I love 7-inch FPV drones and during the last year, they were my most used FPV gear. For freestyle, cinematic, cruising and long-range. I currently own 3 7-inch FPV drones and 4th one in the assembly process. I think of myself as a 7-inch expert. I've wasted enough money and equipment to know what works and what does not work well with 7-inch propellers.
If you are thinking about building your own 7-inch FPV drone, here are some tips that might save you a lot of time and a lot of money on wrongly chosen equipment.
Of course, the frame is the most important part of a build. No matter if you will be building a freestyle or long-range rig, there is one thing that you have to know: Horizontally scaling 5-inch frame by making arms longer is a no-go. Longer arms require more stiffness and they also have to be thicker! The sweet spot is 6mm thick good quality carbon fiber. 5mm thick might work too but forget about 4mm or thinner arms. They will be too flexible and the whole quad will be in a state of a constant wobble!
Kind of obvious, right? The choice of good 7-inch propellers is not that big as one might imagine. Sure, you can buy some "inventions from 4 years ago for cheap" but believe me, they are not worth it. There are 3 types of modern 7-inch propellers that I find worth looking at in late 2019:
- DAL T7056C - good for freestyle and thanks to aggressive pitch they are very dynamic. Can be used no longer flight, but this is not where they really shine.
- HQProp Durable 7x4x3 V1S - a more aggressive version of 7x3.5x3 that in my opinion works much better than original. Slightly more pitch only helps during steady flight. They are rather for cruising than for freestyle.
- GemFan FLASH 7042 - kind of good propeller, but don't get me wrong, you should try 7042 only if DAL or HQProp are too heavy for your motors. So if you have 2207 or 2306 you might try FLASH. If you have bigger motors, just go with DAL or HQProp.
Probably the most often made mistake by the "7-inch virgins" are too small motors. Beginners think that putting the same motors that work great with 5-inch propellers on a 7-inch frame is a good idea. OK, maybe sometimes they lower the KV. This will not work. I know. I tried that by myself and only wasted some money and a lot of time and nerves.
What you need are big motors. In a size of 2506-2507. 2208 might work as well and 2508 is rather the max size that makes sense.
On the KV size, the rule of thumb is that on 4S you should aim at 1500-1500KV and on 6S at 1200KV. Those values give a nice compromise between efficiency and raw power. Of course, this is only a rule of thumb and when needed you might choose higher KV. For example, my 7-inch freestyle drone uses 1700KV on 4S motors.
My recommendation is rather simple: for now, there is only one motor that comes is wide enough KV range and it's called BrotherHobby Avenger v2 2507.
For the rest of the equipment, you can choose whatever you want. Any equipment will work and you do not have to match it to the propeller size. My choices are: