Building ultralight 5" long range FPV quad
The FPV season is over and I’m starting a new build for the winter. I discovered that I enjoy pushing the limits of my quadcopter range more than doing any kind of tricks so I decided to build a quad dedicated to long range flying.
Why choose 5" quad for long range?
Wings and planes are better choices for long range flights but I enjoy the flight dynamics of a quad and the ease of setting it up and launching so I decided to build a quad focused on long range flights. The typical choices for such purpose are 4" or 7" platforms. Larger propellers can achieve higher efficiency, however going up to 7" size increases the cost of all components quite significantly which is why I decided to go with 5".
Components
The main goal when choosing components will be to achieve good efficiency which will enable the long range flights. In order to do that I will focus on lightest components and weight savings.
Frame
There are a lot of lightweight 5" racing frames available on the market but they have some extra thickness to withstand crashes. I decided to risk it and sacrifice crashability to focus on the lightest options out there. It meant going with a single piece low thickness frame. I went with Concept 195 which was the lightest frame I could find. If you want to make your build more versatile you can opt for some heavier racing frame.
| Concept 195 | Hyperbola 501 | |
|---|---|---|
| Weight | 32.3g | 45.9g |
| Arm thickness | 4mm | 4mm |
| Wheel base | 195mm | 229mm |
| Deck | Yes | Yes |
| Mounting options | 25.5×25.5/20×20/16×16 | 25.5×25.5/20×20 |
| Price | $24AliExpress | $35GetFPV |
Motors
There are three common motor sizes that would fit well for this project: 2203.5, 2004 and 2005. The most important feature is their stator volume: 2203.5 is the widest motor but its actually the smallest and 2005 is the biggest. The next most important parameter is motor kV. It defines the unloaded RPM of the motor at 1V of voltage. Higher kV motors will be more aggressive and reach higher RPM while lower kV will be more efficient. At 6S voltage (25.2V) you want 1600kV (40k RPM) to 2000kV (50k RPM) motor, at 4S (16.8V) you typically want 2200kV (37k RPM) to 2800kV (47k RPM) motor. I went with EMAX Eco II 2004 because of its low kV (1600) option and good value.
| BETAFPV 2004 | BrotherHobby TC 2004 | GEPRC GR2004 | EMAX Eco II 2004 | |
|---|---|---|---|---|
| kV | 1700 | 1700/1950 | 2000/2600 | 1600/2000 |
| Shaft | 1.5mm | 1.5mm | 1.5mm | 1.5mm |
| Weight | 16.6g | 14.9g | 16.8g | 18.12g |
| Price | $74BetaFPVGetFPV | $80AliExpressGetFPV | $80AliExpressGetFPV | $66 AliExpress |
Electronics
I was looking for a 25x25 sized all-in-one board to save space. It had to support 6S and have at least 3 UARTs for RX, VTX and GPS. Finally, it had to have a barometer builtin in order to support iNav navigation modes like position hold or return to home. Diatone MAMBA AIO ticked all those boxes.
| MAMBA AIO F740 MK4 | Foxeer Reaper F745 | GEPRC GEP-F722-35A | |
|---|---|---|---|
| MCU | F740 | F745 | F722 |
| Gyro | ICM42688P | BMI270 | BMI270 |
| ESC | BLHeli_32 | BLHeli_S | BLHeli_S |
| Power | 40A | 45A | 35A |
| Barometer | Yes | No | No |
| Serial ports | 6 | 5 | 5 |
| Input Voltage | 3~6S | 2~6S | 2~6S |
| Mounting | 25x25 | 25x25 | 25x25 |
| Weight | 11g | 8.6g | 8.8g |
| Price | $94AliExpressGetFPV | $99AliExpressGetFPV | $89AliExpressGetFPV |
RC link and FPV gear
You can’t go far if you loose your control link or FPV video feed. I use ExpressLRS for all my RC projects. It has the best range in the market and since it’s open source you have a lot of choices of hardware. For this project I got a full diversity receiver BetaFPV SuperD. It has two independent receive circuits each with their own antenna and switches to the one with the strongest signal. I placed my antennas on front and back of the quad to get the best signal no matter which direction I’m flying. I have a transmitter with 1W of output power which should be able to keep the control link going for tens of kilometers.
For a long range video feed I decided to go with an analog video transmitter (VTX). They are usually more powerful, smaller and cheaper compared to digital video systems. I had very limited space on the quad so a small VTX was very important. I decided to pick Rush Tank Solo which can go up to 1.6W of output power and paired it with HGLRC Hammer Mini antenna.
Due to size constraints I was looking for nano size camera and selected Caddx Baby Ratel 2. I previously had the bigger version of Ratel 2 and was impressed with the quality. The Baby Ratel didn’t disappoint either: the quality is good, it is configurable, well priced and readily available.
Update: Foxeer recently came out with a Reaper Extreme which is exactly the same form factor but with 2.5W output. It is a bit more expensive but it also packs more power.
Batteries
For long range flights you need a lot of battery capacity. I’ve decided to go for a Li-Ion pack since they give ~30% more capacity at the same weight compared to LiPo. They are not very common in FPV due to their average discharge performance. However, Sony VTC6 cell based packs are capable of delivering 30A of discharge current which is enough for this size of motors with low kV. It’s pretty easy to assemble these packs at home but there are ready to purchase options by GEPRC, Flywoo and other manufacturers.
How does it fly?
The drone has 2 very different profiles on 4S and 6S. It can be a long range cruiser or a racer. The drone is very efficient on 4S and can fly for over 30 minutes. On 6S it flies like a rocketship. However, it’s one of the those cases when a multi tool can do multiple things but doesn’t excel in any of them.
It is very floaty as a long range drone. It takes a while for small low kV motors to spin up 5" props. This acceleration lag made it really difficult to tune position hold and other autonomous flying modes. The low weight which makes it very efficient also makes it more susceptible to wind throwing it around.
As a 6S racer it’s fast and nimble but it’s also very nerve-racking to fly. You know it will not crash well and you have to be very careful with it. It was cool to try it out on 6S but it’s not something I’ll be doing regularly.
It was a fun build and interesting experience to discover different characteristics of these ultra lightweight drones. However, I think my next long range drone will be a more typical 7" or larger quad. I can understand why they are more popular in long range flying: larger props help cover more ground with the same flight time and more weight makes them more stable.