How much does a drone cost?

[Simon Ritchie]

If you can use a drone to carry Lidar equipment rather than a full-sized plane, that’s obviously going to reduce the cost of the survey, but not all drones are up to the task.  In this posting I look at the practicalities of running a useful drone - how much it will cost, how much it can carry and for how long.

I live in the UK but I quote prices here in US Dollars, because most readers will be familiar with them.  I use DJI drones as examples.  This is because they have a wide product range and it’s easy to find prices and specs for them.  This is not meant to be an ad for DJI.  Other drones are available.

The issues of cost, payload and flight time are related.  You can buy a drone off the shelf for a couple of hundred dollars, but it can’t carry much weight and it can only fly for a few minutes.  The more weight a drone has to lift, the shorter the time it will fly and/or the more it will cost.

A quadcopter flies at around 10m/sec, but to run a survey it has to fly back and forth across the site.  The sensor surveys a strip of the land underneath it.  The width of the strip is called the swath.  How many passes the drone has to make depends on that value.

The drones I’m looking at here are expensive and you can expect high-end features such as a programable autopilot, so you can set them running on a predetermined course.  They should monitor their batteries and return to the take-off point when they are running low.  Many will have automatic collision avoidance. However, you can’t just leave them to it, certainly not in the UK.  The law here requires that you keep your drone in sight at all times while it’s flying, ready to take over if something goes wrong.

The classic drone is a quadcopter.  It has four arms, each carrying a motor driving a rotor.  The rotors develop lift.  If the lift is 1Kg per rotor, the quadcopter can lift 4Kg.  It has to lift itself, its batteries and its payload – the equipment that it carries.

The arms have to be held together by a frame, which has to withstand the stress of carrying all that weight.  More weight needs a more rugged frame, which increases the weight further.  More powerful motors are heavier and need a bigger battery, which also increases the weight.

Once it’s up in the air the drone will be buffeted by the wind.  You need extra power to keep it stable.  One rule of thumb is to keep the total weight to half capacity - if your drone is theoretically capable of lifting 4Kg, keep the total weight down to 2Kg.  If your drone and its battery weigh 1Kg, that leaves 1Kg of payload.  You should bear this 50% rule in mind when looking at the drone manufacturers’ specifications.  

Drones are usually powered by Lithium Polymer (LiPo) batteries.  LiPos have a good energy density (they hold a lot of energy for their weight) but they are heavy.  The battery can be the heaviest component of a drone.  For each model of drone there’s a battery with an optimal combination of capacity and weight.

You can increase the flight time by replacing the flat with a fully-charged one when it goes flat, but if your drone has to fly for a couple of hours to survey a piece of ground and you have to replace the batteries every few minutes, this is not a very practical solution.

For one thing, LiPos are expensive – I paid $70 for a 10 Ampere/hour unit.  For another, they take time to charge.  They only survive a limited number of charges and the faster you charge them, the faster they die.  If your battery will give you half an hour of flight time, you’re advised to spend two hours recharging it.  The upshot of all that is, if you want to run your drone for a few hours, you need five batteries, one set running the drone and four on the ground recharging.  You also need enough chargers and a supply of electricity.

This thread discusses the payload that the DJI Inspire quadcopter can carry and for how long:  https://forum.dji.com/thread-4286-1-1.html:  “I performed a payload test on the Inspire 1 today to determine the impact on flight time.  Adding about 1kg reduces the theoretical flight time of a 4500mA battery from 17 minutes down to 11 minutes. I calculated the normal discharge rate at 4.5mA/s and the discharge rate with a 1kg payload at 7mA/s. I arrived at these values by taking off, holding an altitude of about 10 feet, then landing after 3 minutes.  https://youtu.be/4m8-fYiw2HE.”  Other comments in that thread point out the need for some contingency power.

So, with a fully-charged battery, a DJI Inspire 1 can lift 1Kg of equipment for 11 minutes.  If you apply the 50% rule and restrict the load to 0.5Kg, you can maybe extend that to 17 minutes.  Amazon sell these drones for $3,400 including a camera.  I also saw a used one on there without camera for $1,300.

The DJI Phantom 4 model seems to offer a good compromise of price and performance.  It costs about $1,500 and they claim that it will fly with no payload for about half an hour.  I found a posting that claimed that the maximum payload is 800g.  The DJI spec is here:  https://www.dji.com/phantom-4/info but it doesn’t guote the payload.

DJI’s Matrice range is heavier and designed for “industrial” use.  The Matrice 600 weighs 4Kg and costs $5,000 from Amazon.  To increase the flight time and payload you can fit five batteries (at $200 each!) which increases the weight to 9Kg and the price nearly $6,000.  With five batteries it will carry up to 6Kg of payload.  The spec https://www.dji.com/matrice600/info quotes a flight time of 16-35 minutes depending on the payload.  For continuous running you will need a lot of batteries, chargers and probably a generator.

If you can keep the payload reasonably low you can use fewer batteries, so this drone looks like a good workhorse, but it’s expensive.  Probably not for the enthusiastic amateur.

I’m told that if you want a heavy lifting drone, you will get it cheaper and better by building your own.  There are plenty of websites that sell the parts.  Note that if you order from a foreign website, you will have to wait longer for the delivery.  Here in the UK, Alex Elliot, author of the Build Your Own Drone Manual published by Haynes, runs the Unmanned Tech Shop.  I found them very helpful and prompt.  Other sellers of drone parts are available.

Building your own drone requires a combination of basic mechanical construction, electronic and IT skills.  If you haven’t built one before, I think it could easily take a week or more of full-time work.

To sum up, useful drones are available, ready built or home made.  At present they cost quite a lot of money, but they are affordable by some.  The price depends on the weight of the Lidar sensor.  That’s the subject of another posting.