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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Ooops...FM is always going to be <1 (less than one).  Please forgive the typo.

Hey Brad, maybe a picture will help:

 

12x45 EPP slow fly props arent suitable for craft weighing in excess of 5kgs.......

I have been told that normal props are not meant for Quadcopters, but for planes. Quads are heavier which would cause the props to break. So they really need to make sturdier props specifically for Quads so that more weight can be lifted. We will see how the Graupner props do.

I forgot we had a short video!!

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.

It helps me understand that I must respectfully stand diametrically opposed to your assertion.  The propeller can never have more force upon it (other than its own mass, of course) than it, itself, can generate in aerodynamic thrust.

They're not, but that's an aerodynamic thing.  Consider this: how fragile do the roots of helicopter blades look in comparison to airplane propellers?  It could very well be that in multicopter applications, the RPM limits may be more easily exceeded, and that might be something to pay attention to.

Methinks this issue needs a bit of further discussion, as the diagram you show illustrates a fundamental misunderstanding of Newton's Third Law.  Just to clarify, forces are described as vectors, and the only two attributes of a vector which matter are the magnitude and the direction.  It does not matter what generates the force, only that is has a magnitude and a direction. 

If the forces applied to a mass are in equilibrium, there is no net change in velocity.  This is true for a hovering multicopter or a fixed wing plane zooming along at a constant speed.  In the case of the multicopter, the force equilibrium pairs are thrust and mass.  In the airplane example, they are thrust and drag.  However, the force vectors still cancel each other; they do not add because they are equal and exactly opposite. 

Here is a good illustration of the concepts:

http://www.physicsclassroom.com/class/newtlaws/u2l4a.cfm

Exercise 4 gives the example of Kent pulling a rope away from a wall with 500 N of force, not at all unlike a propeller producing 500 N of thrust in a static test stand.  The second example shows Kent pulling with 500 N of force against an elephant pulling away with enough force to achieve a tug-of-war stalemate, just as a model airplane propeller in a multicopter application would in hovering.  The force exerted by Kent (our propeller analogy) is the SAME in both cases.  You could just as easily substitute total drag in a fixed wing plane for the elephant in our thought experiment and garner the same result.

As a matter of fact, DIY Drones sells the APC "electric slow flyer" props precisely for multicopter applications, and this is entirely appropriate.  While I would argue that there is plenty of room for innovation with regards to efficiency, I do not question their safety or mechanical integrity when used within RPM limits in VTOL aircraft. 

Don't you hate it when that happens?  Yep, that one was either defective or overloaded.

Brad, I thought you had understood the point of the diagram.  I guess not.

Basically, there's no understanding of Netwon's law, here.  In order for a multirotor to hover, the prop has to provide enough thrust to negate the force of gravity.  Since force is not magically conjured up, it is derived from the prop blades pushing against the air.  If the multicopter weighs 5 kg, then the force on the prop blades is 5kg divided by 8 props for an octocopter.  That means  each prop has to be able to hold up 0.625kg or 625g of weight.  This is irrelevant to the rpm which is related to centrifugal force only.

Perhaps another diagram might help:

Sorry for the two right hands.  So, basically, when a multicopter is hovering, it's equivalent to the picture above.  Just as in the picture above, then prop is not moving up or down, and there is no net velocity, but you can see that there's still a downward force from the weight.  The props can only bear so much weight before breaking.   In the multicopter case, this is the weight of the craft.  If the craft weighs 5kg, then this weight is distributed to all the props.  This, again, is regardless how what rpm the props are spinning at.

When the propeller is horizontal, then that weight is not present,or at least not directly, because the weight is a function of gravity. (F=mg)  The wings are bearing most of the weight directly.  And as you mentioned, the props will feel the drag from the air, and inertia during acceleration and deceleration.  There is less stress on the props axially, and they are designed with that amount of stress in mind.  They are not designed to bear the weight of the craft directly.

I hope I'm explaining it clearer now.

(facepalms)

I thought about asking you to take this to PM, but I cannot let this position of yours stand unchallenged.

The minute I saw your diagram I thoroughly understood your erroneous assertion with the greatest of clarity.

A given propeller turning at a given RPM and producing a concomitant amount of static thrust in air will experience the same aerodynamic forces upon the blades absolutely without regard to its orientation.  The notion that there's an extra force if the prop is pointed up due to gravity is patently false.

I will concede that a propeller used in a practical multicopter will likely experience higher throttle settings for a longer duration than in a typical fixed-wing application.  However, if a propeller is operated within the manufacturer's RPM limits in a multicopter and it fails, then said propeller is either defective or improperly designed.

No propeller or helicopter rotor is made "to bear weight".  They are made to produce thrust.  Either this thrust exceeds the the forces aligned against it (either drag or gravity or both), and motion of the system results, or it does not.

As a moderator of this site you ought to know better, as your misinformation is tantamount to accusing the sponsor of this venue of selling propellers that are unsuitable for the application intended. 

Speaking of sponsors, Chris Anderson has a degree in physics from Georgetown.  If you still do not understand why what you say is false, perhaps he can help (with the commensurate authority figure gravitas).  Otherwise, I am done.

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