establishing proper linkage ratios and servo requirements

[JoeAirPort]

	Quote: Originally Posted by DronNq
	Don, my idea was about increasing resolution in stick movements (140% EPA) but decrease servo travel to 100% @ 140% input, so speed and deflection stays the same, but resolution raises, or not? 1" arms would be ok if you have the same length surface horn. That's why I said 1:1 (in my case it will be 1.25:1.25")

If I understand this correctly you would not have the max resolution with that setup. You'd be better just setting your linkage with a higher mech advantage and leave the servo travel at or near 140%. That would maximize your mechanical advantage and resolution. So you'd be better off with a 1.5"/1.25" ratio as an example (surface/servo).

[Flatlandman]

i agree the extra speed gained from the lesser mech setup doesnt come into play when the servos are at or near 0.12 speed over 60°.

[JoeAirPort]

And as Mith smartly pointed out earlier....if you have lots of mechanical advantage, you will not stall the servo and it will actually run at its rated speed !! Well as you can see I'm a total fan of using all the servo angle and getting as much mechanical advantage as possible. I drew up some "ideal" linkage sketches once and saw what happens when it goes from center to full deflection. With less than 1:1 your mechanical advantage gets worse the closer you get to full deflection. With a 1:1 your mechanical advantage stays the same as the surface goes to full deflection. With a more than 1:1 your mechanical advantage acually increases as you get closer to full deflection. This is because since the servo arm is shorter than the control horn, the distance from a straight line along the push rod to the servo axis of rotation gets shorter on the servo side faster than the control horn side as it approaches full deflection. Draw it out on graph paper once and you will see this. Mith, you might have to check my logic here. My EE skills only go so far into the ME arena. Also I'm only taking into account 2 dimensions when our servo arms, control horns, and push rods actually move around in 3D space.

[Mithrandir]

	Quote: Originally Posted by JoeAirPort
	And as Mith smartly pointed out earlier....if you have lots of mechanical advantage, you will not stall the servo and it will actually run at its rated speed !! Well as you can see I'm a total fan of using all the servo angle and getting as much mechanical advantage as possible. I drew up some "ideal" linkage sketches once and saw what happens when it goes from center to full deflection. With less than 1:1 your mechanical advantage gets worse the closer you get to full deflection. With a 1:1 your mechanical advantage stays the same as the surface goes to full deflection. With a more than 1:1 your mechanical advantage acually increases as you get closer to full deflection. This is because since the servo arm is shorter than the control horn, the distance from a straight line along the push rod to the servo axis of rotation gets shorter on the servo side faster than the control horn side as it approaches full deflection. Draw it out on graph paper once and you will see this. Mith, you might have to check my logic here. My EE skills only go so far into the ME arena. Also I'm only taking into account 2 dimensions when our servo arms, control horns, and push rods actually move around in 3D space.

I think you are on the right track......

here is a snap of a Spreadsheet showing the FORCE available from a 300 oz*in servo as a function of arm position. In this instance, I used a arm length of .625"
But it show how the FORCE increases towards the end of the servo travel. Keep in mind, as the rudder deflects, the reverse is happening and the required FORCE gets higher. But when 60 degrees on the servo yields 45 degrees on the rudder, the Available Force on the servo increases at a faster rate then the FORCE required increases.... so essentially, from the geometry, you have more holding force at the extremes then at center.... hey... that is what we want hiuh???

[JoeAirPort]

Very cool, what you show in your spread sheet is basically what I saw in my sketch but I didn't have all the numbers. After seeing that I always wanted more than 1:1 on my linkages. Can you imagine the poor people that have less than 1:1 on a Giant gasser? That's a recipe for disaster. With rudders being 1:1 it also tells you why you need such huge servos.

So your spread sheet is only looking at the servo side with a .625" long servo arm correct? I was going to ask what the control horn length was but it seems like your only showing the servo side. If it's rudder like you said then it's 1:1? That's a very cool spread sheet though. You use Cos for the torque formula to calculate Force. You need Sin to figure out how far in the X direction the ball end moved straight back. Then from one extreme of X to the other is the 1.0825". I gotta save a picture of that to make up my own. Thanks.

[Mithrandir]

	Quote: Originally Posted by JoeAirPort
	Very cool, what you show in your spread sheet is basically what I saw in my sketch but I didn't have all the numbers. After seeing that I always wanted more than 1:1 on my linkages. Can you imagine the poor people that have less than 1:1 on a Giant gasser? That's a recipe for disaster. With rudders being 1:1 it also tells you why you need such huge servos. So your spread sheet is only looking at the servo side with a .625" long servo arm correct? I was going to ask what the control horn length was but it seems like your only showing the servo side. If it's rudder like you said then it's 1:1? That's a very cool spread sheet though. You use Cos for the torque formula to calculate Force. You need Sin to figure out how far in the X direction the ball end moved straight back. Then from one extreme of X to the other is the 1.0825". I gotta save a picture of that to make up my own. Thanks.

Actually, the .625" horn length this was for another effort.... but in practice, I setup the rudder on my AW Su29 such that +/- 60 degrees on the servo yields +/- 45 degrees on the rudder.... so almost like a 4:3 servo-to-rudder ratio.

I will try to finish the spreadsheet to show what the forces available at the rudder horn are at each angle thru the deflection.... using the approximately 4:3 ratio.....

[JoeAirPort]

Dumb question, but you must be talking about a push-pull rudder setup right?

I'd be really interested in seeing the numbers on that updated spread sheet.

[Scott Gerber]

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[QSPILOTCMH]

I am setting up my first gas a 25% SU26, and I learned a lot from this thread. I have it setup for max throw on all surfaces using swb 1 1/4" arms with Hitec hs5645MG servos. The elevator is maxed at 40 degrees, aileron 40 degrees, and rudder 45 degrees. My low rates need to be around 12 degrees elevator for smooth flight which puts me around 40%. Of course the ailerons and rudders need turned down as well.

Do I lose a lot of servo resolution by having such a low rate? I originally set it up for 3D but I am leaning more towards smooth IMAC style flying. I may revisit 3D once I get more time on the plane, however I have a few glow planes that 3D excellent! Should I shorten the arms or just leave the rates turned down?

Secondly how do you determine the required elevator, aileron, rudder throws for imac flying?

Thanks!

[JoeAirPort]

	Quote: Originally Posted by QSPILOTCMH
	Do I lose a lot of servo resolution by having such a low rate? I originally set it up for 3D but I am leaning more towards smooth IMAC style flying. Secondly how do you determine the required elevator, aileron, rudder throws for imac flying? Thanks!

Yes, no doubt you lose resolution setting it up for 3D throws and dialing the rates down for IMAC. But for me I have no choice as I fly more 3D than anything else. If I were competing in IMAC I would set my plane up for IMAC throws and not be able to do 3D. I'd have another plane for 3D.

Usually the plane mfg will post those throws in the manual. And that would be just a starting point. I'd fly through my routine and tweak them from there.

[Flatlandman]

IMAC and 3d rudder throw is about the same

Rudder set to 45° high about half to start for low rates but adjust the low in the air to be just enough rudder to hold a knife edge at 75% throttle

For imac setting the high rate rudder i use for Hammers and Rollers thats why i use the 45°


The ailerons set low rate for about 20° then adjust in the air for best snap roll.

Ele low rate ~12°

[cap231jim]

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[QSPILOTCMH]

Thanks for the feedback!

[Rishelman]

This is a great source of info. Wish I had seen it a little sooner. If anyone is out there, I'd love some input on something I've been wondering about since I built my first 50cc gasser.

I followed the manual for installation of the control horns, and didn't REALLY think about it at the time. Now it seems like I should have thought twice. Here's the issue:

The control surface horn is the typical threaded rod into the hardpoint, with a plastic horn on the end, then ball link to pushrod...yada, yada. But, the rod threads in just behind the bevel of the hinge line. It's just back far enough to get a nut to sit flat at the base of the rod. Now the horn DOES line up over the hinge line. But my rod is nowhere near 1.5" back (to match length of servo arm).

At the time it seemed fine based on what I had read up to that point. To get full deflection I was in the %135 travel area on my JR 9303. I'm new to gassers so I don't have a lot to compare to. The plane seems to fly fine, but as my skill improves I feel I am sensing some blowback.

Bottom line: I just ordered some Hitec 7954SH servos for AIL, and I'm wondering if I should change my mechanical setup with the install of new servos.

Hope this all makes sense, it's easy for me to see in my head, don't know how well it came out for someone who's never seen my plane.

Any thoughts??

Rishelman

[Bill Clark]

the guys will come crawling out when they get the subscribtion "ding". in the mean time seems like a good first step would be to get a throw meter on the surfaces and see what you got including various points in between.