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Old May 30, 2012, 09:09 PM
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Originally Posted by bigroger View Post
Numbers from yesterday, all on the same lipo and not recharged between.
APC 12x6, 10650rpm, 46A, 761W
Vox 13x5, 10110rpm, 42A, 646W
Vox 13x6.5, 9300rpm, 51A, 796W
APC 13x6.5, 9390rpm, 60A, 950W

Today, fresh lipo each time but not same lipo (maybe in IR issue leading to strange numbers).

APC 12x6, 10200rpm, 44A, 686W
APC 13x4, 9930rpm, 53A, 841W
Vox 13x5, 9930rpm, 41A, 615W
Vox 13x6.5, 9300rpm, 55A, 883W
APC 13x6.5, 8850rpm, 56A, 895W

Numbers still feel a bit screwy to me, but you can clearly see that the vox spins at higher rpm than the same size apc prop where you would expect. One would assume this means more thrust on the vox over the apc, but I have no way of measuring thrust apart from flying.
Very interesting.

Are you going to fly each one?

I think is going to be the determining factor.

Thanks for your efforts.

JC
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Old May 30, 2012, 09:49 PM
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Well, yesterday I flew the vox 13x6.5 and loved it, also ran the apc 12x6 and hated it.

I found out today that my spinner was miles out of balance and have since addressed that.

I don't currently plan to fly out the other three props at this time but perhaps over the next few days might give it a try.

Still want a few more flights on the vox 13x6.5 now that I have balanced up the spinner to see if that yields a bit more performance (rate of spool up) than yesterday.
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Old May 30, 2012, 10:39 PM
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Here's how the same props match up, first set and second set. Outside the fact that the 13x4 data makes no sense, the back to back tests are about as consistent as we could expect. Other than the one 13x4 test, there is nothing wrong with this data.

Vox 13x5 @ 10110 RPMs and 42A = 38W consumed and 2.5A (80.16% KvE)
Vox 13x5 @ 9930 RPMs and 41A = 39W consumed and 2.6A (80.7% KvE)

APC 12x6 @ 10650 RPMs and 46A = 139W consumed and 8.4A (78.5%)
APC 12x6 @ 10200 RPM and 44A = 139W consumed and 8.9A (79.8%)

Vox 13x6.5 @ 9300 RPMs and 51A = 187W consumed and 12.0A (72.7%)
Vox 13x6.5 @ 9300 RPMs and 55A = 274W consumed and 17.1A (70.6%)

APC 13x6.5 @ 9390 RPMs and 60A = 323W consumed and 20.4A (72.4%)
APC 13x6.5 @ 8850 RPMs and 56A = 371W consumed and 23.2A (67.5%)
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Old May 30, 2012, 10:48 PM
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Thanks Jim.
It would appear I should retest the APC 13x4.

Also the Vox 13x5 does not seem to produce much value as its lower in watts that the apc 12x6.

But maybe it has more thrust for lower amps than the VOX 13x6.5?

I'l take the 13x5 with me next time I fly and give it a try.
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Old May 30, 2012, 10:56 PM
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Thanks Jim and Roger.

The results of your testing is very interesting.

I'm going to hold off on placing my prop order until i see what the in flight results produce.

Really appreciate your efforts.

JC
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Old May 31, 2012, 12:22 AM
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Originally Posted by Aeroplayin View Post
No, if you are spinning a prop at a specific RPM and on a specific number of volts, then the KvE is determined by the difference in RPM, not Watts. All the numbers there for KvE are calculated by volts, Kv, and RPMs and not Watts, and seem to be correct.
Jim,

We are discussing different things I think which explains the different figures.

I'm talking about'real' mechanical efficiency.. i.e. how efficiently the motor converts electrical input power to mechanical output power at the prop. IMHO this is the only true efficiency. If you accepted the numbers Roger got and you accept your own power calculation then that would be 94% (which is unrealistically high)

'KV efficiency' is AFAIK an abstract concept that I've never heard of anywhere else but this thread, it's not true efficiency.

Weight of the prop should make no difference to the peak RPM, but it would make a difference to the power the motor put into accelerating the prop.. this may well be the explanation.. i.e. the peak figures aren't occurring at the same instant.


Roger,

I agree with you that the numbers still look odd (better than befiore but still a bit strange). FWIW I always use the same battery but top it up between runs when I'm doing testing. Jim's point about prop weight might also be the explanation. Best way to eliminate that is to gently increase throttle so you don't get a big amp spike as the motor works to spin the prop up, which kind of screws the calculations.


Steve
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Old May 31, 2012, 01:07 AM
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Thanks guys.

I am gonna re fly first to compare the vox 13x5 and 13x6.5 then will retest.

Got my first shipment of gensace 25C4s2600 today and will retest on that after I break it in a little but in the mean time its the old turnigy for me.

For acceleration, I normally start the prop on my switch on the Tx then accelerate to about 3/4 pretty quick, wait about 2 seconds then punch to wot.

Is that ok or should I go real gentle all the way to peak?
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Old May 31, 2012, 06:31 AM
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A gentle increase all the way over about 4 seconds or so might be the best way to do it. Thats what i've been boing lately to try to eliminate the high spike that you tend to get if you puch up to WOT quickly.

It had never really accured to me that I might see higher amps during acceleration with heavier props, and I've not really seen any data from my own tests that points to this. But as Jim points out the possibility is there, a gentle increase should at least rule it out.

The 13x6.5 APC data is all the more odd if I compare it to what I got on the same props with my 750kv motrolfly DM2820:
846W
54.8A
15.46V
9570RPM

So less amps, less watts but more RPM. Is the Motrolfly that much more efficient

Steve
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Old May 31, 2012, 07:07 AM
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Originally Posted by JetPlaneFlyer View Post
Jim,

We are discussing different things I think which explains the different figures.

'KV efficiency' is AFAIK an abstract concept that I've never heard of anywhere else but this thread, it's not true efficiency.
Steve, any time we analyze the relationship between the measured voltage, the Kv of the motor, and a prop constant that considers its physical features, any variance of RPMs must be considered. If one test of an 820 Kv motor with a 13x6.5e APC shows 15.8v and 10,365 RPMs, and a second test of an 820 Kv motor with a 13x6.5e APC shows 15.4v and 10,355 RPMs, then we must account for the fact that the first was rotating at an 80% rate relative to an unloaded motor and the second was rotating at a rate of 82% relative to the unloaded system.

Without taking this into consideration, I would question any results that showed a variation. It can also help in explaining load if the same prop constant was used for these two tests result calculations, which it should have since they were the same prop specs.

Whether you've seen this before or not, ignoring this relationship skews the data unless we are using constants as variables, which is not the intent of the constants in the mechanical Watts formula. The reasons for the differences can be many, but as long as I account for it, I don't have to explain it any other way in any statistical analysis. I calculate this variable all the time to make sure the reported volts and Amps make sense. For example, the 13x4 makes no sense because the KvE is something like 62%, so one has to be off and it is probably the RPM reading. A 13x4 should spin much faster at those volts and the volts look okay.
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Old May 31, 2012, 07:21 AM
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Quote:
Originally Posted by JetPlaneFlyer View Post
The 13x6.5 APC data is all the more odd if I compare it to what I got on the same props with my 750kv motrolfly DM2820:
846W
54.8A
15.46V
9570RPM

So less amps, less watts but more RPM. Is the Motrolfly that much more efficient

Steve
If I used a 1.03 pK for your data using the 13x6.5e APC, then at 15.46v, 9570 RPMs is 82.53% of the RPM potential.

9570 RPMs = (750Kv x 15.46v) * 0.8253

Plug the pK and the RPMs into the mechanical Watts formula and you get 663 Watts-out.

Roger’s values on the 820Kv Torque:
APC 13x6.5 @ 9390 RPMs (72.4%) and 60A = 323W consumed and 20.4A
APC 13x6.5 @ 8850 RPMs (67.5%) and 56A = 371W consumed and 23.2A

Your values on the 750Kv Motrolfly:
APC 13x6.5 @ 9750 RPMs (82.5%) and 54.8A = 183W consumed and 11.9A
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Old May 31, 2012, 08:49 AM
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Looking for the right size motor and ESC to run a 14x7 prop on my new EF Edge. I have the recommended setup on my EF Extra and want to try a different setup on my Edge.
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Old May 31, 2012, 09:28 AM
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Originally Posted by collinss View Post
Looking for the right size motor and ESC to run a 14x7 prop on my new EF Edge. I have the recommended setup on my EF Extra and want to try a different setup on my Edge.
In one month, Motrolfly is reintroducing their DM2820-680Kv motor. I have one -- actually, it's on my son's Edge -- and I think many here will agree that it is the best motor for the 14x7 prop at this time. Here's the video that should answer most questions.... wait for the first 50 seconds because there's a lot of wind, and the camera man (me) was having trouble following the plane. The pilot is obviously better at what he does than the cameraman.

http://www.youtube.com/watch?feature...&v=YvhrwrCEWfQ
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Old May 31, 2012, 09:35 AM
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Originally Posted by Aeroplayin View Post
In one month, Motrolfly is reintroducing their DM2820-680Kv motor. I have one -- actually, it's on my son's Edge -- and I think many here will agree that it is the best motor for the 14x7 prop at this time. Here's the video that should answer most questions.... wait for the first 50 seconds because there's a lot of wind, and the camera man (me) was having trouble following the plane. The pilot is obviously better at what he does than the cameraman.

http://www.youtube.com/watch?feature...&v=YvhrwrCEWfQ
Great, Thanks!
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Old May 31, 2012, 10:01 AM
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Originally Posted by Aeroplayin View Post
In one month, Motrolfly is reintroducing their DM2820-680Kv motor. I have one -- actually, it's on my son's Edge -- and I think many here will agree that it is the best motor for the 14x7 prop at this time.
Agreed. I have the 2820-750 (683kv) on my Edge with the 14x7 also. Amazing combo.
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Old May 31, 2012, 10:23 AM
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Originally Posted by -= GYRO =- View Post
Agreed. I have the 2820-750 (683kv) on my Edge with the 14x7 also. Amazing combo.
Brings up a good point... I think the motor will still be listed on the motor page as the DM2820-750 but the different Kv options will be listed in the chart like the other motor options are now, I think. Seem the 750 and the 960 are the only ones listed there now.

http://www.subsonicplanes.com/Motrolfly_Motors.html
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