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Old 02-19-2013, 04:38 PM   #1 (permalink)
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Default Pushrod or no pushrod?

Today I found myself wondering about the tail push rod and if it's strictly necessary to have one in the first place. A possible solution could be to mount a servo so that the servo arm is the tail pitch arm one would normally connect to the push rod.

There is at least one possible con:
A servo that far out on the boom would shift CoG. Question is will it have to be too much weight too far aft with a servo powerful enough to control tail pitch?

Possible pros: Fewer links that could pop and with fewer things to adjust like push rod length there would be a simpler mechanical setup.

I say possible and would like to hear what you think. Would the cons out-weight the pros and would there be pros in the first place? Would it be technically feasable and would it offer any benefits?
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Old 02-19-2013, 09:25 PM   #2 (permalink)
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A servo is pretty heavy, placed all the way out at the end of the boom and you are looking at quite a bit of weight.

A 60g servo 5x the distance from the CoG compared to the ballast (battery usually), would require 300g to balance it. The tail could be even further out than that so the weight adds up pretty fast.

It would also reduce the flip rate (for a given cyclic input) as the mass is now farther from the axis of rotation. It is best to keep the tail as light as possible.
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Old 03-01-2013, 09:11 AM   #3 (permalink)
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great information, but I could try this first
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Old 03-02-2013, 01:53 PM   #4 (permalink)
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Good thinking.

But also ever tail tap might result in a jacked up tail servo, that might get expensive real quick.


But also I have a forward CoG now, so maybe moving the tail servo back some would center the overall weight? Hmm.
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Old 04-15-2013, 12:38 PM   #5 (permalink)
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I did it on my old Raptor 60. It worked and there was very little flex in the pushrod (because it was about a half inch long. I just ran a long extension back to the boom with some zipties. It looked different and seemed to work fairly well. As long as you put the servo underneath the boom so the blades hit the boom and not the servo first .

That being said, I got rid of my Raptor and now have an Avant with the servo where it is meant to be, running a pushrod
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Old 05-26-2015, 06:17 PM   #6 (permalink)
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I've done this on a camera-ship that needs the weight back there anyway. Works well.
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Old 05-28-2015, 10:14 AM   #7 (permalink)
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minuta de hierba.
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Old 07-30-2016, 08:41 PM   #8 (permalink)
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It depends on your goal.
If wild, smack style 3D is the goal, you want as much mass centrally located as close to the center of rotation on all three axis
The center of rotation on a conventional helicopter is the main rotor head block for cyclic and the main mast for rotation.
That's why the motor, batteries and servos are mounted close to the main blades.

If you want sedate, then just replace the tail drive, tail servo and all the mechanical stuff with a brushless motor and the tail rotor ESC. Like the small helis.
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Old 11-22-2016, 11:32 AM   #9 (permalink)
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vote for pushrod
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Old 02-08-2017, 02:33 AM   #10 (permalink)
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Ho fatto questo su una fotocamera nave ha bisogno del peso indietro comunque. Funziona bene.
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Old 02-20-2017, 08:51 AM   #11 (permalink)
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If it was a good idea all the Heli manufacturers would do it. The fact that most the best designs do not should answer your question.


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Old 02-20-2017, 09:09 AM   #12 (permalink)
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Don't see any real benefit, not like the pushrod is a problematic part.

Sticking it on the end of the tail will decrease mechanical gain and increase the chances of it getting damaged in a crash, struck by the blades or stripping the gears which almost never happens with the current locations manufacturers use, all to gain.....well, nothing really??
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Old 02-20-2017, 09:21 AM   #13 (permalink)
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Sticking 100g out on the tail would destroy tail performance. Horrible whipping. Bad stops.

When designing an airframe you want as little mass as possible out on the tail..few grams for CG adjustment is fine. Whole servo is catastrophic.
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Old 02-20-2017, 09:26 AM   #14 (permalink)
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in theory push rods have a drag coexi. so if the drag is reduced to direct link, it will have a better response... i recall the same thing happening to servo geometry.. the links got shorter as the servos are closer to the axis its controlling. i think that made a big diff and as long as you can get it to work practically its just a reworking with mild benefits...i can only see 3d smack pilots noticing.

push rods are sometimes weak, snap, and have drag under extreme load i can see the benefit for those guys hammering piro moves and stressing the system. my align push rod wore out after 300 flights, has a 10 degree bend, its the thinnest thing.
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Old 02-24-2017, 10:14 AM   #15 (permalink)
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Opening shot - big servo out on the tail:

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Old 03-05-2017, 09:06 PM   #16 (permalink)
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That works fine for that application because he is flying ti very mildly with zero maneuvers emphasizing the tail. You would not have to deal with any pushrod issues, but shouldn't be in the first place. I have never heard of an instance where a pushrod on the tail is a limiting factor. Having any weight that far out on the tail would destroy tail performance. Stops, starts, and piro consistency would go down the drain. Does not matter even if you had the best gyro/servo combo you could get.
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Old 03-29-2017, 10:38 PM   #17 (permalink)
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So funny, I thought about this idea years ago. Yeah, you'd eliminate any flex/slop/friction of the rod, but the weight would be a killer.

Just to give you an idea of what kind of "magnification" occurs when adding mass/extending the tail--I extended the tail of my gaui x5 65mm. That took almost 5oz of "ballast" in the front (a bigger lipo) to compensate.
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