### CORRECT PROPS for CORRECT WEIGHT and MOTORS

We have an Octo Copter with Turnigy 810 kv motors. We got composite props which we thought would be fine. It went up in the air and one broke from what we think was a stress fracture.

We have now gotten Graupner props which look much sturdier.

I am wondering why there are no specifications for  Props when it comes to the weight and size of the motors?

#### Replies

• Wow guys. I didn't think my post would cause so much typing. Thanks, I am now fully fully educated :)

• One way to test how much actual weight a prop can handle is to hang weights off it and see what the breaking point is.  Multiply that by about 8 for your Octo, since the weight is distributed over 8 props.

• @Karla: Based on what I've seen, much of the knowledge base in this area is garnered from trial-and-error.  Most of the effort (and frankly, the glamor) is in the control system and not so much in the aerodynamics.  Experience does rule, of course, but static thrust testing of model airplane propellers is a bit esoteric.  Perhaps one should either go completely with a proven solution (i.e. get a kit) or be prepared to experiment.

@Graham: No offense intended, but those calculators don't provide good results for static thrust.  They all tend to over-predict because they're using mathematical models that fall apart at low advance ratios (no forward speed).  If you want to do some comparisons, here's a database of model airplane propellers tested in a windtunnel by Dr. Selig's crew at the University of Illinois:

The props tested are just the sort most multicopter enthusiasts employ.  Unfortunately, there's a bit of math involved to get to the point of comparing the static thrust performance of one to another.

The term "figure of merit" or FM is used by helicopter aerodynamic engineers to compare the hovering efficiency of one rotor to another.  I'll spare you the long description, but it's a ratio of performance of an ideal rotor to the one in question, so it's always going to be >1.  Here's the formula (in spreadsheet notation) which you'll need to use to calculate it from the data in the "static" table:  =((CT)^1.5)/(CP)*0.707))

You'll know you've done it correctly when the FM numbers range from 0.25 to 0.55 or so.  Enjoy!

• Moderator

What brand for the composite props? I don't think the manufacturers take 'copters into account when designing props so usually it's just a RPM limitation. I've had a few props break in flight but it's always been the cheap ones.

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