Replies to This Discussion

Permalink Reply by Johannes Huss on November 6, 2011 at 11:40pm

Hi guys, your comments helped me a lot in finding a clue on how this all could happen. So this is the final crash report.

 

First I need to state: It's all my fault. After many broken acrylic motor mounts I decided to attach the motors directly on the arms, like many other constructors (e.g. MK or Jakub) are doing it.

 

But the problem is that the thickness of official kit arms is way too thin! Like I said, I had many trouble-free flight hours with that setup, but tension and vibrations did their job well and attacked structure of the arm slowly but powerful. This is what I found after the crash:

I assume that it broke in flight. How do I come to that conclusion? Wires of the motor are twisted heavily, also their insulation is hurt badly. And I cannot believe that the injury on the arm is caused by the crash itself. So I think it happened like CyberCrash stated: At the first cracking noise, the motor broke off, maybe only partially. This could explain why the copter still looks like to be flying stable. But after some seconds it drops out completely. And this leads to the not Hollywood-like ending of the story.

 

So thanks again for your comments! I'm kind of glad that it was all my fault, this makes me still believe that ArduCopter is a great product. :)

• ▶ Reply

Permalink Reply by bmarshall5253 on November 7, 2011 at 1:17am

Johannes,

Sorry for your loss.

I noticed how thin the slow flyer props are near the hub and I looked up on APC's site the recommended max prop speeds

for a 10" Slow Flyer prop the max is 6500 rpm (65000/10")

At full throttle a 850 kv motor wants to turn around 10,710 rpm on a 3S lipo at full charge (12.6 volt)

9,520 rpm at 11.2 volt rated voltage of 3s which is still almost 50% over APC's recommend max RPM for a slow fly prop.

I know a copter is not operating at full throttle most of the time but it looks like anything above 6500 rpm would be asking for trouble and operating outside the designed range of a Slow Flyer prop. Even a full throttle static test might over stress the prop causing a later failure in flight.

Perhaps others have measured the actual rpm of the Arducopter setups and can confirm if the recommended rpm limit is being exceeded.

APC Suggested RPM Limits

1. Glow Engine and Speed 400 Electric Props
   
   Maximum RPM=190,000/prop diameter (inches)
   (For example, a 10x6 glow engine prop should be limited to 19,000 RPM)
   
   
   
2. Thin Electrics and Folding Electric Props
   
   Maximum RPM=145,000/prop diameter (inches)
   
   
3. Slow Flyer props
   
   Maximum RPM=65,000/prop diameter (inches)
   
   
   
4. Racing Props
   
   8.75 N,W and 8.8 series 40 Pylon props
   Maximum RPM=225,000/Prop diameter (inches)

 

• ▶ Reply

Permalink Reply by Johannes Huss on November 7, 2011 at 1:35am

This is interesting. Since I'm using 4S Lipos, maximum rpm would be 14280. Can those props suitable for higher rpm be used on a multicopter?

• ▶ Reply

Permalink Reply by Johannes Huss on November 7, 2011 at 1:39am

I just disassembled what was possible. Had to use a cutter many times. Here's a picture of the outer endings of the arms. One can see much bending and cracking here which supports my theory.

What a bad idea to mount motors like this! :-(

 

• ▶ Reply

Permalink Reply by steve F11music.com on November 7, 2011 at 3:27am

I think it's a bad idea to mount with the acrylic supports also. They also eventually crack or the screws break. I switched to the 3dr frame and it's much better. They mount directly but use thicker aluminum.

And I've had my share of broken props! I don't know if they broke under rpm stress or from being slightly damaged from prior crash but I'm looking for stronger props.

Why are we using props at their rpm limits? Do we have to go to cf props or are there other cheaper options? The prop on my skywalker plane is so much stronger. Is there some reason we use the slow fly props. I had 12" cf props but they were hand made and expensive.

So what options are there for better props? It's a shame to have a crash because of the failure of a $4 part :-/ and to think I was flying with a 4s sometimes, though never got close to maxing the throttle.

• ▶ Reply

Permalink Reply by Johannes Huss on November 7, 2011 at 5:39am

I'm also highly interested in some really solid props. Maybe we should open another thread for that.

• ▶ Reply

Permalink Reply by steve F11music.com on November 7, 2011 at 6:39am

Yes lets. It's a shame to have such a critical part be a weak link in the system.

 

The best I found so far are these:

http://www.quadcopters.co.uk/?PageID=ShowItem&Item=Pair%20of%20...

 

Just can't find them anywhere else and prefer to source them from the US if possible.

 

Maybe these would work also as a 10" replacement? They are not CF but they say:

 

Suits 750-800kva on 4S

 

Which equates to about 12,000rpm.

 

http://www.multiwiicopter.com/products/pro-photography-props-set-9x...

• ▶ Reply

Permalink Reply by steve F11music.com on November 8, 2011 at 2:56am

http://www.quadcopters.co.uk/?PageID=ViewBasket&Zone=Worldwide&...;

It convinced me to buy a few sets :)

• ▶ Reply

Permalink Reply by Johannes Huss on November 8, 2011 at 3:01am

I'm looking forward to see your experiences with that props!

• ▶ Reply

Permalink Reply by steve F11music.com on November 8, 2011 at 7:02am

Me too :) We should have more and better options for props. I've spent $100s in replacement parts costs because of props breaking and now seeing their rpm limit, I see they are minimal for what we should be using.

• ▶ Reply

Permalink Reply by Tomas Soedergren on November 7, 2011 at 4:58am

Really seems like metal fatigue to me.

Interesting observations regarding prop types and speeds. I think the electric thin types might be good, but might be difficult to find matching push & pull props.

I share your loss Johannes. Really. Crashed my rather new hexa into my flag pole, yesterday.

Of course I have excuses, but in the end, well... my fault.

• ▶ Reply

Permalink Reply by Johannes Huss on November 7, 2011 at 5:37am

Here's another shot of the arm which I suspect to be the reason for the crash. Do you see those marks? I'm sure they are by a freely spinning motor and prop. It rotates clockwise on this arm so everything makes sense.

 

• ▶ Reply

• ‹ Previous
• 1
• 2
• 3
• Next ›
• Page