Given all the fuss about weight thanks to the new FAA regulations, I thought I’d take a little bit of time to write up my approach to multirotor design. Seeing brand new pilots buying huge 2208 motors “to win races” with their first quad or wanting to build something like Zevi’s monstrous 2315 Alien build without any experience always makes me cringe a little. Following this guide should expose overweight quads and help you slip under that magic 250g mark if need be. ( If you need a parts list, check out the ET160 build showcase a brushless build under 130g)
Speccing out motors and props for multirotors has always been somewhat of a dark art. I recently did a little work at mttr.net on the Micron prototype (flickr) in optimization of a lightweight 10″ propulsion system and found there is no accurate way to theoretically calculate efficiency (though plenty of calculators claim to) because of variation in prop design, motor quality, and environmental conditions. I might have set the record for longest flight time on 10″ props (48 mins) but progress was made only by manually testing different propulsion setups. Over the last few years I think I’ve learned a thing or two about putting together power systems, so here are some guidelines tailored for mini quads (but generally applicable to almost anything from 100-1000g).
I base my calculations off motor size and weight (mwt = weight of 1 motor). Pick a motor, propeller, and battery size and build the rest of your system around that:
Motors and Props
2204 and 1806 motors generally weigh just about 25g. 2204 & 1806 motors at around 2300kv are good for 4″(4s), 5″ (3/4s) and 6″(3s). Any lower KV (1800-2000kv), try to stick to 6″ props only (especially on 3s). Any higher KV(2500-3200kv), look for 4″ (3/4s) and 5″(3/4s). Motors larger than 2204 quickly reach diminishing returns on 5″, so you are better off with larger propellers if you are using 2206-2208 motors. 2204 and larger motors are best if you are plan to often take HD video from a GoPro.
1306 motors are around 13g a pop. 2300kv versions work best on 5″ (2300kv 3s) 31ookv versions are perfect for 4″ (3s) and 5″ (2s). Going up one more cell is definitely possible, but shouldn’t be necessary because the low weight means great acceleration.
Bullnose props should be treated as the equivalent of standard propellers of one size up (albeit with a little lower efficiency). For example, 4045BN, would be comparable to 5045 standard props. This means the 2204 and 1806 which work great on 5″ props (3/4s) will also work great with 4″ BN props (3/4s).
I source 90% of my equipment from TMotor, Scorpion, RCX(RCX>DYS any day), Cobra, Emax, Airbot, Gemfan, and HQProp. Oh, make sure to buy locknuts for your motors and toss the stock ones.
Find a nice, carbon frame that is about 4-6mwt. Larger photo ships like the Blackout 360 and Hovership Superbeast will be on the heavier end, closer to 8 or 9mwt, while acro quads like the Warpquad 200 will be closer to 2-3mwt. Going up in frame weight generally means a larger(bigger props) and more durable frame, along with extra functionality. Lower frame weight generally means lower repairability, but a clean design (see QAV210). Low weight and support for large props are a great a mix for endurance.
Some of my favorite frames are from TDrones, BoltRC, ImpulseRC, ShenDrones, Diatone, MyRCMart, and Lumenier (though the new knock off QAVs are pretty sick)
Pick up ESCs that run about 1/3-1/2 mwt. Make sure to pick up 4s capable escs even if you only plan on using 3s, the overengineering comes in handy when things get hot. Generally motors under 18g are fine with 10a ESCs and anything under 30g should be fine on 12-20a ESCs. Anything 40-60g (not for miniquads, please) should run 20a ESCs, and for 70g+ go with 30a.
Stay away from KISS escs, they tend to explode (spectacularly). Most of my ESCs are Xrotor, Rotorgeeks, and RTFQuads. Go for BLHeli or SimonK if available. The ones you see on ebay work ok, but make sure to get extras, they tend burn out often.
FC + RX should be 5-15g. Advanced builders can remove the casing from their FC/RX to decrease weight. Just remember to insulate any exposed conductors. When building quads under 150g, switching from full size F1/2/3 boards to the mini counter parts can save a valuable 4-5g. Switching to single cable SBUS or CPPM also helps cut down the several cables needed for PPM connection. Spektrum satellites also use a single cable and are dirt cheap too (though you will need to solder a bit).
While the new F3 boards have lots of great features, Naze32, CC3d, and other F1 boards will be relevant for atleast another year or two. Watch out for ebay boards, sometimes the bootloaders are screwed up.
Radio side I run my Futaba 8fg + DSMX module(best setup EVER) with a Lemon SAT/Naze32 copter side, but anything FRSky, LemonRX, or OrangeRX should be great. Keep in mind the Spektrum compatible LemonRX is significantly less expensive than FRSky ($6 vs $30 per RX). Go for the Taranis if you want a radio with loads of features or DX6/i for fast out of the box fun.
Video Gear + Odds and Ends
Your props, battery strap, PDB, and all wiring should be under 50g. Matek makes some awesome PDBs with voltage regulation for dirt cheap. PDBs can cut down on wiring and decrease weight.
FPV equipment should be no more than 20g + 10g antenna (squeeze this down to 15g total on micro quads) Most small VTX’s are under 10g. If you need any additional weight savings, removing the SMA/RPSMA jack and direct soldering your VTX to your antenna can save a couple more grams (experts only).
Immersion RC VTX’s are the best, but they are also pretty big & heavy. Skyzone and Aomway both make fantastic 32ch 5.8 gear that is compatible with Fatshark receivers – their 200mw VTX’s are usually under 10g. Aomway antennas are also the best you’ll find under $40, please don’t buy the shitty IMRC Spironets. For the lazy bunch, the Spektrum VA1100 is a decent short range combo VTX/Camera option.
There are a plethora of cameras available, but make sure you pick up one that matches the mounting options on your frame and the voltage output of your PDB. Remember that you can steal power from the 5v rail of your flight controller for your camera. Ditch metal cased cameras for decased or plastic cams for lower weight.
If you plan on using a HD recording camera, try to find one under 3mwt, the lighter the better. Check out the new Runcam2, Polaroid Cube, and GoPro Session cameras.
Your battery should be a little heavier than your frame itself, around 4-8 mwt. (Lighter end for acro and heavier end for endurance) Remember to watch for the C rating. Higher C batteries do have more amps available at their disposal, but they do weigh and cost more. Generally 20-40c batteries are great for cruising and beginners, but 60-80c is better for intense sessions. Switching to XT30 instead of XT60 plugs can save a few grams as well.
My favorites are VCANZ, Bonka, Tattu, Nanotech, and Zippy. Multistar lipos are a great choice for energy density.
Yay – you made it to the end!
In summary, a quick solution for a target weight (including FPV gear, but minus battery) would be the following for most builders:
Dry weight < Motor weight * 13, or 13 mwt
AUW (Dry weight+ battery) < 20 mwt
That is a dry weight around 325g for 2204 and 1806 builds and 170g for 1306’s.
Advanced pilots should drop that by 2-3 mwt for best performance, which gives around a 5+ thrust/weight ratio. Your motors will be pushing about double the AUW to properly hover/maneuver, so estimate each motor pulling about 9-10 mwt. This averages mixed/low alpha flight times of around 7 -10 minutes and racing fpv sessions of 4-7 mins. Light quads can break 20 mins in the air, and heavier ones will struggle to stay up for longer than a few mins. Weight is everything!
If you found this guide helpful, please consider donating a few bucks through paypal – paypal.me/mvphysengclub. All proceeds go to purchasing new gear for our school’s RC club. We are supported by TMotor, TDrones, Scorpion, and VCANZ, so shout out to them for their support and continued innovation in the hobby.
Check out the ultimate 250 guide for more info on your first build.
If you were wondering if your system is up to snuff, leave me a comment here or join us on r/multicopter.
Check out the latest build showcase – the 130g Diatone ET160!