Prop Talk, APC's 14 vs 15s flight performance.

[maxwelltub]

Hello er body, have a few questions about APC props. I am flying a Carbon Core 950 Hex with 6s power, Sunny Sky 320kv motors and Zenmuse gimbal. I am on the quest for some good props so I am kind of running the gamut through some popular brands until I find the right ones for me.

Looking at the motor efficiency and thrust chart it looks like the APC 15x4s should be a good combo. However I found them to turn my hexa into quite a jumpy machine, almost as if I was flying in manual even though I was flying in Alti mode. I thought maybe my z axis had been off, or my gains needed adjusting, but after messing around for a little bit I just switched props to the 14x4.7 and it flew much nicer.

So was this an issue of gains on my WKM? If I dial in the gains more will I get a better result?

Ultimately I would like to switch to wood xoars. I flew the 14's today and they were nice with dead wind, but kind of felt like I was driving on flat tires with the amount of flex to them once it got a little gusty around mid day, plus the high 90 degree weather didn't help. I don't really want to invest in xoars until I have a good prop and pitch combo. This may or may not be a flawed rational but its seems to make sense.

Any insight is welcome, maybe I am not understanding the chart on the motor page, maybe I need to spend more time dialing in the gains to get the 15s to work properly.

 

[Electro 2]

So was this an issue of gains on my WKM? If I dial in the gains more will I get a better result?

I don't really want to invest in xoars until I have a good prop and pitch combo. This may or may not be a flawed rational but its seems to make sense.

I think it very much is. My experience is that one brand of a particular size doesn't equal another brand of the same size. Gains and such have to be tweaked for optimal performance with each.

 

[maxwelltub]

I guess I should say I definitely toyed with the gains for about 10-15 minutes with my TX, but I guess I just need to speed a little more time with it. And thanks for the tip with the prop size vs different brads. I kind of figure that was the case, but I was hoping to figure out if I should use 15s or 14s. I guess I just need to mess with it some more next week.

 

[Electro 2]

I guess I should say I definitely toyed with the gains for about 10-15 minutes with my TX, but I guess I just need to speed a little more time with it.

Ha! I've been tweaking PIDs for about 3 months on a new build, over a 100 flights. And, am just now getting to tuning the automated functions (HH, AH, PH, RTH). "A little more time", is an understatement if you want it/them dead-on.

 

[maxwelltub]

Haha, well I've been flying Helis and MR for a little bit so I know what you mean about the endless tuning. What I am talking about is the the hexa being way out of wack with the 15"s, and rotating the gain knob was having little to no effect on the symptoms. I thought my center if gravity measurements had somehow gotten messed up in the assistant software, that's the way it was acting. But it seems clear the general consensus is more tuning of the gain functions for altitude mode. Anything else to adjust in the electronics end of things? Thanks for the feed back

 

[Electro 2]

With MW the tuning "pecking order" is, in order, gyro P, gyro I, gyro D, accel P, accel I, accel D, mag hold P, mag hold I, mag hold D, alti hold P, alti hold I, alti hold D, pos hold P, pos hold I, pos hold D, Then the RTH items in the same order. Some of it is interactive ans will cause you to return to a previous setting and bend it again. Particularly true of the prime axes functions. I'm up to alti hold at present and haven't gotten to the pos hold or RTH stuff yet. Since RTH is 6 items, I'm expecting it to be tough, though not as squirmy as alti hold, which is proving to be a bugger. Since the automated functions all cascade on one another's correctness, it can get ugly. God forbid I should ever change props and have to do this all again. "No Mr. Bill, Noooo"

 

[maxwelltub]

Thanks for the reply. The wkm doesn't allow for adjusting those parameters as far as I know. The most you can dive into fine tuning is with motor mixing and gains on each channel. But I will look more deeply to see if there is a wkm order of tuning. Thanks again,

 

[Electro 2]

Motor mix is a function of frame type and isn't "tuned", the WKM provides preconstructed tables that are perfect for normal frames. The WKM and all closed source FCs reduce the adjustments available to the user to simplify setup. Far easier to deal with than an open-source FC which has a tremendously wide range of adjustments to accomodate all possible applications. Only a few things to tweak on the DJI stuff, and very small range to tweak them over. You'll be cool in a short period of time.

 

[PeterLester]

IMHO Of the PID parameters, the I and D for the WKM are contained in the advanced tuning section. P - proportional is the only parameter configurable from the Basic and Attitude tuning sections. And there are no options to tune a separate RTH PID loop.

Thanks for the reply. The wkm doesn't allow for adjusting those parameters as far as I know. The most you can dive into fine tuning is with motor mixing and gains on each channel. But I will look more deeply to see if there is a wkm order of tuning. Thanks again,

 

[maxwelltub]

So we have come full circle. In my original post I stated that I was getting very erratic behavior when I switched props on my wkm system. After adjusting the one parameter I have control over I was not able to improve the symptoms. I learned something about open source FC but not much about what I'm working with.
I have read in other post people have had to intentionally set z axis to different values to achieve the best performance. The way the MR is acting with the 15"s is that of a bad z axis.
So now I ask is it possible that changing props may require you to mess with z axis setting? I do understand that the CG does not change in reality, just change the settings in the FC?

 

[maxwelltub]

I think I may have figure it out and its a real dummy move, which is often the case when I have these brain busters, but non the less talking it out indirectly lead to the possible solution. I don't know exactly how much the bird ways but I would say between 10-15 pounds, I will check today when I get to my shop. I think that the 15"x4 props are quite simply not generating enough thrust at lower throttle levels. theoretically the thrust should be double the weight if I am not mistaken. That would explain the wonky instability if the motors are working too hard with not enough extra thrust to balance out the flight controls.

 

[Electro 2]

I think I may have figure it out and its a real dummy move, which is often the case when I have these brain busters, but non the less talking it out indirectly lead to the possible solution. I don't know exactly how much the bird ways but I would say between 10-15 pounds, I will check today when I get to my shop. I think that the 15"x4 props are quite simply not generating enough thrust at lower throttle levels. theoretically the thrust should be double the weight if I am not mistaken. That would explain the wonky instability if the motors are working too hard with not enough extra thrust to balance out the flight controls.

Yep, I agree. And, yes, 2-to-1 is a good thrust-to-weight ratio for a multi. If your conjecture is true, the flight controller doesn't have enough "headroom" to increase the lift when it needs to and this configuration won't ever be "OK". Should show up as an unusally high hover throttle point in manual mode, as well. With a fresh pack on board, right out of the gate, I hover at roughly 45% of full throttle. As the pack sinks over the flight duration, it ends up being about 55% at the end.