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May 21, 2012, 05:45 PM
Tarot 550/600 & OMPHobby UK
Smoggie's Avatar
Here's the numbers for 13" props on my DM2820750 (750kv):

With the 13x7 Xoar:
983W
64A
15.35V
8910 RPM
Thrust 3.5Kg

With 13x6.5 APCe
846W
54.8A
15.46V
9570RPM
Thrust: 3.04Kg

Personally I don't think the Xoar 13x7 is a bad option at all.. yes it pulls almost as many watts as the APC 14x7 but it also gives not far away from the same thrust but with more pitch speed than the big APC.. So it's a option worth considering IMHO. in fact this is the prop I now intend to use on my motor due to the structural problems I've had with the 14" APC (assuming I can repair the motor)

The only time the Xoar looks bad is if you work on purely theoretical power out calculations based on assumed prop constants rather than work on actual measured data. My conclusion based on actual observed data is that the 13" Xoar has a high prop constant making it in terms of power absorbed and thrust produced perform in a way that's not far removed from the larger APC. This is the only explanation that fits the observations.
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May 21, 2012, 08:00 PM
Registered User
Aeroplayin's Avatar
Quote:
Originally Posted by JetPlaneFlyer
Here's the numbers for 13" props on my DM2820750 (750kv):

With the 13x7 Xoar:
983W
64A
15.35V
8910 RPM
Thrust 3.5Kg

With 13x6.5 APCe
846W
54.8A
15.46V
9570RPM
Thrust: 3.04Kg

Personally I don't think the Xoar 13x7 is a bad option at all.. yes it pulls almost as many watts as the APC 14x7 but it also gives not far away from the same thrust but with more pitch speed than the big APC.. So it's a option worth considering IMHO. in fact this is the prop I now intend to use on my motor due to the structural problems I've had with the 14" APC (assuming I can repair the motor)

The only time the Xoar looks bad is if you work on purely theoretical power out calculations based on assumed prop constants rather than work on actual measured data. My conclusion based on actual observed data is that the 13" Xoar has a high prop constant making it in terms of power absorbed and thrust produced perform in a way that's not far removed from the larger APC. This is the only explanation that fits the observations.
Yeah, you're probably right, but it only takes 575 Watts to turn a 13x7 Xoar to 8910 so the 64A on 15.35v is something that would not be an option in my climate considering the relatively low upside. The RPMs is just not worth all the work being performed by the motor.

Comparatively, the same 750 motor on the same number of cells turned the 14x7 APC only 30 RPMs less and your measurement for Watts-in and Amps was about the same... 984W and 64.1A. The Watts-out for this example is 784. So the bottom line for me is that the electrical consumption is the same for the Xoar 13x7 with much less relative power on the out side.

Now, moving to the 13x6.5 APC at 9570 RPMs.... this is much more attractive as a viable option. That's 663 Watts out at virtually the same 59 mph and 42 seconds more flight time. This is right there with the 600Kv on 5S with the 13x6.5 APC except the 600 on 5S reads 15A less and an increase in relative flight time of a full minute even with a 2200mAh battery.

So for me, the 13x6.5 APC on 750Kv with 4S is much better than the same motor with the 13x7 Xoar, but the 600Kv motor with the 13x6.5 APC on 5S with a 2200mAh battery is better still.
May 21, 2012, 09:57 PM
Registered User
Aeroplayin's Avatar
stgdz... did you have any problem mounting the Motrolfly in the MXS? A friend of mind went back north for the summer and is building one now and said that with about 1/16 gap between the spinner back plate and the cowl, he's only getting about an 1/8 inch of the cowl overlapping the fuse.
May 21, 2012, 10:03 PM
Volts>amps
stgdz's Avatar
Quote:
Originally Posted by Aeroplayin
stgdz... did you have any problem mounting the Motrolfly in the MXS? A friend of mind went back north for the summer and is building one now and said that with about 1/16 gap between the spinner back plate and the cowl, he's only getting about an 1/8 inch of the cowl overlapping the fuse.
yeah I have a gap on all the motrofly motors. Its about 1.5mm but I run the cowl so that it lines up with the A former.


My bigger problem is with the motor mount, do you guys use washers with your screws? I can't fit washers on my mount so the screw doesn't have a lot of meat to hold onto.
Last edited by stgdz; May 21, 2012 at 10:19 PM.
May 21, 2012, 10:19 PM
JKasprak
propchief's Avatar
I went from a Torque to a Motrolfly on my MXS. Bolt holes lined up and I put the G10 (slim) spacer behind the motor. No issues with the spinner gap and the overlap is same as with the Torque. I did find some very small washers in my stock that gave a little extra meat on the mounting bolts.

Hope that helps.

John
May 22, 2012, 01:58 AM
Tarot 550/600 & OMPHobby UK
Smoggie's Avatar
Quote:
Originally Posted by Aeroplayin
Yeah, you're probably right, but it only takes 575 Watts to turn a 13x7 Xoar to 8910
Jim,

We have been through this one before and I don't want to drag it up again but I still say that your 'power out' calculation is based on assumed prop constant and is therefore not proven for this particular prop.

The 13x7 Xoar produces almost as much static thrust as the APC (3.5kg vs. 3.72Kg) and based on claimed pitch (If we can believe that) and measured RPM it has a little bit more pitch speed than the APC. So according to my measurements (taken same day with same hardware) it's transmitting not far from the same power as the APC 14x7 regardless of what the power out calculations might say

Anyway.. the answer to this one might be for someone else to do a test on the Xoar 13x7 and see what they find.

Steve
May 22, 2012, 02:03 AM
Tarot 550/600 & OMPHobby UK
Smoggie's Avatar
PS,

I went flying with my re-built Motrolfly using a 13x6.5 prop.. It only lasted two flights before the bearing tube came loose from the back plate again

I contacted Ken and he can fix it but that would mean return shipping from the UK, two way. Anyway he advised that the way to fix it is to use Black Loctite (basically super glue with rubber powder in). I'd just been using the standard medium strength thread locking blue type which obviously isn't strong enough.

So I'll clean it all up and have another go at repairing.

Steve
May 22, 2012, 02:07 AM
Just flitting about!!
bigroger's Avatar
Steve and Jim,
you two guys need to sit down in a little room with your computers and spreadsheets and have a showdown.

I think from personal experience it all comes down to the stick work. As long as your not burning up motors/ esc's/ lipos/ and your happy with the way it flys then that's all that should matter.

I'd rather be out burning packs advancing my flying skill set then being worried about Xoar 13x7 versus APC 14x7.

I have noticed that Vox props pull more amps and more thrust/rpm than APC props in the 12, 13, 14" sizes until you go into the 15" size. At that point the apc burns more electrons but doesn't seem to generate more thrust and definitely lower rpms.

haven't tried Xoar as yet, I have a Xoar 16x7 for the 60" edge and a Vox 16x7 so that will make an interesting comparison, but I think its already been covered in the blind study the Kitt clan covered some months back.
May 22, 2012, 02:09 AM
Just flitting about!!
bigroger's Avatar
Quote:
Originally Posted by JetPlaneFlyer
PS,

I went flying with my re-built Motrolfly using a 13x6.5 prop.. It only lasted two flights before the bearing tube came loose from the back plate again

I contacted Ken and he can fix it but that would mean return shipping from the UK, two way. Anyway he advised that the way to fix it is to use Black Loctite (basically super glue with rubber powder in). I'd just been using the standard medium strength thread locking blue type which obviously isn't strong enough.

So I'll clean it all up and have another go at repairing.

Steve
Steve, this sounds like what happened to my 2820-750 when it pulled out after a blender and I was using the apc 15x6 prop.

I guess I count myself lucky considering your did the same thing almost and on a 14x7 as well.
May 22, 2012, 04:28 AM
Tarot 550/600 & OMPHobby UK
Smoggie's Avatar
Quote:
Originally Posted by bigroger
Steve, this sounds like what happened to my 2820-750 when it pulled out after a blender and I was using the apc 15x6 prop.

I guess I count myself lucky considering your did the same thing almost and on a 14x7 as well.
Yep, its the same issue no doubt. When i read of your problem I tightened up the set screws on my motor and I think that's all that prevented the stator pulling out just like yours did. The intereference between the stator mount tube and the backplate is all fretted away, the only thing holding the motor together was the two set screws.

It's something to watch on these motors. It may in fact be a good idea for anyone pushing these motors hard to Loctite the two parts together from new?.. I appreciate not many people would be up for pulling apart a brand new motor but it's something I might do with my next DM2820 just to be sure.

Steve

PS.. In my email exchange with Ken he said that the 14x7 prop would be too much for the 750kv version of the motor, which backs up my recent experience.
May 22, 2012, 07:51 AM
Registered User
Aeroplayin's Avatar
Quote:
Originally Posted by JetPlaneFlyer
Jim,

We have been through this one before and I don't want to drag it up again but I still say that your 'power out' calculation is based on assumed prop constant and is therefore not proven for this particular prop.

Steve
Steve,
Yes, we’ve been through this before and if I had agreed with you, we would now both be wrong. Additionally, I would now have a 750Kv motor that I’d be considering another home for since running a 14 inch prop with it is out of the question in Florida.

When I bought the Edge, I started with a 13 in prop setup on 3S with a 950Kv motor and didn’t like it. The 14 inch was out of the question because it was cooking my system and ran out in 3.5 minutes. I definitely saw the advantage of the 14 so I then spent a lot of time finding another power combo and eventually bought the 638Kv. That handled the 14 just find on 4S but I was not getting the punch I was looking for at the top end, so when Tom got his Edge, we went to the 683Kv on the 4S with the 14 and found the sweet spot.

When you had reported the 984W on the 750Kv, I was seriously considering it until I ran the numbers and found that setup to be over-worked and 14 Amps were going to something other than turing the prop. There is no qualification for me to have a motor that needs 64A to spin a 14x7 8800 RPMs. Not where I live and fly.

The important thing for me is to separate the load that the motor is carrying from the performance it is delivering. This is what it was all about with the original prop study. Just because a meter tells me that I’m getting more Watts doesn’t mean anything in terms of power at the prop. Finding the right mix between power-out and heat is my main concern flying during the summer months here in Florida.

What is clearly evident is that the 750Kv, 683Kv, 638Kv, and 600Kv motors are all possible option for generating over 6 lbs of thrusts on some combination of battery and prop. What is also now evident is that turning a 14 inch prop on 4S on a 750Kv is not a good idea -- or a 14 on the 600 with 5S.

If I take all the data from the 750Kv, 683Kv, 638Kv, and 600Kv, and I eliminate the 14 inch prop combos from the 750, and the 14 inch prop from the 600Kv on 5S, I have a few remaining options for delivering both the thrust and speed I'm looking for, and do it with under 50A.

They turn out to be the 683Kv motor on 4S with the 14x7 APC at 48 Amps, and the 600Kv on 5S with the 13x6.5 APC at 40A. The 600Kv on the Xoar 13x6.5 with 5S is next, and the 638Kv on 4S with the 14x7 is right at the tipping point of thrust and mph, but at only 36A and over 8 minutes of flight time.

Now if a 14 inch prop is a preference, or if 4S is a preference, the 683Kv and the 638Kv are the remaining options with the 683 having a clear advantage.

For me, the 750 is no longer an option but I still intend to try the 638Kv on 5S with the 2200mAh battery and the 13x6.5e APC to test the 14 inch against the 13 inch one more time. Other than that, and because of the number of 4S batteries I have, and how much I like the 14 inch prop, the next 48 EXP will get another DM2820 with a 683Kv custom wind.
May 22, 2012, 07:58 AM
Registered User
Aeroplayin's Avatar
Quote:
Originally Posted by stgdz
yeah I have a gap on all the motrofly motors. Its about 1.5mm but I run the cowl so that it lines up with the A former.


My bigger problem is with the motor mount, do you guys use washers with your screws? I can't fit washers on my mount so the screw doesn't have a lot of meat to hold onto.
Thanks for the info, and I'll pass it along.

I also do use washers between the mounting screw and the cross mount on all of the Motrolfly motors and all have the G10 cross mounts now.
Last edited by Aeroplayin; May 22, 2012 at 08:04 AM.
May 22, 2012, 08:47 AM
Volts>amps
stgdz's Avatar
How did you get the washers on? For me the screw heads are touching the back of the motor where the mount is screwed into.

Even though its slotted I can't get the washes on no matter how much I twist and turn.



Steve, for those grub screws. Blue loctite isn't sufficient is it? It looked like red was on it but it broke free really easily.



Also Jim you are now saying 750 and 14" props are nono now. Wonder what ken will end up doing.
May 22, 2012, 09:31 AM
Registered User
Aeroplayin's Avatar
Quote:
Originally Posted by stgdz
Also Jim you are now saying 750 and 14" props are nono now. Wonder what ken will end up doing.
The Subsonic Web site shows the 14x7 on 4S with the 750, so I have no idea, but for me, flying in Florida summers, 64A at WOT with a 14x7 APC is something I donít need when 45A to 48A is only 400 fewer RPMs on the 683Kv. Steve has already mentioned that the 13 is where he is going now from what he has experienced with the 14 and I think it's a good idea with all things considered.

So for me, if I want the 14 on 4S, I already know what motor to use. Tomís videos with the 48 Edge are all on the 683Kv on 4S with the 14x7 APC and Iíve yet to see anything more aggressive until I saw your 600Kv on 5S. Plus you are only generating 35A and the difference between in and out is only 100W compared to the 200W on the 750.

If the 9500 RPMs with the 13x6.5 on my 638 changes my mind about the 13 inch prop, then I should be able to put 5S in my existing motor, which makes the 750 something Iím not interested in anymore.

Quote:
Originally Posted by stgdz
How did you get the washers on? For me the screw heads are touching the back of the motor where the mount is screwed into.
I donít really remember if I had an issue with the washers so I just looked at both planes and I donít see any washers. I guess we donít need them considering the abuse.
May 22, 2012, 10:51 AM
Tarot 550/600 & OMPHobby UK
Smoggie's Avatar
Stgdz,

I think most people are coming to the conclusion that the 14" APC prop is just too much for the 750kv version of the DM2820. Personally I had no issues at all with overheating, the motor never got too hot. It also gave good flight times so efficiency in actual use was IMHO very good. The limit that has shown up is basic structural integrity of the motor over medium to long term use. If you had asked me a few days ago I’d have sworn that the motor was fine with the 14” APC… but we all live and learn.

Ken at Subsonic also confirmed to me by email that his opinion was that the 14" prop was too much for the 750kv wound motor. The info on the Subsonic site is based on the old 638kv motor and probably should be updated.

It's not Loctiting the grub screws that's the issue, blue or red Loctite will be ok for that. It's the permanent bonding (with Loctite 410 or 480 toughened CA) of the bearing tube into the back mount that's what I’m faced with doing.

Steve


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