### 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.

This reply was deleted.

### Activity

DIY Robocars via Twitter
RT @SmallpixelCar: Wrote a program to find the light positions at @circuitlaunch. Here is the hypothesis of the light locations updating ba…
yesterday
DIY Robocars via Twitter
RT @SmallpixelCar: Broke my @HokuyoUsa Lidar today. Luckily the non-cone localization, based on @a1k0n LightSLAM idea, works. It will help…
Thursday
DIY Robocars via Twitter
@gclue_akira CC @NVIDIAEmbedded
Wednesday
DIY Robocars via Twitter
RT @luxonis: OAK-D PoE Autonomous Vehicle (Courtesy of zonyl in our Discord: https://discord.gg/EPsZHkg9Nx) https://t.co/PNDewvJdrb
Wednesday
DIY Robocars via Twitter
RT @f1tenth: It is getting dark and rainy on the F1TENTH racetrack in the @LGSVLSimulator. Testing out the new flood lights for the racetra…
Wednesday
DIY Robocars via Twitter
RT @JoeSpeeds: Live Now! Alex of @IndyAChallenge winning @TU_Muenchen team talking about their racing strategy and open source @OpenRobotic…
Nov 20
DIY Robocars via Twitter
RT @DAVGtech: Live NOW! Alexander Wischnewski of Indy Autonomous Challenge winning TUM team talking racing @diyrobocars @Heavy02011 @Ottawa…
Nov 20
DIY Robocars via Twitter
Incredible training performance with Donkeycar https://www.youtube.com/watch?v=9yy7ASttw04
Nov 9
DIY Robocars via Twitter
RT @JoeSpeeds: Sat Nov 6 Virtual DonkeyCar (and other cars, too) Race. So bring any car? @diyrobocars @IndyAChallenge https://t.co/nZQTff5…
Oct 31
DIY Robocars via Twitter
RT @JoeSpeeds: @chr1sa awesomely scary to see in person as our \$1M robot almost clipped the walls as it spun at 140mph. But it was also awe…
Oct 29
DIY Robocars via Twitter
RT @chr1sa: Hey, @a1k0n's amazing "localize by the ceiling lights" @diyrobocars made @hackaday! It's consistently been the fastest in our…
Oct 25
DIY Robocars via Twitter
RT @IMS: It’s only fitting that @BostonDynamics Spot is waving the green flag for today’s @IndyAChallenge! Watch LIVE 👉 https://t.co/NtKnO…
Oct 23
DIY Robocars via Twitter
RT @IndyAChallenge: Congratulations to @TU_Muenchen the winners of the historic @IndyAChallenge and \$1M. The first autonomous racecar comp…
Oct 23
DIY Robocars via Twitter
RT @JoeSpeeds: 🏎@TU_Muenchen #ROS 2 @EclipseCyclone #DDS #Zenoh 137mph. Saturday 10am EDT @IndyAChallenge @Twitch http://indyautonomouschallenge.com/stream
Oct 23
DIY Robocars via Twitter
RT @DAVGtech: Another incident: https://t.co/G1pTxQug6B
Oct 23
DIY Robocars via Twitter
RT @DAVGtech: What a great way to connect why @diyrobocars community is so valuable and important! Have to start somewhere @IndyAChallenge…
Oct 23