is it worth it?

[Longbaorder23]

Hey all,
im looking into bigger batteries for my quad for longer flight time. i currently use two batteries, a 5000 and a 2200. i use the 2200 strictly for acrobatics. it gets me about 6-7 minutes. the 5000 which i use more conservatively, gives about 15-16 minutes. im getting into FPV and I think ill want more like 20 minutes for some real enjoyment. question is, is it worth it to simply add a larger battery? Ive been doing some math about it, and it seems with 3s lipos available, these batteries are roughly 12mah per gram of battery total weight. if the cells are high quality then they can be around 13mah per gram. not the most accurate way to look at the matter, but its my take. I know that when it comes to adding batteries of high capacities to a model, the flight time is a bell curve. Meaning, if you put a battery much too heavy for the model (but it can still fly) the flight time is greatly reduced. if you put a battery very lightweight and small on there, the flight time will also be reduced. my quad is big, and 3.5 lbs without accessories or FPV gear. how high up of a battery capacity do you think i can put on it? is 5000 on the growing or shrinking side of the flight time bell curve? maybe a 6000 mah battery will do better? Any help would be appreciated. Thank you!

 

[Craftydan]

If your power system can handle it, move to the next curve.

Bump up to a 4S pack, and the added power will allow you a higher optimal AUW, allowing you to add larger packs at less of a penalty. If you're too far along the curve that the 5000 is more weight than flight time, you might see a satisfying increase in flight time.

 

[Longbaorder23]

Hmm, my escs are rated at 4s, and they're 25 amps, quite over kill probably, so I think they'll be fine. My motors are rated at 3s though... They are balanced and always use balanced props of course. I'm also confused on props, if I go up in voltage to 4s, what dimensions of my props would I change? Currently using 11x4.7 props. 1020kv motors I believe. What would be suitable? Do I really have to change? I'd rather not. Thank you!

 

[Craftydan]

You're already looking at new batteries, and changing props are cheap . . . motors not so much. It really depends on your motors -- bearings are your limiting factor, but look around and you might find reports good/bad from those who've stepped up to 4s.

For your props, you'll have to drop the width at least an inch to 10" -- maybe even to 9", but the 10" will run at lower RPM, taking it a little easier on overspeeding your bearings. For your pitch, slower is always better, but the really slow props are sometimes hard to find.

 

[Longbaorder23]

why would lower pitch props be even more ideal?
also, yeah some of the bearings in my motor dont sound so perfect, and have had a couple bits of dirt in them before, so i may just go buy some quality bearings for them. another note, my motors when i fly acrobatic or even general flight are barely warm, so i think with proper precautions and setup 4s will be fine, but yeah ill check in with others.
Is 4.7 pitch a lot for a large quad? whats ideal? it flies just fine as is. thank you!

 

[Craftydan]

"Flies fine as it is" is a good thing

Prop pitch is set by the ideal speed the propeller needs to move forward so it always has fresh air, in inches-per-revolution -- that 11x4.7" prop needs to move forward 4.7" within the time it takes the prop to spin once in order to get fresh air and perform optimally. The greater the mismatch between the forward speed and the *ideal* forward speed, the less thrust the prop will generate because it's, well, beating the air -- turbulent air

See the problem yet?

How fast is the prop's linear motion in a hover? The steeper the prop on on a multirotor, the greater the disparity, and the less efficiently it will run . . . but wait, there's more!!!

The faster the RPM, the faster it will need to travel forward, so . . . moving from 3S to 4S will further increase this by making the props spin roughly 33% faster, making the pitch problem that much worse.

In the end, you make it as little of a problem as you can by getting the shallowest pitch you can . In reality it'll be hard to find something shallower than 4-ish inches, and the extra power form the extra RPMs will more than make up for the loss in prop efficiency . . . just don't go up in pitch

Actually it's the barely warm you're trying to keep. You're boosting your voltage so you can get more power with the same or less current. it's the current that generates the waste heat and burns out the coils, so pushing the voltage upward should let you keep your cool while putting a LOT more energy into the air.

The bearings . . . well, good clean bearings just make everything fly nicer . . . .

 

[stay-fun]

How fast is the prop's linear motion in a hover?

Now that, is a super interesting question! After all, in a hover, the whole quad is in a vortex of air, which is going down. So you could definitely say the quad, or at least the props, have a certain 'air speed'. How much is the airspeed? Hmmm... That all has to do with the weight of the quad, and the amount of air that is being pushed towards the ground, and how fast that air is being pushed (action is reaction - Newton's 3rd law). Here, if you push less air but faster, or more air but slower, has the same force upon the quad - but slower and more air is probably more efficient.

So uhh, I don't have a clear answer, obviously. Just wanted to point out how complicated you can make this

 

[Craftydan]

So uhh, I don't have a clear answer, obviously. Just wanted to point out how complicated you can make this

Indeed, but sucked air isn't clean standard pressure air a tractor prop would encounter in forward flight and even then it's not going to be the optimal airspeed. It's good enough, but they're still eating turbulent air if the flow isn't enough to clear the air before the next blade hits, especially if it's descending even the slightest bit.

Simple or complicated, steepeing the prop makes it worse, so for fixed pitch, it's best to keep the pitch shallow.

 

[Longbaorder23]

ah that makes much more sense! when i add more gear to the quad ill most definitely look into a bit shorter and lower pitch props. but just to check, lets say i go from:
5000 mah 3s
11x4.7 props with 1020kv
to:
5000mah 4s (for example)
10x4 props with 1020 kv

would i see an increase in flight time? (in theory)
thanks for all the info!

 

[Craftydan]

you should, if for no other reason your motors should hover at a lower throttle setting.

Now how much depends on how well suited the 3s5000 pack was -- if it was far up or over the curve, you should see a substantial improvement in flight time. Even if the 3s pack was still suitable, because you're "moving left" on that bell curve as you step up to the next curve, your motors should be moving closer to an efficient AUW/mAh capacity . . . so yes, it will increase. Unfortunately, predicting how much with accuracy is a bit tricky . . . unfortunatly, experimentation ain't cheap

 

[stay-fun]

Simple or complicated, steepeing the prop makes it worse, so for fixed pitch, it's best to keep the pitch shallow.

Define 'shallow'. I mean, would you recommend a 1" pitch? 0.5" (when that was possible)?

There's also the rpm of the prop. The higher the rpm, the more power draw. I'd like to compare this to helicopters (as I'm more used to those ): I can run my 450 helicopter at 2400 rpm headspeed, or 3200 rpm. The 2400 rpm will require a steeper pitch to sustain hover than the 3200 rpm. So will I increase my flight time when I go to 3200 rpm?
My experiment says no

Now you probably have more experience in the MRs (I have had a tricopter though), so you can say I'm talking BS , but I'm just trying to understand

 

[Longbaorder23]

interesting point stay-fun. I think that theres also a sweet spot with such things, and with regular helicopters theres a much different set of goals to meet with headspeed, and im assuming higher headspeed yields higher performance, but shorter battery life. but i think also the very nature of mulitrotors and helicopter rotor speeds are very different, though having some shared properties.
I think in my case, when i go to add weight to my copter, like, significant weight, maybe a gimbal for my camera(s) or a larger flight controller system, ill upgrade to 4s and shallower and shorter props.
but this sparks interest into me, how do those people go out and fly multirotors for like an hour at a time? im getting into fpv, and i have a feeling im gonna want to start stretching my wings and go out further and further. i know i can do things to make my entire setup lighter, but is getting like hour long flights just perfecting the flight time curve? i dont think i want to fly for an hour, but if i could get like 40 minutes at some point, that would be my dream setup. Thanks for all the help!

 

[Craftydan]

hmmm. . . that is an interesting point. To extend it to the limit (which can point out problems in the logic) a flat prop would be the gas-mileage equivalent of idling the car, so you're right, there is such a thing as "too shallow".

From what I've seen, the slower props perform better, and as I mentioned earlier, it's hard to find a fixed prop with less pitch than 4" . . . perhaps that's for the best . . .

 

[rifted]

Quick tip also, lower RPM with big props and 4s also work great, but it is also price. I fly a Tarot 650 with 580 RPM 12.5 props and 4s 5000 and fly's 13 min, with time to spare to land. I also try with 2 3.3 making a total of 6600 and fly 16 min with time to land. I do not like to push my batteries to the limit (dont like to drop my Tarot into to ground because of a "emty fuel" problem lol) but if you realy want to go nuts with flight times. Crafty Dan says it, "experimentation ain't cheap."

Go with 15 -17 inch props 6s 5000 to 10000 and low RPMS 320ish ... lol

Good luck!
Ed

 

[Longbaorder23]

im pretty sure my frame simply wouldnt allow props of such monstrous size haha. but on the website for your quad, the tarot, it says for carrying heavier weight payloads, go up in pack voltage, down in kv, and up in prop diameter. pitch change not noted. isnt that the opposite of what you said, craftydan?

 

[Craftydan]

LB,

What I've recommended was based on keeping your power system the same.

"All else remains the same" trends:

- voltage up, prop diameter down
- KV up, prop diameter down

Riffed's setup would push voltage up and kv down . . . see the competition in the trends? In this case, moving to his setup the KV drops *MUCH* faster than the increase in voltage in order to use a bigger, more efficient motor. Prop size goes way up for kv, but down a little for voltage . . . yielding a net gain in prop length.

If you replace the motors (not a bad idea, just expensive) you need to get the appropriate props for those motors at that voltage. If you're not up to flying with such a long prop, don't go that deep. For 4S, An 800kv motor will likely fly in the neighborhood of a 10-11" prop, and a 700kv will likely fly 13-15", depending on the size of the motor.

 

[rifted]

im pretty sure my frame simply wouldnt allow props of such monstrous size haha. but on the website for your quad, the tarot, it says for carrying heavier weight payloads, go up in pack voltage, down in kv, and up in prop diameter. pitch change not noted. isnt that the opposite of what you said, craftydan?

yea, well as Craftydan said, he did recommend a good set up to keep it cheep with what you have so no big expenses are made.

 

[Longbaorder23]

ahh! okay that makes much more sense now. installing my fpv gear tomorrow morning and flying it fpv for the first time! so ill see how it goes! but the fpv gear is so light compaired to my quad anyway, so no upgrades yet. but if i decide to add ill definitely do the upgrade. thanks for the help guys, ill keep this advice in mind!