the objective of this build is to fly at least 7-9kms out (and back!) utilizing the lightest li-ion pack possible which i guess could be 4S 21700 or 6S 18650. it must have motors spec’d to run on 6S with low enough KV to efficiently operate on 4S also.

1. lithium-ion pack 21700 or 18650 in whatever cell count to accomplish the task
   
2. apex 7in frame (165g)
   
3. emax eco2 2207 or 2306 1700-1800kv motors to drive 6 or 7in props
   

Well, the whole thing seems more ultralight than light to me. I would rather start with what it's supposed to be for, if it's supposed to fly somewhere on a flat plain, where no altitude climb is expected and no wind is expected. That might work. The 4s21700 is already on the limit for a 6-inch, the same goes for the 6s18650, mainly because of the current, you definitely don't want these batteries to draw more than about 15A for a long time, better around 10A. Ri is significantly dependent on temperature, the stated current values are well manageable even when flying in mountains below freezing point. If it should carry a full-size GoPro, it will really be on the edge. A 7-inch with such small batteries will not fly 20 km, maybe if it is really slowly crawling along, but probably not.

And as for the ImpulseRC APEX7 LR, it's designed by someone who doesn't really do LR frames. The APEX7 LR is such a weird frame, it has little internal space, it has little space for batteries, it's just a remake of a freestyle frame. The APEX7 LR is not actually made to last 20+ min in the air. Realistically it will take up to 12-15 minutes depending on the battery, speed and flight style.

hello MB!

your wealth of information is a most precious resource. now i can avoid all the guesswork on pack size and come up with realistic expectations with the present equipment available. i suppose i will need larger motors than 2207. someone did recommend i consider size 2507 1500kv for more headroom. then i guess 6S 21700 should make sense as well for the added capacity.

my use case is basically flying on level ground and over water to a nearby lake. no action camera for the lightest AUW. ambient temperatures 27-35deg C all year round.

as for the apex7 and its limitations, my other option is the TBS source one V5 DC 7in frame. it is lighter and seems to have more space than the apex7. although i am a bit anxious about the DC frame. but it does have 6mm thick arms.

Check out this video from DaveC, it's his Rekon6. This is a really extreme use of the 4s21700. I think it will help to understand a lot of things and then it is possible to continue further ...

For the short distance you want to attain, Hawk, you don't need to go with lithium ion  and all the negatives that come with Li-Ion.  If you are already committed to Li-ion, then I guess stay with it.  I prefer the punch of Lipo and like I said you lose nothing with Lipo, unless you are trying to stay below a governmental weight limit like some try to do here in the States. 

My favorite 7 inch linked to here on the forum to my build below.  It would be considered an old frame now, as it was in vogue in 2019.  But it is a solid frame, with stiff arms so as to not induce vibration, and plenty of room for your components.  Like we talked about before, if you go with bottom mount for the battery that leaves the entire top deck for stuff, i.e. GPS puck, GoPro, in my case a spot for my Marco Polo transmitter, and maybe other things you might want.  I made legs for my bottom mount as I live in the desert which is very rocky and not healthy for a Lipo to land on.  But I believe you live in an area where the ground is softer with vegetation, so you wouldn't need to make legs for yours.

I have used the 399 as my mid-range quad, doing what I call mountain surfing, running up and down ridgelines, canyons, etc.  This quad was used mostly for 3-5 miles (4.8km to 8.0km) out but I have been further out.  This is with a 4s lipo, but the quad is 6s capable.

As most of the old timers on the forum know, I prefer a 10 inch quad for my long range stuff.  Below is my build of the FR10G.  The build thread is incomplete, as I didn't post final pics, but you can get an idea what it is like.  I love this frame so much I have 4 more unassembled frames that I had plans for a pylon racing drone, an acro drone, and 2 other long range versions.  But I got burnt out on quads and went to planes, and now cars.  

Anyway, maybe my builds might give you some ideas.  But my best advice is if you are only looking at what I would call mid-range, 7-9kms, ditch the Li-ion route and go lipo.  If you start doing extreme long range with a quad, 25km+ and are ok with a boring cruise, then Li-ion is the mac-daddy.  

Pyrodrone for the 399 Super G +



399 Super G + 


FR-10G