I'm finishing up construction of my Gee Bee R3. wing span 62". weight 63 oz. Battery 6 cell  P28A 18650 2800mAh 35A. I have a 45 and 65 amp ECS. I have a BadAss 3520-650Kv motor. I was thinking of getting a 560 KV instead. What would the guys on Stunt Hanger think?

Quote from: Donald R Olson on December 20, 2025, 09:04:28 PM
I'm finishing up construction of my Gee Bee R3. wing span 62". weight 63 oz. Battery 6 cell  P28A 18650 2800mAh 35A. I have a 45 and 65 amp ECS. I have a BadAss 3520-650Kv motor. I was thinking of getting a 560 KV instead. What would the guys on Stunt Hanger think?

Why the lower KV?   I have a 650 and a 710 and I cannot say that I notice any difference.  Maybe you will get one of the real experts in this to chime in.

Ken

Looking at there prop data chart it looks like the lower KV can swing a bigger prop with the higher voltage without over amping. the

I'm using a 685 kv and a 720 kv motor.  Most guys now use the 710 kv BadAss.  I wouldn't go by that prop chart. 

The one thing the prop charts are good for is to give an idea of the max rpm for a particular motor/voltage setup for various prop sizes. What you need to look for is that at the rpm you want to fly in level flight has head room for the Kv rating of the motor. Generally, want 20 - 30% headroom.

Best,    DennisT

Thanks guys, I appreciate the advice. I was trying to come up with a way to figure out how to use the prop charts for CL. Thanks Dennis, the 20-30% makes total sense.

Not that the opinion of some unknown flier from Utah means anything, but my recommendation would be to not drop down to 560kv. The 3520 650 and 3515 710 are what many are using with LiIon. I use 3515 in my old plane and will try the 3520 650 in my new plane your size. I do have one plane using 3520 790. I use 6s with 30B and 28B cells in those planes.

One challenge for me using LiIon is finding something that can handle the voltage drop in the big demand maneuvers at the end of the pattern. When I build a plane that is very different in size or for my new twin, I use the charts to make sure my motor will still work at 3-3.1 volts when the batteries get lower. One of the changes I made when going from LiPo to LiIon, was changing the ESC cutoff voltage from 3.2 to 2.8. I learned the hard way when my motor, set at 3.2, cut off during a flight. I also use active timers, so I try and make sure there is enough rpm at 3v under load to add 1000+ for the active boost.

I switched a 575 sq inch 55 oz plane to LiIon when the 5S2200 LiPos I was using did not have enough capacity at lower altitude. I left in the same motor that ran great on the LiPos. The new set up it worked fine at 5000 ft, but at 1000 ft it sagged in the overhead and clover. It had plenty of capacity with 5s3000 batteries finishing at 3.65v. The 780kv motor was sagging when voltage dropped during later maneuvers. The 2820 780 was replaced by a 910kv I had in a similar sized plane I ran on 4s LiPo. The 780 does work on 6s. I tried it but don't want to add an extra 50 grams if not necessary.

I also tried the 3515 580 when they first became available. I tried it in my very big profile that consumes a lot of battery finishing around 3.4-3.5v with a 3515 710. With the 580kv I had the same issue of sagging in late maneuvers. It did not last long in that plane that worked fine with 710 kv. I am now running 3520 790 and have no issues even with low finishing voltage. After a crash repair to rear fuse, I also needed more weight in the nose.

Igor uses LiPo but said if he went to LiIon he would use higher KV to avoid the sag.

I now look at the next lower cell count for what rpm the motor will be able to produce. Yellow is fine when including the boost since it doesn't last that long. If Im using 6S, I look at the chart for 5S and see how the motor will perform. The 6S chart doesn't mean much for me since the esc/timer is restricting rpm lower than maximum. Often it is quite a bit lower. If I want to use 5s, I look at 4s charts. For my twin, after running on 6s and 5s with different motors, I want to try 4S so I look at 3s to see what may work.

Page

Quote from: Page Peterson on December 21, 2025, 01:00:45 PM

Igor uses LiPo but said if he went to LiIon he would use higher KV to avoid the sag.

I have been trying to figure out how to do that since switching to Li-Ion.  The old wives told me that a lower KV was better for stunt since it has more torque at a given RPM.  I am starting to question them.  Can any body explain the relatonship between KV and battery consumption.

Ken

Quote from: Ken Culbertson on December 21, 2025, 03:52:27 PM
I have been trying to figure out how to do that since switching to Li-Ion.  The old wives told me that a lower KV was better for stunt since it has more torque at a given RPM.  I am starting to question them.  Can any body explain the relatonship between KV and battery consumption.

Ken

Sure.  KV = RPM per volt.  So a high KV motor needs and wants a tiny prop.  Under-revving a motor drives the amps & watts through the roof and depletes the battery.

What Page said about having adequate head room makes a lot of sense.  For efficiency you want to pick a prop which will run at about 80% of the max RPM that the motor can deliver for a given voltage (Kv x Pack voltage).  I think that many will run the calculation based on a freshly charged pack which is not what the battery will be able to deliver half way through the flight.  Running out of head room simply means that the ESC or timer can no longer govern the RPM because the voltage has dropped to the point where the motor cannot deliver the RPM. I fly regular lithium packs and I normally run the calculations assuming a pack voltage of 3.3 volts per cell or so under load (freshly charged:  4.20 v per cell).

The other thing that matters quite a bit is prop selection and the RPMs you use. Not all props are created equal.  I will probably get some flack for saying this, but in my experience (for a non carbon prop) XOAR is best, baddass is OK., falcon is not great and APC's stink.... Over this past summer I ended up testing a whole bunch of props on two of my models and that is what the data suggested.  The test was pretty simple, set the RPM so that I would get the same lap time as the current prop I was using and then go fly the pattern. I normally kept the diameter but varied the pitch depending on what was available for that particular brand. It would normally take a couple of flights to set the lap time and then I would fly the sequence on two separate follow on flights.  I also made a note of how the model felt during the flight.  My packs are numbered so it was easy to keep track of which pack flew which flight and they are also the same age.  Since they are numbered, I try to balance their use (the all accumulate cycles at roughly the same rate).  Then at home I would charge the packs and see how much energy I put back in.

I saw variations in consumption ranging from about 5% to almost 10% depending on the prop used.  XOAR always came ahead and APC always last.  I have also played with larger props (say 13" on a .40 sized model) and they also appear to be more efficient than a smaller prop.  It did not seem to affect the corners but then again, I am just an average pilot. I do not use an active timer, just a CC in governor mode while being careful with the settings.  I have found that using the higher switching frequency rates seems to help.

Good luck

Bill,
Interesting observation on the different props. It would be interesting to see how props of the same pitch set for a specific rpm performed then see how they did when the same props were set up for the same lap time. I haven't been using wood props on my electric's because they can stop with the prop vertical and can easily break them, I have used the APC's, MAS. The MAS's are quite flexibles (like the old TopFlite nylons) and so are the APC grays. APC introduced a line of black props that seem to be 20% stiffer than the MAS and old grays. Also, there is a difference between the flat back props and the electric undercamber have you tried both?

I also use the simple fixed rpm mode in the Castle ESC's with all rpm sections set to the desired rpm. I use a simple Zton timer (set at full power) to set the motor start delay and flight time. You mentioned that you increased the frequency - to what level (I have been using the default 8hz)? I understand that the outrunner auto setting uses the 8hz as the base but increase it as needed although I don't know what triggers the increase. Does this give extra pull through the top part of the maneuvers?

Best,   DennisT

Dennis,

I just took a quick look and on my 40 sized model I am running with 16khz PWM.  What I discovered was that some motors tended to screetch a little on the 8 Khz setting making me think that the prop was unbalanced. I had bench tested the motor before putting it on the plane and it did not make the noise.  It took me a little while to figure out that the PWM setting I used on the bench test was 12Khz rather than the 8 Khz used on the plane (I fly R/C as well so I had a bunch of controllers sitting around so I have one dedicated for bench testing).  How I ended up using 16khz on the latest set-up escapes my memory.  A couple of my planes used the 50 amp esc which allowed for data logging and while maneuvering it was not unusual to see the current spike by about 30-40%.  Although the controller is trying to hold the RPM steady as the airspeed changed (say due to going "vertical") the power delivered increased by more than I imagined.

My experience has been that wood props tend to be underpitched; what I mean by this is that the RPM needed to achieve the desired lap time is much less than what I would calculate just based on prop pitch.  I do not have a pitch gauge so I do not know for certain but suspect that with the wooden blade being a little thicker the blade airfoil becomes more important. I try to run lower pitch props (say 4" to 5") so I was not always able to get a one for one comparison across the different brands.  XOAR props are limited to 12K RPM and on lines longer than 60' (center to center) I have to run 5" pitch not to over RPM the prop.  My set-up mimics a tuned pipe set-up in that I try to use a lower pitch running at higher RPM. My impression is that this helps keep the plane from over speeding on the downlines. 

I use a simple timer just as an on and off switch and rely on programing the ESC to achieve what I want.  Compared to when I used to fly glow, this set-up is much better but probably not as good as using an active timer.  Given my skill level I would say that it is good enough.

I hope this helps.

Bill,
Thanks for the information. I have had a motor/prop combo that did screech a bit on take-up then settled, wonder if it was the 8hz PWM. Castle has an "Outrunner" option on the PWM that I am told starts a 8 but moves it up as needed could this work?

What did you change the PWM to that stopped the screeching?

Best,   DennisT

Coming back to the "what do I buy" question since I am about to buy another motor.  I am plagued with the sag in battery in the clover.  I am running the 3515-790 and 3520-710 motors.  The 3515 has less sag than the 3522.  I am running 2800 6s Li-Ion batteries.  It usually finishes with an average cell count of 3.1.  I can run the 3515 on a 2800 5s Lipo with no sag but not with the head room you need with a Lipo. Would going to the 880 or 970KV motors as Igor hints at perhaps be the right way to go?  I could just buy 2 Tempest motors and try both for the same price.

While the Tempest motors cost less, they are also less efficient. Lower efficiency isn't going to improve average current or sag and may not give you the answers you're looking for.

Learned some today.  After conferring with Inov8tive Designs I ordered the 3520-650.  What I learned was that the 880 would have to be choked down nearly 50% to be happy and that my old standby the 710 was really a 5s motor.  Another trip back from the ledge.

Ken

Ken, I know you have some 5S 45B or 50B packs from your twin. The 710 is very happy with those packs. I had a big amp draw heat issue with 6S 30B packs and 710 kv. I needed some extra nose weight due to some back half repairs, so I went to 3520 790. It ran well, but the packs got even hotter due to higher amp draw. I have a 650 for the plane I am working on, but have not tried it. I went to the heavier 5S 45B and 50B with the lighter 710 and now nothing gets hot and plenty of power / head room. This is a 760 sq in plane on 70ft lines doing 5.3 lap times.

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