racer or become a qualified drone pilot, stick with 2.4 GHz. It isn’t going anywhere soon. Traditional controllers take patience and practice to master, but it will be worth it. With perseverance you will be able to pilot any RC craft put in front of you. Wi-Fi is best if you have a need to be fulfilled and don’t really want the hassle of having to spend your time piloting a drone. This particularly applies to all the extreme sports fans who simply want a drone to capture whatever it is they do at their best.
Another metric to consider is which additional sensors the quad includes.
The most popular and necessary would be GPS sensors which allow for a lot of different functionality. With this drones can essentially be 'automated' by programming Latitude/Longitude waypoints that it will fly to without user control. It also allows for safety features such as ‘Return Home’ and ‘Auto-Landing’
For automatic altitude hold a drone will need to be equipped with some form of an altimeter. This can be a barometric sensor that measures atmospheric pressure, or an ultrasonic sensor that uses high frequency sound to measure distances. Compasses are also becoming popular on the current range of drones and provide 'Go Home' functions without the expense of GPS sensors.
One of the most important things to consider that is often overlooked is the skill level that is required to pilot a given drone. Not all drones are created equal.
While the quads accelerometers and gyros do a good job of keeping it balanced in air, controlling a quad is still a skill that needs to be attained through practice.
It takes time to get used to the controls and operating in a 3D flight zone, controlling altitude, direction, and speed all at the same time. Newer flyers should stick to the smaller quads that are ready to fly (RTF) out of the box and won't do too much damage in crashes.