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Oct 30, 2019, 08:43 PM
Build Fly Crash - Start Again!
onewonders's Avatar
Thread OP
Question

Two Elevators vs One


So I've been working on my Harvard build; designed to have one servo with an elevator joiner.

But after reading many notes about elevators and servos and joiners, I decided to go with two servos one for each side. I am using modified strip ailerons pieces from dubro

So instead of a hs-625MG rated 6v 6.8kg-cm weighing in at 55.2g or two HS5087 rated 6v 3.6kg-cm weighing in at 21.9g each

i see a weight savings with slightly better torque, but .02sec slower response . Another golden rod will be used so there goes the weight advantage

Seems to me that two servos are the winner with better control for trimming . Am I right or wrong? Thoughts?
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Oct 30, 2019, 09:51 PM
Scott
Pylonracr's Avatar
Well, the weight is a wash, the speed difference will never be noticeable, and the redundancy will give you piece of mind if nothing else. If you were ever to have a servo failure it would probably save your plane. If you never have a servo failure you will never know the difference.

Do the math.

Scott
Oct 30, 2019, 09:58 PM
N = R* fp ne fl fi f
Azoic's Avatar
I have used both twin and single servo's in models. It's usually the size that dictates the need. I still trust a single 4.8kg servo and 60cm rod on the elevator for my 72" Astro Hog. It has never needed more, even when it was on floats. I do run 2 x 20kg for the elevator on my 30cc Yak and the same on my .120 Size Decathlon. Both for response and adjustment. Weight is another factor, the heavier the model, the more control effort needed usually. Some will disagree with all the above, opinions will no doubt vary.

Some would say one servo is more than enough for any model if it's the right size, but then, I am the same individual that installed 2 x 40kg servo's for the elevator on his .46 size Decathlon......responsive is NOT THE WORD !!!!!
Oct 30, 2019, 10:43 PM
If it flies, I can crash it.
rocketsled666's Avatar
Absolutely needing more than one servo per control surface is primarily a function of the size of the control surface (and how fast the plane is flying). It's all about deflection. You apply a force at one point. The surface deflects more at that point than at points further away from where the force is applied. if the surface is big enough, and the plane is going fast enough, the difference in deflection across the surface could be significant.

For a single-sided elevator linkage, the side that doesn't have the linkage deflects less than the side that does. So with high elevator deflections you might also get some roll from the asymmetry.

There's also speed. The servo has a specific amount of power to move the control surface. That means that the more force the servo has to exert, the slower it has to move. A bigger servo would work the same as two smaller servos. But for a model designed for one smaller servo, adding a second on the other side of the fuse is often pretty easy.
Oct 31, 2019, 10:25 AM
Build Fly Crash - Start Again!
onewonders's Avatar
Thread OP
Thanks guys, my assumptions were right...I will continue with two smaller servos for each side of the elevators!
Oct 31, 2019, 02:48 PM
Registered User
I seem to remember that using two servos can be a little problem because a simple'Y' connector will make them move in opposite directions. You can use a servo reverser or another channel and mix them, if you have that extra channel.
Oct 31, 2019, 02:56 PM
AndyKunz's Avatar
I'm pretty sure he's talking about separate channels, so they can be trimmed independently (sub trim, travel, and balance settings are per-servo).

Andy
Oct 31, 2019, 03:12 PM
Registered User
A simple Y connection is all I ever use and I only use one channel. Both controls will go in the same direction if the arm on the servo is on the same side.

I use two servos for elevator control almost exclusively. Saved a plane once when it had one side control linkage break down.

Bedford
Oct 31, 2019, 04:02 PM
N = R* fp ne fl fi f
Azoic's Avatar
Quote:
Originally Posted by bp4rc
A simple Y connection is all I ever use and I only use one channel. Both controls will go in the same direction if the arm on the servo is on the same side.

I use two servos for elevator control almost exclusively. Saved a plane once when it had one side control linkage break down.

Bedford
I can beat that.....I lost the entire right hand stab, elevator and servo on my 30cc Extra when the factory bolt locations let go. Still got the model down in one piece, with little effort, it flew ALMOST the same with one side of it's bum fluttering down out in the boonies......I got the stab back, fixed the dodgy mounts, refitted it and it's still flying.

If it can take two servo's......fit it with two servo's. Seems a simple solve to me. My models are usually worth more than a single servo cost, so even the math cannot win...maybe I need some of that NEW math they always talk about.....
Oct 31, 2019, 10:38 PM
'Douglas' to his friends.
Hmm... This topic is making me think. I'm building a Curare 20 (from short kit...); this 'plane has separate( anhedral...) elevators, and I've installed a single servo with a 'Y' coupler for the control cables. So far, indications point to the C of G being too far forward (although it's early days...); it may be a helpful option to double up the elevator servos, instead of this coupling, thus adding useful weight towards the rear. Two birds with one stone, eh..? Hmm... I'm glad I read this topic...
Nov 01, 2019, 10:32 AM
Build Fly Crash - Start Again!
onewonders's Avatar
Thread OP
Quote:
Originally Posted by AndyKunz
I'm pretty sure he's talking about separate channels, so they can be trimmed independently (sub trim, travel, and balance settings are per-servo).

Andy
Actually I was only thinking of a Y-cable, but this is a great idea, then yes I would have better trim controls. I was going to ensure trim was mechanically perfect, but once this fuselage is sheeted there won't be any access to the tail and the elevator connection to the control rods. (unless I want to open it up) hmmm maybe an access panel...Harvards had those all over the place.

Thanks
Doug
Nov 01, 2019, 03:21 PM
N = R* fp ne fl fi f
Azoic's Avatar
Simple solution to the sticky issue of access after covering....
Cover the entire model in Cling Wrap........it's thin enough to blow a hole in just looking at it, rolls and folds on itself and cannot be layed out flat again for love nor money, tears in the wrong place when your trying to get it off the roll, etc.
BUT !!! You wouldn't have to GUESS where, IF anything goes wrong, as the model would be see through.
And you'd be the HIT of any BBQ at the field, as you'd be the only one with a pre-packed ready to eat model.......

Just an Idea......
Nov 01, 2019, 04:19 PM
Build Fly Crash - Start Again!
onewonders's Avatar
Thread OP
Always interested in ideas crazy as it sounds I just might try it. Does it come in trainer yellow? This is a Harvard (not a texan)
Nov 02, 2019, 02:46 AM
N = R* fp ne fl fi f
Azoic's Avatar
"Does it come in trainer yellow?"

If you wrap a Ham and Mustard sandwich in it first it just might......


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