Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

Surface Damping

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Old 06-05-2012, 12:16 PM
  #1
tomfiorentino
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Default Surface Damping

All things remaining equal, do you get a better damping effect on the control surface horn using a hole closer to the surface or further from the surface?

I am having trouble rationalizing this one because while I understand the torque and leverage issues, I'm not so sure about the other components when the harmonics come into play.

I'm pursuing more of a precision set-up on my 260 so I am wrestling with the whole issue of maximizing the ATV of the servo for purposes of resolution while at the same time controlling the amount of surface throw.

Anyway...who has a tid bit to share...

Thanks,
Tom
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Old 06-05-2012, 01:50 PM
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Default RE: Surface Damping

max out the servo rotation when surface throw is at desired max
watch geometry such that travel is fairly even -either direction
do not allow flex in push rod or other linkage bits.
and NO drag in it !!!!!
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Old 06-05-2012, 04:14 PM
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Default RE: Surface Damping

Tom, RMH is pretty much the master at setups as he has built airplanes for the best. Not much to add except: Servos, servos servos LOL. I'm not sure what size your 260 is but outfit it with premium servos. No matter how well setup the airplane is, it will be of no use if you experience servo blowback or poor centering. A good example of this is my current project. A 50cc Laser 200. The servos planned at this point are Hitec 7955's. Power wise these are overkill but it's the precision I am after. There are even better options out there for precision but I have found the Hitecs a great value oveall. For aerobatics I like a servo no slower then .15 sec. 60 degree. Once you have quality servos, good linkages and surfaces that move freely you are about 1/4 of the way there. The rest is all setup. Like RMH states you will want to keep the ATV and DR settings high and set throw mechanically. The caviat here is that I like to run a servo arm no shorter then 1". Any shorter and you effectively reduce the speed in which the surface moves. Some will disagree with this but it is what has worked well for me. Rule of thumb is I usually set up with 1.25" servo arms and 1.5 to 1.75" control horns. This gives enough throw for light 3D work but allows you to do an IMAC setup without killing too much resolution. For airplane setup, step one is to measure the airplane and make sure it is strait. If not make corrections. We can go for pages about the actual setup but the best thing to do is get the airplane flying, set the CG where you want it and then ask questions.
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Old 06-05-2012, 05:49 PM
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Default RE: Surface Damping

ok guys thanks for the comments.

It is my 50cc Aeroworks and I have new Hitec 7955TG's (I had the jittery issue with my other set of servos and Hitec wonderfully replaced them for me).

On my original setup I was chasing my tail trying to do the whole 3D setup AND precision set up. But as has been discussed here before, when you set the ATV to 100% there is way too much throw and I cant get rid of it mechanically. I can mitigate by going way out on the control surface horn, and then started thinking about the dampening effect (pro or con) of that kind of setup.

Ultimately, I don't think torque ends up being an issue with these servos on this size airplane regardless of what I do mechanically.

Since I am doing a re-install of the servos, I'll start with connections that are 1" from the output shaft on the servo side and 1" out on the control horn side; set the ATV to 100 % and see where I am...maybe I will follow up with you guys at that point.

Which reminds me. From a resolution standpoint, is setting ATV at 100% with say 50% exponential equivalent to setting ATV at 50%? Short of the expo effect of the expo...is resolution adversly affected?

Not that I would be able to tell worth a hoot...I just like to break this stuff down and understand it.

Thanks again guys...

Tom
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Old 06-05-2012, 06:03 PM
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Default RE: Surface Damping


Quote:
ORIGINAL: tomfiorentino

ok guys thanks for the comments.

It is my 50cc Aeroworks and I have new Hitec 7955TG's (I had the jittery issue with my other set of servos and Hitec wonderfully replaced them for me).

On my original setup I was chasing my tail trying to do the whole 3D setup AND precision set up. But as has been discussed here before, when you set the ATV to 100% there is way too much throw and I cant get rid of it mechanically. I can mitigate by going way out on the control surface horn, and then started thinking about the dampening effect (pro or con) of that kind of setup.

Ultimately, I don't think torque ends up being an issue with these servos on this size airplane regardless of what I do mechanically.

Since I am doing a re-install of the servos, I'll start with connections that are 1" from the output shaft on the servo side and 1" out on the control horn side; set the ATV to 100 % and see where I am...maybe I will follow up with you guys at that point.

Which reminds me. From a resolution standpoint, is setting ATV at 100% with say 50% exponential equivalent to setting ATV at 50%? Short of the expo effect of the expo...is resolution adversly affected?

Not that I would be able to tell worth a hoot...I just like to break this stuff down and understand it.

Thanks again guys...

Tom
Actually, if you go too short on the control horn at the surface you can create flutter regardless the servo power.

Travel and expo have little to do with one another. What has worked well for me in my precision setups is to only use enough travel to get through the sequence. The exceptions to this is a flight mode for spins and rollers ( More elevator and rudder less aileron) and snaps ( less elevator and rudder, more aileron ) Use expo to smoothen out inputs around center. When I let guys who are just getting into IMAC fly my airplane they usually comment on how soft the controls feel but ultimately are able to fly it better then their setups.

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Old 06-06-2012, 04:49 AM
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Default RE: Surface Damping

A couple of generalizations -which may be handy

when measuring relative servo arm length and control surface horn length
measure from center (hinging point)
the servo arm length of aprox 75% of surface horn length is a rough starting point- for a good 3D setup
rudder/el/ ail -then readjust servo horn length (shorten it if possible). This isall about maximizing force available
at speed I have seen guys using $$$servos on 42% models complain and demonstrate aileron blowback at cruise speed -
Bigger models have far more airload problems (has to do with RN) and servos just simply may not be up to the task.
Iyt is hard to believe the 300"ounce stuf (3 per ailerons) cant do th e job - but seen it and the only fix is spades . just like full scale
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Old 06-07-2012, 10:09 AM
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Default RE: Surface Damping

RMH has it right. And you're worrying about the wrong things.

I'm not sure what you mean by "dampening" other than your spelling of the word involves water. "Damping" is the act of reducing motion. If you're looking at maintaining control over the surface and avoiding flutter then the most travel at the servo for angle and then to mechanically reduce the throw at each end of the pushrod is the proper method.

If you can't limit the control surface travel by using the pushrod at the outer hole then switch the output arm on the servo to a shorter one or use a more inward located hole.

The pins or wires in the end connections of the pushrods should be a snug but not tight fit in the holes. You want to reduce any play to the minimum.

And by using the full 100% travel provided by the maximum setting of the ATV and then mechanically reducing the travel via the correct pushrod and end horns you also tighten up on the centering consistency of the surfaces. All in all using full servo travel and proper mechanical setups is a win-win.
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Old 06-07-2012, 12:19 PM
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Default RE: Surface Damping

OK thanks guys.

Too funny on the dampening mistake. Of course!

Im going to get this hooked up and I will be back most likely. I see what you are saying though....

Appreciate the responses guys.

Tom
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Old 06-12-2012, 08:01 AM
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