Multirotor prop/motor sizing application guide

[powdermnky007]

My suggestion for the next flite tip.

You've given us a video on how motor kv rating works and a video on how prop sizing works. Now we need one that helps convert that knowledge into practical wisdom of how it functions in the real world. I know David has the knowledge, because he has tried them all. I've looked over David's Tricopters that hes built in the past to see how he sized his motors/props. Starting with the newest and going to the oldest we have:

vtail quad
sk3 2822 1275kv motor with 8" props
or 2826 1200kv with 9" front & 8" back (for camera lifting)


I think these were on the batbone
Turnigy Park300 Brushless Outrunner 1380kv
8045 SF Props 2pc


2.6 hv deluxe
T-Motor 2216-12 800kV
Graupner 9×5 E-props.


2213N 800Kv Brushless motors
10*4.7 APC props


DT750 750kV Motors
GWS 10*4.7


I see David has moved from a low kv/big prop (lower rpm, more torque, more efficient) to a high kv / small prop (higher speed, less torque, less efficient) which brings the slowfly props out of their design range (which is a trend I see happening CONSTANTLY with these multicopters).

I'm about to build a multirotor for the first time (aren't we all?) and one part of me wants the efficiency and one part of me wants the speed. Planning on using the anycopter platform due to its future upgradeability and I'm starting out with a tricopter. I plan on advancing to fpv/gopro hauling in the future (don't we all) so also need the power to lift those later on and don't want to have to purchase new motors.

So David, why the faster and faster motors? Why risk a prop breaking in flight? With your massive popularity there are more and more people being inspired by you and designing their rigs around what you are flying. Please provide an insight to your shift in motor/prop selection and let us know how they differ in the real world.

Also David, you need more evil remarks. If you don't make a flite tip on this subject, I will make using sip ties illegal and all your rigs will crash!!!

 

[powdermnky007]

Maybe show the difference between using 900kv motor w/10" prop vs 1380kv motor with 8" prop. Lets say each one produces the same amount of thrust. Will they fly the same? Does thrust = thrust?

 

[Crashpilot1000]

Why don't you use ecalc and find the appropriate motor/prop combination for your needs YOURSELF. Ecalc is the perfect reference for ALL your motor/prop/multicopter questions. More agility or more flighttime it's up to you. David is not god - he does things his way and has some good points, I do them differently and you probably will do them differently as well. Zipties are fine when you use UV resistant stuff. Those normal chepo zip ties will break sooner or later in the sun - no way around that fact. You will want to double secure the motors with non uv resistant zip ties and change them regulary.
Cheers
Kraut Rob

 

[powdermnky007]

I'll check out ecalc thanks for the info. I don't have anything against zipties, there were just a few flite tips where David made an evil comment at the end of the episode. Like "if you don't scratch build a plane, your wife will leave you and you will die alone". I just thought those comments were hilarious. Every time he ends an episode I hope he's gonna say something. But he hasn't in a while

 

[Crashpilot1000]

@powdermnky007: Ok, didn't know about that wife and zip tie stuff . I think that ecalc stuff should be written in big red letters above any planning of an aircraft. There is so much information about all the combos(prop/motor/esc) and you can play through different multicopter setups. I myself ecalced my quad before building it and the flighttime is spot on what ecalc told me before. Even the predicted Amperes for hovering were right (measured with Frsky FAS100, displayed on DHT-U Module). Once you get the hang of that webinterface you can get lost there for hours and it never gets boring. And the best thing is, that it gives you warnings when you overstress one component (RPM Warning on desired prop, Motoroverheat, esc overpower etc). I know some people build their setups in the red areas of the ecalc curves, assuming that their quadmotors will never go 100%. Warthox does this (running 3S motors on 4cells etc) but he is sponsored by a well known german RC store and I doubt that some of his setups last longer than a few flights for the video. Well, I am not that type of guy, I like my setup run within specs and I land mostly with over a minute flighttime left on the lipo. That gives me very long living lipos and the next lipo is swapped in in no time. Waiting for some standard lipowarner going off will reduce their lifetime drastically. Maybe they should make an episode on ecalc - planning a copter for your needs (flighttime/speed/lifter or a mixture etc).

Cheers
Rob

 

[powdermnky007]

I just spent some time on ecalc. You are right, man that thing is AWESOME! Yay math! Thanks again for the great resource!

Also found this tid bit, which helped my understanding as well.
Smaller props are lighter and have less wind resistance which means that they can change their speed quicker, resulting in grater stability. Always use as small props as you can. It should hover around mid-stick. If it’s under that you can probably use a smaller prop.

Longer arms also improves stability as the props are moved further from the center of gravity. This means that the props much travel a further distance to bank the arm to the same degree.