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DJI is able to achieve ~30 minute flight times with their dones. Whereas, a typical FPV drone is only ~7 minutes

I see this as a huge achievement by DJI, I am interested to understand how they were able to achieve this advancement

  • The flight characteristics of these two crafts are quite different. A DJI drone is flown at much slower speeds than FPV drones So, if a FPV drone was flown slowly and cautiously, would the battery extend to be comparable with DJI's?

  • I have read elsewhere that the energy cost of FPV drones is due to the small blades, the motor needs to spin faster to compensate. This would imply that if you increased the blades (and the arm length) you would get longer flight times...?

  • Or has DJI really advanced that much in ESC performance, motor performance, battery chemistry to reduce energy losses?

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There's no magic technology, it's just a difference in design goals. FPV drones are designed for speed, performance and robustness, DJI drones are designed for duration and carrying a gimbled camera.

Larger, slower turning rotors are more efficient at producing lift but have a lower top speed. That's why helicopters can hover and propeller planes with the same size engine can't - but the plane is faster. Photography drones (such as DJI) use large rotors and exploit that efficiency for duration. Race drones use smaller rotors because they want to go as fast as possible.

The other factor is that photography drones typically carry larger batteries for longer duration, while race drones want to keep their weight down for maximum agility and acceleration. A photography drone will fly so long as it can produce more thrust than its weight, typically they have less than a 2:1 thrust-to-weight ratio and can't maintain altitude when angled more than 45 degrees from horizontal. Race drones can have a 10:1 thrust to weight ratio, and commonly fly at pretty extreme angles.

So long as a race drone has enough duration to complete the race, any extra battery capacity is just dead weight, slowing it down. For their pilots, changing the battery every few minutes is a small price to pay for the performance.  

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    $\begingroup$ Do DJI's drones not fall into the category of FPV drones? $\endgroup$ – user253751 Oct 1 at 12:58
  • $\begingroup$ @user253751 In the case of the DJI Mavic Mini, at least, I would say no. The view from the drone is displayed on the controller (via your phone), whereas an FPV drone is meant to display the camera image in goggles that the operator wears. $\endgroup$ – JYelton Oct 1 at 20:25
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    $\begingroup$ I don't understand the second paragraph - if larger rotors are more efficient at producing lift, wouldn't they be more efficient at going fast, which is just lift in a different direction? $\endgroup$ – Wossname Oct 2 at 1:17
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    $\begingroup$ @Wossname - the thrust produced by a propeller drops off as the aircraft goes faster, and is limited by how fast it spins. It's a bit like gears on a car or bicycle, you can trade force for speed. There's more detail (and maths) in the link. $\endgroup$ – Robin Bennett Oct 2 at 8:27
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    $\begingroup$ @Wossname: Large slow rotors are more efficient at producing lift at low airspeeds than small and fast ones. The efficiency drops rapidly as airspeed increases relative to the speed of the rotor blades. $\endgroup$ – Ilmari Karonen Oct 2 at 9:01
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This is a good question. In short, While DJI drones certainly have a lot of engineering and design put into them, the way they achieve long flight times is no secret. The DJI drones' long flight times are largely due to the high capacity batteries, motors, and propellers used. Although, there are many smaller things that also affect flight times.

Probably the most significant of these is the battery. A typical "FPV drone" uses a LiPo battery with a relatively low capacity and relatively low-quality cells in order to reduce the all-up-weight and cost. DJI uses more expensive LiPo cells that have a higher energy capacity per weight. The DJI batteries have noticeably more capacity, but they are heavier and quite expensive making them less than desirable for usual FPV flights because batteries are often destroyed in a crash. However, you certainly could use these very high quality battery cells on an FPV drone if you wanted longer flights. Another way that some FPV pilots can get longer flights without spending too much is by using Li-ion battery packs. They cannot deliver nearly as much current nor are they as durable, but they store a lot of energy and normally can get 20-30 minutes of flight time.

There are many FPV pilots who custom build Li-ion packs out of 18650 cells or buy pre-built Li-ion packs to get longer flight times. One FPV drone I can think of that works well with Li-ion is the Flywoo Explorer LR. I've seen people getting ~40 minutes of flight time with it.

Other big things that contribute to the DJI drones' long flight times is the type of motors and propellers used. Drones that are designed for long flight times, like DJI drones, or some long range FPV drones, have motors with less powerful magnets or a wider air gap between the stator and magnets which results in more energy efficient operation. High performance motors, which are seen on most FPV drones, have very powerful magnets and tiny air gaps to increase performance and torque at the expense of some efficiency. As for propellers, when you look at drones designed for long flight time, you see two blades, low pitch, and longer blades. But when you look at High performance FPV drones, you see steeper pitch, more (usually three) blades, and shorter blades so that the motors can change the prop speed faster. More blades on a propeller usually decreases efficiency but increases stability. Having blades with higher pitch increases the consumption of current, so a lower pitch is often more energy-efficient. And the size of propeller also makes a significant impact as larger propellers are more efficient.

There are a lot of factors that impact the flight time of drones. Certain drones, like the DJI drones, are just designed with efficiency in mind while other drones, like a typical "FPV drone", are designed with performance in mind.

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  • $\begingroup$ Most of the DJI batteries I have used are LiPo (example); although, I can see where you've got confused as LiPo are more correctly "Lithium-Ion Polymer"! See Wikipedia $\endgroup$ – Kralc Oct 1 at 7:14
  • $\begingroup$ @Kralc thanks for pointing that out! I must have missed that. I've updated my answer accordingly. $\endgroup$ – Jacob B Oct 1 at 7:51
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    $\begingroup$ FPV lipo packs are not cheaper (in fact if you pull out the expensive branding they're probably more expensive cells) - the chemistry is different and designed to be able to deliver higher current (lower internal resistance) and not high capacity relative to size/weight $\endgroup$ – Brydon Gibson Oct 1 at 18:27
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    $\begingroup$ @BrydonGibson that's sort of what I was trying to say. Most people use cheap 15-25 dollar batteries for their FPV quads. But they could opt for more expensive batteries designed for either extreme performance or extra high capacity if they want to get longer flights. $\endgroup$ – Jacob B Oct 1 at 23:09
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I have built and owned plenty of racing and photo drones. DJI is one of best for photo drones.There is a big difference in a racing drone that uses ESCs and motors that take a lot of power to fly at high speeds and for quick maneuverability and a photo drone which does no need to use as much power. They have designed the camera drones to use lower RPM speeds with bigger props to handle the weight and to fly for longer time. An fpv racing drone used to have a very short flight time but that is changing fast. A good example of this increased flight time is the new DJI Titan DC5. The flight time has increased dramatically over older FPV drones.

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