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Multirotor Propeller rules of thumb

AKCub

Junior Member
#1
I'm mostly done building one of David's Tricopters and one of the things that still isn't very clear to me is the thought process that goes into prop selection for a particular motor/battery setup.

It might become clear when I fly the tricopter and experiment with different props.

For instance, using a DT-750 motor and a 2200 mah battery if I change the prop from a 9x4.7 to a 10x4.7 slow fly what sort of affects will I see? More/less duration? Better/worse stability? This might be tied into stability but will different size/pitch props affect flight characteristics?

What about the difference between a 10x4.7 and a 10x4.5 prop on the same system?

Also, what about the difference between a slow fly prop and a multirotor specific prop?

These questions are related to my specific platform but I think they are applicable to any of the new folks building multirotors. Are there any rules of thumb for guessing what a new prop will do in a particular system in regards to the above questions (and any other you feel are relevant)?

Would it be possible to cover this in a portion of an episode?

Thanks for everything,

Scott
 
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#2
Hi Scott, did anyone every answer your questions?
I think going from a 9" to 10" prop will cause the battery to drain faster, however you'll get a slight bit more lift. If you have a light tricopter you probably don't need the 10".

I'm starting to experiment too with David's tricopter. One thing I'm running into is the yaw issue others have mentioned. I fixed this by putting two 3.8 pitch counter rotating props on the front and a 4.7 pitch on the back.

I just recently had my motor that holds the 4.7 pitch prop burn out or something and didn't know if it was caused from having different pitched props on the same tricopter. Anyone have experience with this?
 

AKCub

Junior Member
#3
No answers but I have been flying a fair amount with the 10" props.

I like flying the 10" APC MR props because they took less to balance and they seem quieter. I also have 9 x 4.7 Gemfan props but I am reserving them for spares, mostly because they seem noisier and they are less balanced out of the box. I haven't noticed a significant difference in battery life between the two props.

With no FPV gear, my copy of David's Tri weighs 1051 grams with a 3 cell 2200 mAh battery. I get about 12 minutes of flight time to my low voltage cutoff of 10.5 volts. I did lengthen the arms from David's (front 39.5cm, rear 41.5cm). The front props are counter rotating but based on what I see I will go to all 3 props same direction as I haven't noticed a significant difference in hover yet and the CR props seem to be a little harder to find. (I'm in Alaska, everything is harder to find). I am also using stock firmware on the KK 2.1.5 board (1.6).

I did have to deal with some of the tail servo shimmy/wobble but I did three things to mitigate it. Not sure which one solved the problem but it is almost gone now. First I checked and corrected the balance. Go figure that the only prop that required any corrections was the tail prop. Next I checked the mounting on the tail servo mechanism and found that I could make it move a bit by pushing on it so I re did the zip ties and snugged them down a bit more. Finally I upped the servo smoothing number from the stock number to 75. I will have to double check that later but I'm almost sure that's where I ended up. Now the wobble is non existent in hover and cruise. I do see a little in a full power pop from an hover out of ground effect but it goes away as soon as power is reduced.

I have about 20 flights now on it. Most around the yard but I have been to the park a couple times to get it up to speed. I added a piece of tape to tail boom for perspective. Next go-around I will paint tail boom a different color and add lights.


photo 1 (1).JPG photo 2 (1).JPG

Hi Scott, did anyone every answer your questions?
I think going from a 9" to 10" prop will cause the battery to drain faster, however you'll get a slight bit more lift. If you have a light tricopter you probably don't need the 10".

I'm starting to experiment too with David's tricopter. One thing I'm running into is the yaw issue others have mentioned. I fixed this by putting two 3.8 pitch counter rotating props on the front and a 4.7 pitch on the back.

I just recently had my motor that holds the 4.7 pitch prop burn out or something and didn't know if it was caused from having different pitched props on the same tricopter. Anyone have experience with this?
 

stay-fun

Helicopter addict
#4
Hm, I'm not very experienced in multirotors, but what I understand from David's HV tricopter, is that if you go with smaller props at higher voltage (and higher rpm), your 'air columns' are smaller but have more thrust. This gives you more stability when descending. His HV (on 4s batteries) runs on 9x5 props.