Trouble with APC 13X6.5

I

iconflyer

Member
Hello,
I have a heavy lift hexa copter based on rusty's frame with 16mm booms, fitted with AXI 2814/22 and AV200-360 gimbal. The copter is capable of take-off at about 60% throttle with an AUW of about 5000grams with APC 14X4.7. I know that I'm running the APC SFs at way above their recommended safety limit so I wanted to switch to APC Thin electrics 13X6.5. The problem I'm having is whenever I install the 13X6.5 and about the point the copter is about to take one or more of the booms break. I haven't balanced any of my props(neither the SF or the Thin Electric) so I assume it's vibration related issue but on my du-bro balancer neither set of props seems severely out of balance. No idea what's going on...any experience with these props? Any idea what's going on?
 

DucktileMedia

DucktileMedia

Drone Enthusiast
Can only be 2 things. Either massive vibration or the booms are too weak to handle the thrust/vibration. most heavy lifts use much thicker arms than 16mm. My hex has 7/8" arms. lifting a av200/360 with a camera that would be appropriate for that gimbal is pushing the limits of a 2814. The pics I have seen of the few hex helis lifting that much all use AXI 4120's. But forst and foremost you should always balance your props, especially APC!
 

I

iconflyer

Member
IrisAerial said:
Can only be 2 things. Either massive vibration or the booms are too weak to handle the thrust/vibration. most heavy lifts use much thicker arms than 16mm. My hex has 7/8" arms. lifting a av200/360 with a camera that would be appropriate for that gimbal is pushing the limits of a 2814. The pics I have seen of the few hex helis lifting that much all use AXI 4120's. But forst and foremost you should always balance your props, especially APC!
Click to expand...
Thanks for the reply!! yeah I figured since it flies ok with the 14X4.7 and it seems sturdy enough I could use the heavier 13"s ... I guess their extra weight makes the spool up vibrations too much for the arms?

I'm only planning to use my nex5n or panasonic gh2...the 5000grams is really the upper limit of my testing with the 7D. I would expect my maximum AUW to be around 4000grams.
What kind of frame are you using that has 7/8" arms?
 

DucktileMedia

DucktileMedia

Drone Enthusiast
I build all my own frames. try balancing. if you are breaking arms from vibration your videos are going to look like hell.
 

I

iconflyer

Member
IrisAerial said:
I build all my own frames. try balancing. if you are breaking arms from vibration your videos are going to look like hell.
Click to expand...
i have no other option either that or go back to the 14"s...you're right about the vibes but the strange things it that the 14" seem fine vibration wise(that is before balancing, I always balance for the job)...the thin electrics must just need a lot more work.
What props/motors do you use and what kind of mounts/cameras do you fly if I may ask? :)
 

J

jes1111

Active Member
What are these 16mm booms made of? "Break"? - as in: snap, crack, fold up, shatter? I'd agree they sound too thin, but really that depends entirely on the material and the wall thickness. I'd say that a quality 16x14mm pulwinded CF tube would be just dandy at 5kg. A cheapo 16x14mm pultruded CF tube - no.

Have you entered your current numbers in eCalc? Since you have a verification reference of 60%throttle to hover, it should be easy to then try out alternative props to see what happens. The danger with 13x6.5 APCs (other than breaking your booms) is that they will pull a whole lot more current than your 14x4.7 props. Alternative 14" are available in wood from JXF via HiModel - same factory that makes the identical Xoar props but without the scary prices.
 

I

iconflyer

Member
jes1111 said:
What are these 16mm booms made of? "Break"? - as in: snap, crack, fold up, shatter? I'd agree they sound too thin, but really that depends entirely on the material and the wall thickness. I'd say that a quality 16x14mm pulwinded CF tube would be just dandy at 5kg. A cheapo 16x14mm pultruded CF tube - no.

Have you entered your current numbers in eCalc? Since you have a verification reference of 60%throttle to hover, it should be easy to then try out alternative props to see what happens. The danger with 13x6.5 APCs (other than breaking your booms) is that they will pull a whole lot more current than your 14x4.7 props. Alternative 14" are available in wood from JXF via HiModel - same factory that makes the identical Xoar props but without the scary prices.
Click to expand...

Thank you very much for the info..I'm using 16mmx15mm CF T-Rex 600 tail booms from Hobbyking...they seem like good quality...I would really appreciate it if you can direct me to a reliable online source for 16X14mm pulwinded CF tubes?

on the amp numbers...you're right, I did run everything on ecalc....14" provide the best lift current for such a heavy weight...both the 13" and 14" exceed the maximum the AXI2814 can handle but I'm practically I will never come close to the maximum on these motors. The only reason I'm considering the 13" is the RPM limit on the 14" which are holding well despite me passing their recommended limits. I'll check out the JXF props, they maybe my solution!!
 


I

iconflyer

Member
Thanks jes...the ones I have are pretty much the same...I think the 13" props I have just need balancing...the type of breaking is random(combination of folding and shattering) and usually near where the motor is mounted so i think the vibrations are just to much for the CF arms to handle. I wonder if the stiff JXF props will have the same issue...I assume they are as heavy as the thin electrics...are the usually well balanced/drilled?
 

E

Elliott000

Member
From experience from my heli background... I have a trex 550 and have gone through a few cf booms, they are only wrapped very thinnly with cf rest is aluminum and they have no strength in comparison to the HL booms on my droidworx... Id go a different route for arms personally
 


O

ovdt

Member
I'm not sure the cause of this problem is prop balancing. I have used APC 13x6.5 with weak arms in combination with MK3638 motors and didn't have any problems. I didn't balanced it either.

I also use JXF wood props, which I'm quite happy with. They are very light, stiff, resistant to temperature changes, easy to balance and cheap.

HiModel is having problems stocking with JXF wood electric props. They are out of stock most of the time and their notification system doesn't work.

They have Gemfan wood props in stock, I haven't seen anyone using them but I took the risk and order some batch to try. They're heavier than JXF wood props.

http://www.himodel.com/search.php?psearch=14x4&Submit=Search
 

I

iconflyer

Member
ovdt said:
I'm not sure the cause of this problem is prop balancing. I have used APC 13x6.5 with weak arms in combination with MK3638 motors and didn't have any problems. I didn't balanced it either.

I also use JXF wood props, which I'm quite happy with. They are very light, stiff, resistant to temperature changes, easy to balance and cheap.

HiModel is having problems stocking with JXF wood electric props. They are out of stock most of the time and their notification system doesn't work.

They have Gemfan wood props in stock, I haven't seen anyone using them but I took the risk and order some batch to try. They're heavier than JXF wood props.

http://www.himodel.com/search.php?psearch=14x4&Submit=Search
Click to expand...

Weak CF or Alu arms? I also find it strange because I've never had any issues with APC props before...so what else could it be?

BTW any experience with these http://www.himodel.com/plane/14x4.7_Carbon_Fiber_Propeller_Set_CW_CCW.html?
 

J

jes1111

Active Member
ovdt said:
I'm not sure the cause of this problem is prop balancing. I have used APC 13x6.5 with weak arms in combination with MK3638 motors and didn't have any problems. I didn't balanced it either.

I also use JXF wood props, which I'm quite happy with. They are very light, stiff, resistant to temperature changes, easy to balance and cheap.

HiModel is having problems stocking with JXF wood electric props. They are out of stock most of the time and their notification system doesn't work.

They have Gemfan wood props in stock, I haven't seen anyone using them but I took the risk and order some batch to try. They're heavier than JXF wood props.

http://www.himodel.com/search.php?psearch=14x4&Submit=Search
Click to expand...
I could be wrong, but I think every time I mention JXF props from HiModel on this or any other forum, they go out of stock ;-)
 

I

iconflyer

Member
Just an update of some crazy testing I've done today: After I had an arm shatter yesterday during an attempted take off, took out the motor and broken arm, installed the motor/mount on the same arm and hooked it up to my thrust bench testing setup...the crazy thing is that I held the arm in my hands to see how much vibrations are being produced...to my surprise, at the upper limit of the motor(the motor was pulling 20amps at 4S) the motor/prop were rock solid...just sound vibrations radiating through the arm but nothing severe at all to cause shattering of the arms!! I'm very confused. Appreciate any feedback.
 


I

iconflyer

Member
I think I did :)
At this point the only thing I can think of is somehow the slight vibrations produced during the spool up match the harmonic frequency of the arms setup and that's what's causing the shattering...I'm replacing the bearings on some of the motors now and I will retest by mounting the motors closer to copter body...
 

KopterX

KopterX

Member
Shorter arms are always better when comes to vibrations. I would put the engine as close is possible to center and have about 1 cm clearance between propellers, less than one 1cm may allow a propeller strike if the arms are weak.
 

I

iconflyer

Member
I think i figured out what the problem is: Torsion! Apparently and because the thin-e props from APC are stiffer than SFs, slight variations in the disc angle cause too much torsion stress on the arms...the arms are not the best quality but with the 3KG weight and the single mounting point using rusty's derlin blocks, the stress was causing the arms to bascially snap as they twist beyond what they can handle. I'm going to try to use two boom block per motor rather than one to distribute the force more evenly and see if that makes a difference.
 

J

jes1111

Active Member
ah! that makes sense - cheap pultruded CF can be remarkably poor in torsion. Not all CF is created equal. There's a number of different ways to form a tube of reinforced carbon fibre. The cheapest way is pultruded which involves pulling straight fibres through a resin bath and then through a die which forms the tube - this yields a tube with all the fibres running parallel along the length of the tube. This pultruded tube is actually optimal for some applications since it's immensely strong in tension and is very stiff (against bending) but (as you've discovered) it's a very poor performer in torsion and is easily crushed, too. The next step up is to encase one of these pultruded tubes with a single layer outer sleeve of 0º/90º woven CF - this is largely just for cosmetic appearance but it does slightly improve the torsional strength and crushability. The best CF tube for this application is what's called "pulwinded" - similar to pultruded but instead of running the fibres parallel to the length, they are spiralled around a forming tool in opposing directions, resulting in a finished tube that performs well in torsion, crushing, stiffness and tension/compression. One more step above that is to finish the pulwinded tube with an outer sleeve of 0º/90º weave, again largely for cosmetic reasons. Obviously, in addition to these wide variations in performance, there's a wide variation in price, too, since these different processes have different raw materials costs, machinery costs and finishing costs.

Furthermore, there can be wide variations between brands in material and construction quality. The performance of finished carbon fibre depends not only on the fibres themselves but also the binding resin. Cheapo factories will use cheapo carbon fibre and cheapo resins. They'll also bias the "fill" in favour of resin rather carbon fibre since resin is obviously considerably cheaper by weight/volume than carbon fibre. The optimum ratio is around 60% carbon fibres and 40% resin. Less CF and more resin will result in the same cosmetic appearance but much less performance. Also, for ultimate performance, the resin needs to be of the type that requires a combined heat/pressure curing cycle - adding further to the manufacturing costs.

My own conclusion is that, unless you buy high quality CF tube you might as well stick with aluminium, which is likely to out-perform the cheapo CF tubing, is certainly cheaper and has a surprising similar weight.
 

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