Replies to This Discussion

Permalink Reply by Brad Hughey on March 17, 2012 at 6:24pm

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

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Permalink Reply by Karla Jean Fripp on March 17, 2012 at 12:24pm

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.

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Permalink Reply by Brad Hughey on March 17, 2012 at 2:08pm

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.

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Permalink Reply by Brad Hughey on March 18, 2012 at 8:44am

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.

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Permalink Reply by Ellison Chan on March 17, 2012 at 12:49pm

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.

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Permalink Reply by Karla Jean Fripp on March 18, 2012 at 12:55pm

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

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Permalink Reply by Brad Hughey on March 18, 2012 at 5:59pm

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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