Last blog post we wrote about why we're revisiting using a quadcopter for a bat drone and mentioned some of the options available to either buy or build your very own.
In this post we're going to be writing about the new quad we've designed, what we're hoping it'll be able to achieve, and providing the specs. We get a little technical in the middle of this post, so if you're not interested in all that side of the project then we've put a summary at the end.
The last quad we flew with was successful in recording bat calls but could only fly for about 10 minutes. The main aim for the new quad is to increase the flight time to something more usable. We'll be very happy if we can achieve a flight time of 30 minutes on a 'normal' capacity battery and potentially longer using a larger battery or even a battery with a higher energy density.
When designing a quad that can fly for what's considered a long time (30 minutes or greater), lightness and efficiency are key. To this end, we decided to build the frame from carbon fibre tubes and plates. Carbon fibre offers excellent properties as a building material as it's light and strong, which is just what we need. We've also chosen light weight options for other components like lights (we're only using 4 small LEDS) and high efficiency motors and props.
Here's a few photos of the new quad:
And here's the spec sheet:
We know the weight of all the individual components of our quad so can take a guess at the All Up Weight (AUW) of the quad. This comes to 1418g.
How long will it fly for?
An increased flight time over what's commercially available (between 20-25 minutes) at this price point (circa £600) is one of the major reasons for building our new quad. It's fairly straight forward using basic maths to give us an educated guess for the flight time. Obviously, the best way to tell how long it flies for is in real world testing but this is how we worked out how long we thought it would fly for during the design phase.
In order to do calculate theoretical flight time, the first thing we need to know is what efficiency the motors are at when the quad is hovering (ie when the thrust is equal to the weight). We can use the thrust table from T-motor's website for this.
Using the data from the thrust table we've plotted total thrust (of all 4 motors) vs. the efficiency; that looks like this:
Now, we can use the equation from the trend line to work out the efficiency of the motors at the actual weight of the quad (1418g). The equation is in a small font and it's: y = -0.0021x + 17.995. Working through this, we calculate the efficiency to be 15.02g/W at 1418g of thrust.
Great stuff! We now have everything we need in order to take a reasonable guess at how long we can stay in the air for. Here's the info we've gathered:
First, let's work out the battery's Watt hours = 4 x 14.8 = 59.2 Wh
Now, let's see how many Watts it take to produce that 1418g of thrust = 1418 / 15.02 = 94.4W
Finally, the flight duration possible = (59.2 / 94.4) x 60 = 37.6 mins
This would completely drain the battery; we're going to aim to leave at least 20% battery capacity remaining as this will help maintain the life of the batteries and also provide some spare juice should we need it. We can simply take 80% of the time calculated to work this out; so it's 30.1 mins.
We can expect this to be a bit less in reality for a number of reasons, such as, the quad won't be able to stay perfectly still so will use greater than 1418g of thrust to maintain position; the motor manufacturers like to be optimistic with their efficiencies; we've extrapolated the efficiency; we've used an estimation of the AUW.
How can we fly for longer?
There's a great post about this on a blog called Capolight that covers most of the main options.
For us, the simplest way is to add further batteries. In fact by using two of the same batteries (the Multistar 4Ah Lipos) we can already increase the flight time to a safe 47 minutes (with 20% battery capacity remaining).
There's a variant of this which is to use a different type of battery with a higher energy density. We're currently using Lipo batteries with an energy density of about 185 W/kg. A Li-ion battery has an energy density of around 250 W/kg. There's a supplier called Titan that we'll be trying in the future. With a 7Ah Li-ion from Titan we can keep the weight of the quad down and increase the flight time to 49 minutes (again with 20% capacity remaining).
Another method we might try is to invert the motors (as in putting them on the bottom of the arms) as this is meant to increase efficiency by up to 10%. It improves the down-wash airflow by removing the arms and edging closer to an ideal laminar flow. We're already using round profile arms which have improved airflow over square profile arms so we don't expect to see a huge jump in efficiency from this option.
Finally, a really simple way to increase flight time is to actually provide some lateral movement of the quad. This increases something called translational lift due to the additional airflow over the propellers. This means it adds to the total lift meaning the propellers don't have to work as hard to produce the same amount of lift. Obviously, this is only up to a particular speed at which the increased lift is cancelled out by the motors working harder to obtain that speed.
A clear example of this is the flight times given for the DJI Mavic - see the specs on their website. The maximum flight time is given as 27 minutes with a flight speed of 15.5 mph. The hover time however is 24 minutes at stationary hover as there is no additional lift from the increased airflow over the props.
Next post we'll let you know how we've got on with the maiden flight of the new Bat Quad mkII. We'll hopefully be able to confirm some real numbers for flight times as well as info from testing the methods of increasing flight times.
If you've been following the blog for a while (and maybe even follow us on twitter - @project_erebus) then you might know that we've tried both successfully and unsuccessfully to use a quad in the past and are building a new quad for the project. In fact, it's the one flying drone that we've been able to record some really good bat calls from (see the 'successfully' link above).
In this post we're going to briefly cover problems using quadcopters and how we're now overcoming these issues. We've written about what quadcopters are capable of, and finally an overview of what's on the market to buy and a bit about building your own. If you've ever considered getting into bat droning then this will hopefully help you find one that works for you.
Originally, we struggled to make quads work for us. We had problems such as a swinging load issue; short flight times of about 10 minutes and ultrasonic interference. Recently, with the Peersonic detector and the Soundtrap detector, we've been able to overcome the ultrasonic interference - by placing the detector further from the quad - and overcome the swinging load issue - the detectors are now much lighter and don't affect the flight of the quad to such a calamitous extent.
However, the quad we were using for this testing (see here) is quite heavy and has an average efficiency meaning relatively short flight times. Happily, technology has come on somewhat in the time we've been progressing the project and we're now ready to re-visit the quad as a capable vehicle that we think probably has the best chance of succeeding as a flying 'bat drone'. After we posted about the pendulum detector issues we hypothesised that a quad would be a very capable bat drone though we didn't have the funds to capitalise on this at the time.
Fortunately, we've had a very kind donation since then from Belos Ecology and along with our own funds we've been able to put together a new quadcopter which we hope will provide us with the platform to collect some strong data and make progress with the project. We'll be posting about the build and design of this soon.
What are multirotors capable of?
We're focusing on flight times and ease of use as we feel these are the most important factors for new users to consider. We want you to be able to get out there and get bat droning with ease so we've put together a few options that should help you with this.
You should also be aware that there are important laws that users must obey regarding the use of drones. For commercial users in the UK there are some requirements that you have to fulfill in order to use a drone; have a look at our blog post here. For recreational users (such as ourselves) there's a great website put together by the CAA (civil aviation authority) and NATS (national air traffic services) which can be found here; of particular interest is the drone code.
There are a number of multirotor options available, such as quadcopters, hexacopters, octacopters and more niche designs such as X8, tri-quads and co-axial copters. We're going to focus on standard layout quadcopters as these are an elegant, simple and proven design.
With regards to capability, multirotors have relatively short of flight time. You'll be looking at a flight time of 20-25 minutes per battery for a normal quadcopter like a DJI Phantom series and up to an hour for specifically designed and built quads.
This is a very fast paced area for development and flight times are constantly improving. The record flight time for a small quad for example is just under four hours though this uses a specialist hydrogen fuel cell power supply.
Something worth noting is that the flight times given for models are really hover times rather than a measure of the 'useful' time you can fly for. The quadcopters are holding position and not spending any excess energy on making any changing lateral movements. A further note is that if the quad is travelling in a straight line there is additional wind flow over the propellers resulting in more lift - which is called translational lift. This will increase efficiency and flight time, although only up to a point, which is where the energy put in to gather the speed outweighs the efficiency gains from the translational lift.
Buy or build?
The first question to ask is, 'What do you need in order to capture bat calls from a drone?'. Let's take a look below:
For most people we think buying a quad is going to be the easiest route into using a bat drone. If you're into DIY projects and maybe even have a little soldering experience or would like to learn about this sort of thing then by all means have a look at our guide to building and/or designing your own below.
So, what's a good flight time for a quad that can be bought or built? It depends on the weight of the payload, in our case the bat detector, but generally if you can achieve greater than 20 minutes useful flying time then you're doing well. If you're achieving 30 minutes then be very happy and anything over 45 minutes of useful flight time is exceptional. For example, DJI make a claim of 27 minutes maximum flying time for their Mavic quad which is very impressive. It does seem this flight time was aided by a flying speed of 15 mph thereby providing further translational lift; as evidence of this, the maximum hover time is lower at 24 minutes.
In terms of ease of use between bought or built it really depends on the 'brains' of the drone called the flight controller. This is the clever electronics which can enable autonomous operation and all sorts of interesting flight modes, such as altitude hold, loiter, follow me and follow waypoints. DJI make great flight controllers which you find in their quads or buy separately to use in your own build. There's also a community based flight controller based on Ardupilot firmware which we use. It runs on various hardware and we're using a 3DR Pixhawk flight controller. Again, this allows a lot of interesting flight modes which suit our needs. We would suggest at a minimum you ensure the flight controller has GNSS (like GPS and/or GLONASS) and the ability to follow waypoints.
Buying a bat drone? See below!
For the majority of users this is going to be the easiest way into working with a bat drone. There are a number of great multirotors on the market which offer excellent reliability and build quality as well as great flight times and very intuitive user interfaces.
Which one suits you may well depend on the cost. Generally, as you pay more you increase the flight time and the payload the multirotor can carry. Starting costs are realistically between £300-400 and you can spend up to several thousand pounds.
The most recognisable manufacturer of ready to fly multirotors is DJI. They make quads such as the Phantom series and Inspire series. We've been experimenting with a Phantom 3 recently and found it to be very capable. There's a lot of really great features and nice little touches, such as screw on propellers, smart batteries (which discharge themselves to a storage voltage after a set time), and a fantastic user interface (which is an amazing selling point). Plus, they're really easy to fly quads that can get you flying great straight out of the box with a simple setup procedure. If you want a really straight forward way into flying your own bat drone then you can't go wrong with DJI. You'll be looking at a flight time of around 20 minutes and a cost of between £300 for a second hand Phantom 2 on eBay to £3000 for a new Inspire 2. Downsides are that additional batteries are quite expensive (relative to DIY options) though they are the 'smart' batteries as mentioned above so at least this lets you feel a bit better about it!
We would also recommend a company called 3DR. They manufacture a quad called the Solo and although they've re-focused on commercial applications recently, and re-branded and re-priced the Solo for this market, you can still pick one up second hand. 3DR also make the Pixhawk flight controller that we use in all of our drones. You can find a second hand Solo on eBay for around £350 and should be looking at a flight time of just under 20 minutes.
Although we haven't used a Solo it runs off an adapted version of the same flight controller firmware we use so it's also very easy to fly and offers all the necessary features required.
If you're looking for something that'll fly for a bit longer then this is certainly possible with a ready to fly quad. DJI make a heavy lift (larger payload) hexacopter called the Matrice 600 and a smaller quad version called the Matrice 200. It's designed as a professional filming platform and can fly for just under 40 minutes with a very light payload like a bat detector. The reason its a hex rather than quad is for redundancy as a hex can fly with a damaged motor/prop. The only issue is that the pro version costs £5200 and you'll need the optional extra improved batteries to maximise the flight time.
What we'd recommend is that you contact someone at heliguy.com or uavshop.co.uk. They'll be able to put together a custom build for you. We've dealt with them in the past for some advice and they're very knowledgeable, helpful and friendly. It's also the easiest option for a ready to fly long duration multicopter, though certainly not the cheapest!
Build and/or design a bat drone
This section will only be of interest if you're into DIY projects. If you would like to give it a go yourself then read on!