P-38 Lightning Brotherhood
06-27-2015, 01:39 PM
#1102
Hi Chris!
I have an engineering type question. I'm still not understanding the relationship between Kv, volts, rpm and thrust.
I get Kv being rpm per volt.
For my question, let's use Eflite 90/325's. Assuming correctly sized props, which will give more power/thrust:
8s with a smaller prop/higher rpm or 6s and a larger prop/lower rpm? Or am I still completely off the trail?
Thanks in advance!
I have an engineering type question. I'm still not understanding the relationship between Kv, volts, rpm and thrust.
I get Kv being rpm per volt.
For my question, let's use Eflite 90/325's. Assuming correctly sized props, which will give more power/thrust:
8s with a smaller prop/higher rpm or 6s and a larger prop/lower rpm? Or am I still completely off the trail?
Thanks in advance!
That said, yes KV is RPM/Volt but there are many ways to get to the same result power wise.
In your set up your trying to balance the Power Triangle between the Motor ESC and Battery Pack.
My personal target RPM set up is 11,000RPM because I compensate for the systems deficiencies with an assumed 20% loss. So doing the math I find the right size motor for torque and the right KV that gets us as close to 11K as possible for whatever voltage it's rated to and space allowance in the model. After that the ESC is pretty obvious. Then in use I find it's usually as expected and there's room to play with props.
Brushless motor torque comes from two things primarily; length and diameter. For an in-runner the longer the magnets the more torque it can generate. For an out runner the larger the diameter the more torque it can generate. Add length or diameter to either within its model class and the same is true; OR motor with longer magnets makes more torque than its shorter version for the same diameter.
Then theres KV to Torque. Lower KV makes more torque than higher KV. High KV motors draw more Amps because they lack torque typically unless the physical design of the motor generates mechanical torque thru a gearbox or its inherent design.
I designed a large OR boat motor for a 55in catamaran that was both large in diameter and long with high enough KV to reach 30KRPM on 10S. It's a monster motor and can sustain high loads as expected for boat use. It would run at over 3KW no problem at low temps and low Amps but the boat would hit speeds of 50mph.
Thrust comes from RPM, diameter, and pitch. You can reach the same speed in different combinations of the three. It's model dependent and power system limited. For a given RPM you can really affect the Amp load by prop diameter or pitch. Change one at a time and see on a bench what the numbers are then try them in the plane. It's going to change slightly in the air and by weather on the day you fly due to Altitude Density. At higher AD conditions sometimes you have to change props to get more bite but watch the loads and temps.
Shoot for lower Amps for lower temps in your set ups and you can't go wrong.
What plane are you setting up is it a P38?
06-27-2015, 01:59 PM
#1103
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Wow! Thank you Chris!
This will take awhile to absorb!
My setup is Eflite 90's with Castle Edge 100's and 8s batteries. I have this for both my Top Flite Corsair 60 and VQ Lightning. Neither are completed, but just need to install the retracts in the Corsair and it's ready to fly. The Corsair's AUW is 11lbs. I'm hoping to keep the Lightning to about 19.5lbs.
I fly from about a 300' grass field. My idea was to have extra power to get it airborne, then to throttle back for cruise. My goal is to fly scale-like, rather than for speed.
This will take awhile to absorb!
My setup is Eflite 90's with Castle Edge 100's and 8s batteries. I have this for both my Top Flite Corsair 60 and VQ Lightning. Neither are completed, but just need to install the retracts in the Corsair and it's ready to fly. The Corsair's AUW is 11lbs. I'm hoping to keep the Lightning to about 19.5lbs.
I fly from about a 300' grass field. My idea was to have extra power to get it airborne, then to throttle back for cruise. My goal is to fly scale-like, rather than for speed.
06-29-2015, 01:22 PM
#1104
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[QUOTE=Chris Nicastro;12062129]In electric power systems for RC voltage is king and more efficient. So go with the appropriate diameter and try differnt pitched pops to see the load live on a Watt meter. If you don't have one you must get one, it should be required! At the power your testing at its too dangerous not to. We have one!
In your set up your trying to balance the Power Triangle between the Motor ESC and Battery Pack. - The Eflite 90's are rated at 50a/65a burst. So I'm thinking I'm pretty safe on the esc's. How important is battery C rating in this triangle?
In your set up your trying to balance the Power Triangle between the Motor ESC and Battery Pack. - The Eflite 90's are rated at 50a/65a burst. So I'm thinking I'm pretty safe on the esc's. How important is battery C rating in this triangle?
06-29-2015, 08:52 PM
#1106
[QUOTE=sudstad;12063119]
C rating is just the batteries ability to deliver max voltage under load. Its also a way to try and get the most voltage instantaneously under load but thats for a racing application where every tenth of a volt counts. Typically you dont need very high C rates on regular planes. Anything in the 25-35C is fine and cheaper! Helicopters flying 3D need high C rates to maintain head speed under high loads and rapid pitch changes. RC cars need high C rates to be competitive. Your scale models and sport planes do not.
C rate related to charging is also very important. Charge your packs at 1C, so 5000mah / 1000 = 5Amps
Also an interesting thing about packs capacity and C rate. You can acheive the same amount of power with different packs.
So for example if your tight on space or want more run time you want to understand these numbers.
Capacity divided by 1000 x C rate = max output.
Burst rate on packs or ESC's are only for about 30 seconds or less of use. Its not a value you want to tune to.
5500mah x 35C = 175A
3500mah x 15C = 52.5A
2500mah x 20C = 50A
1428mah x 35C = 50A
1000mah x 50C = 50A
This is just a basic break down of how the numbers interact so you can imagine the space and power the packs can supply.
Its better to have head room in your battery pack by about 20% over what you need so the pack is never maxed out to be safe. Its called a Margin of Safety. Same for the ESC, you want a bit more than the motor is rated for. With the packs available today its not nearly as critical as it used to be and the prices have come down but you need to know what your looking at.
To sum up; the motor maxed out cannot draw more than the ESC can handle. The ESC maxed out cannot draw more than the Battery can handle.
In electric power systems for RC voltage is king and more efficient. So go with the appropriate diameter and try differnt pitched pops to see the load live on a Watt meter. If you don't have one you must get one, it should be required! At the power your testing at its too dangerous not to. We have one!
In your set up your trying to balance the Power Triangle between the Motor ESC and Battery Pack. - The Eflite 90's are rated at 50a/65a burst. So I'm thinking I'm pretty safe on the esc's. How important is battery C rating in this triangle?
In your set up your trying to balance the Power Triangle between the Motor ESC and Battery Pack. - The Eflite 90's are rated at 50a/65a burst. So I'm thinking I'm pretty safe on the esc's. How important is battery C rating in this triangle?
C rate related to charging is also very important. Charge your packs at 1C, so 5000mah / 1000 = 5Amps
Also an interesting thing about packs capacity and C rate. You can acheive the same amount of power with different packs.
So for example if your tight on space or want more run time you want to understand these numbers.
Capacity divided by 1000 x C rate = max output.
Burst rate on packs or ESC's are only for about 30 seconds or less of use. Its not a value you want to tune to.
5500mah x 35C = 175A
3500mah x 15C = 52.5A
2500mah x 20C = 50A
1428mah x 35C = 50A
1000mah x 50C = 50A
This is just a basic break down of how the numbers interact so you can imagine the space and power the packs can supply.
Its better to have head room in your battery pack by about 20% over what you need so the pack is never maxed out to be safe. Its called a Margin of Safety. Same for the ESC, you want a bit more than the motor is rated for. With the packs available today its not nearly as critical as it used to be and the prices have come down but you need to know what your looking at.
To sum up; the motor maxed out cannot draw more than the ESC can handle. The ESC maxed out cannot draw more than the Battery can handle.
06-30-2015, 05:52 AM
#1107
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Thank you again for the great info/insight!
In theory, I think my setup is in the ballpark. Testing will tell the tail.
EDIT:
Eflite 90 - 50a cont, 65a burst
CC Edge - 100a
4000mah/30c - 120a?
Without having done any testing at all... it would "seem" to be a safe setup... unless I over-prop.
Something that I and others have noticed is there isn't much power difference between around 60% throttle and WFO. Is that normal for electrics or an indication of something out of balance... motor/esc/battery/prop?
PS - Looking forward to your video!
In theory, I think my setup is in the ballpark. Testing will tell the tail.
EDIT:
Eflite 90 - 50a cont, 65a burst
CC Edge - 100a
4000mah/30c - 120a?
Without having done any testing at all... it would "seem" to be a safe setup... unless I over-prop.
Something that I and others have noticed is there isn't much power difference between around 60% throttle and WFO. Is that normal for electrics or an indication of something out of balance... motor/esc/battery/prop?
PS - Looking forward to your video!
Last edited by sudstad; 06-30-2015 at 09:18 AM.
06-30-2015, 10:53 PM
#1108
If you see no difference then it could be the load is too light. Check the amps. It could be reaching full load early in the TH range. Add more load if you can until you reach 50A.
Be sure your EPA and ESC are 100% so you know for sure they are linear and equal.
With your Watt meter you should see some good info. For best results use a Data Logger so you get more data points. Thats more accurate and will expose the batteries true performance as well as the whole system.
Watch the Wattmeter and the motor temp so that the load is about 50A on the bench and the max motor temp is under 180F. Any hotter and you risk the motor. In flight it will cool better. On the bench is the hardest condition typically.
Yes 120A max amps.
Temps are a big thing to watch for.
ESC - 140F max
Battery Pack - 120F max running temp, never hotter while under load
Motor - 180F max
The system will exotherm in a manner called Heatsoak after the system is shut down. Thats why in your set up its important to know operating temps because when you shut it down the temps go up before they go down. Its more dangerous for the battery packs than the rest. A hot running pack can over heat and fail after its run.
Your set up on paper is OK, just try props and get to know what effects what.
Me too, I need to talk with Bob and see if he has posted it to YouTube
Be sure your EPA and ESC are 100% so you know for sure they are linear and equal.
With your Watt meter you should see some good info. For best results use a Data Logger so you get more data points. Thats more accurate and will expose the batteries true performance as well as the whole system.
Watch the Wattmeter and the motor temp so that the load is about 50A on the bench and the max motor temp is under 180F. Any hotter and you risk the motor. In flight it will cool better. On the bench is the hardest condition typically.
Yes 120A max amps.
Temps are a big thing to watch for.
ESC - 140F max
Battery Pack - 120F max running temp, never hotter while under load
Motor - 180F max
The system will exotherm in a manner called Heatsoak after the system is shut down. Thats why in your set up its important to know operating temps because when you shut it down the temps go up before they go down. Its more dangerous for the battery packs than the rest. A hot running pack can over heat and fail after its run.
Your set up on paper is OK, just try props and get to know what effects what.
Me too, I need to talk with Bob and see if he has posted it to YouTube
07-01-2015, 07:13 PM
#1111
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So much to learn. Luckily, that's part of the fun!
07-01-2015, 07:15 PM
#1112
I had no idea what EPA was either...LOL
I would do fixed end points, instead of auto calibrate. Really quick and easy to do. With auto cal, I have had the ESC reach full throttle with only 3/4 stick input many times. It will act as you described earlier....throttle does not have any effect above a certain point.
I would do fixed end points, instead of auto calibrate. Really quick and easy to do. With auto cal, I have had the ESC reach full throttle with only 3/4 stick input many times. It will act as you described earlier....throttle does not have any effect above a certain point.
07-02-2015, 06:00 AM
#1114
There is something us twin fliers absolutely should do that I know is counter intuitive for most guys, myself included, and that is read the instructions that come with the ESC.
There are a lot of potential setups but most will have two ESC's and trust me, identical ESC's can come from the factory with different characteristics. In my admittedly short and limited experience with electric twins I have run into this several times already and in one case on a setup that had been flying fine and something caused a change in one of the ESC's.
Point being, it is not hard to do by following the steps laid out in the instructions (I should probably mention I fly Turnigy Plush) and I was floored by the difference it made in the throttle response mentioned above. Among other things the process sets/matches the end points (to use an example also from a previous post) on your throttle stick. It was the first thing I thought of when Sudstad posted
Something that I and others have noticed is there isn't much power difference between around 60% throttle and WFO. Is that normal for electrics or an indication of something out of balance... motor/esc/battery/prop?
Chris answered with an absolutely appropriate pitch suggestion but as we all know rarely is there only one variable to consider.
Another two cents.... setting the end points on the ESC,s helps ensure the props start at the same time and at the same rpm.
I hope I helped
There are a lot of potential setups but most will have two ESC's and trust me, identical ESC's can come from the factory with different characteristics. In my admittedly short and limited experience with electric twins I have run into this several times already and in one case on a setup that had been flying fine and something caused a change in one of the ESC's.
Point being, it is not hard to do by following the steps laid out in the instructions (I should probably mention I fly Turnigy Plush) and I was floored by the difference it made in the throttle response mentioned above. Among other things the process sets/matches the end points (to use an example also from a previous post) on your throttle stick. It was the first thing I thought of when Sudstad posted
Something that I and others have noticed is there isn't much power difference between around 60% throttle and WFO. Is that normal for electrics or an indication of something out of balance... motor/esc/battery/prop?
Chris answered with an absolutely appropriate pitch suggestion but as we all know rarely is there only one variable to consider.
Another two cents.... setting the end points on the ESC,s helps ensure the props start at the same time and at the same rpm.
I hope I helped
07-02-2015, 08:42 PM
#1116
One detail about ESC and TH stick relationships on auto calibrate is that the full range of steps or resolution built into the ESC is spread out thru the full stick range. It makes it smoother in my opinion.
As for the difference in output vs stick position at the prop its true there are a few variables. The Wattmeter or data log is where to look for answers. You must know the numbers including temps to know where to push and adjust.
In a perfect set up world there are three things that come together at the same time when running.
Max power is acheived, max temp is maintained under control and max run time is reached all at the same time. This is harder than it sounds but its an interesting approach to getting your set up right.
As for the difference in output vs stick position at the prop its true there are a few variables. The Wattmeter or data log is where to look for answers. You must know the numbers including temps to know where to push and adjust.
In a perfect set up world there are three things that come together at the same time when running.
Max power is acheived, max temp is maintained under control and max run time is reached all at the same time. This is harder than it sounds but its an interesting approach to getting your set up right.
07-03-2015, 07:08 AM
#1120
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Clearly, I need to do some research/reading on both the transmitter and esc's to understand how to best set them up and to match the esc's!
07-03-2015, 05:43 PM
#1121
Did the maiden, and a 2nd flight on the Yellow rebuild today. Flew great, zero trim needed. Two year project is coming to an end, time to fly it
https://www.youtube.com/watch?v=IbuZPSLzAQs https://www.youtube.com/watch?v=q1mUG8XEMjA https://www.youtube.com/watch?v=RWD4t6kMIJA
https://www.youtube.com/watch?v=IbuZPSLzAQs https://www.youtube.com/watch?v=q1mUG8XEMjA https://www.youtube.com/watch?v=RWD4t6kMIJA
07-04-2015, 04:37 AM
#1123
Thanks Chris
Still waiting on your video...LOL!
I used elastic thread and fishing hooks, just cut hook part off. Drilled a couple small holes in top of verticals, and they just slip in (no glue). 3rd hook passes thru back on canopy frame, and is bend over and CA'd inside (see pic). I didn't go too tight, as wires tend to get pushed around when moving the plane
I only got one picture, but one of the guys posted a couple on the other site.
Still waiting on your video...LOL!
I used elastic thread and fishing hooks, just cut hook part off. Drilled a couple small holes in top of verticals, and they just slip in (no glue). 3rd hook passes thru back on canopy frame, and is bend over and CA'd inside (see pic). I didn't go too tight, as wires tend to get pushed around when moving the plane
I only got one picture, but one of the guys posted a couple on the other site.
Last edited by 70 ragtop; 07-04-2015 at 04:40 AM.
07-04-2015, 01:12 PM
#1124
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