How to Choose a Propeller for a Multi-rotor Drone
Drones are a marvel of modern engineering. In such a small package, drones manage to pack all the motors, sensors, and propellers that they need to lift off the ground and even come with high-quality cameras and other data collection equipment.
Nowadays, the vast majority of mainstream drone users prefer the simplicity and versatility of multi-rotor drones. They are small, can take off without a runway, and can hover in place. One of the components that play a vital role in how multi-rotor drones can fly is their propellers. How do these propellers work? What should you consider if you’re buying propellers?
How does a propeller work?
Much like a helicopter, multi-rotor drones rely on the lift that their propellers generate when they spin. As soon as propellers spin, their unique and deliberate shape allows them to pull air from the surroundings and push them into the space below. The speed at which propellers spin is controlled by the motors they are connected to, which in turn are controlled by an electronic speed controller (ESC).
You may be wondering, why do drones need multiple propellers? What will happen if one propeller fails? Drone pilots with extensive flight experience can probably answer the second question – the drone will likely spin out of control before eventually crashing to the ground.
This happens because the spinning motion of the motors tends to exert a similar rotational force on the body of the drone itself. In a multirotor drone, all motors run at the same time to cancel the rotational force of each other. For this reason, the ESCs of each motor in a drone have to constantly work together, each motor running at just the right speed to counteract the rotational forces from all the other motors.
As a side note, a helicopter only has one large rotor, but the rotational force exerted by this rotor is counteracted by the small tail rotor.
Aside from providing lift, the propellers of a drone also power its forward motion. This is done by pitching the drone – making the rotors at the back rotate faster to create more lift relative to the front rotors. This action angles the drone in such a way that the lift generated by the rear rotors pushes the drone forward. Doing a roll is pretty much the same thing, except it involves the left and right-side propellers to affect sideways motion. Thus, the rotors of a multi-rotor drone do double-duty: they provide both lift and propulsion.
There may be a couple of variations in how a propeller performs according to its diameter, thickness, and shape. Although not all propellers are made equal, they all work using essentially the same aerodynamic principles. Knowing how crucial propellers are to the performance of a drone, you can now better appreciate how valuable it can be to get the best propeller for your drone if you want to tweak how well it flies.
Factors to consider when choosing a propeller
Most drone manufacturers sell their proprietary propellers for their drones. While these propellers should perform well enough, you also have the freedom to play around with propellers from third-party vendors. This is probably more familiar ground for drone racers who are used to mixing and matching drone components from various manufacturers.
In any case, you should always make sure to buy your propellers as a whole set. As we have mentioned, propellers work together to counteract the rotational forces they exert on the drone. Using propellers that were never meant to work together will create an imbalance, making control of the drone much more difficult.
If you want to get the best flight experience out of your drone, you need to understand how each characteristic of a propeller affects the drone’s performance. Often, the choice is a compromise between two extremes. The choice of which propeller is all up to you and your priorities as a drone pilot.
Spare propellers are typically available in either plastic or carbon fiber. There are also fiberglass-reinforced and wooden options, but these are less common. For our purpose, let’s just look at the relative advantages and disadvantages of both plastic and carbon fiber propellers.
Right at the onset, it’s hard not to notice that carbon fibers are at least twice as expensive as plastic propellers. This immediately puts the burden of proving its superiority on carbon fiber propellers. Given that they are much more expensive, are carbon fibers better? There are several, equally valid ways of looking at it.
Carbon fiber propellers are much more rigid compared to their plastic counterparts. This makes them more durable, so your carbon fiber propellers should be able to withstand a couple more crashes. Since they are not as flexible, carbon fiber propellers also produce less vibration while they are spinning, making them a little quieter. This lack of vibration also makes carbon fiber propellers more apt for high-RPM high-kV motors.
However, many drone pilots have noticed that flight stability gets compromised when flying with carbon fiber propellers. This is likely due to the rigidity of the carbon fiber material. In this regard, plastic propellers can flex and adjust to varying weather conditions. Think of it as carbon fiber propellers bashing through wind resistance, while plastic propellers weave through it. Aside from poor stability, the resistance of carbon fiber propellers to flex also makes the propeller motors work harder, resulting in noticeably shorter flight times.
Although carbon fiber propellers are durable, their lack of flexibility also means that they are brittle. At best, they can develop small cracks when you crash your drone. At worst, their rigid properties could mean that shockwaves propagate through it, ultimately causing damage to the propeller motors. We consider this one of the biggest disadvantages of using carbon fiber propellers since propeller motors are much more expensive and difficult to replace.
2. Length and Pitch
These two propeller characteristics are so closely related that it makes sense to analyze them at the same time. The length of the propeller, sometimes referred to as the diameter, is simply a measure of how long the blades are. On the other hand, the pitch of a propeller is defined as the distance that the propeller would travel in a single revolution. A longer propeller has a higher pitch, so the effects on flight performance are basically the same.
As you know by now, multi-rotor drones can fly and move using the lift generated by the propellers. You can probably imagine how having larger propellers will generate more lift. This is the primary advantage of longer and higher pitch propellers – a more powerful lift and drones that can move faster. This can be advantageous for racing drones or for drones designed to carry payloads.
Longer propellers are also heavier, which means that the propeller motors have to work harder. The higher power output needed by high-pitch models will result in a noticeably shorter flight time with the same battery. The increased mass of longer propellers also increases their rotational inertia, such that motor speed adjustments are not as immediate. This can make your drone feel unresponsive to sudden changes in direction and will likely also increase your drone’s braking distance.
Shorter propellers with lower pitch cannot generate as much lift. This means that your drone will be slower, and generally unsuitable for carrying a payload. However, the reduced load on the propeller motors means that you can fly your drone longer since the motors draw less power from the battery.
For us, the biggest advantage of using a shorter propeller is the increased responsiveness. Since shorter propellers are lighter, they carry much less rotational momentum. This means that speed and directional changes are more pronounced. If you are a professional drone photographer who needs to fly through tight spots or near obstacles, you will probably appreciate the increased sensitivity that comes with using shorter propellers.
The choice of whether to use shorter or longer propellers often comes down to a balance between power and responsiveness. Drone racers are probably very familiar with this conflict, as they often need to hit just the perfect blend of these two flight characteristics. As with the other factors in choosing a propeller, there is no right or wrong choice here – just a choice of which one works best for you.
3. Shape and surface area
Shape and surface area are two more characteristics that need to be discussed at the same time since they’re very closely related. The surface in this context refers to how thick the propellers are due to their particular shape. As you could expect, a propeller with a higher surface area can generate a powerful lift. However, increasing the surface area of a propeller also increases its drag. This means that the extra lift generated by the propeller requires a disproportionate amount of power from the motor, resulting in a higher current draw from the battery and overall lower power efficiency.
When it comes to shape, the most distinct difference of propellers is in the shape of the propeller tip. A propeller can either have a pointed nose, a bull-nose (BN), or a hybrid bull-nose (HBN). A propeller with a pointed tip has the least surface area. Thus it generates the least amount of thrust. However, its shape also reduces drag, resulting in better power efficiency and longer flight time.
On the end of the spectrum are the BN propellers. The tip of a BN propeller is essentially flat, giving it a much larger surface area. This generates more thrust at the price of similarly increased drag. This type of propeller is appropriate for applications that require a large amount of lift, such as when your drone needs to carry a payload. However, you will need to recognize that your flight time will suffer.
The hybrid bull-nose (HBN) propeller, as its name implies, combines the designs of both pointer and BN propellers. It’s a middle-ground alternative that has a somewhat pointed end that can help cut through wind resistance, but also has a large surface area for increased thrust. If you’re not happy with using the two extremes of propeller shapes, then perhaps this hybrid alternative will work for you.
Again, the choice of which propeller shape to get depends on your application. For leisurely flying, pointed propellers are great, so your drone can stay in the air for longer. If you want to go faster, then you’ll have to switch out for HBN or BN propellers. The added lift given by HBN and BN propellers may be valuable if you’re flying at higher elevations, such when you’re taking off at the top of a mountain. However, your flight time will be seriously compromised so it will be best to have a couple of spare batteries ready to go.
4. Number of blades
This may be a huge departure from the usual image of multi-rotor drones, but it’s actually possible to buy propellers with more than two blades. Casual drone pilots probably won’t need to bother with this aspect, but drone racers might be interested. As you might expect, a propeller with three blades will generate much more thrust than the standard two-blade propeller. However, this is effectively just another way of increasing a propeller’s surface area. This means that the additional thrust still comes at the price of increased drag.
Propellers with three blades are incredibly common in the world of drone racing because of the speed increase that they deliver. However, racing drones only need a short burst of power – they are designed for speed and performance, not longevity. If you fly drones as a hobby, or even as a profession, using propellers with three blades will probably be overkill. You will also lose quite a bit of flight time, which is always a drag (in the figurative sense).
The great thing about increasing propeller blades, and why it’s very popular among drone racers, is that it increases the thrust of the drone while retaining a short propeller pitch. This means that responsiveness does not suffer, allowing racing drones to negotiate through the curves and turns of any racing course.
The drone racing world has not exactly been known for moderation, so you can even buy propellers with four or up to six blades. The concept of increasing thrust while also increasing drag also applies here, so a six-blade rotor is often considered hilariously inefficient.
Propellers are generally very cheap, but there could be a significant difference in the prices of propellers from different manufacturers and drone models. For example, a complete set of propellers for the DJI Phantom 4 can cost between $15 to $20. The redesigned (low-noise, gold-tipped) propellers that came with the Mavic Pro Platinum are even more expensive, costing up to $30 for a complete set. Propellers for other popular drone brands, such as Yuneec and Parrot, cost pretty much in the same range.
Replacement propellers for toy drones, such as the Syma X5 and the Hubsan X4, are much cheaper. You can probably get a complete set of propellers for a toy drone for $5 or less.
Third-party brand propellers – the ones usually used for racing drones – are also quite cheap. Drone racers go through these propellers pretty quickly, so they are often sold in large bundles, usually in packs of 16 pieces. Purchased in bulk, these third-party propellers typically also cost less than $5 per complete set.
The propellers are some of the essential components of a drone, but drone pilots rarely give a thought to how they work. Aside from the lift that these propellers generate, they also provide the thrust that allows drones to move forward and maneuver once they are up in the air. By understanding the interplay of thrust, drag, inertia, and momentum, we can make better decisions on which propeller is best-suited for our particular circumstances.
For most of the casual drone pilots, their options for propellers are typically limited to those that are offered by their drone brands. However, most of these brands still offer some options, such as the DJI Mavic Pro being compatible with both the quick-release 8330 and the low-noise 8331 propellers. For drone racers, understanding propeller dynamics is a crucial skill that may be the difference between a drone that can win races and one that will crash on the first sharp turn of the racing course.