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Pushrod Geometry

Old 11-13-2011, 07:19 PM
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Poffit
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Default Pushrod Geometry

Hopefully I can get an answer once and for all on pushrod setup. After searching for hours on the Internet I still haven't come up with a black and white answer.

Even though i and Windows Paint don't get along very well I muddeled up a couple drawings tohelp with my question.

In this example, the dual servos are in the fuse and the pushrods are crossing each other. The pushrod angle in the drawing is a little severe just to help get my question across.

Most of what I've read says that for the elevator to have the same amount of travel the servo arm should be parallel with the elevator hinge line

It seems to me that to have the same amount of travel up and down that the servo arm should be 90 degrees to the pushrod.

Which diagram is correct for the same amount of travel both up and down.
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Old 11-13-2011, 07:25 PM
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Default RE: Pushrod Geometry

your first picture is right. The servo arm and the pushrod must be a 90 degrees when the surface is at neutral.
Old 11-13-2011, 07:51 PM
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Default RE: Pushrod Geometry

Ditto
Old 11-13-2011, 08:32 PM
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Default RE: Pushrod Geometry

OK, now we have a concensus. I wish that I could get on the Internet and delete all those 10's or 100's of pages that said the servo arm should always be parallel with the hinge line. So if the first one is correct then my next question is about the setup for the ailerons. If it helps I will get back on Paint and make a couple more diagrams, but I should be able to describe my question.
The servos are mounted in the bottom of the wing on their side using a hatch. So the servo arm sticks out of the bottom of the wing through a slot cut in the hatch. The pushrod is going to run at an angle to the chord line of the aileron, when the aileron is at neutral. Forgetting about differential for now, to have the same travel up and down the servo arm should be at 90 degrees to the pushrod. Correct?
Like my first question, there is a lot of information out there that says the servo arm should be perpendicular to the aileron.
Old 11-13-2011, 08:33 PM
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Default RE: Pushrod Geometry

Well, in a perfecr world, it SHOULD be parallel with the hinge line. But then, if we lived in a perfect world...
Old 11-13-2011, 08:45 PM
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Default RE: Pushrod Geometry

Yes, with the servo arm poking through a slot, the arm should still be perpendicular to the pushrod when the surface is at neutral.
Old 11-14-2011, 03:22 PM
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Default RE: Pushrod Geometry

Assuming pic 1, to correct the geometry at the control horn end and actually get pretty equal (mechanically set) throw when there is a modest height differential, you may need to shim the horn attach point with a wedge to make the horn holes (roughly) perpendicular to the rod.

I make little beveled plywood shims all the time to do this sort of geometry correction.

When the height difference is extreme (ex. Elevator on a T-Tail plane), it may make sense to introduce intermediate 90 degree bellcranks to attain the desired right(ish) angles.

People frequently introduce undesirable differential by simply bolting their horns to a tapered aileron with no shim angle corrections. When the horn and servo are bottom of wing mounted, the induced differential from that flat mounting on a tapered aileron will typically be causing slightly more mechanical down deflection than up even when the servo horn/rod are at a perfect 90 degree angle. More down deflection that up can promote a tip stall when you're low and slow at higher AOA.

[The exception here would be an undercambered wing where the aileron angle may be enough to pitch the aileron horn tip forward rather than rearward.]

Old 11-14-2011, 04:45 PM
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Default RE: Pushrod Geometry

Okay Cutaway, obviously I am a little weak in the geometry area. I want to make sure I understand what you're saying about the elevator horn. If I understand correctly, the hole that the pushrod connects to on the elevator horn should have an imaginary line that is perpendicular to the pushrod and should extend through the center pivot of the hinges on the elevator.

Do I have this right.

Pat
Old 11-14-2011, 06:02 PM
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Default RE: Pushrod Geometry

I believe the getting the push rod a s close to 90 with the servo AND the control horn is the way to go. Do some snooping on the helicopter sites and you will see how its done on that side of the fence.

Go here... http://www.helifreak.com/showthread.php?t=41692

Watch the CCPM set up to see how complex it can get. I've been there and done that so airplane set up is a breeze!!

There is a lot of good info in that thread. Some good radio reviews, while biased towards heli's still good reviews. Some neat stuff like the CSM carb smart that has applications in planes also. LOTS of good info on electric too, batteries, charging and such.

Ken
Old 11-14-2011, 06:17 PM
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Default RE: Pushrod Geometry

On the kinds of planes that beginners fly, how important is it that the elevator travel be exactly equal in both directions? My guess would be, on a scale of one to ten, about zero. Rudder travel, sure. And for ailerons, you usually want differential.
Old 11-15-2011, 02:02 PM
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Default RE: Pushrod Geometry

wait one second here I think the first picture is wrong. the arm should at center be parallel to the hinge line. That way when the arm moves it has equal up and down movement. the push rod should be 90 degrees or perpendicular to the hinge line. the second picture is right but it looks off because the pushrod is not perpendicular too the control arm
Old 11-15-2011, 05:12 PM
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Default RE: Pushrod Geometry

Now we don't have a consensus. Okay Rlipsett, lets say I am going to use a stiff carbon fiber pushrod that has very little flex. There is no way to run the pushrod 90 degrees to the hinge line.
If the servo is mounted in the fuse and the control horn is on the elevator the pushrod is going to run at a slight angle to the hinge line. Not as much of a angle as in my drawing, which I exaggerated to get my point across. What do you say now?

Pat
Old 11-15-2011, 06:15 PM
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Default RE: Pushrod Geometry

if the servo is in the fuse mount the control arm verticle up or down depending on positioning. run the push rod straight back 90 degrees to the control horn which should be straight back behind the servo arm. they should be inline



anything that is out of alingment creates torsion force which tries to break things. my rudder is a little off as you can see. optimally the control horn should be slid up more on the rudder so all things would be equal
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Old 11-17-2011, 10:38 AM
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Default RE: Pushrod Geometry

IMO... The first picture in your first post is pretty darn close to what I would call ideal. Assuming the hinge line is at the top of the control surface, If the rod were extended just a bit it, the pivot point ON THE ROD and the hinge line would form a nice 90 degree angle to the rod. That would give equal throw each direction. I've gotten caught in the trap that the push rod pivot point has to be at a 90 to the control surface. It does not need to be. If you took the picture a step further and moved the servo directly below the control surface, the pivot point of the rod would be above the control surface, again assuming the surface is hinged at the top.

You can simulate this with a couple of pieces of balsa on a piece of wire. One will be the control surface and the other the control horn. On the horn use a stick with a pin to make your push rod. With the control surface staying in the same spot relative to the rod try moving the control horn around the rod keeping the push rod at a 90 degree angle to the control horn. You will see if the horn/rod relationship is always 90 you will always have equal control movement. No the only question is "where do i hang the servo?"

This makes sense to me, I hope everybody else see's what I'm trying to get at.

Ken
Old 11-18-2011, 08:55 AM
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Default RE: Pushrod Geometry

ORIGINAL: Poffit
Okay Cutaway, obviously I am a little weak in the geometry area. I want to make sure I understand what you're saying about the elevator horn. If I understand correctly, the hole that the pushrod connects to on the elevator horn should have an imaginary line that is perpendicular to the pushrod and should extend through the center pivot of the hinges on the elevator.

Do I have this right.
Right. Whenever you have trouble visualizing something like this, think of the extreme case, like where the elevator horn is raked back at say a 70 degree angle. The ramifications of geometry issues are more apparent visually when you take a situation to extremes.

Mechanical models made of cardstock with pin pivots might help sort things out too. I'll often mockup a linkage when I'm doing some new original design so I can make sure stuff is going to work and fit OK within structural constraints.
Old 11-21-2011, 12:39 PM
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Default RE: Pushrod Geometry

cutaway look where how he drew the line that is parrallel to the hingeline. the hinge line is not like the top of the control surface.
Old 11-22-2011, 01:38 AM
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Default RE: Pushrod Geometry

Hi!
The first picture is right! Not the second!
Look at it like this. The pushrod is what you should use as a reference line!
Then you imagine 90 degree lines to the servo center and to the pivot line (Hinge line) where the elevator is attched to the stab. Simple as that!
Old 11-24-2011, 09:05 AM
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rlipsett
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Default RE: Pushrod Geometry

I agree with setting the push rod 90 degreesto the servo arm and control arm but for maximum throws the arm has to also be parallel to the hinge line which the first picture is not even close too
Old 11-24-2011, 05:05 PM
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Default RE: Pushrod Geometry



If the push-rod is free to move, for instance if it's a long piece of balsa rod, then the rod should be arranged so that it's perpendicular to the hinge line. At neutral the rod should also be perpendicular to the servo arm  and should be parallel to the chord line of the control surface. The shape and rake of the control horn don't matter, what matters is that the attachment point of the rod to the horn is perpendicularly "below" the hinge line of the control surface. This will give equal throws and remove the tendency for the control rod to rip off the horn.

If you're using Bowden cables or "Gold'n rod", then the cable needs to be attached to the servo so that at neutral it's at 90 degrees to the servo horn. At the surface, the cable needs to be perpendicular to the hinge line and parallel to the chord line for best effect.

Old 11-25-2011, 07:49 AM
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Default RE: Pushrod Geometry

Hi!
You must have misunderstood where the elevator hing line is!
The elevator hing line is in the middle of the rear line on elevator picture drawn. If the control rod is 90 degrees to the servo arm center... and at the same time....rod is 90 degrees to the elevator hinge line ...the trows are equall!

The first picture is right!

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