Servo Horns - they are so... 80's

[founder]

I don't know much about engineering, but maybe someone here does and can tell me why in these days we can't get a solid servo arm that will fit on a servo without the need for subtrim. Why do we have to fish through and find the "Neo" of servo horns?

Is it because it's really hard to make servos and horns that line up? Or is it because easier/better to just get as close as you can and subtrim it out?

Get with the times, Servo Horn!

Little side questions:
a) What are the ills of subtrim that makes it so unappealing to most?
b) How much sub-trim is too much (I have a DX7 and X9303 if you want to give numerics).

[CTG1129]

it all depends on the heli,on my 600 i got the align servo horns and they worked just fine no subtrim,cant say the same for the 450 or 500

[founder]

But is it because they always work, or did you luck-out? Will they line up on say... JR servo's, or Futaba?

[HydroJoe]

I think there is a reasonable answer.....All servos are different. Even like servos have different neutral positions as well as different throws. It's not like we are flying the space shuttle.....sometimes the tolerences arn't exact.

I have used up to 29 clicks on subtrim with no ill effects. That was after trying EVERY combination of horns to servos.

I can say that with the Align JR horn on my 600, I never use subtrim. Mabey on high priced servos, they pay attention to the gear placements.....I don't know

[pvogel]

@founder -- with modern digital servos it is possible to use a servo programmer to set the center point of the servo *precisely* and eliminate all subtrim.

Subtrim, in the radio, is considered "bad" by some because you aren't actually changing the center point of the servo, you are shifting the signal sent to the servo when your stick is at center in the radio, that means you have more available travel on one side of center than the other. In most cases a *small* amount of subtrim isn't going to hurt anything because the radio is not using the full 900-2100 micro-second signal (that's why they can go up to "150%" travel... so sliding the travel window by a few percent doesn't hurt and you still get equal amounts of travel on either side of the subtrimmed "center".

Let's put this another way -- 900-2100 usec is the signal range, for a total range of 1200usec, divide that in 1/2 and you get the center point of 1500usec. So, with no subtrim, at center stick your receiver is putting out a 1500usec pulse to command the servo to center. On my JR radio, I can adjust the high and low endpoints of travel from 100% (default) up to 150% (max), that means that 100% travel is using 2/3 of the available 600usecs on either side of center, or 400usec. So if my travels are at 100% low and 100% high, the receiver is putting out a signal from 1100usec to 1900usec, leaving 200usec to "spare" on either side of center for subtrim. If I put in subtrim such that I've shifted by 100usec, then at center stick my receiver is putting out 1600usec, and at max 2000usec, min is 1200usec, I still have equal travel on either side and the subtrim didn't hurt.

On the other hand, if I set my travel volume to 150% of high and low (assuming that is possible without binding) then I WILL see a difference in travel volume between center and high stick vs. center and low stick. In that situation subtrim would be "bad".

Similarly, if I'm needing to put in a LOT of subtrim (i.e. because I'm lazy and don't want to fix things mechanically) then I could swamp that default 200usec of subtrim and again cause a differential between center/high vs center/low. That could cause my roll rates to be inconsistent left/right, etc.

So, get it as "right" as possible mechanically, then use a *little* subtrim --or-- a servo programmer to get it "perfect".

I'm still researching the servo programmers, but my impression is that if you set the center with a servo programmer, you will always have the full 120-180 degrees of travel available to you mechanically and electronically and your radio subtrims can always be 0. Does anyone else have experience with digital servo programmers?

Peter+

[founder]

Peter+! You have expanded my understanding. I owe you big time!

I only found a programmer for Hi-Tec (http://www.servocity.com/html/servo_...er_tester.html).

If what you say is true, then I don't think it's that big a deal basically i'm cutting off what I don't use.

... Spoke too soon: http://www.spektrumrc.com/Products/D...?ProdID=SPMDSP For JR servos... and reasonably priced if it does the job - reading manual.

[MrMel]

Even worse:

Throughout the whole range of JR/spektrum radios, there is a bug.

That is: For 120 degree swash you SHOULD have EXP=ACT in the swash menu activated, else you get interaction, PERIOD. (it takes out the round movement of the servo-horn)

BUT: Its a bug in the software, so when you have EXP activated AND using subtrim you get unlinear movement of the pitch, so if you for sample have 70 in subtrim on any servo, you will have a pitch curve that looks like -13 0 +12
No problem to adjust by lowering your lower pitch by a few points, but annoying nevertheless.

(its present even on the 12X flagship)

I say +-30 subtrim is OK.

What I wonder is how long it takes for some "geared" CCPM servos to come out, no servo horn instead a "spiral" gearbox, pushing straight up/down, made to fit our helis.... *hint* *wink*

[founder]

Originally Posted by MrMel

What I wonder is how long it takes for some "geared" CCPM servos to come out, no servo horn instead a "spiral" gearbox, pushing straight up/down, made to fit our helis.... *hint* *wink*

I've wondered that too. So that you get a sliding movement instead of an arc?

[heli-cuzz]

Not familiar with JR servos, but the futaba servo horns on a futaba servo can be turned to achieve a perfect 90 degree at center with no sub or trim.

[pvogel]

Originally Posted by heli-cuzz

Not familiar with JR servos, but the futaba servo horns on a futaba servo can be turned to achieve a perfect 90 degree at center with no sub or trim.

I believe that's true with any servo, IF you have a starred servo horn, but, for example the Hitec MG servos come with a metal servo horn that has only one arm, so it can be impossible (without something like the HFP-21 or the HFP-20) to eliminate the need for subtrim. I've never needed more than 20 "clicks" of subtrim to center at 90 degrees though).

Peter+

[founder]

From the manual of the Specktrum digital servo programmer:

[Once everything is hooked up] "To adjust the center point, slightly turn the servo arm, being careful not to over-twist and strip the servo gears."

Just seem like overkill now though.

[saltyzoo]

Here's a really crappy linear servo that's been around for years. http://www3.towerhobbies.com/cgi-bin...&I=LXCJG6&P=FR

It really shouldn't be that hard to put a rack and pinion on a good servo to make it linear.

But what we really need are small, light, efficient linear actuators. Maybe in another decade....

[Finless]

Originally Posted by MrMel

Even worse:
That is: For 120 degree swash you SHOULD have EXP=ACT in the swash menu activated, else you get interaction, PERIOD. (it takes out the round movement of the servo-horn)

BUT: Its a bug in the software, so when you have EXP activated AND using subtrim you get unlinear movement of the pitch, so if you for sample have 70 in subtrim on any servo, you will have a pitch curve that looks like -13 0 +12

That is why I do not use EXP activated and adjust the end travels for level travel of the swash. Even when not using subtrim I never liked the feel with EXP=ACT. It felt like it put expo in the feel of pitch.

Bob

[MrMel]

Originally Posted by Finless

That is why I do not use EXP activated and adjust the end travels for level travel of the swash. Even when not using subtrim I never liked the feel with EXP=ACT. It felt like it put expo in the feel of pitch.

Bob

Hows your barrel rolls doing, impossible to not get them...

Btw on the 12x the exp=act default. Its funny because ive met people selling their crappy flying heli because they DIDNT enable it

Btw, it hasnt really something to with pitch, but to disable the circular move of the servo at full aileron + pitch, if you take a swash plate bubble-tool you will see what it does.

Granted, you need to adjust your pitch curve afterwards, up 15-20% to get same "feel" but thats the price for nice rolls, tictocs and whatever.

[Thrasymedes]

Originally Posted by founder

I don't know much about engineering, but maybe someone here does and can tell me why in these days we can't get a solid servo arm that will fit on a servo without the need for subtrim. Why do we have to fish through and find the "Neo" of servo horns?

Is it because it's really hard to make servos and horns that line up? Or is it because easier/better to just get as close as you can and subtrim it out?

Get with the times, Servo Horn!

Little side questions:
a) What are the ills of subtrim that makes it so unappealing to most?
b) How much sub-trim is too much (I have a DX7 and X9303 if you want to give numerics).

This is why I'm a fan of the Futaba star wheel or solid disk. You don't have to hunt for the right one, they will work and the disk is even better. I've used metal arms and do not like them at all.

http://www3.towerhobbies.com/cgi-bin...?&I=LXH362&P=7

http://www3.towerhobbies.com/cgi-bin...?&I=LXFAG8&P=7

[MrMel]

Its Ok with the wheel, but for example BLSxxx users with only HD arms, its an issue (or anyone want to use the HD arms)

No wheel is delivered with the BLS servos.

[Thrasymedes]

Originally Posted by MrMel

Its Ok with the wheel, but for example BLSxxx users with only HD arms, its an issue (or anyone want to use the HD arms)

No wheel is delivered with the BLS servos.

You sure about that? Looks like they come with a set..

http://www3.towerhobbies.com/cgi-bin...?&I=LXUZE9&P=0

If not its a cheap order. If the pros can use them to spank hard 3D out, they are plenty strong enough.

[Finless]

Originally Posted by MrMel

Hows your barrel rolls doing, impossible to not get them...

No problems what so ever! Just watch some of my maiden flight videos like the Protos and Atom I just did. They roll like they are on rails.

Bob

[MrMel]

No problems what so ever! Just watch some of my maiden flight videos like the Protos and Atom I just did. They roll like they are on rails.

I can make you an video if you want... try a bubble level and see for your self.
Newer helis like Protos, Align T700, Aurora have very very low swash mix, that takes out alot of that interaction, the higher swashmix values you have the greater the interaction there is.
Tradeoff is that you need great servos for those helis (slop will show directly)

IMHOP it shouldnt even be possible to disable it with 120 degree swash, since there aint no "non-circular-moving-servos" out there.

[MrMel]

You sure about that? Looks like they come with a set..

http://www3.towerhobbies.com/cgi-bin...?&I=LXUZE9&P=0

They migth have changed that for the new dedicated Heli servos, the BLS451 didnt come with it (got a few myself)