Motor Swapout Data...very good info and very interesting...

[helloman1976]

I have a DJI F550 knock-off and it weights in at about 3.5-4lbs fully loaded. I was using 750Kv 250W Turnigy NTM motors from Hobbyking and I have recently replaced those with much faster 1350Kv 310W Turnigy NTM motors also from Hobbyking and here's what I've discovered. I'm spinning the exact same APC 10"x5 pitch propellers and I used the same ones in each test. I found the following...

The 750Kv
More lifting power and more "punch" under weight
They never, ever, ever even got luke warm
Significantly less power draw (max wattage was 950W with a burst high of 1280W)
Flight time with 8,000mah battery was about 12-15 minutes

The 1350Kv
More speed hands down
Much more nimble
Excellent wind recovery and can fly in 35mph gusts with little corrections other than into the wind somewhat
Uses more power (max wattage 1,400W with burst high of 2440W)
Flight time is lower now to around 10-12 minutes
They get warm to hot but never burning hot and I tested them under severe conditions of up and down max to low power over and over to heat them up as much as possible....they get pretty warm like that though.

My thoughts....

I think with the 750Kv they weren't working very hard and since they are able to spin MUCH bigger blades they were basically idle but at the same time I was not able to pull their max power no matter what because of that so I shorted myself some flight power.

With the 1350Kv they are almost too powerful and I believe are working too hard with the 10"x5 props and would probably do better with 9x5 really.

My theory is that the 1000Kv motors are the way to go because they should have more torque than the 1350Kv but use less power like the 750Kv because they are able to spin 10x5 with no problems. I'll try to keep this post updated and refresh it when I get the 1000Kv's installed.......Hope that helps someone.

 

[ChrisViperM]

Cool info...thanks.

Chris

 

[kloner]

the 750 kv are made to spin 12 and 13" props, if your not loading them up it's because your frame is too small for the weight and your unable to prop up to the task. mine use 12" props and are power houses on 4s, 2 seperate rigs both same weight as you describe


my coax is 12x5 and12x7, flat 6 is 12x6

 

[helloman1976]

the 750 kv are made to spin 12 and 13" props, if your not loading them up it's because your frame is too small for the weight and your unable to prop up to the task. mine use 12" props and are power houses on 4s, 2 seperate rigs both same weight as you describe


my coax is 12x5 and12x7, flat 6 is 12x6

I can't use 12" because they wouldn't fit on my Hexa and 10" is the max so I'm basically at the point where I'm tuning the motors to the 10"x5 prop versus the other way around. I had NO data at all when I selected my motors, only basic math calculations. How do you know a 750kv is for a 12-13" prop? Is there a chart or something? Also are you talking about 28's in the 750Kv rpm range? I saw that the 1000Kv motors are tested with 10x5 props, that's probably a good indicator though isn't it I wish I had known. I'll likely be selling some brand new 750Kv and 1350kv motors soon! lol.... I really liked the 750Kv, they pulled very little power, never got hot AT ALL EVER NO MATTER WHAT, and were able to lift a bulldozer I'm hoping the 1000Kv motors, which are about 60W stronger, will be a good balance between the INSANE speed of the 1350's and the torque/energy efficiency of the 750kv. Think so? I'd go back to the 750Kv motors but they are clearly under prop'd and since I can't use bigger props I'm thinking 1000's...does that make sense? Thanks!


EDIT : Also what about reducing the prop size to 9x5 or making the Hexa lighter so there's less load on the motors? Just some ideas....I've heard people say they went to a smaller prop and got longer flight times in situations similar to this.

 

[bruce365]

Hi helloman, if you are looking for a good calc program try this one:-
http://www.ecalc.ch/xcoptercalc_e.asp
Regards - bruce

 

[helloman1976]

Hi helloman, if you are looking for a good calc program try this one:-
http://www.ecalc.ch/xcoptercalc_e.asp
Regards - bruce

Thanks Bruce, I've used that thing a dozen times and it always says "no enough power to hover" no matter what I put in there. Even if I put the weight of my Multi at 0 it still says it won't fly and I'm using only built in options. Doesn't matter what I put in, even if I look it all up manually so I wouldn't trust that thing as far as it works

 

[gtranquilla]

Granted there several counter-intuitive aspects of ECalc for Multicopter such as:
a) xcopter weight with and without "drive"..... (doublecheck with the bottom reported AUW - all up weight to gain perspective and lot this data correctly at the get go)!!!
b) erroneous data presented as fact when you don't put in all the date required for it to do proper calculations
d) confusing Motor manufacturer/type KV in rpm/volt.... etc/.....work thru it carefully!!!
e) Inability to load some new motor data no available torque curves or data from new manufacturers.
f) calculate button may or may not do anything whatsoever!!!!

But if you fight your way through these issues you gain tremendous insight into which way to go..... Here is a good example:
View attachment 9896





View attachment 9897

Thanks Bruce, I've used that thing a dozen times and it always says "no enough power to hover" no matter what I put in there. Even if I put the weight of my Multi at 0 it still says it won't fly and I'm using only built in options. Doesn't matter what I put in, even if I look it all up manually so I wouldn't trust that thing as far as it works

 

[helloman1976]

Granted there several counter-intuitive aspects of ECalc for Multicopter such as:
a) xcopter weight with and without "drive"..... (doublecheck with the bottom reported AUW - all up weight to gain perspective and lot this data correctly at the get go)!!!
b) erroneous data presented as fact when you don't put in all the date required for it to do proper calculations
d) confusing Motor manufacturer/type KV in rpm/volt.... etc/.....work thru it carefully!!!
e) Inability to load some new motor data no available torque curves or data from new manufacturers.
f) calculate button may or may not do anything whatsoever!!!!

But if you fight your way through these issues you gain tremendous insight into which way to go..... Here is a good example:
View attachment 12716





View attachment 12717

I'll give it another shot and when I do I'll post like you did. Maybe you can spot what I'm doing wrong.

 

[helloman1976]

Ok so here's an update. I haven't posted the videos yet but I will soon. You can REALLY hear the difference too. I ended up going back to my 750Kv's because I like the torque over the speed or so it seems. I was able to hit 65mph across the field, which is about 640 feet long, in just a matter of maybe 12 seconds-ish or so, it was amazing. I was able to go up to around 400 feet in just a matter of a few seconds but it was so wild and out of control there's no way I could enjoy flying like that so I swapped back. The big difference was that where the 750kv could stop me mid-fall the 1350Kv's really had to work to stop my fall from 400 feet and I almost smacked into the ground. The 750kv motors were able to grab me to a dead stop almost from a fall from 400 feet to almost an instantaneous stop in the air and I missed that enough to swap back. These 750kv motors will pull 900W burst at 14.5V@62A or about 10A per motor and they are rated at a maximum of 18A. When done flying with the 750Kv they are ICE COLD and I mean ICE COLD as if you had grabbed them from your refrigerator. The 1350kv's had a hard time grabbing me in mid flight and I'd fall an extra 20-30 feet beyond what I wanted to do as they worked to stop my fall. With the 1350's I was able to fly more like an airplane and didn't really have to tilt as much. I should mention too that all of this was done with the same props, same setup etc etc 10"x5pitch APC props. After a flight with the 1350's, and I worked them HARD, they were only warm to the touch and the ESC's were a little warm too whereas again with the 750kv they are also ice cold always. The 1350's were hard to fly too, they never wanted to come down and you had to basically cut the throttle completely to bring her down. It was really hard to fly in manual with the 1350's too, it was all over the place sideways and up-and-down 20-40 feet at a small flick of the throttle. Landing in manual was not possible really. Hope this data helps someone... I'm glad I could discover this but I'm done with motor swapping for now. If I were to upgrade I'd go to the 1000Kv @ 315W NTM Turnigy version.

 

[bruce365]

Hi helloman, the F550 from DJI comes with stock 920kv motors at about 120 watts each motor. My virgin F550 flew best with 8 inch graupner props (smoother than with 10") and 4s battery (5amp). When I added weight, landing and gimbal, telemetry, gopr2, FPV gear I had to go to 10 inch graupner props to carry the extra weight (2.5kg).

DJI obviously had done their research and decided a 920kv motor was best for the rated weight and prop size. What made you therefore decide to use a 1350kv motor with 10" props?

Ecalc works well if you feed it with appropriate numbers.
Regards - bruce

 

[AustinFPV]

Try the new DJI 10-3.8s. You are the perfect tester with lots of APC 10-5 experience on two different motors.

 

[gtranquilla]

Have you considered adding on the aluminum hex arm extenders or using the RCTimer 800 arms which are designed specifically for upgrading DJI 550? These enable larger prop diameters so that your motor prop combos and matching batteries stay in the right ballpark. In any case there is significant multi-disciple engineering hidden inside these off the shelf MRs such that significant changes to payload weight, motor and props means re-engineering is in order and eCalc is a good tool for that purpose IMHO.

Ok so here's an update. I haven't posted the videos yet but I will soon. You can REALLY hear the difference too. I ended up going back to my 750Kv's because I like the torque over the speed or so it seems. I was able to hit 65mph across the field, which is about 640 feet long, in just a matter of maybe 12 seconds-ish or so, it was amazing. I was able to go up to around 400 feet in just a matter of a few seconds but it was so wild and out of control there's no way I could enjoy flying like that so I swapped back. The big difference was that where the 750kv could stop me mid-fall the 1350Kv's really had to work to stop my fall from 400 feet and I almost smacked into the ground. The 750kv motors were able to grab me to a dead stop almost from a fall from 400 feet to almost an instantaneous stop in the air and I missed that enough to swap back. These 750kv motors will pull 900W burst at 14.5V@62A or about 10A per motor and they are rated at a maximum of 18A. When done flying with the 750Kv they are ICE COLD and I mean ICE COLD as if you had grabbed them from your refrigerator. The 1350kv's had a hard time grabbing me in mid flight and I'd fall an extra 20-30 feet beyond what I wanted to do as they worked to stop my fall. With the 1350's I was able to fly more like an airplane and didn't really have to tilt as much. I should mention too that all of this was done with the same props, same setup etc etc 10"x5pitch APC props. After a flight with the 1350's, and I worked them HARD, they were only warm to the touch and the ESC's were a little warm too whereas again with the 750kv they are also ice cold always. The 1350's were hard to fly too, they never wanted to come down and you had to basically cut the throttle completely to bring her down. It was really hard to fly in manual with the 1350's too, it was all over the place sideways and up-and-down 20-40 feet at a small flick of the throttle. Landing in manual was not possible really. Hope this data helps someone... I'm glad I could discover this but I'm done with motor swapping for now. If I were to upgrade I'd go to the 1000Kv @ 315W NTM Turnigy version.

 

[helloman1976]

Hi helloman, the F550 from DJI comes with stock 920kv motors at about 120 watts each motor. My virgin F550 flew best with 8 inch graupner props (smoother than with 10") and 4s battery (5amp). When I added weight, landing and gimbal, telemetry, gopr2, FPV gear I had to go to 10 inch graupner props to carry the extra weight (2.5kg).

DJI obviously had done their research and decided a 920kv motor was best for the rated weight and prop size. What made you therefore decide to use a 1350kv motor with 10" props?

Ecalc works well if you feed it with appropriate numbers.
Regards - bruce

I don't have a DJI F550 and I actually own the knock off version which I hand built from the ground up. I had no point of reference and the eCalc only seems to work for those who have an engineering degree or those who wrote the code as I can't get it to work for the life of me even with manufacture based data or the already installed drop down boxes. If I enter 0 weight it still says it won't fly and that's using ONLY drop down menu items. Regardless, I have flow with the 750Kv motors before and I had though that the DJI motors were 1350Kv and I was told wrong on another forum so that's why I went with a "matching" but higher amperage motor. The craft was flyable like that but not for what I needed, it was wayyyy too hot for me but it sure was fun for those 3 flights. I'll be posting the videos in the next couple days, you can hear that the motors sound differently...almost like a turbine

 

[bruce365]

Hi helloman, I did read in your first post that you had a F550 clone (I assumed the differences between genuine and knock off would be very minor if any). If you were given wrong information I can understand how you ended up with the wrong kv motors. What figures are you tying to feed into e-calc?
I look forward to your video.
As mentioned earlier you can get extensions or longer arms for your hexa to enable you to fit larger props for your 750kv motor, but that would not be necessary unless you intend to increase the total weight of your hexa to 3kg or above.
Regards - bruce

 

[helloman1976]

Hi helloman, I did read in your first post that you had a F550 clone (I assumed the differences between genuine and knock off would be very minor if any). If you were given wrong information I can understand how you ended up with the wrong kv motors. What figures are you tying to feed into e-calc?
I look forward to your video.
As mentioned earlier you can get extensions or longer arms for your hexa to enable you to fit larger props for your 750kv motor, but that would not be necessary unless you intend to increase the total weight of your hexa to 3kg or above.
Regards - bruce

First of all, thanks for your help and I truly appreciate it. I won't use extenders because it will no longer fit in my travel case if I do and I'd have to remove them and that's a huge pain. I swapped back to the 750Kv motors and they are a dream to fly compared to the old ones. It's interesting that you say that the stock motors are 920kv and 120W as these are 750Kv and 250W. I can pull a maximum of about 90A through these motors with this prop setup and I can do that all day long with ZERO heat, the motor cans are like ice cubes from the air temp when I'm done flying versus the 1350's which were warm to the touch along with the ESCs. If I went above 3kg I'd go with a deeper pitch prop and I'd be willing to be it would be problem solved, 10"x5 to 10"x7 for example) so I really like this nice torquey motors now that I've flow on the faster versions. I was able to get eCalc to work just now, I looked at your screen and I think the issue was some of the numbers were already added and included in some of the other steps. I changed it to without total drive weight and it came up just about correct. Thanks!


EDIT : One more thing, Kloner was right...the 1350Kv's are way too fast, the 750Kv's are great and I am now also starting to think that the 1000's are too much. Kloner also recommended going up to the 800Kv if I wanted an upgrade and in the eCalc they look like a perfect fit so +1 to Kloner, I'm getting tired of admitting he's right though lol...call me hard headed but I'll learn...

 

[gtranquilla]

IMHO - One thing to keep in mind is that your prop tips must never reach or exceed the sound barrier.
And there are also design constraints on the props such that there is a finite safe maximum speed for each prop design which is typically quite a bit below the speed of sound.
Unfortunately this data may not be available from all prop manufacturers. Some also sell very high performance props that are designed for much higher speeds should the need arise....
The fundemental starting point for all this is the torque/speed curves.... Once you understand where the sweet spot(s) are for optimum lift and/or speed rrequirements, the rest should fall in line.
The number of poles in the motor sets the precedence for the motor/pro design speed range... From there one has to get the battery voltage needed to allow for most or all of that speed range.

I don't have a DJI F550 and I actually own the knock off version which I hand built from the ground up. I had no point of reference and the eCalc only seems to work for those who have an engineering degree or those who wrote the code as I can't get it to work for the life of me even with manufacture based data or the already installed drop down boxes. If I enter 0 weight it still says it won't fly and that's using ONLY drop down menu items. Regardless, I have flow with the 750Kv motors before and I had though that the DJI motors were 1350Kv and I was told wrong on another forum so that's why I went with a "matching" but higher amperage motor. The craft was flyable like that but not for what I needed, it was wayyyy too hot for me but it sure was fun for those 3 flights. I'll be posting the videos in the next couple days, you can hear that the motors sound differently...almost like a turbine

 

[helloman1976]

IMHO - One thing to keep in mind is that your prop tips must never reach or exceed the sound barrier.
And there are also design constraints on the props such that there is a finite safe maximum speed for each prop design which is typically quite a bit below the speed of sound.
Unfortunately this data may not be available from all prop manufacturers. Some also sell very high performance props that are designed for much higher speeds should the need arise....
The fundemental starting point for all this is the torque/speed curves.... Once you understand where the sweet spot(s) are for optimum lift and/or speed rrequirements, the rest should fall in line.
The number of poles in the motor sets the precedence for the motor/pro design speed range... From there one has to get the battery voltage needed to allow for most or all of that speed range.

Man, I WISH I could find data on these props from a wind tunel complete with CAD designs and all that but I think The Chinese are more interested in selling them to us then sharing their design specs. I use the APC props which are very rigid and tapered at the end. The cord is thick but thins out as it gets closer to the tip whereas the profile also flattens out. These props have been tested to 20,000rpm for 5 minutes and as long as they are balanced they handle that fine. A 750Kv motor, assume 4S, will spin these around 9,000rpms and a 1350Kv motor will spin these around 16,200 rpms. They are 10" wide so if you want to pull out your calculator and tell me when they break the 760mph sea level speed of sound mark or 330m/s, by all means share that data. Better than doing the math is to listen for the sounds of the prop breaking the sound barrier which should be more than obvious if you are standing close to it and it's quiet out and it'll sound a lot like fluttering. Also keep in mind the pitch of your blade will change with the centrifugal force flattening it out. There are many, many, many other factors as well like altitude and air temperature but I think the basic idea is to follow what you've seen work and listen for the side effects of the things that don't work and go from there.

So, given that propeller Mach = diameter (inches) x prop rpm / 255871.5 my outcome would be

Information is an estimate based on 80% theoretical rotational maximums and 4S power (15VDC).

750Kv
0.35 mach = (10" x 9,000rpm) / 255871.5

1350Kv
0.64 mach = (10" x 16,200rpm) / 255871.5

 

[gtranquilla]

http://www.ehow.com/how_7445569_calculate-prop-tip-speed.html (may not always be the case for MRs)

On helicopters the tips will begin to delaminate when hitting the speed of sound due to cavitation and related factors.

Calculating tip speed:
http://www.pponk.com/HTML%20PAGES/propcalc.html

 

[helloman1976]

<a href="http://www.ehow.com/how_7445569_calculate-prop-tip-speed.html" target="_blank">http://www.ehow.com/how_7445569_calculate-prop-tip-speed.html</a> (may not always be the case for MRs)<br>
<br>
On helicopters the tips will begin to delaminate when hitting the speed of sound due to cavitation and related factors.<br>
<br>
Calculating tip speed:<br>
<a href="http://www.pponk.com/HTML%20PAGES/propcalc.html" target="_blank">http://www.pponk.com/HTML%20PAGES/propcalc.html</a>

<br>
<br>
Helicopters? Are you spinning 26" blades? <img src="http://www.multirotorforums.com/images/smilies/smile.png" border="0" alt="" title="Smile" smilieid="1" class="inlineimg"> You're probably right there though. I'm most happy that my Computer Science and Engineering degree paid off and my math was right lol... I was expecting someone to fact check me and do the math and then correct me if I'm wrong but that online calc proved I was dead on so that's a good feeling at least. Now quick, someone as

 

[Phil550]

First of all, thanks for your help and I truly appreciate it. I won't use extenders because it will no longer fit in my travel case if I do and I'd have to remove them and that's a huge pain. I swapped back to the 750Kv motors and they are a dream to fly compared to the old ones. It's interesting that you say that the stock motors are 920kv and 120W as these are 750Kv and 250W. I can pull a maximum of about 90A through these motors with this prop setup and I can do that all day long with ZERO heat, the motor cans are like ice cubes from the air temp when I'm done flying versus the 1350's which were warm to the touch along with the ESCs. If I went above 3kg I'd go with a deeper pitch prop and I'd be willing to be it would be problem solved, 10"x5 to 10"x7 for example) so I really like this nice torquey motors now that I've flow on the faster versions. I was able to get eCalc to work just now, I looked at your screen and I think the issue was some of the numbers were already added and included in some of the other steps. I changed it to without total drive weight and it came up just about correct. Thanks!


EDIT : One more thing, Kloner was right...the 1350Kv's are way too fast, the 750Kv's are great and I am now also starting to think that the 1000's are too much. Kloner also recommended going up to the 800Kv if I wanted an upgrade and in the eCalc they look like a perfect fit so +1 to Kloner, I'm getting tired of admitting he's right though lol...call me hard headed but I'll learn...

I've been following this thread with interest as I may have to upgrade the motors on my 550.
It currently weighs 2330g with standard dji motors, esc's, aeroxcraft landing gear, gimbal etc but I would like to upgrade the gimbal, which would push the auw up to about 2600g.
I would like to keep the Graupner 10x5 props and my 5000mah 3S lipos so which motors should I consider - the 800kv or 1000kv? Are the Turnigys a good upgrade - they seem very cheap...?

Thanks

Phil