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Sep 13, 2020, 02:58 PM
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Mismatched Servo Travel


Hi guys,
just finishing up my latest build which is coming in on the heavy side. Iíd planned to use Emax ES3302ís all-round but as the ounces piled on I decided to opt for Corona DS-239MGís on flaps. I havenít stripped many flap servos in the past, even though I always fail to retract them on touch down, but I hoped this might spare me some grief in the future.

Hereís the issue, the end points on the servos are completely out of sync. Iíve fixed this by adjusting the end points of each servo individually (the 2 Coronas have different EP as well). With a lot of fiddling, Iíve gotten to the point where the servos work in unison when coupled in normal flight and still achieve similar crow angles on landing.

Is this the way itís usually done? Just seems messy. In retrospect, Iíve realized a similar result could have been achieved by adjusting the flap to aileron mix. In future Iíll just stick with one brand of servo. Perhaps even venture into the ďrarifiedĒ air of KST or such Ö. Yes Iím cheap or rather can justify the expense based on my limited flying skills. Any suggestions, should I retrace my steps and change the setup? Thereís time I havenít maidened yet as the winds probably wonít pick up till October.
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Sep 13, 2020, 04:25 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by crashbuilder
...........................
Hereís the issue, the end points on the servos are completely out of sync. Iíve fixed this by adjusting the end points of each servo individually (the 2 Coronas have different EP as well). ............................
It's not quite clear to me what you mean here. Do you mean that the angles of rotation of a similar pair of servos (in this case Corona) are different to each other?
Or that your control surfaces are arriving at differing deflections for a given tx. command signal?

Quote:
flap to aileron mix. In future Iíll just stick with one brand of servo................................
Do you not mean the reverse? That is: aileron to flap mix? Or are you refering here to butterfly mix? (otherwise known as Crow).
The fact that you mention this as a possible "fix" suggests that the problem is not the servos delivering differing angular rotation for a given signal, but what happens after the servo. (ie: your mechanical linkage set up).

Jim.
Sep 13, 2020, 05:30 PM
Registered User
Thread OP
Quote:
Do you mean that the angles of rotation of a similar pair of servos (in this case Corona) are different to each other?
Yep, and even though both the Coronas and the E-Max are rated at 40 degrees the Coronas seem to have more throw ..... Just went back and checked the Corona horn is about 0.5 mm longer than the E-Max.

Quote:
Do you not mean the reverse? That is: aileron to flap mix? Or are you refering here to butterfly mix?
Yep, if I remember correctly, on the DX8 you do a aileron to flap mix at 100% to achieve full span ailerons and camber.

It all seems to work, Crow, differential, snap flaps and all 3 flight modes. I just ended up doing it this way because I had to adjust end points to ease aileron binding in crow mode. So I just kept going ...
Sep 13, 2020, 06:45 PM
Registered User
mlachow's Avatar
Those problems are probably a combination of the angle the servo is at neutral, horn angles, and horn lengths. Left to right side differences can show up this way.

What you need to look for in your transmitter is the Balance mix. That let's you match up left and right side travel with a curve. Your solution is not just endpoints
Sep 13, 2020, 08:11 PM
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Thread OP
I’ll take another look at the manual. I got the impression that balance was for multiple servos driving the same surface. Like I said there’s time. The end point fix seems to work .... the flap and aileron stay in alignment though the entire arc.
Sep 13, 2020, 10:12 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by crashbuilder
Yep, and even though both the Coronas and the E-Max are rated at 40 degrees the Coronas seem to have more throw ..... Just went back and checked the Corona horn is about 0.5 mm longer than the E-Max.
..................................
Your have refered to degrees of rotation and length of servo horns in the same answer.
The confusion continues.

When communicating via this medium, it is important to take care with specifics.
To properly answer my previous question, you would need to place two servos of the same make and model along side each other, symmetrically oriented, then operate them and compare the angles of deflection/rotation.
Ignore the servo length. This has nothing to do with angle of rotation.
Have you done that?
Sep 14, 2020, 02:20 PM
Registered User
Thread OP
Thank you all for your time and effort.
Before Tx programing begins in earnest, I attempt to do the following

1. Zero trim servos
2. Set end points to 100 (default)
3. Install servo arms and sub trim them to appropriate angle (matching that angle for corresponding servos)
4. Install identical length control arms on corresponding flight surfaces
5. Ensure that said arms are equidistant from hinge point.

Notwithstanding human error, from this point on significant deviation in flight surface deflection can be attributed to servo rotation …. Yes?

Perhaps I should restate the question. If your physical flap and aileron servo setup (on a given wing) results in differing control surface deflection how would you compensate (at the TX) to achieve synchronized full span ailerons?

Or is this even desirable? I’ve seen setup notes that have ailerons deflecting less than flaps in launch mode.
Sep 14, 2020, 04:38 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by crashbuilder
........................If your physical flap and aileron servo setup (on a given wing) results in differing control surface deflection how would you compensate (at the TX) to achieve synchronized full span ailerons? ...............
I question the underlying premise. What do you mean by "full span ailerons"? Do you mean they both operate in equal deflections when aileron commands are provided?
If so, you are not setting them up for optimum effect in the roll axis. Doing so, obviates one of the benefits of having both control surfaces installed and not just ailerons.
To do so, you need the flaps to deflect around 30% to 50% aileron deflection.

Quote:
Or is this even desirable? ...................................
No. See above explanation.
Sep 14, 2020, 06:09 PM
Registered User
Thread OP
I guess we all approach things differently. I find it quite useful to couple flaps and ailerons for:
Max roll response
Full span camber - when lift is iffy
Full span reflex for increased speed when lift is good

The setup I’m using (courtesy of Speedster Den) decouples flaps and ailerons for:
Snap flaps, mine is set more for loops than banging turns - kicks in after 50%
Crow on landing - opposing flaps and ailerons

Like I said to each their own, still curious about how people approach syncing flaps and ailerons when using different servos for each function.
Sep 14, 2020, 06:42 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by crashbuilder
I guess we all approach things differently. I find it quite useful to couple flaps and ailerons for:
Max roll response.......................
The irony is that you will achieve "Max roll response" by mixing ail to flap as I described and not by setting them up to obtain the same deflection when aileron commands are sent.

Quote:
Full span camber - when lift is iffy
Full span reflex for increased speed when lift is good................
These are achieved via different mixes and have nothing to do with what I have suggested.

Quote:
....................... how people approach syncing flaps and ailerons when using different servos for each function...................
This question is still not clear. It's as garbled as ever. We do not appear to be communicating!

Edit: I am about to unsubscribe.
Sep 15, 2020, 12:13 AM
DS Junkie
Screamin' Eagle's Avatar
I can't say I'm totally clear on the question, but I will tell you that when wing servos don't match or have the same amount of travel, chaos will ensue. I mean they don't match out of the box. Three best answer is not the one you want to hear, which is replace the wing servos so they all match. Otherwise, while it is possible to maybe fix it all in the radio, you will wish you hadn't.
Sep 15, 2020, 01:54 AM
F3Foamie Pilot
Quote:
Originally Posted by Screamin' Eagle
I can't say I'm totally clear on the question, but I will tell you that when wing servos don't match or have the same amount of travel, chaos will ensue. I mean they don't match out of the box. Three best answer is not the one you want to hear, which is replace the wing servos so they all match. Otherwise, while it is possible to maybe fix it all in the radio, you will wish you hadn't.
+1

Remember the "KISS" principle.
Sep 15, 2020, 02:04 AM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by Screamin' Eagle
...................... replace the wing servos so they all match. ..........................
My understanding is that as long as they are fitted in matched pairs, all is ok.
I do this and commonly fit a matched pair of stronger, tougher servos to the flaps, and less powerful (matched pairs) in the ailerons.
I don't see a problem with this.
Correct or not?
Sep 15, 2020, 02:33 AM
Registered User
Not sure if this is what you mean but,

If you’re trying to get the values on the monitor screen to match, don’t.

As long as mechanically the surfaces move the same through their travel left/right, and the max throws are where you want them, then you have what you desire.

There’s also no balance feature on the DX8, the DX9 has it.
Last edited by i_am_mark_evans; Sep 15, 2020 at 02:45 AM.
Sep 15, 2020, 09:32 AM
DS Junkie
Screamin' Eagle's Avatar
Quote:
Originally Posted by Jim.Thompson
My understanding is that as long as they are fitted in matched pairs, all is ok.
I do this and commonly fit a matched pair of stronger, tougher servos to the flaps, and less powerful (matched pairs) in the ailerons.
I don't see a problem with this.
Correct or not?
Matched pairs are fine. I don't mix flaps to ailerons on my planes for various reasons but given that the flap geometry is going to be different anyway, as long as both aileron and both flap servos are the same that will eliminate a lot of heartache.


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