Can any .dat legend ease my concerns

drdikky · Aug 17, 2020

Hi Guys (& Gals)
I am a relative newbie. I crashed my drone into some bushes on the side of a mountain last week. having read many horror stories about prop damage affecting motor speeds I decided to fit a new set of props to my mini. They aren't DJI props but aftermarket ones (the initial ones were DJI). After I found the mini and got home I decided to just do a hover indoors to give me a .dat file to look at and compare with the same scenario after the prop change.
Can one (or more) of you fine people have look at the before and after .dat file and tell me if it looks as though the props were damaged in the crash and needed replacing and if after replacing them things look OK now please
2020-08-12 is the after crash file but with the crash props fitted
2020-08-17 is with the replacement props fitted

Thanking you in advance
Richard

Starz · Aug 17, 2020

I’m certainly not the file expert, but the props are fairly easy.
If they show any damage whatsoever, toss them. Could have even bent one of the screws.
As for the non DJI props, I’m not a fan. No pun intended.

old man mavic · Aug 17, 2020

welcome to the forum,i am not the one to look at files ,but i am sure there will be someone along soon who will be able to help you
@Starz said i only use OE DJI props ,so cant comment on the quality of the aftermarket ones

DrizzyDrones · Aug 17, 2020

Dji props are best but I believe a cpl aftermarket sellers do possibly have good product.. i was interested in opinions on the quick release adapter that allowes props to be removed without removing scrwes I would love to be able to pop em off when not in use for longer periods.. and I think u can use dji props with the setup but I didn’t get much feedback as people haven’t tried them on here

drdikky · Aug 18, 2020

On the aftermarket props theme, having been involved in engineering/production for the last 43 years, in my experience, no firms make all their own component parts as it is uneconomical for them to do so. They sub-contract and merely brand things as their own. That is why I have no problem using aftermarket parts. It wouldn't surprise me to find that the props I purchased were made in the same plant as the DJI ones. Tooling is very expensive but once made the resultant parts it produces are cheap to manufacture. That is why I don't necessarily use price as a guide to quality.
For the price of 1 "O.E.M." set of props I can have 6 sets of aftermarket ones.

Deleted member 103366 · Aug 18, 2020

If you down load the software CsvView and 'give it' a dat file it can produce charts of your motor speeds, they can be interesting even if we can not draw from them the understanding gained by the "DAT legends".
From memory of a quick look at the relevant charts your new props did change the speeds of your motors but that's as far as I can go.

old man mavic · Aug 18, 2020

drdikky said:
On the aftermarket props theme, having been involved in engineering/production for the last 43 years, in my experience, no firms make all their own component parts as it is uneconomical for them to do so. They sub-contract and merely brand things as their own. That is why I have no problem using aftermarket parts. It wouldn't surprise me to find that the props I purchased were made in the same plant as the DJI ones. Tooling is very expensive but once made the resultant parts it produces are cheap to manufacture. That is why I don't necessarily use price as a guide to quality.
For the price of 1 "O.E.M." set of props I can have 6 sets of aftermarket ones.

the tooling might be the same ,but the plastic used in the mold can be of a different quality, what you have to remember is, that along with the battery ,the props are the most important parts that keep the drone in the air ,and failure of either of those two things in particular ,will result in a rapid descent to terra firma

drdikky · Aug 18, 2020

old man mavic said:
the tooling might be the same ,but the plastic used in the mold can be of a different quality, what you have to remember is, that along with the battery ,the props are the most important parts that keep the drone in the air ,and failure of either of those two things in particular ,will result in a rapid descent to terra firma

There is nothing to suggest that an inferior grade of plastic is used in the manufacture of the aftermarket props. Unless we go down the road of analysing the make up of the plastic in each we will never know. We have to rely on the in-house quality control to even ensure that the "O.E.M." material make up is consistent.
From my experience, regularly the same materials are used in the manufacture of items which will be branded differently for different clients, including injection moulding as different materials will give different shrinkage rates resulting in non spec items.
I would suggest that the list of things to give a rapid descent to terra firma would have to include the motors as well
I am not trying to be argumentative, merely voicing my opinion.

Zbip57 · Aug 18, 2020

Crashed props.

Aftermarket replacement props.

For comparison, here is a set of new DJI prop blades on my Mini, all except the (red) right-front motor which still has the original prop blades since last December. At hover the motors should all be spinning at nearly the same speed around 9600 rpm.

drdikky · Aug 18, 2020

Zbip57 said:
Crashed props.
View attachment 110927

Aftermarket replacement props.
View attachment 110928

For comparison, here is a set of new DJI prop blades on my Mini, all except the (red) right-front motor which still has the original prop blades since last December. At hover the motors should all be spinning at nearly the same speed around 9600 rpm.
View attachment 110929

That's great, many thanks for your time and the comparison. Food for thought !!
Cheers !!

Zbip57 · Aug 18, 2020

drdikky said:
That's great, many thanks for your time and the comparison. Food for thought !!
Cheers !!

Did you do your hover tests indoors?

I didn't want to disturb our house guests so I once did the test outside on a day that I thought was dead calm. I got a fright when I saw the motor speed plot and again the left-rear (blue) motor was spinning significantly faster than the others. It turns out the very slight breeze was enough to work the left-rear motor harder than the others. I repeated the test indoors and again all four motors were back to spinning at the same speed. Phew.

If you are doing the test outdoors, you could repeat it four times turning the Mini 90° to face North/East/South/West. That should clearly show any influence of a breeze.

I'm just thinking the spread in your motor speed plots might be caused by wind influencing the attitude of the drone...

drdikky · Aug 18, 2020

Zbip57 said:
Did you do your hover tests indoors?

I didn't want to disturb our house guests so I once did the test outside on a day that I thought was dead calm. I got a fright when I saw the motor speed plot and again the left-rear (blue) motor was spinning significantly faster than the others. It turns out the very slight breeze was enough to work the left-rear motor harder than the others. I repeated the test indoors and again all four motors were back to spinning at the same speed. Phew.

If you are doing the test outdoors, you could repeat it four times turning the Mini 90° to face North/East/South/West. That should clearly show any influence of a breeze.

I'm just thinking the spread in your motor speed plots might be caused by wind influencing the attitude of the drone...

View attachment 110934

Hi there,
it was hovering indoors
Just a bit more info, I have a skin on it and some landing gear. This brings the weight up to 271g. Don't know if that makes any difference ?
Thanks for your patience and help
Richard

Zbip57 · Aug 18, 2020

drdikky said:
Don't know if that makes any difference ?

For sure extra weight will make the motors run faster to generate the required extra lift. Here's the plot (indoors) on the same day as my first plot above, but this time with the prop guard cages installed. You can see the increase in the average motor speeds compared to running without the prop guards. But again, with fresh prop blades, the motors are all running at relatively the same speeds.

drdikky · Aug 18, 2020

Zbip57 said:
For sure extra weight will make the motors run faster to generate the required extra lift. Here's the plot (indoors) on the same day as my first plot above, but this time with the prop guard cages installed. You can see the increase in the average motor speeds compared to running without the prop guards. But again, with fresh prop blades, the motors are all running at relatively the same speeds.

View attachment 110935

I have got a full set of DJI props in the case, I will swap out the aftermarket set for those as soon as I have chance and try a hover test so I can upload a new .dat file
Thanks for your help
Richard

drdikky · Aug 18, 2020

Zbip57 said:
For sure extra weight will make the motors run faster to generate the required extra lift. Here's the plot (indoors) on the same day as my first plot above, but this time with the prop guard cages installed. You can see the increase in the average motor speeds compared to running without the prop guards. But again, with fresh prop blades, the motors are all running at relatively the same speeds.

View attachment 110935

this is at hover indoors in my garage with a brand new set of DJI props fitted . Not a lot different to my aftermarket props.
Any ideas ?
Thanks in advance

old man mavic · Aug 18, 2020

even in still air, all the motors will not run exactly at the same speed ,the slight variances are because of the way a drone flies,,it relies on the speed of any pair of motors to control the drone and keep it in the hover, even if inside there is wind produced by the props, which reacts with the surroundings as it bounces off the floor and walls ,and this will cause subtle speed changes to the motors as they react to the slight movement ,and the info that the IMU is giving to the ESC's

Zbip57 · Aug 19, 2020

drdikky said:
this is at hover indoors in my garage with a brand new set of DJI props fitted . Not a lot different to my aftermarket props.
Any ideas ?

Averaging all four together gives pretty much the correct amount for new props at hover. But why the difference in the individual speeds?

The left-rear (blue) and right-front (red) are the counter-clockwise (CCW) spinning motors, versus right-rear and left-front clockwise (CW). If your landing gear extensions made the drone nose-heavy or tail-heavy you'd expect to see both front or both rear motors spinning slightly faster to compensate. But on a diagonal?

Spinning the CCW motors faster than the CW motors normally should create torque to induce a yaw to the right for the aircraft. If it is actually holding steady in a hover like this, it would suggest that the controller is compensating for a tendency of the drone to yaw left.

Is it easy to remove the landing gear? It would be interesting to see if that makes any difference to the motor speeds in another hover test.

Zbip57 · Aug 19, 2020

Hmmm, just had another thought.

Is it possible that your crash may have twisted an arm out of alignment?

It really doesn't look like a major difference in motor speeds. If you're not getting the Motor Speed or Propeller Speed error message in the latest firmware while flying, it's probably not enough of a difference to worry about. It's just curious that there is such a consistent difference in speeds.

drdikky · Aug 19, 2020

Zbip57 said:
Averaging all four together gives pretty much the correct amount for new props at hover. But why the difference in the individual speeds?

The left-rear (blue) and right-front (red) are the counter-clockwise (CCW) spinning motors, versus right-rear and left-front clockwise (CW). If your landing gear extensions made the drone nose-heavy or tail-heavy you'd expect to see both front or both rear motors spinning slightly faster to compensate. But on a diagonal?

Spinning the CCW motors faster than the CW motors normally should create torque to induce a yaw to the right for the aircraft. If it is actually holding steady in a hover like this, it would suggest that the controller is compensating for a tendency of the drone to yaw left.

Is it easy to remove the landing gear? It would be interesting to see if that makes any difference to the motor speeds in another hover test.

Landing gear is easy to remove, I'll try your suggestion and post results. By eye and as good as I can measure, nothing seems twisted and I don't have any error messages. I will go and try things now.
Thanks for your continued help
Richard