Crashed today...

[eaglegoaltender]

The overall better suggestion would be to learn to read the flight log data. The .txt flight log file will need converted to a .csv file. The current converter that is best for this now has fee's attached in order to convert them. However, there should be a New FREE one ready for use in the next 7-10 days hopefully. Stay tuned for that news.

Excellent point "Frank" as to applying the "flight log data" as part of the overall maintenance plan!

Just wanted to point out there will always be heat variances in the motors. Each motor will turn a different rpm based on flight pitch & roll alone. The temps can be confusing when the a/c has a heavy side wind while in route.

My point of checking by touch is – if one or more motor frames has a very noticeable difference as to touch it warrants further investigation as to winding insulating integrity. By noticeable difference I mean hot versus warm since these rotating frames don’t get beyond the warm point to the touch after flying.

You are correct that by touching the frames of the motors there will be variances as to temp due to unequal loading during flight with the throttle, wind directions etc. (slight differences) - all valid points to consider. It will be difficult also to determine internal winding temps this way without doing an actual temperature measurement at the windings. You may feel a slight difference when touching closer the bottom of the motor frame – however the rotating housings/frames won’t give a very accurate reading as to temps since they feel just “warm” to the touch due to internal clearances and an open frame design, even using IR spot thermometers that only measure surface temps wouldn’t show as being high or very accurate at the frames.

With regard to motor temperatures; it really is imperative to know the motor temp rise and max allowable temp for the motor windings regardless of variances in temperature between motors or flight conditions etc. Since a motors’ winding insulation is the true determining factor with regard to its life span it is important to realize the temp rise with respect to windings and insulation value.

(A Motors temperature rise is based on the change of temperature within the motor when operated at full load or if you like full throttle over a specific period of time that allows for motor temp to settle/remain steady + using a standard of 40’c ambient air temp. The difference between the motors starting temp and its final increased/steady temp in that ambient temp = motors temp rise – winding temperature. An allowance of 10’c is added to the calculated rise to take into effect the internal windings hot spot).

I have no idea what the temp rise is for these motors but I did read on a DJI forum somewhere the max safe operating temp for the 960kv motor is 51’c (seems a little low). So by doing a little reverse calculation we could estimate the motor temp rise. If we applied the standard used for air ambient temp at 40’c, and by using that estimated max operating temp of 51’c (assuming that is the steady final increase in temp we see during say a "static testing") we would subtract 40’c from that 51’c for a total of 11’c motor temp rise. We then add a 10’c variance/allowance for "hot spot" internal windings temp that gives us our final allowable motor temp rise at 21’c (as stated, seems low).

e.g. If you are flying with a 25’c air ambient temp and the rise is calculated at 21’c our operating temp for the motor including the estimated temp rise could be in the *46’c range (5’c lower than the calculated/estimated max allowable safe operating motor temp suggested by someone at that 51’c). Doubtful when flying with the cooling from the props and the motors ability to dissipate heat with the style of frame it has (open) we would even reach that temp.

*other variables to include would be amount of time operating at full load/throttle – not very likely those internal temps are met with a 20 minute flight time with prop down draft/wash for cooling and combined with an open frame motor. It’s the down time when taking a break during fly times, or changing batteries when the internal temps of the windings can be adversely affected due to no cooling effect from the props and sitting idle.

“Rule of thumb”; the life (L10) of motor winding insulation (i.e. motor) will be cut in half for every 10’C of operation above the rated insulation temperature – conversely, if motor is operated at 10’c or more below the rated hot spot temp, insulation life can be doubled (when calculating the unused capacity of 10’c).

I noticed on a DJI forum they mentioned the life (L10) of these motors was around 200 hours. That is ridiculously low for them IMO and must be a mistake on reporting of same – I think perhaps they may have meant the life of the bearing and even that is low for the bearing. The neodymium/Iron used in the magnets don’t even feel any negative effect till around the 80’c + mark and we won’t be flying anyway with temps that high since all the lubricant will be long gone from the bearings. DJI”s latest motors on the P3 use double layer, single wire stator, this also allows for quicker heat dissipation from the motor windings – hence, a further extension of windings insulation integrity and life well beyond that 200 hours I read about. That had to be a mistake!

 

[flyNfrank]

Excellent point "Frank" as to applying the "flight log data" as part of the overall maintenance plan!



My point of checking by touch is – if one or more motor frames has a very noticeable difference as to touch it warrants further investigation as to winding insulating integrity. By noticeable difference I mean hot versus warm since these rotating frames don’t get beyond the warm point to the touch after flying.

You are correct that by touching the frames of the motors there will be variances as to temp due to unequal loading during flight with the throttle, wind directions etc. (slight differences) - all valid points to consider. It will be difficult also to determine internal winding temps this way without doing an actual temperature measurement at the windings. You may feel a slight difference when touching closer the bottom of the motor frame – however the rotating housings/frames won’t give a very accurate reading as to temps since they feel just “warm” to the touch due to internal clearances and an open frame design, even using IR spot thermometers that only measure surface temps wouldn’t show as being high or very accurate at the frames.

With regard to motor temperatures; it really is imperative to know the motor temp rise and max allowable temp for the motor windings regardless of variances in temperature between motors or flight conditions etc. Since a motors’ winding insulation is the true determining factor with regard to its life span it is important to realize the temp rise with respect to windings and insulation value.

(A Motors temperature rise is based on the change of temperature within the motor when operated at full load or if you like full throttle over a specific period of time that allows for motor temp to settle/remain steady + using a standard of 40’c ambient air temp. The difference between the motors starting temp and its final increased/steady temp in that ambient temp = motors temp rise – winding temperature. An allowance of 10’c is added to the calculated rise to take into effect the internal windings hot spot).

I have no idea what the temp rise is for these motors but I did read on a DJI forum somewhere the max safe operating temp for the 960kv motor is 51’c (seems a little low). So by doing a little reverse calculation we could estimate the motor temp rise. If we applied the standard used for air ambient temp at 40’c, and by using that estimated max operating temp of 51’c (assuming that is the steady final increase in temp we see during say a "static testing") we would subtract 40’c from that 51’c for a total of 11’c motor temp rise. We then add a 10’c variance/allowance for "hot spot" internal windings temp that gives us our final allowable motor temp rise at 21’c (as stated, seems low).

e.g. If you are flying with a 25’c air ambient temp and the rise is calculated at 21’c our operating temp for the motor including the estimated temp rise could be in the *46’c range (5’c lower than the calculated/estimated max allowable safe operating motor temp suggested by someone at that 51’c). Doubtful when flying with the cooling from the props and the motors ability to dissipate heat with the style of frame it has (open) we would even reach that temp.

*other variables to include would be amount of time operating at full load/throttle – not very likely those internal temps are met with a 20 minute flight time with prop down draft/wash for cooling and combined with an open frame motor. It’s the down time when taking a break during fly times, or changing batteries when the internal temps of the windings can be adversely affected due to no cooling effect from the props and sitting idle.

“Rule of thumb”; the life (L10) of motor winding insulation (i.e. motor) will be cut in half for every 10’C of operation above the rated insulation temperature – conversely, if motor is operated at 10’c or more below the rated hot spot temp, insulation life can be doubled (when calculating the unused capacity of 10’c).

I noticed on a DJI forum they mentioned the life (L10) of these motors was around 200 hours. That is ridiculously low for them IMO and must be a mistake on reporting of same – I think perhaps they may have meant the life of the bearing and even that is low for the bearing. The neodymium/Iron used in the magnets don’t even feel any negative effect till around the 80’c + mark and we won’t be flying anyway with temps that high since all the lubricant will be long gone from the bearings. DJI”s latest motors on the P3 use double layer, single wire stator, this also allows for quicker heat dissipation from the motor windings – hence, a further extension of windings insulation integrity and life well beyond that 200 hours I read about. That had to be a mistake!

I have not followed this thread so I'm not real sure why there is such a concern on motor heat. But I do know that the main reason dji put any references towards motor heat, is due to the body only. They switched to more efficient motors that also cooled themselves and the body per rpm's. Without a doubt they more then paid the price for attempting to go down the least expensive route. The bodies would become heat stressed followed by brittle then comes the cracks.

 

[TheRealNick]

Here is a plot of the left back motor commanded and actual speed. You can see it increased just before the altitude crosses what was the starting ground point. It is weird that the altitude continues to drop, but I think the crash might have messed up the sensor. Left back motor load is not shown, but it also drops at the same time. This indicates the left back prop came off and then the Phantom crashed shortly after.

The other weird motor behavior (right front) I mentioned appears to be after the crash (or at least after the altitude drops below the starting point, which according to Steve did not occur) as shown in this chart below:

And finally I found an ESC stall message and added a red line at that point, which is after impact and before the strange motor (right front) behavior

 

[eaglegoaltender]

I really think DJI needs to emphasis that the props are not self tightening and need to be tightened firmly by hand....

Couldn't agree more as to the props and the poor choice of the phrase "self tightening" - fact is once you spin on a prop till it stops - the shoulder on the prop hub seats itself tight/flush on to the top of the motor frame and you are very limited or lucky rather to tighten it any further - but you must snug it up finger tight after spinning it on regardless.

Once the motors are started - the props rotation does lock the prop in position but certainly by definition alone is not "self tightening".

Some do and some will take that phrase quite literally and assume that once the prop is just started on to the threads it will take care of the tightening all by itself.

Another excellent analysis and report concerning the prop issue and ultimate crash - very nice job Nick as to "Root Cause Failure Analysis" - you do enjoy this kind of "forensics" don't you!

 

[TheRealNick]

Couldn't agree more as to the props and the poor choice of the phrase "self tightening" - fact is once you spin on a prop till it stops - the shoulder on the prop hub seats itself tight/flush on to the top of the motor frame and you are very limited or lucky rather to tighten it any further - but you must snug it up finger tight after spinning it on regardless.

Once the motors are started - the props rotation does lock the prop in position but certainly by definition alone is not "self tightening".

Some do and some will take that phrase quite literally and assume that once the prop is just started on to the threads it will take care of the tightening all by itself.

Another excellent analysis and report concerning the prop issue and ultimate crash - very nice job Nick as to "Root Cause Failure Analysis" - you do enjoy this kind of "forensics" don't you!

I do enjoy it! Mostly to try to eliminate future crashes, I took that part out of my post as I have been messaging with Steve and I do not think prop tightness was the issue in this case. So I am not sure why his prop came off, but I don't think it is because it was not tight enough or because of an ESC issues....still a puzzle....

 

[eaglegoaltender]

So I am not sure why his prop came off, but I don't think it is because it was not tight enough or because of an ESC issues....still a puzzle....

If not improper tightening and I realize he has stated they were tight prior to flight, in a post he sent me he stated he was pretty "****" when it came to pre-flight checks and since it sounds like he has had past experience with RC planes - I don't doubt his word.

Since they were tight I will relate back to my own post with regard to thermal expansion rates of the metal thread versus plastic hub threads combined with the fact plastic fasteners when exposed to a thermal load or constant mechanical load will 'creep" and can become loose due to stress and expansion. In particular when we are talking about two different materials (plastic and metal) the thermal expansion rate (if any) is not equal.

Any fastener - has what is called a "clamp load" and usually, this is not a concern if both the male and female connectors are made of the same material.

Albeit, the metal threaded motor shaft shouldn't see a lot of heat under normal conditions - however if the motor is having issues or a bearing is heating up, the metal shaft can be affected enough to create some thermal expansion (considering the diameter of that rod/shaft it wouldn't take a lot to do this).

That expansion only has to be minimal IMO to have a negative affect on the plastic hub expansion rate, hence - "creeping" and loss of "clamp", all of which could equal a loose fit, which in turn and over time may cause a fly off of the prop.

The biggest fear I have and IMO the issues with props that have been in service for some time and are flying off - is my concern with regard to the stress and/or strain strength curve for plastics since they are very temperature dependent. Any long term thermal expansion and contraction at the Prop hub could be of concern over time.

Creeping will most definitely create a loss of "clamp load"! One might argue that since the props are "self tightening" the loss of clamping is negligible - however, I would say one must keep in mind that loss of clamping from creeping is also a product of over loading - and since these are "self tightening" the load is constant while rotating.

 

[TheRealNick]

If not improper tightening and I realize he has stated they were tight prior to flight, in a post he sent me he stated he was pretty "****" when it came to pre-flight checks and since it sounds like he has had past experience with RC planes - I don't doubt his word.

Since they were tight I will relate back to my own post with regard to thermal expansion rates of the metal thread versus plastic hub threads combined with the fact plastic fasteners when exposed to a thermal load or constant mechanical load will 'creep" and can become loose due to stress and expansion. In particular when we are talking about two different materials (plastic and metal) the thermal expansion rate (if any) is not equal.

Any fastener - has what is called a "clamp load" and usually, this is not a concern if both the male and female connectors are made of the same material.

Albeit, the metal threaded motor shaft shouldn't see a lot of heat under normal conditions - however if the motor is having issues or a bearing is heating up, the metal shaft can be affected enough to create some thermal expansion (considering the diameter of that rod/shaft it wouldn't take a lot to do this).

That expansion only has to be minimal IMO to have a negative affect on the plastic hub expansion rate, hence - "creeping" and loss of "clamp", all of which could equal a loose fit, which in turn and over time may cause a fly off of the prop.

The biggest fear I have and IMO the issues with props that have been in service for some time and are flying off - is my concern with regard to the stress and/or strain strength curve for plastics since they are very temperature dependent. Any long term thermal expansion and contraction at the Prop hub could be of concern over time.

Creeping will most definitely create a loss of "clamp load"! One might argue that since the props are "self tightening" the loss of clamping is negligible - however, I would say one must keep in mind that loss of clamping from creeping is also a product of over loading - and since these are "self tightening" the load is constant while rotating.

I would like to know the type of plastic used. I focus when I install my props and try to feel for any difference in force or amount of angle after finger tight. I think you could feel creep or at least a change prior to failure. My props have been on and off probably 35-40 times....thinking about just replacing them...

 

[Phantom751874]

Spin on then finger tight here as tight as I can get them. I'm not real strong plus it's a bit of a weird grip on the top of the motor with your thumb and index finger. The groved top part of the motors kinda gets more painful as you tighten up so I use the pain meter to stop. I take the props off with my DJi supplied metal wrench (v1 motors).

One thing to note is never snug up the props while they are still hot. They'll tighten up more but you're introducing more potential play over time in the prop threads if you do that.

 

[eaglegoaltender]

I would like to know the type of plastic used.

Just a "semi educated guess" but I think they are likely a Thermoplastic (Nylon) material since that material is widely used for plastic fasteners (nuts, bolts, washers, screws) and I have even seen plastic/nylon gears, hubs and even bearings. It also has a high resistance to heat as to damage or melting (but will thermally expand) and will experience creep under thermal/mechanical loading.

I think you could feel creep or at least a change prior to failure

Yes you are correct as to feeling the result of any "creep or loss of clamp load" once the prop hub threads begin to wear or have been stretched via thermal expansion and contraction you should be able to feel a slight looseness when rocking them while installing or lifting slightly once installed (any clearances would be minimal but enough to make you change them). Unfortunately if the creeping or loss of clamp load was occurring in the air.....well not a lot we could do about that.

Constant installing and removal will over time have an effect on the fit - another reason for Thermal Plastic material is the "memory" as to threads keeping their form (somewhat anyway). I have started to use just a piece of printer paper that I lay the corner of on the top of the motor frame - spin on the prop and the hub shoulder grabs the paper and won't pull out. If the paper seems easier to pull out or is loose after spinning the prop on - your prop is not fitting/seated properly and likely time to replace it.

 

[Jussaguy]

I tightened my props as I always do - spin them on then tighten just past snug point.

I hold the motor and spin them hard so they catch and that's been good enough. It's more of a twist with the Inspire.

To the OP, sorry about your bird! Sucks.

 

[David Cohran]

I had the same thing happen, meaning crash. Except I don't know if I flew it into, the wind took it, or it malfunctioned but it hit the tree 20' up. My heart sunk being it was only my third flight. I walked up to it and the camera was laying separate from the craft. I though immediately there goes my drone. Expensive mistake. I then started doing research and found this had been done before, what I broke was the yaw arm and ribbon, and it can be fixed for $45 worth in parts. I watched a great how-to video on youtube and are waiting the parts to have it back in the air.

 

[SteveVaus]

I've posted separately about my current status but will add here: My P3A crash and DJI service - Crashed on 2/17. Shipped to DJI on 2/18. DJI acknowledged receipt Friday, 2/19. I just checked online and status says "Repair Cost Assessed & Quoted." I have heard nothing from them yet directly, but am optimistic because the online repair status also shows "Payment Status - Free of Charge". If I'm interpreting this correctly any knocks on DJI service are outdated. Thus far they've been lightning fast and reached the appropriate conclusion re the crash. Fingers crossed!

 

[eaglegoaltender]

I then started doing research and found this had been done before, what I broke was the yaw arm and ribbon, and it can be fixed for $45 worth in parts. I watched a great how-to video on youtube and are waiting the parts to have it back in the air.

Gotta love all those "how to videos" on YouTube!

Give the bird a good going over as to any structural cracks or looseness, if you don't open it up at least a little "shake" won't hurt to see/hear if anything is loose inside.

Check your prop hubs for any cracks and the motor metal mounting thread for any bent or damaged areas - probably a good idea to replace the props after a 20ft drop.

Good luck on the repair job.

 

[David Cohran]

I've posted separately about my current status but will add here: My P3A crash and DJI service - Crashed on 2/17. Shipped to DJI on 2/18. DJI acknowledged receipt Friday, 2/19. I just checked online and status says "Repair Cost Assessed & Quoted." I have heard nothing from them yet directly, but am optimistic because the online repair status also shows "Payment Status - Free of Charge". If I'm interpreting this correctly any knocks on DJI service are outdated. Thus far they've been lightning fast and reached the appropriate conclusion re the crash. Fingers crossed!

I spoke to a DJI rep today and in my case, he was quick to say pilot error (as expected). I don't want to send it in and it cost to repair half of what I paid for the device. I asked what about it being a malfunction of the craft and he said the can tell what happened with the use of some program. I do know that before my flight, it kept saying something like there was some sort of interference. I was in an open area with no power lines around. Then it displayed green for safe to fly. I am not 100% sure I had full control of the bird. When it got close to the trees, I took my hands off the controls and it continued to the tree. Thought maybe wind but aren't the suppose to hover in compensating for a small breeze?

I was flying with carbon Fiber props (for the first time) so it broke two of the props. I will check for stress cracks elsewhere before flying it again.

 

[TheRealNick]

I spoke to a DJI rep today and in my case, he was quick to say pilot error (as expected). I don't want to send it in and it cost to repair half of what I paid for the device. I asked what about it being a malfunction of the craft and he said the can tell what happened with the use of some program. I do know that before my flight, it kept saying something like there was some sort of interference. I was in an open area with no power lines around. Then it displayed green for safe to fly. I am not 100% sure I had full control of the bird. When it got close to the trees, I took my hands off the controls and it continued to the tree. Thought maybe wind but aren't the suppose to hover in compensating for a small breeze?

I was flying with carbon Fiber props (for the first time) so it broke two of the props. I will check for stress cracks elsewhere before flying it again.

If you post your .dat on dropbox I can take a look at it...

 

[SteveVaus]

UPDATE Wednesday 2/24: Spoke to DJI. Confirmed it was approved as a warranty repair. Quad has been assigned to a tech for repair; then to QC; then to testing; then back to me. I asked if the record reflected the specific cause of the prop loss and crash... no.

 

[David Cohran]

If you post your .dat on dropbox I can take a look at it...

I would be don't know how to do that. I haven't even figured how to look at videos I've taken. Just have been learning how to fly it.

 

[David Cohran]

If you post your .dat on dropbox I can take a look at it...

Therealnick, I looked at my track and right before it struck the tree the mode went to Opti then atti. My controls were neutral. It drifted into the tree. I'm not sure what you mean by .dat or how to load any data on here.

 

[TheRealNick]

Therealnick, I looked at my track and right before it struck the tree the mode went to Opti then atti. My controls were neutral. It drifted into the tree. I'm not sure what you mean by .dat or how to load any data on here.

The directions on how to pull a .dat file are earlier in this thread:

Crashed today...

from your description if it was under trees then it may not be able to get GPS, it will then switch modes and drift with the wind...

 

[David Cohran]

The directions on how to pull a .dat file are earlier in this thread:

Crashed today...

from your description if it was under trees then it may not be able to get GPS, it will then switch modes and drift with the wind...

Thanks for you help. It was not under the trees, out in the open. The whole flight from exiting my truck to end was strange and abnormal. to start I turned the bird and controls on and it was difficult to get calibrated. The controller kept saying something I couldn't understand. After 3 attempts, setting it on the ground, and debating weather to pack up and leave, I receive a safe to fly green. When I took off, it seemed to be a little less stable than previous times flying. I thought it was due to flying with carbon props for the first time. I was in a wide open area. when I reached the distance I was traveling, it seemed real unstable. This is when I took my hands off the controls and it drifted into the tree.