It’s a question that we often ask ourselves as drone flyers after a close call or an accident. Can a dragonfly with three propellers? If you’ve had a nasty incident or you are just the inquisitive sort you may have already experimented with this question. In this article we are going to have a look at the science and experiments that have been done online which have tried to answer this exact question.

In a hurry? Here’s the short answer:

Yes, a drone has been shown to be able to fly with three propellers by scientists. However, this requires special algorithms to be implemented into the drone recovery software. A typical commercial drone does not have these algorithms and so will not be able to fly with only three propellers.

There are some drones that have been designed with only three propellers and the difference between this and a four propeller drone is that they have been designed that way and have been balanced from the start.

Let’s take a look at one of the most interesting studies about whether or not a drone can fly with the loss of one, two, or three propellers.

What the science says

A study was published in 2014, that looked at this exact question. It was presented at the International conference on robotics and automation (ICRA). The paper looks at a four propeller drone maintaining a height around a position in space despite having lost a single, two opposing, or three propellers.

In each of the situations in order to control the drone the drone had to control around a primary access. This meant that it is possible for a drone to fly with three propellers if it has been specifically programmed to then rotate around and around upon losing a propeller.

This is what it looks like when a drone loses a propeller and has been programmed to spin around an axis and then automatically soft land. It’s pretty impressive that the drone can act on the incident so quickly:

This algorithm initially uses the remaining propellers to put the drone in a hovering horizontal spin. After securing the stability of the drone by selectively altering the thrust of each propeller, it steers the quad core by tilting the angle of rotation. It then decreases the thrust slowly and eases the drone down to a controlled landing.

Interestingly, the algorithm also works if only one propeller is operational. Here is the video of the drone also losing to propellers:

The methods that the scientists talk about could also be a applied to design rotating vehicles that could be designed to have as few as one propeller. This would make it much cheaper to produce than it drone with four propellers at the cost of not being able to control the full vehicle altitude or stability.

Drone that can fly on one propeller

The same group of researchers that we have discussed above had their research reported on the BBC. In this video they demonstrate the drone that was designed to fly on one propeller. They really wanted to push the boundary of what they could do with drones and there is a video in the link, above, which shows you how much the drone has to spin to stay stable with only one propeller.

Credt: BBC

So, drones can be programmed to fly (well…spin) when they only have three propellers functional. Why exactly do drones need four propellers? Let’s take a look at that question now.

Why do drones have four propellers?

Drones are deceptively hard to keep in the air. They require a huge amount of engineering and software control to keep them stable. The reason we have four propellers on a drone is because this is one of the most economical ways to keep the drone balanced and also stable throughout the range of conditions that drones encounter.

You need to have an even number of propellers spinning clockwise and counter clockwise to stop the aircraft from spinning out of control. By having four separate points that have variable thrust the drone is also able to overcome any sideways, front and back, or rotational instability by ramping up or down the speed of each of the four motors.

This does not happen as efficiently with two propellers and so the next even amount of propellers is four. This is why drones have to have four propellers to stay stable and balanced throughout their movements.

What about more propellers?

Drones can also have six or eight propellers but they follow the same rules as above:

  • an even number of propellers
  • arranged symmetrically around the drone body
  • an equal amount spinning clockwise and anticlockwise
  • able to be controlled individually for movements.

The only limit to the number of propellers that you have around a drone is the physical space that you are able to give each propeller. If they come too close to each other the drone will not be able to have efficient airflow across the surfaces of the propellers and it will be significantly more unstable.

It is all a balancing act to keep the drone in the air with the lowest amount of weight (motors and propellers) possible.

On the less sciency and of the spectrum, there has been a YouTube experiment with a DJI Phantom for. Let’s take a look at that experiment now – even though it is not based in science it is still very interesting.

YouTube experiment with a DJI Phantom 4

In this video the premise was relatively simple. The experimenter took a DJI Phantom for with four full propellers and slowly chopped away at the length of one of the propellers. They took off about 1 inch from either side of the propeller until there was very little left.

From the video you can see that even though the blades were cut with a saw and they leave a ragged edge it isn’t until the majority of the blade has been removed that there is an issue with stability.

The key takeaway’s are that the drone spins around its axis because of the unbalanced clockwise and counter clockwise movements of the propellers. This is a signal that the drone is at the limits of its stability as taking off anymore will flip the drone rather than spin it.

What you can do if you break a propeller during a flight

If you are flying and you accidentally chip or otherwise damage a propeller there are a few things that you can do. However, you cannot fully recover a DJI drone if you completely lose a propeller. It doesn’t have the mechanisms by which to stabilise itself all the algorithms which allow it to land safely and securely like the drones that we have mentioned in the science experiment, above.

Here, will have a look at what you can do if you accidentally chip or damage a propeller while flying:

  • hands off the controls – if your drone has an auto stabilisation feature the best thing you can do is give the drone a chance to if you notice any issues with the drone stability or it has come into contact with something take your hands off the controller unless you need to steer away from imminent danger.
  • Look for spinning – if your drone is spinning after coming into contact with anything in the immediate environment it is an indicator that your drone is almost about to drop out of the sky and not be able to fly anymore.
  • Return to land if it’s safe – if you chip or break a propeller during a flight and the drone is still in the air you may be able to manoeuvre it safely back to the landing spot. If you are unable to do this land immediately in the closest safest area and go to the drone to collect it.
  • Replace the blades – it goes without saying that if you have damage blades (no matter how little they are damaged) you should replace them. Like tires on a car a drone propeller is the only thing that is really keeping in the sky and even just a small hairline fracture can easily turn into more during the forces of rotation while flying. If in doubt, replace all of the propellers and dispose of the old ones.
  • Do not try to catch the drone – if you are flying above yourself and the drone is falling toward you get out of the way and do not try to catch the drone. That is because the propellers are spinning at a very high rate which means that they can easily cause soft tissue damage. Particularly if they have sharp edges due to damage.

If the drone is out-of-control it can be relatively dangerous. Check out my other article – are drone blades dangerous? [Full blade safety article) – click here.

Are drone blades dangerous?

A broken propeller is much sharper than a typical drone propeller so any advice would be to make sure that you do not come into contact with the spinning drone blade if it has become damaged or dislodged. Drone blades can be very dangerous because they spent a very high number of revolutions and if they are broken they will be spinning faster than usual. The sharp leading edge of the blade can easily cut and a soft tissue and cause a laceration that they cannot cause significant damage so that a finger will be taken off, for example.

If you want to protect your blades you can use propeller guards – most drones come with propeller guards and they will protect the drone blades as well as anything in the environment. You should do this if you are flying in a new environment or ones with overhanging or close proximity obstacles.

Conclusion

Drones stay in the air because they have been designed and engineered to stay perfectly balanced while flying.

Four propellers is the minimum at which a drone can stay stable enough to fly in a controllable way. There are some three propeller drones on the market but quad propeller drones are the most economical in terms of weight and stability.

If a drone loses a propeller it can remain stable as long as there are special recovery algorithms programmed into the drone. In order to stay in flight the drone is required to rotate around a primary axis so that it can remain stable and land securely and safely.