Ask the pro's.. elevator servos
08-07-2007, 06:08 AM
#1
Ask the pro's.. elevator servos
There is a thread here in the beginners forum about two elevator servo's. There are a couple of posts that suggest putting the two servo's on, one on each side, using the up vertical servo arm for one elevator side and the down servo arm for the other elevator side.
I'm sure people like RC Ken, Bruce, and MinnFlyer (Mike) (and, not to slight others, but I'm sure there are more) have done this at least once in their RC career..
and has something to say about this.
My question is this: Will this cause some sort of differential in the operation of the elevator, favoring the up side of the servo arm with more movement than the down side? (consider the angle that the control rod from the servo hits the control horn)
Dick.
I'm sure people like RC Ken, Bruce, and MinnFlyer (Mike) (and, not to slight others, but I'm sure there are more) have done this at least once in their RC career..
and has something to say about this.My question is this: Will this cause some sort of differential in the operation of the elevator, favoring the up side of the servo arm with more movement than the down side? (consider the angle that the control rod from the servo hits the control horn)
Dick.
08-07-2007, 06:32 AM
#2
Senior Member
RE: Ask the pro's.. elevator servos
It appears that configuration works best when you place the two servos vertical relationship to each other such that the two pushrods come back from the elevator horns as nearly on the same line as possible. That means that one servo is tucked up as close to the horizontal stab as possible, while the other is much lower on the fuselage. The arrangement will drive the two pushrods as nearly the same as can be done when one is being driven from below while the other is being driven from above. The reason the above/below is important is the arc the two servo arms follow. One arc will follow the arc the elevator horn follows. The other arc does not. They will then lose a very tiny bit of uniformity of motion.
The limitations to this are the size of the fuselage back there and the size of the servo arms. Also, you have to know beforehand which hole in the servo arm you're going to use for the pushrod connection. And of course, you have to accept that it's still not going to be absolutely perfect. No matter how you orient where the two different connection holes, the arcs going in opposite directions causes different movement.
Given the tolerances in our hardware, the differences are not always of any consequence.
The limitations to this are the size of the fuselage back there and the size of the servo arms. Also, you have to know beforehand which hole in the servo arm you're going to use for the pushrod connection. And of course, you have to accept that it's still not going to be absolutely perfect. No matter how you orient where the two different connection holes, the arcs going in opposite directions causes different movement.
Given the tolerances in our hardware, the differences are not always of any consequence.
08-07-2007, 06:36 AM
#3
RE: Ask the pro's.. elevator servos
I have never installed tail servos with one arm up and one down. One only needs to look at the geometry to see that differential would occur. One side will most likely get more up than down and the other will get more down than up.
EDIT: in the narrow confines of the tail it somtimes helps to stagger the vertical poisitions like da rock mentioned to allow them to fit. This would reduce the difference side to side.
EDIT: in the narrow confines of the tail it somtimes helps to stagger the vertical poisitions like da rock mentioned to allow them to fit. This would reduce the difference side to side.
08-07-2007, 07:03 AM
#4
RE: Ask the pro's.. elevator servos
Thanks.
What I have is a Seagull Super Star. This plane has an arrangement with one rudder servo and two elevator servo's under the horizontal stab. I opted to make the rudder system as pull-pull which eliminates one servo back there. On the elevator, the holes in the fuselage that were already there, have the servo arms on the same horizontal plane. That makes the choice to have differential or use a Y with a reverser, which is what I did.
The reason I brought this up was that I was somewhat concerned about the potential differential (confirmed by Bruce) that will occur with the up-down setup without a Y Reverser. I was looking to see if the DX7 had some sort of mixing scheme to put the elevator servo's on a separate channel then have them operate that way, but thought that it would be much simpler for the elevators to be connected via the Y Reverser.
And, as I said, in another posting, there is the mention of this up-down setup and I was thinking of the differential posed by such an arrangement.. as of course could be corrected as Da-Rock suggests. But, that means that different cutouts must be made in the fuselage to accomodate such an arrangement.
So, it seems that the simplest design would be to use the Y Reverser.
A third choice, of course, would be to put the servo's in the wing saddle area (there is certainly plenty of room in the Super Star for this) and use either rods or pull-pull. My Excelleron 90 has pull-pull elevators AND rudder. That's a lot of cables going back to the tail.
Dick.
What I have is a Seagull Super Star. This plane has an arrangement with one rudder servo and two elevator servo's under the horizontal stab. I opted to make the rudder system as pull-pull which eliminates one servo back there. On the elevator, the holes in the fuselage that were already there, have the servo arms on the same horizontal plane. That makes the choice to have differential or use a Y with a reverser, which is what I did.
The reason I brought this up was that I was somewhat concerned about the potential differential (confirmed by Bruce) that will occur with the up-down setup without a Y Reverser. I was looking to see if the DX7 had some sort of mixing scheme to put the elevator servo's on a separate channel then have them operate that way, but thought that it would be much simpler for the elevators to be connected via the Y Reverser.
And, as I said, in another posting, there is the mention of this up-down setup and I was thinking of the differential posed by such an arrangement.. as of course could be corrected as Da-Rock suggests. But, that means that different cutouts must be made in the fuselage to accomodate such an arrangement.
So, it seems that the simplest design would be to use the Y Reverser.
A third choice, of course, would be to put the servo's in the wing saddle area (there is certainly plenty of room in the Super Star for this) and use either rods or pull-pull. My Excelleron 90 has pull-pull elevators AND rudder. That's a lot of cables going back to the tail.
Dick.
08-07-2007, 07:05 AM
#5
RE: Ask the pro's.. elevator servos
Whenever I have set up a twin servo elevator I have avoided the very situation explained above. Why? I wanted to avoid different geometry for the pushrods. Whether this really would happen I don't know, but I didn't even want to face the chance of it happening. When you connect pushrods to different parts of the two servos you have a real chance of the movement being different for the elevator as the servos move through their range of motion. Even if the throws are identical throughout the motion of the servos you could also be faced with a situation where one servo will lose a mechanical advantage and not have the strength that the other servo has. Because of this I prefer to set up the servos using a computer radio in order for them to be reversed properly.
Ken
Ken
08-07-2007, 08:00 AM
#6
RE: Ask the pro's.. elevator servos
G'day Dick,
How's it going, I have 2 elevator servos mixed with my DX7, in fact I am using it with the 3 planes I use with my DX7, easy to do, & no need for "Y" leads or reversers or whatever, just plug right Elev servo into elevator channel & the other into Aux2, & mix them, using either mix 5 or 6, trim include mixes, you will need to INH, the flap switch & slider, check out Bob's DX7 page for any hints you may need, but it is the best & simplest way, & you have full control over each servo.
Keep em flying Dick.
How's it going, I have 2 elevator servos mixed with my DX7, in fact I am using it with the 3 planes I use with my DX7, easy to do, & no need for "Y" leads or reversers or whatever, just plug right Elev servo into elevator channel & the other into Aux2, & mix them, using either mix 5 or 6, trim include mixes, you will need to INH, the flap switch & slider, check out Bob's DX7 page for any hints you may need, but it is the best & simplest way, & you have full control over each servo.
Keep em flying Dick.
08-07-2007, 03:14 PM
#7
Senior Member
RE: Ask the pro's.. elevator servos
ORIGINAL: CGRetired
Thanks.
What I have is a Seagull Super Star. This plane has an arrangement with one rudder servo and two elevator servo's under the horizontal stab. I opted to make the rudder system as pull-pull which eliminates one servo back there. On the elevator, the holes in the fuselage that were already there, have the servo arms on the same horizontal plane. That makes the choice to have differential or use a Y with a reverser, which is what I did.
The reason I brought this up was that I was somewhat concerned about the potential differential (confirmed by Bruce) that will occur with the up-down setup without a Y Reverser. I was looking to see if the DX7 had some sort of mixing scheme to put the elevator servo's on a separate channel then have them operate that way, but thought that it would be much simpler for the elevators to be connected via the Y Reverser.
And, as I said, in another posting, there is the mention of this up-down setup and I was thinking of the differential posed by such an arrangement.. as of course could be corrected as Da-Rock suggests. But, that means that different cutouts must be made in the fuselage to accomodate such an arrangement.
So, it seems that the simplest design would be to use the Y Reverser.
A third choice, of course, would be to put the servo's in the wing saddle area (there is certainly plenty of room in the Super Star for this) and use either rods or pull-pull. My Excelleron 90 has pull-pull elevators AND rudder. That's a lot of cables going back to the tail.
Dick.
Thanks.
What I have is a Seagull Super Star. This plane has an arrangement with one rudder servo and two elevator servo's under the horizontal stab. I opted to make the rudder system as pull-pull which eliminates one servo back there. On the elevator, the holes in the fuselage that were already there, have the servo arms on the same horizontal plane. That makes the choice to have differential or use a Y with a reverser, which is what I did.
The reason I brought this up was that I was somewhat concerned about the potential differential (confirmed by Bruce) that will occur with the up-down setup without a Y Reverser. I was looking to see if the DX7 had some sort of mixing scheme to put the elevator servo's on a separate channel then have them operate that way, but thought that it would be much simpler for the elevators to be connected via the Y Reverser.
And, as I said, in another posting, there is the mention of this up-down setup and I was thinking of the differential posed by such an arrangement.. as of course could be corrected as Da-Rock suggests. But, that means that different cutouts must be made in the fuselage to accomodate such an arrangement.
So, it seems that the simplest design would be to use the Y Reverser.
A third choice, of course, would be to put the servo's in the wing saddle area (there is certainly plenty of room in the Super Star for this) and use either rods or pull-pull. My Excelleron 90 has pull-pull elevators AND rudder. That's a lot of cables going back to the tail.
Dick.
The up/down arrangement works "good enough" for some folks, but usually only when the pushrods are aligned. There will be a very tiny bit of difference, and you'd better pray that you don't need to change what holes the two pushrods are connected to in the servo arms.
The reversing Y will work perfectly as long as the two servos are matched for neutrals and throws. I'd bet the odds are good of finding two that do. But haven't had to do it. soooo
A computer radio will also work perfectly with each servo plugged into it's own recepticle, AND it'll probably let you adjust neutrals, throws etc for each to get very equal outputs. But then......... what if you want to mix rudder>elevator. Radio probably will do it but could take a rocket scientist to set the radio up. That's just conjecture. I never had the radios I've got today back when I used two elevator servos. Back then I had to rewire one servo.
08-07-2007, 07:54 PM
#8
RE: Ask the pro's.. elevator servos
This is good stuff, guys, and I really appreciate it. Allan, I will take a close look at your setup because I am using the DX7 for the Seagull.
Ken, that differential in the geometry is what I was concerned about. Especially noting that a very small change in the elevator causes a visible change in the flight path of the aircraft in flight. So, if one side deflects more than the other side, then the result is obvious.. the plane will probably veer to one side or the other, undesirable for input to elevator caused reaction.
One gentleman sent me a pair of small lasers to use as alignment tools. They take a 4.8 volt NiCd battery and put out a focused beam that can be directed to a ruled scale. I am going to use it to check deflection of control surfaces. I can tell what is going on with the alignment and movement of the ailerons, elevators (both sides) and the rudder. Neat gadget for centering and measuring throw.
DaRock, I currently have a wired Y reverser installed. I am going to play with that just to see how much, if any, differences exist.. then I am going to use the computer radio, as Allan suggests, to get them right.
Thanks again, guys.
Dick.
Ken, that differential in the geometry is what I was concerned about. Especially noting that a very small change in the elevator causes a visible change in the flight path of the aircraft in flight. So, if one side deflects more than the other side, then the result is obvious.. the plane will probably veer to one side or the other, undesirable for input to elevator caused reaction.
One gentleman sent me a pair of small lasers to use as alignment tools. They take a 4.8 volt NiCd battery and put out a focused beam that can be directed to a ruled scale. I am going to use it to check deflection of control surfaces. I can tell what is going on with the alignment and movement of the ailerons, elevators (both sides) and the rudder. Neat gadget for centering and measuring throw.
DaRock, I currently have a wired Y reverser installed. I am going to play with that just to see how much, if any, differences exist.. then I am going to use the computer radio, as Allan suggests, to get them right.
Thanks again, guys.
Dick.
08-07-2007, 08:10 PM
#9
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RE: Ask the pro's.. elevator servos
My problem is that I have a computer radio, but this specific radio (Futaba 7 CAP) does not have the capability of two elevator servos. You would think they do this, but...
As stated in the other thread, I'll probably go with the reversed y harness.
As stated in the other thread, I'll probably go with the reversed y harness.
08-07-2007, 08:44 PM
#10
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RE: Ask the pro's.. elevator servos
it seems to me there is alot of over complication in this issue by simply fliping one of the servos and using a Y harness while carefully aligning the separate elevators the amount of throw and deflection is negledgable.
not needing either a comp.radio or servo reverser, simply a adjustment in pushrod length for the flipped servo.
the plane will fly fine.
not needing either a comp.radio or servo reverser, simply a adjustment in pushrod length for the flipped servo.
the plane will fly fine.
08-08-2007, 06:27 AM
#11
RE: Ask the pro's.. elevator servos
ORIGINAL: bigtim
it seems to me there is alot of over complication in this issue by simply fliping one of the servos and using a Y harness while carefully aligning the separate elevators the amount of throw and deflection is negledgable.
not needing either a comp.radio or servo reverser, simply a adjustment in pushrod length for the flipped servo.
the plane will fly fine.
it seems to me there is alot of over complication in this issue by simply fliping one of the servos and using a Y harness while carefully aligning the separate elevators the amount of throw and deflection is negledgable.
not needing either a comp.radio or servo reverser, simply a adjustment in pushrod length for the flipped servo.
the plane will fly fine.
08-08-2007, 08:37 AM
#12
RE: Ask the pro's.. elevator servos
Dick, unless you're flying very presision stuff, I wouldn't worry about it. In fact, the Pacific Aeromodels Gee Bee "Y" Racer uses this method - this is a 180 4-stroke size airplane which you can see a review of [link=http://www.rcuniverse.com/magazine/article_display.cfm?article_id=132]HERE[/link].
Now on The Gee Bee Y, they DID have one servo higher than the other, and that's a big help, but if you look at the image below you'll see that even with some pretty radical throws, there's not a LOT of difference.
To describe what the image points out:
A) The distance from one side to the control horn is shorter on top than on bottom.
B) The bottom pushrod has been made longer to compensate.
C & D) As you can see, the top pushes a little more in one direction and pulls a little less in the other than the bottom - If the two servos were aligned so thatone was higher or lower than the other, this difference would be even less.
Bottom line, If it were me, I'd stagger the heights of the servos and I wouldn't lose any sleep over it.
Now on The Gee Bee Y, they DID have one servo higher than the other, and that's a big help, but if you look at the image below you'll see that even with some pretty radical throws, there's not a LOT of difference.
To describe what the image points out:
A) The distance from one side to the control horn is shorter on top than on bottom.
B) The bottom pushrod has been made longer to compensate.
C & D) As you can see, the top pushes a little more in one direction and pulls a little less in the other than the bottom - If the two servos were aligned so thatone was higher or lower than the other, this difference would be even less.
Bottom line, If it were me, I'd stagger the heights of the servos and I wouldn't lose any sleep over it.
I will probably just keep it the way it is now and just fly the thing.. ha..
09-08-2007, 05:36 PM
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RE: Ask the pro's.. elevator servos
One thing to think about when moving the servos inside the plane verses putting them in the tail is the balance will be all messed up and you are going to be putting some serious weight in the tail to make up for the servos that are not there. It was designed for the servos to be in the tail and the balance will come out close that way. If you put them up front along with the receiver, you are going to be very nose heavy. Also, the Super Star is very touchy on pitch. If you get one elevator going up more then the other, they will induce roll into the plane and it is going to be fun trying to do loops and such. This plans is pitch sensitive anyway, so take care when you start placing servos and hook ups that are different then what is called for.
Ed
Ed
09-09-2007, 07:26 AM
#15
RE: Ask the pro's.. elevator servos
ORIGINAL: MinnFlyer
Dick, unless you're flying very presision stuff, I wouldn't worry about it. In fact, the Pacific Aeromodels Gee Bee "Y" Racer uses this method - this is a 180 4-stroke size airplane which you can see a review of [link=http://www.rcuniverse.com/magazine/article_display.cfm?article_id=132]HERE[/link].
Now on The Gee Bee Y, they DID have one servo higher than the other, and that's a big help, but if you look at the image below you'll see that even with some pretty radical throws, there's not a LOT of difference.
To describe what the image points out:
A) The distance from one side to the control horn is shorter on top than on bottom.
B) The bottom pushrod has been made longer to compensate.
C & D) As you can see, the top pushes a little more in one direction and pulls a little less in the other than the bottom - If the two servos were aligned so thatone was higher or lower than the other, this difference would be even less.
Bottom line, If it were me, I'd stagger the heights of the servos and I wouldn't lose any sleep over it.
Dick, unless you're flying very presision stuff, I wouldn't worry about it. In fact, the Pacific Aeromodels Gee Bee "Y" Racer uses this method - this is a 180 4-stroke size airplane which you can see a review of [link=http://www.rcuniverse.com/magazine/article_display.cfm?article_id=132]HERE[/link].
Now on The Gee Bee Y, they DID have one servo higher than the other, and that's a big help, but if you look at the image below you'll see that even with some pretty radical throws, there's not a LOT of difference.
To describe what the image points out:
A) The distance from one side to the control horn is shorter on top than on bottom.
B) The bottom pushrod has been made longer to compensate.
C & D) As you can see, the top pushes a little more in one direction and pulls a little less in the other than the bottom - If the two servos were aligned so thatone was higher or lower than the other, this difference would be even less.
Bottom line, If it were me, I'd stagger the heights of the servos and I wouldn't lose any sleep over it.
Exactly! I run this stuff on several planes. No big deal. Maybe in precision aerobatics obviously but I don't see an issue here at all. One rod is a tad longer at the same angle, so what? Works fine
