Top tips for long-range FPV
When talking about the long-range FPV, 7-inch quadcopters are the first choice. After all, due to bigger propellers, they are both more efficient and have more lift for bigger batteries. But, like many things in life, this is not that simple. Bigger propellers are not the only factor that affects range. If you want to fly far and back, here are some tips on how to increase the range of an FPV quadcopter.
Kind of obvious, right? To fly far, propellers need to generate enough thrust to keep the drone in the air and consume as little power as possible. It's an entirely different goal than for racing or freestyle. What matters is not the maximum thrust, but the efficiency of delivery.
This is why:
- Bigger propellers usually have higher efficiency than smaller ones
- Propellers with lower pitch often have higher efficiency than high-pitched, aggressive, props
- Two bladed propellers should be more efficient that 3 or 4 bladed ones: vortexes at the tips on the blades consume a lot of energy. Fewer blades mean fewer vortexes. Fewer vortexes mean less energy lost
- For the same reason, propellers with "pointy" blade tips should be more efficient than the "bull-nose" style.
- Propellers with narrow blades are better for long-range than broad, "paddle" like props. It's because an airfoil with a high aspect ratio (narrow but long) has less induced drag than a low aspect ratio airfoil (broad but short). After all, it's not a coincidence that gliders look like they look.
Just like with propellers, the art of choosing correct motors for long-range FPV is to get just enough power to get the job done. In most of the cases, this comes down to less than more KV while keeping the size at reasonable levels. Since we are talking mostly about 7-inch long-range quads, this brings us to the 2507 motor size (25mm stator diameter and 7mm stator height) at around 1200KV.
I don't think I should explain the motor size. KV, however, needs a few extra words. 1200KV is a practical choice for both 4S and 6S. In theory, on 6S LiPo 800KV would be the equivalent of 1200KV on 4S. For an unknown reason, such low KV motors in sizes suited for 7-inch props are almost possible to get. 1200KV is usually the lowest rating on the market, and this is the only reason I recommend 1200KV also for 6S.
Bear in mind that lower KV rating does not automagically make the setup super-efficient. Yes, lower KV motor needs less current to generate the same torque as the high KV, but torque is not what keeps it in the air. It is power as torque times angular velocity. Assuming the same weight and propellers, you will need the same power for both low KV and high KV motors. If we forget about losses, then:
current * voltage = torque * angular velocity
Lower KV motor will need less current to generate the same torque, but higher voltage to deliver the same angular velocity. So KV does not affect efficiency directly.
The argument that lower KV and higher voltage causes less battery sag is also not wholly valid. Yes, the current will be smaller, but the 6S equivalent of 4S battery will have smaller cells, lower capacity in mAh, and lower C rating. So what we gain thanks to lower currents, we lose in smaller battery cells. Of course, we could put a bigger battery, but then we compare apples and oranges.
The only real-life gain from using 6S over 4S is less voltage drop on wires.
Another obvious tip? Yes and no. Yes, in a way that you need a bigger battery to hold more energy to fly further. No, in a way, that bigger and heavier battery is not always better. In general, increasing battery size increases the range, but the growth is not linear, and at one point, the whole design will be too heavy to fly, motors will be working outside the efficient zone and overheat.
Let's assume a 7-inch quadcopter weight 500g without battery, 1.5Ah battery adds another 200g, and in such configuration, it flies 10 minutes.
Increasing battery to 3Ah will not double the flight time as weight increases by 28%. So in the best-case scenario, the flight time will increase to around 17 minutes. Usually, less.
Another aspect is the battery technology. LiPos are suitable for high current applications like racing or freestyle. The energy density and minimum safe discharge voltage are far from perfect. Not only high energy LiPo will be heavy, but it also can not be discharged too deep, as it will damage the battery. 3.3V per cell is usually the lowest you can go.
The answer are LiIon batteries. Not only they pack more Wh in one gram of weight (almost twice as much), most of them can be safely discharged even down to 2.8V-3.0V. The downsize of LiIon technology is relatively low current. You can get Sony 18650 VTC6 rated for 20A continuous discharge, but it's 20A that will cause the battery to catch fire! The practical max discharge current is around 10-12A. Above this value battery starts to sag. This is why you should always connect 18650 in 4S2P for 4S. For 6S you might be able to live with 6S1P.
The key is to find a good compromise between weight and range.
Keep it as lightweight as possible. Throw out all the elements that are not required. For example, arm protectors. Do you need them? Replace GoPro Hero with a Split style camera. Instead of 4 separate ESCs use 4-in-1. Use slightly shorter battery wires. And if you really want to go hardcore, you might replace steel screws with aluminum ones.
Throttle and stick management
It is one of the essential skills when piloting a long-range FPV drone. The more gentle you are on the sticks, the less energy will be wasted on not needed movements. For example, when flying forward, fly forward and only forward. Keep constant altitude, constant speed, and straight-line. If you want to gain some altitude, do it slowly while still flying forward. Think ahead, and instead of making a rapid turn in the last moment, start it slowly in advance. In the end, you will be surprised how much extra range can be gained only by adjusting the flying style.
Optimal cruising speed
Drones have both limited flight time and range. The faster you fly, the less time you will need to get somewhere. It the same time, the drone will need more energy to overcome drag. This is why every moving object has its own "economy speed" allowing the most extended range. In other words, it is the speed that guarantees the most efficient conversion of energy into the distance. If you fly slower or faster than "economy speed", max range will decrease.
The simplest way to do it is to measure efficiency. For example, INAV has an OSD element called "Efficiency" that shows how much energy in Wh is currently required to travel 1km. The lower you can get, the further you will go. You cut it 20% and you get an extra 20% of a max traveled distance!