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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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

Graham, you're spot on.  They would never quote a maximum weight because it's an irrelevant number.  They might quote a maximum thrust, however, at a given RPM, but that too might be meaningless, because the thrust will change based on the inflow. 

Karla, you may have missed this because the thread with Mr. Chan sorta ran off the rails.

Whomever told you that "normal props" are not meant for quads is wrong.  A prop doesn't "care" what it's used for, it only cares how hard it's being driven.  That's why there are maximum RPM ratings on propellers.

You can easily determine the maximum RPM of your motor by multiplying the KV rating (in your case 810) by the maximum voltage of your battery.  If it's a 4 cell LiPoly, then that's 4.2 * 4 or 16.8 Volts.  In this case, your motor at full throttle and unloaded can reach a maximum RPM of 16.8 * 810 or 13,608.  If the "composite" props you used were the APC Slow-Flyer series as sold by DIY Drones, then yes, you exceeded the maximum RPM for which they're rated and that's why they broke.

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.

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

Oh, wait!  There's more where that came from!  You haven't heard about blade element theory, the Kutta-Joukowski theorem of airflow circulation, the effects of Reynolds numbers on the coefficients of drag...or...or....(lol)

Seriously, I'm glad there was some value for you embedded in there somewhere.  :-)

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