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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@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?

Hey Brad, maybe a picture will help:

 

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.

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. 

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.

P.S. Chris graduated from George Washington, not Georgetown (my mistake).

P.S.S. The APC Slow Flyer series props are made for very light aircraft applications.  If you examine the root of the blades (near the hub), they are obviously far less robust in this area than propellers meant for heavy duty use (such as gas-powered planes).

You can calculate your maximum RPM by taking your fully charged battery voltage and multiplying that by the KV rating of your motor.  For example, an 880 KV motor with a 5 cell LiPoly pack (assuming 4.2 volts per cell) will turn 18,480 RPM (an unloaded maximum).  That is FAR beyond the rated specs of the APC Slow Flyer series, which is 65,000 divided by the prop diameter in inches.  So the 10-inch APC SFP should never, ever see an RPM more than 6,500.

There is also a technical advisory on the APC website with regards to mounting this series of propellers:

http://www.apcprop.com/v/html/tech_support_notice.html#slofly

Brad we seem to have a difference in opinion on the stresses put on a horizontal versus vertically mounted propeller.  No problem.

However, I need to point out that I am not accusing anybody of anything.  I simply point out that rpm ratings are only an indication of how much centrifugal force a propeller can withstand.  It is not an indication of how much weight or thrust a propeller can bear.  That force is called thrust bending force.

Mounted vertically, and for a single engine,  this thrust bending force will seldom exceed the thrust rating of a propeller.  However mounted horizontally, and in a multi-engine configuration of a multirotor, this is not always the case.  When the multirotor is tilted towards one engine, the weight and thrust of the other engines will concentrated on that engine.  The amount of thrust bending force on that one engine will exceed that created by one engine alone, and hence the extra stress.

Propeller ratings do not take multrotor aircraft with horizontally mounted propellers into account.  Realize that this is all new to everyone, and we have to come up with new ideas.  Sometimes these ideas seem counter-intuitive.  There is no bible written for these things yet.  What applied to fixed wing aircraft do not always carry forward intact to horizontal multirotor aircraft.  I would love to see a guide that outlines the requirements of propellers to be used for multirotors, but no such guide exists yet.  Perhaps I may do some experiments and present them in a blog as a guide to choosing propellers for our multirotor aircraft.be 

Btw, there is no technical test for becoming a Moderator, we are simply the maintenance people of the web site.  I do not claim to be an expert on aerodynamics.  I'm interested in this stuff, and like to discuss them.

Presumably, propeller manufacturers do not test their products for RPM tolerance in a vacuum.  If there is air present, then aerodynamic forces exert themselves upon the blades in a manner consistent with the laws of physics, which are well known and documented.

Again, it is true that VTOL applications do spend more time at higher power than fixed-wing, but that should not make any difference in the ultimate mechanical integrity of the prop.  There is certainly no "extra force" on the blades of a propeller simply due to direction.  Therefore, unless a manufactured offers a disclaimer akin to "not intended for sustained full-throttle use" (which should make one seriously doubt the quality of the materials or design), then there IS NO DIFFERENCE.

If I cannot persuade you, then a simple agreement to disagree will have to suffice.

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

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

I forgot we had a short video!!

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