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.