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Old Nov 27, 2011, 07:49 PM
They Call him Dead!
YellowJacketsRC's Avatar
United States, SC, Pawleys Island
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I understand, but if the motor is able to get overheated with WOT before you dial back you throttle end points, then that means you are going to be running your motor with less efficiency. The motor "wants" to spin at a certain rpm when power is on. All the esc is doing is switching power on and off very quickly (it is not dialing in more or less power with throttle, it just clicks on and off very quickly for fast and slowly for slow).

So, every time the esc clicks "ON" the motor tries to spin at a certain rpm as determined by the winding and the volts. If the motor is creating too many amps at wot on a 16x 8, that is a sign that the prop is too big for that motor. Dialing the wot back does keep the amps down, but the motor is still trying to spin at the rated rpm every time the esc switches on. This causes the motor to run less efficiently no matter where the throttle is. When a motor runs less efficiently, you are going to get more heat and less thrust per lipo pack. It will also cause more wear and tear on the motor over time regardless of you over all amps.

A 16 x 7 or 16 x 6 would give you the same or possibly better performance without the need for dialing back the throttle AND your motor would run more efficiently and last longer....
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Old Nov 27, 2011, 09:18 PM
Not as Good as The Kid
Aeroplayin's Avatar
South Pasadena, FL
Joined Sep 2009
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Well, this new wrinkle is something different than dialing back the endpoints thinking that it will reduce the current. As we know, the current is the same when the FET’s are open, and the variation in the Amp reading from part throttle to WOT is nothing more than a representation of the volume of current over time. Think of it this way:

I have 5 gallons of water in a gravity tank with a gate valve. In the first example, I open the valve for one second, close it for one second, open it for one second, then close is for one second. In the second example, I open the value for four seconds, and then close the valve.

In these examples, the gate valve represents the FET switches. The first example represents half throttle for four seconds and the second example represents WOT for four seconds. At WOT, twice the volume of water ran out of the tank, but the same amount of water was moving out of the tank during each example when the valve was open.

With this understanding, what Tom is doing by dialing back the throttle may be much different. I'm still working on this data and it may take a while considering this new issue, but here is what I'm thinking about:

[1] I have a motor that delivers almost 1600 Watts and 11.94 pounds of thrust on 65A with a 16x8 Xoar at about 83% throttle (5/6th throttle setting).
[2] At WOT with the 16x8 Xoar, it will deliver 1800 Watts and 12.32 pounds of thrust at about 82 Amps.
[3] 1600 Watts and 11.94 pounds of thrust on 65A is enough power for this plane and the 16x8 inch disk delivers the extra torque advantage at lower RPMs.

Question: If at 83% of my throttle setting, I am getting 100% of my power and torque requirements, is there any benefit to adding 17 Amps to the system if I'm only getting 6 more ounces of thrust?

My hypothesis is NO.

Now if you think the percentages are different with the 15x8 props, or from one motor to the other, they are not. All data samples seem pretty consistent when I run a relative analysis.

Tom is flying for 6.5 minutes on a 6S Gens Ace and putting back 75% of the battery’s capacity (2475mA) which is 380mA per minute or 0.38 Amp per minute or 22.8 Amps per hour.

A reading of 25.22 Amps is the average for half-throttle during the 15x8 and 16x8 Xoar tests, so it is safe to say that Tom is averaging about half throttle overall. There is no way I can think of, subjectively, that would qualify using the remaining 1/6th of the throttle curve considering this data, therefore dialing the inefficient part of the curve out should be of no negative consequence.
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Old Nov 27, 2011, 09:30 PM
VOLTS > AMPS
stgdz's Avatar
United States, MN, Buffalo
Joined Jul 2011
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AP what esc are you because I swapped a motor over this weekend that had a force esc on it before to my exp with airboss. I swear to god watching the motor o the airboss it looks and sounds like the airboss has a much slower acceleration then the k-force esc.

This is the K-force setup in a different plane
http://www.youtube.com/watch?v=u2lott-VDC0
This is the same motor as the k-force with the airboss esc. It seems like it takes a bit longer to ramp up to full speed, of course I didn't have it mounted in the same position tonight because I am a moron from time to time but I will see if I can get a video of the esc's later this week.
http://www.youtube.com/watch?v=tO2l8ZmJHZo
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Old Nov 27, 2011, 10:04 PM
Not as Good as The Kid
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South Pasadena, FL
Joined Sep 2009
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............. Good question. I actually had to take a 100A HV OPTO ESC out of an old plane I was running 8S on because I was still deciding on which one to use. Some guys were raising some questions about the Castle, which is the one I have in mind. A friend offered to let me try his .... when he gets it back from CC. In the meantime, I did have some timing issues with the K-Force, which is a Hobbywing equivalent.

I was thinking about the ZTW too, but wanted to go with the 100A OPTO, but for $9 more, I can also have the CC 100A. So while I was making up my mind, and waiting to try the CC, I took the Super Brain Turnigy 100A out of a Band-Z and it seems to do everything we want, including soften the throttle response a bit, which may have its advantages too. It's not cheap... $100, but if you never visited the product page on HK, they will give you their one-time offer for $89 and change. I'm not recommending them, just stating the facts... I knew it worked, ran it successfully for about 50 flights, tried it on the 60 inch, and it resolved the issues I had with the other off-brands while I decided between the CC and the ZTW. It's is not light... 4 oz. I think the ZTW is 2.8 oz in comparison.
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Old Nov 28, 2011, 10:50 AM
They Call him Dead!
YellowJacketsRC's Avatar
United States, SC, Pawleys Island
Joined Jul 2003
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AP. The point I am trying to make is that running the 16 x 8 and dialing it back is less efficient and causes the motor to work harder for the same amount of thrust as using the same diameter prop but with a lower pitch and allowing the rpms to go higher.

Your example of the water ignores one aspect of the equation. The motor's demand for affects the amp draw. The motor will ask for more power with a 16 x 8 than with a 16 x 7 at WOT. Also, and MOST IMPORTANTLY, the motor is designed to run at a certain RPM. If the prop is grabbing too much air, the motor will be working beyond its ability to turn the prop every time the fet switches on, REGARDLESS of throttle setting. The overall effect is that a lower pitch prop will give the same thrust without dialing back the throttle curve as the 16 x 8 when it is dialed back AND it will not cause the motor undue stress AND it will lose less mah over time due to the more efficient use of power.

One way to verify my assertion would be to run a 16 x 8 at 68 amps for 10 seconds and then measure to temp of the motor. Then run a 16 x 7 at 68 amps and measure the temp of the motor. My guess is that the motor will run at lower temp on the 16 x 7 in this case which means that less power is being lost to heat.

Also, with the higher rpms, you are going to get more centrifugal force which will help stabilize the plane in hover.
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Old Nov 28, 2011, 10:52 AM
They Call him Dead!
YellowJacketsRC's Avatar
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Also if my assertion is incorrect, then you would be able to run a 20 x 8 prop on the same motor by dialing the throttle back further. We all know that would not work well....
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Old Nov 28, 2011, 12:24 PM
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vienquach's Avatar
Chicago, IL
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Quote:
Originally Posted by dead View Post
AP. The point I am trying to make is that running the 16 x 8 and dialing it back is less efficient and causes the motor to work harder for the same amount of thrust as using the same diameter prop but with a lower pitch and allowing the rpms to go higher.

Your example of the water ignores one aspect of the equation. The motor's demand for affects the amp draw. The motor will ask for more power with a 16 x 8 than with a 16 x 7 at WOT. Also, and MOST IMPORTANTLY, the motor is designed to run at a certain RPM. If the prop is grabbing too much air, the motor will be working beyond its ability to turn the prop every time the fet switches on, REGARDLESS of throttle setting. The overall effect is that a lower pitch prop will give the same thrust without dialing back the throttle curve as the 16 x 8 when it is dialed back AND it will not cause the motor undue stress AND it will lose less mah over time due to the more efficient use of power.

One way to verify my assertion would be to run a 16 x 8 at 68 amps for 10 seconds and then measure to temp of the motor. Then run a 16 x 7 at 68 amps and measure the temp of the motor. My guess is that the motor will run at lower temp on the 16 x 7 in this case which means that less power is being lost to heat.

Also, with the higher rpms, you are going to get more centrifugal force which will help stabilize the plane in hover.
I think this is true.

On my setups, I try to avoid dialing back the throttle EP. Instead, I prop it wrt. motor spec and kv.

If you want to run a larger prop, it's better to choose a lower kv motor.
On 6s, 480-500kv is good for Xoar 15x8. 450-480 is better for Xoar 16x8.

Vien
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Old Nov 28, 2011, 12:29 PM
They Call him Dead!
YellowJacketsRC's Avatar
United States, SC, Pawleys Island
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Originally Posted by vienquach View Post
I think this is true.

On my setups, I try to avoid dialing back the throttle EP. Instead, I prop it wrt. motor spec and kv.

If you want to run a larger prop, it's better to choose a lower kv motor.
On 6s, 480-500kv is good for Xoar 15x8. 450-480 is better for Xoar 16x8.

Vien
Yup, or just use a lower pitch 16" prop. You are still going to get the same large area of prop wash and the same amount of air moving. But the rpms will be higher and the system will be more efficient.
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Old Nov 28, 2011, 02:27 PM
Facts, Logic, 3D
Tom K.'s Avatar
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Yeah... okay, that makes sense now. I didn't understand it at first. I get what you're saying... but the stuff comes down only slightly warm and the amps are within range and it's workin'.

--Tom K.
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Old Nov 28, 2011, 03:07 PM
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Originally Posted by Tom K. View Post
Yeah... okay, that makes sense now. I didn't understand it at first. I get what you're saying... but the stuff comes down only slightly warm and the amps are within range and it's workin'.

--Tom K.
It's still "working" but make sure your ESC can handle the amp at WOT.
Although you dial down the throttle EP, the amount of current that the ESC sees is still the same at WOT without EP reduced (pulsing).

Vien
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Old Nov 28, 2011, 03:51 PM
Not as Good as The Kid
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Quote:
Originally Posted by dead View Post
AP. The point I am trying to make is that running the 16 x 8 and dialing it back is less efficient and causes the motor to work harder for the same amount of thrust as using the same diameter prop but with a lower pitch and allowing the rpms to go higher.
Right. My testing now is to see if the efficiency curve will verify this. From what I am seeing, the top end adds 17 Amps but only 6 ounces of added thrust. If the performance curve is better on the lower pitch, we'll know soon because Frank handed me a 16x6 and a 16x7 Vox yesterday to test this. Thanks, Frank. The only comment I can make now is that the West Coast testing on the 15x8 (Thanks Lee) displays similar results from 50% throttle to 100%, so we’ll need to test the 15x8’s again at 5/6th settings (83%) to try for the linear results Vien suggested – which was a good one that moved this test to whole new level. Thanks Vien.

Quote:
Originally Posted by dead View Post
Your example of the water ignores one aspect of the equation. The motor's demand for affects the amp draw. The motor will ask for more power with a 16 x 8 than with a 16 x 7 at WOT.
Sure, but so far, we don't see much difference until we get up into the range where there is a clear drop in efficiency, and I'm not sure yet if this is any better with the smaller diameters or with less pitch. I have some graphs prepared that are just about ready to publish on the Web site, so please take a look and let me know what you think. I'll let you know when it's there.

Quote:
Originally Posted by dead View Post
Also, and MOST IMPORTANTLY, the motor is designed to run at a certain RPM. If the prop is grabbing too much air, the motor will be working beyond its ability to turn the prop every time the fet switches on, REGARDLESS of throttle setting. The overall effect is that a lower pitch prop will give the same thrust without dialing back the throttle curve as the 16 x 8 when it is dialed back AND it will not cause the motor undue stress AND it will lose less mah over time due to the more efficient use of power..
Right now, we're seeing no stress on the Motrolfly with the 16x8 because it will easily run up to 8400 RPMs at 80 Amps, 1800-plus Watts, and 13-plus pounds of thrust with no restrictions. We're not dialing back to protect the motor from the big prop because these readings display a 75% efficiency at WOT and 78 to 80 percent is what we'd expect with the 15x7. So not bad at all.

Quote:
Originally Posted by dead View Post
One way to verify my assertion would be to run a 16 x 8 at 68 amps for 10 seconds and then measure to temp of the motor. Then run a 16 x 7 at 68 amps and measure the temp of the motor. My guess is that the motor will run at lower temp on the 16 x 7 in this case which means that less power is being lost to heat.
Well the only problem with this is that Tom is running the 16x8 for 6.5 minutes and everything comes down cool, even when he runs the 25C battery -- which will find some limitations in the 82A range, while the 30C Sky will not see the same limitations until the 98-100A level. This tells me that the efficiency range is valid and we're not losing Amps to heat because of overloading the motor.

Quote:
Originally Posted by dead View Post
Also, with the higher rpms, you are going to get more centrifugal force which will help stabilize the plane in hover.
I agree but the gyroscopic precession is certainly a torque-induced effect, and the slipstream force is another one and I’d rather ignore these vectors at this point. We don’t seem to notice the left vector from precession when punching out of a hover so far, but I’ll make Tom aware of it so he can comment on any noticeable pull from sudden throttle shifts.
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Old Nov 28, 2011, 04:02 PM
Not as Good as The Kid
Aeroplayin's Avatar
South Pasadena, FL
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Quote:
Originally Posted by vienquach View Post
It's still "working" but make sure your ESC can handle the amp at WOT.
Although you dial down the throttle EP, the amount of current that the ESC sees is still the same at WOT without EP reduced (pulsing).

Vien
Right, and this is why we went to the 100A ESC instead of the 85A. The slight voltage sag verifies that there is little IR from overloading. I can get 82 Amps from the 3300 battery with a 30C rating and a 100A - 120A ESC so we're not missing any Amps sneaking into heat. Very low temps after 6.5 minutes of normal flights by Tom, which you can see on the video a page back or so. At one point, you can see him come in low, and snap to a hover, let the tail slide, then punch the throttle. The clip is not long enough to fully see the pullout, but you will get the idea…. it was awesome.
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Old Nov 28, 2011, 04:06 PM
They Call him Dead!
YellowJacketsRC's Avatar
United States, SC, Pawleys Island
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Ok,
So let's say you are not holding the throttle EP back due to overloading the motor. Let's say it is only because you don't need that last bit of amp draw because the plane has WAY more power than it needs.

I still assert that it would be better to use a lower pitch 16" prop at higher rpms to achieve the same thrust and amp readings. The point would be that it would be easier on the motor due to the KV rating and it would spool up faster and should give more punch out of a stand still hover.

Thoughts????
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Old Nov 28, 2011, 04:11 PM
Facts, Logic, 3D
Tom K.'s Avatar
Florida
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We're already getting 8200-8400 RPM's dialed back.
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Old Nov 28, 2011, 04:35 PM
Not as Good as The Kid
Aeroplayin's Avatar
South Pasadena, FL
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Quote:
Originally Posted by dead View Post
Ok,
So let's say you are not holding the throttle EP back due to overloading the motor. Let's say it is only because you don't need that last bit of amp draw because the plane has WAY more power than it needs.

I still assert that it would be better to use a lower pitch 16" prop at higher rpms to achieve the same thrust and amp readings. The point would be that it would be easier on the motor due to the KV rating and it would spool up faster and should give more punch out of a stand still hover.

Thoughts????
What will provide the definite evidence, one way or the other, is if the lower pitch straightens out the Amp and Thrust graph plots. As they are now, they go up sharply, then flatten out at the top. If the 16x6 shows relatively the same leveling off, then we're better off riding the 16x8 to 83 percent than the 16x6 to 83 percent. If the curve stays at virtually the same angle all the way to the top, then the 16x6 would be the better prop option. If it was not raining out today, I'd have put the 16x6 on it and reported back by now, but we'll all have to wait.
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