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?

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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.

They were the grey composite ones from J Drones, 12 x 4.5 epp. I don't say it's the manufacturer but someone should put together some specs and say which props can be used for which weight ect ect.

APC quote "Slow Flyer props - Maximum RPM=65,000/prop diameter (inches)" (from this link) so 5416rpm max for 12", I doubt they would ever quote a max weight though. A full power climb at 600W is a massively different "weight" experienced by the prop than in a gentle hover at 180W.

The plane was in a flat hover when the prop broke.

Maybe use a prop tachometer to check the RPM?

How will measuring the speed of the props help to know whether they will handle the weight of your copter?

Normally max RPM will be exceeded long before max carrying weight (or thrust, to give it it's correct term) is exceeded. If a prop breaks before max RPM is achieved then there's something wrong with the prop. 

A 12" prop spinning at 5400 RPM should theoretically be able to produce about 770g of thrust so for 8 props that's 6.1kg, how heavy is your Octo?

So the only way to tell if the known specs of the prop were exceeded is to measure it's RPM.

@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:

University of Illinois Wind Tunnel Small Prop Test Database

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!

@Brad. Thanks for the info, very interesting. As I am still learning I didn't take much notice of the props and only started to ask questions when props just kept breaking and they were supposed to be the good ones!

Why don't they have specific props for the specific weight of the copter.

This is all new but I know they set specs for other parts of the copter like the thrust of the motors in proportion to the weight of the copter so they should do that with the props??

Someone should really take the time to do it. I'm sure it would help everyone and alot of money and time would be saved. I'm sure in the next few years some engineer will do it. In the meantime I guess it will just have to be trial and error.

It is all good and well getting a kit but you don't learn as much and sometimes you need it tweaked for the job you doing.

It's all part of the learning process :)

You guys do realize that none of the prop stress ratings apply to multicopters. The rpm ratings refer to how much centrifugal force that the props can handle, which is all fine.  But the major stress on multicopters is along the axis of the prop, which is perpendicular to that.  Mounted horizontally, that force is just equivalent to the force of inertia, which is smaller and only significant at take off.  Mounted vertically on a multcopter, the prop has to be able to bear the full weight of the quad, which is several pounds for a Octo, and increase during hard manuevers .  Most props simply are not built for that kind of stress.

@Karla: Well the props are not supposed to break, regardless of the application, unless they strike something other than air (and even then, many an engine has been ruined by the crankshaft/armature taking the beating force during a crash that the prop probably should have absorbed).

@Ellison: With all due respect, help me understand what you're thinking here.  There's the centrifugal force pulling the blades apart which is tangential to the shaft.  Then there's the axial aerodynamic force of the blades accelerating the air, which would be the same regardless of what the prop was attached to or how it was oriented.  Granted, a multicopter might stay at nearly full throttle more than a fixed wing model, but as a propeller designer, would I assume that a fixed wing pilot doing 3D aerobatics wouldn't shove the throttle forward and keep it there?  There's the dissymmetry of lift in forward flight due to the difference in airspeed between the advancing and retreating blades, but Karla's description of the failure context as "went up in the air" doesn't sound like lateral speed was a factor. 

Dr. Paul Pounds at Yale has stated that even rigid model airplane blades "flap" in response to DoL, but I'm a bit skeptical that it's more than a negligible effect.  However, it would indeed produce a torque moment proportional to the square of the relative velocities.  Given that the tip speed of an 11" prop turning at 5000 RPM is around 163 miles per hour, a multicopter would have to be rippin' fast sideways before this force becomes material to mechanical integrity.  There might be a few other coriolis-type forces here, coupled with a bit of gyroscopic stress, but these would be the same as any fixed-wing model (certainly with aerobatic maneuvers) would encounter.

Why would a prop that fails on a multicopter not simply be a bad prop?

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