Safe speed to avoid motor cracks?

[solentlife]

I have always been of the opinion that hard braking and torque is the biggest cause.

When you separate the shells - you find that it relies on the two bolted together to stiffen and support. Separated - each shell is surprisingly flexible. Looking at the way its put together ... we have two flexible plastic shells that secure together at the weak narrow point ... then there's motors bolted only to the bottom shell ... then a single small screw securing the outermost two shells together.

My question is - is that good engineering ?

Nigel

 

[Mark The Droner]

True, the wings are very flexible, and strengthened when sandwiched together, but the motor mount area has little vertical support beam moldings connecting the four mounts and also extending outwards to the edge. I have a shell right here next to my chair, and the motor mount area actually does flex even with the little I-beam molding. That kinda surprises me. I don't see any reason why those little I beams can't be made wider/thicker with a minor change in the top wing.

Edit: I should say, couldn't have been made wider - past tense. It's too late now.

 

[solentlife]

We all know that stiffening or restricting can increase problem.

A company I was a senior manager in had an NDT dept. that I had to work with quite often. Quite often I would hear about structures / designs that were inherently unable to sustain long service life - but could be produced to service a given period. To me as an observer - it was victim of technical analysis design ...

For literally no weight increase - those arms could have CF introduced to still allow flex to dissipate stress, but keep it together. If the CF then from strengthening arms then was spread as mat where motor mounted - you get a cure without compromising the design shape or flight.

Many substantially lower priced models use CF strategically introduced into the structure, no reduction in RF range, no increase in weight, in fact for manufacturer the advantage of strength for less cost is attractive. Yes - intro of CF to a structure means less traditional material and design requirements, reducing costs. But the model is significantly strengthened.
Look at Hyperion ... Extreme Flight ... Precision Aerobatics .... YT ... just to name a few ...

DJI could revamp their shells and at same time reduce the replacement issues, costs ... even with the discontinued P3 models (S and SE of course as still current) ....

Nigel

 

[Numone]

None thus far, I'm only 8 flights in but still my confidence is low hence why it sat there a while unused, I told myself from the word go to fit the reinforcement plates this time round but why should I have to adapt a brand new out the box phantom just to gain a small amount of assurance that isn't 100% gaurenteed, I haven't heard of the problem happening in p4's only p3's, if my finances at the time would have allowed a p4 I would have got my money back & purchased one but why should dji get away with selling a known product that has had numerous accounts of the same fault? Shell aside my p3a has been faultless to be fair

You shouldn't, but I would advise you to check the 4 bolts that hold the motor to the bottom shell and the 2 others in front of the motor and the 1 at the end of the arm. If they're difficult to undo, they're over tightened which, I believe, crushes and distorts the shell, which can lead to the formation of stress cracks. Before I even switched on the AC for the first time, I installed OE Prop guards, which necessitates the removal of the motor bolts. I the found that mine were massively over tightened, so I reinstalled these to a little over finger tight, together with the other 3 shell bolts too. I removed the prop guards after my first few flights and installed the original bolts. Following 00's flights since, I've had no issues nearly two years on....

 

[Numone]

Completely agree solentlife, dont get me wrong it's a really impressive piece of kit once in the air filming but 100% there is definitely an issue either regarding the design or quality of "some" shells that DJI should accept fill responsibly for imo

In the vast majority of cases, they have replaced the shells under warranty.

 

[Numone]

We all know that stiffening or restricting can increase problem.

A company I was a senior manager in had an NDT dept. that I had to work with quite often. Quite often I would hear about structures / designs that were inherently unable to sustain long service life - but could be produced to service a given period. To me as an observer - it was victim of technical analysis design ...

For literally no weight increase - those arms could have CF introduced to still allow flex to dissipate stress, but keep it together. If the CF then from strengthening arms then was spread as mat where motor mounted - you get a cure without compromising the design shape or flight.

Many substantially lower priced models use CF strategically introduced into the structure, no reduction in RF range, no increase in weight, in fact for manufacturer the advantage of strength for less cost is attractive. Yes - intro of CF to a structure means less traditional material and design requirements, reducing costs. But the model is significantly strengthened.
Look at Hyperion ... Extreme Flight ... Precision Aerobatics .... YT ... just to name a few ...

DJI could revamp their shells and at same time reduce the replacement issues, costs ... even with the discontinued P3 models (S and SE of course as still current) ....

Nigel

Hmmm, well flexing of the arms would result in horizontal movement of the motors which wouldn't be good. I'm pretty sure DJI would've spent 000's man hours designing, testing, redesigning the shell.

 

[gingerbloke]

In the vast majority of cases, they have replaced the shells under warranty.

I was effectively told to FO when I complained to a UK dealer (Heliguy) about my phantom cracks.

 

[Numone]

I was effectively told to FO when I complained to a UK dealer (Heliguy) about my phantom cracks.

Nice. Was it still under guarantee? Even if it it wasn't complain to Trading Standards - that's a terrible way to act toward a customer under any circumstances. Also, the Sale of Goods Act (if it was purchased before 1 Oct 2015) or Consumer Rights Act from Oct 1 2015 state items must be of suitable quality - yours ain't!

Here you go

 

[gingerbloke]

Nice. Was it still under guarantee?

Yes, ten flights on it - mostly hand catches. Was told not covered by warranty. Pretty unhelpful and makes a mockery of using UK dealer rather than going for the best internet price.

 

[Meta4]

Yes, ten flights on it - mostly hand catches. Was told not covered by warranty. Pretty unhelpful and makes a mockery of using UK dealer rather than going for the best internet price.

Just because some wally told you, doesn't mean he's correct.
The number replaced under warranty would suggest he has no idea.
But you might have to deal with DJI rather than your dealer.

 

[Meta4]

I have always been of the opinion that hard braking and torque is the biggest cause.

Is it speed, is it braking, is it torque? Is it something else altogether? (like poor materials quality control)
When you have no idea what the cause is, it's pretty difficult to take appropriate preventative action.

When you separate the shells - you find that it relies on the two bolted together to stiffen and support. Separated - each shell is surprisingly flexible. Looking at the way its put together ... we have two flexible plastic shells that secure together at the weak narrow point ... then there's motors bolted only to the bottom shell ... then a single small screw securing the outermost two shells together.

My question is - is that good engineering ?

Given the number that never have a problem, it's probably quite good engineering.
Flexing is not necessarily a problem. Look out the window at the wings of a 787 or an A380 sometime.
Or at your DJI props.

 

[Numone]

Yes, ten flights on it - mostly hand catches. Was told not covered by warranty. Pretty unhelpful and makes a mockery of using UK dealer rather than going for the best internet price.

You didn't say when this occurred but if its still under warranty, as I said previously, take it up with them again and i Trading Standards if you get the same response - Trading Standards are there to protect the consumer. Incidentally, EU law states that many goods are protected for 6 years, based on product life expectation. Thats why Apple will fix your 5 year old MacBook FOC or, if you argue enough, all motor manufacturers will provide "goodwill" on non-consumables, in other words, all major mechanicals.

 

[gingerbloke]

I repaired it myself, partly because (at the time) the forums were full of people moaning about slow DJI returns. I bought the P2V+ to fly it and couldn't face a long period waiting for it to return from repair. If the UK dealer had replaced it with a new one I would have been happy but otherwise I just wanted to fly it.

FWIW it still flies (a friend owns it) and has 200+ flights on it.

ps I came into this hobby from R/C (build and rtf) and the idea that there was any sort of warranty on something that flies still seems amazing to me.

 

[solentlife]

Is it speed, is it braking, is it torque? Is it something else altogether? (like poor materials quality control)
When you have no idea what the cause is, it's pretty difficult to take appropriate preventative action.

Given the number that never have a problem, it's probably quite good engineering.
Flexing is not necessarily a problem. Look out the window at the wings of a 787 or an A380 sometime.
Or at your DJI props.

Flexing when designed in is good because it avoids localising stress. FYI - my Father was around when Comet Airliners were being stressed tested to find out why incidents occurred.

But if flex is not a design feature - then problems can occur. The P3 relies on the two shells being fixed together to form a structure - much like a bridge or building does. But that thinning of the arm before the motor area along with motor only bolting to the bottom shell - to me is an area that could have been better thought out. That's my opinion after taking apart P3's and seeing just how flexible it really is.

As to warranty - yes DJI do replace cracked shells as long as within warranty period (that period validity varies depending on country bought of course) - but there are various posts on the web citing where DJI refuses to replace a second time.

At times I have sat and thought about discussing with other Phantom Owners I fly with a 'kit' or list of items to fix the cracks or prevent them occurring. Problem is of course - that already we know that DJI have a habit of rejecting warranty claims even when claim has nothing to do with any modification made by owner. So any 3rd party remedy / solution must be regarded as an After Warranty Period matter. I may just pursue this ... not to prove anything, but to provide possible help to those afflicted once warranty runs out.

Nigel

 

[N017RW]

+1 with gingerbloke.

These are toys.

Model aircraft are exposed to extreme stresses which do not scale down with size.
All my field kits contain 5min epoxy and cyano glues.

It goes with the territory.

 

[Mark The Droner]

Shoot, I only have four. I need to step up my game...

 

[joeyjan19]

I recently talked to a engineer about these cracks and he questioned the torque or tightness of the nuts on these connectors. Over tightening by who or whatever is used to tighten the connections could be starting the cracks in some cases.

 

[solentlife]

I recently talked to a engineer about these cracks and he questioned the torque or tightness of the nuts on these connectors. Over tightening by who or whatever is used to tighten the connections could be starting the cracks in some cases.

I don't think anyone denies that over-tightening can incur problems.

My opinion is that it is more than that ... it is only one of many possible factors.

Lets look at a production line ... and where a 'set of screws' are fitted. The old way or in cheap labour - a human will fit and tighten. They learn at start of their work the level of tightness and get a 'feel' for it over time. They sit there day after day screwing / tightening.
The modern way is where a machine inserts screws, tightens to a set torque ... unit moves on.

Ever seen a car production line where wheel nuts are fitted ?

mmmmmm not much room for change unless someone 'resets' workers 'feel' or the machine.

Nigel

 

[Numone]

Ever seen a car production line where wheel nuts are fitted ?
Nigel

Yep - every one I've been to, they're fitted by a machine, which is held by a human, but I'm sure that most of these are now fully automated. But regardless of the method of the delivery, they are nuts or bolts, which are compressing steel-to-steel; they are not steel bolts and screws being delivered into styrene plastic, so there's completely different levels of torque required. Given the cost of labour in China, I'm sure it will be a human inserting these and, with the vast variations in the level of tightness I found on my 3A, I very much doubt if they're tightened using a torque measurement device either.

 

[solentlife]

I have business connections in China .... and spent quite a bit of time over there over years and watched China Labour Market change. The comments about wages and use of are old hat ... China is not the cheap labour base today that some think it is. It is why even the Chinese are investing in Vietnam / Cambodia etc. Chinese factories of moderate to large - many have installed automatic systems and human intervention is greatly reduced. I can even tell you that our business has been hit not only by labour changes but also by Govt cracking down on Environmental issues - closing many factories. But Western World hears very little about this because it does not suit media to report. They prefer to keep the 'old pot' boiling.

As to fitting screws in plastic vs steel ... makes no odds - the machine is dialed to the job and away it goes.

I do not say DJI use such machinery. Just trying to clear a point.

As to differing tensions / torques ... ever used a torque wrench to wind up nuts / bolts ? How inaccurate they are ... slightest bit on threads can cause a false stop to torque wrench. Its why on my Race Car - we used 'torque' washers. Special washers that crush when torqued up ... you measure the thickness of washer to get correct accurate figure.
Now apply that to our subject ... any difference in thread / screw ... and the machine tops out. Result - different torque results we see.

Sorry to be a pain on this ... but the whole subject of the cracks is so full of assumptions and no real hard facts for / against / why ...
Those that slack of the screws and survive crackless are convinced they found the answer. Others who fit plates think they have. Those that do nothing and survive think that its not so much a problem. Its the world and its ways ..

Nigel