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Dec 04, 2015, 05:43 AM
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Joseph3LK's Avatar
Quote:
Originally Posted by pavalige
and guess who's going to fly one pretty soon
Puppy face

That's teasing
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Dec 04, 2015, 06:48 AM
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I agree! I dont have one either. All the protos to date have gone to better fliers than myself.

Quote:
Originally Posted by Joseph3LK
Puppy face

That's teasing
Dec 04, 2015, 08:16 AM
Multirotor Promotor
RaymondM's Avatar
Alright, another innovative idea, bring it on!
Dec 04, 2015, 08:35 AM
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Simple Quad Arm Stress Analysis


The picture below depicts the simulated stress of 20 newtons of force applied to the flat side of the motor mount of a quad arm. The arm simulated is a typical H quad 6in arm anchored by two bottom plates. The arm is 18mm wide and 3mm thick with no feathering at the root.

The simulation roughly equates to a 500 gram quad hitting an arm flat on at 100km/hr. In real terms, it is common to experience catastrophic failure from a fail safe crash. ie. fall out of the sky and your quad rolls uncontrollably and an arm smacks the ground first.

Note the stress is concentrated at the root with considerably less stress at the neck of the motor mount. Disappointingly, this design fails to spread the stress uniformly along the length of the arm. Accordingly, there is a bucket load of heavy CF doing very little.

As you can guess, the longer the arm, worse this chart looks. This issue is a particular challenge for frames that mount their arms centrally as their arms are even longer.
Last edited by boltrc; Dec 04, 2015 at 08:46 AM.
Dec 04, 2015, 09:19 AM
Registered User
Well in my experience this had been a point of failure on arms, but I've also broken arms close to the motor mount, where the arm gets slightly thinner. How do you explain failures there, when the graph seems to indicate very little force is applied at that spot ?
Dec 04, 2015, 09:39 AM
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Subbed.
Dec 04, 2015, 09:55 AM
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BobbyC1's Avatar
Not all crashes are the same. Testing/designing for one type of impact doesn't make it unbreakable.
Dec 04, 2015, 05:06 PM
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@Nicoduf - correct. Based on the feedback from our customers on our existing frames, this is the most common point of failure but as you and Bobby have highlighted, arms do break for different reasons and different places.

Two other failure points we have commonly observed are the neck and also through the inside motor mounts. For example, our first design of the BOLT250 arms suffered several breaks of the 4mm arm through the inside motor mounts. Something we did not see with the same design with 3mm CF arms or in the park. We deduced that there was not enough CF left behind after cutting the motor slots out. As the 4mm arm was thicker and stronger at the root, it applied more stress to this small area and if you hit concrete (not a tree).....bang. We subsequently added about 0.2 grams of CF and the problem went away.

Another common point of failure is the neck. If the arms are not adequately filleted (curves) between the motor mount and the arm, the stress can focus on narrow bands or points in this neck area which is often narrower.

As BobbyC1 highlighted, our tests showed that if you hit motor mount in various locations from various angles, you can increase the stress to this area. Although not modeled here, this is fairly straight forward to risk manage with decent filleting.

The ultimate goal is not make it unbreakable, rather to get more from less whilst maintaining an acceptable mean time between failure in the intended application.
Dec 04, 2015, 05:21 PM
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BobbyC1's Avatar
Good to hear you guys are actually engineering and stress testing the frame rather than just designing something that looks pretty .
Dec 04, 2015, 05:57 PM
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RaymondM's Avatar
+1 Yes!
Dec 04, 2015, 06:37 PM
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Definitely, it's the only frame designer / manufacturer that I have seen publish that sort of information. Admittedly I haven't been looking hard, but other frames usually claim "strong design " sort of arguments without any information backing it up
Dec 04, 2015, 06:49 PM
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HelliYea's Avatar
Super exciting stuff...
Dec 04, 2015, 07:14 PM
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Quote:
Originally Posted by Nicoduf
Well in my experience this had been a point of failure on arms, but I've also broken arms close to the motor mount, where the arm gets slightly thinner. How do you explain failures there, when the graph seems to indicate very little force is applied at that spot ?
That'll be caused by loads being applied mostly horizontal, or in plane with the picture above. Essentially you're putting the arm into compression, and the rapid change of cross-sectional area where it transitions from the motor mount to skinny arm causes it to buckle and/or yield.

Bending loads, like Mark is simulating there, are difficult to deal with from a strength point of view because you have a large leverage affect. Stress goes up with the square of length, and deflection with the cube of length. So a beam doubled in length will have four times more stress, and deflect eight times more.

Bending is not always a bad thing though, the deflection is quite effective at absorbing energy which is a pretty good property to have in a quadcopter arm.
Last edited by delaminated; Dec 04, 2015 at 07:20 PM.
Dec 04, 2015, 07:38 PM
Registered User
Thanks delam - a real structural engineer! Please keep chirping up and adding some proper science to our witchcraft.

You are certainly right about the bending. Our 210 has the most flexible of arms in our range and is proving to be very resilient. The design sees the deflection fairly constant down a good length of the arm. In fact, this even deflection has been a major driver in our designs since.

Cheers
Mark
Dec 04, 2015, 07:41 PM
Registered User
Hey HY, I presume you mean in a nerdy kind of way BTW, your back up bottom plates should ship to you on Monday. Hope that remains in time for your departure. Keen to hear what Juz thinks of the 210 when you get back.

Cheers
Mark

Quote:
Originally Posted by HelliYea
Super exciting stuff...


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