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Direct Drive Servos

Old 11-29-2007, 07:43 PM
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Default Direct Drive Servos

Did any of you guys read the article in AMA's magazine about installing and using Direct Drive Servos? Looks pretty interesting.
Old 11-29-2007, 10:08 PM
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Default RE: Direct Drive Servos

Yes I read the article.

I thought it was very interesting, but I believe this technique is only applicable where there is a foam core sheeted w/balsa.

This is to give the servo case a very tuff anchor.

MR G
Old 11-29-2007, 10:10 PM
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Default RE: Direct Drive Servos

When I said 'the servo case' I was refering to the servo housing the servo is mounted in.

MR G
Old 11-30-2007, 11:42 AM
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Default RE: Direct Drive Servos


ORIGINAL: MR G

When I said 'the servo case' I was refering to the servo housing the servo is mounted in.

MR G
I hear you. It would not be a thing that you could put into an ARF without major surgery. I think that you could use built up wings if you planned it during the construction stage.
Old 11-30-2007, 12:00 PM
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Default RE: Direct Drive Servos

mrgigg,

I really like the direct drive idea and would like to try it, but you are right about the ARF situation.

MR G
Old 12-24-2007, 04:22 AM
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Default RE: Direct Drive Servos

Here's an installation in a Great Planes Eagle ARF...takes about 30 minutes per servo...
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Old 12-24-2007, 08:43 AM
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Default RE: Direct Drive Servos

Would be curious to hear from anyone using the system on 35 and 40% planes.

I'm wondering if the mechanical advantage of 1:1 is enough for the big guys?
Old 12-24-2007, 09:57 AM
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Default RE: Direct Drive Servos

Rick Byrd has one and flew it at the Tucson Shootout... seems to work quite well
Old 12-24-2007, 10:27 AM
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Default RE: Direct Drive Servos

Well...

Allrighty then [8D]
Old 12-25-2007, 12:01 AM
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Default RE: Direct Drive Servos

If the plane can be setup for the proper installation, a direct drive will always be superior for many reasons. For one, no linkage to have play in, or fail. Also the servo and surface have a 1:1 linear ratio of degrees of motion at all points of travel. This means that the surface travels the same speed at all points of surface throw. It also means you don't need exponential since the surface will move the same amount with stick motion....at all points of stick/servo/surface position. For example on a linkage system the surface moves most per stick motion at stick center than at end points. With direct drive not so...it's the same at center, at end points, and like ZZ Top says "all points in between". If you ever take a close look at linkage systems (I did) you will be surprised how non ideal they are (non-linear). I drew hand sketches of three different linkage systems on graph paper. They were all different mechanical advatages...one was like 2:1, one was 1:1, the other was 1:2 (for example 2:1 means 2" control horn with a 1 inch servo arm). It was amazing how the relationship between the servo and surface changed with surface throw. There's nothing constant about that relationship. It changes with every degree of motion of the servo and surface. And only two of those systems I listed should be used. The third one with the worst mechanical advantage gets worse the farther the surface moves.....now that is not a good system to use. The best is the 2:1 which most IMAC and scale plane flyers use. The more your surface moves the better the mechanical advantage gets...because the control rod gets closer to the servo axis of rotation faster than the control rod gets closer to the surface axis of rotation (hinge line). One person in particular argued with me that mechanical advantage is constant but it isn't. The only thing constant once your setup is complete is the ratio of control horn height to servo arm length.

Even with best linkage setup, which is used on the Extreme Flight Yak 87"/88" elevators. The servo axis and hinge axis of rotation are perfectly parallel. Also with a 1:1 setup the push rod ends up pefectly parallel with the centerline of the horizontal stab. That's as good as a linkage system gets. But even with this setup exponential is needed to soften the feel of the center...which moves much more with stick motion than the end points. Because of that constant relationship between stick position and servo/surface degrees, and the elimination of slop, the direct drive will always feel better and control the plane better.

One problem I can see is because the slop is eliminated, ganged servos might be hard to do since the motors are essentially direct coupled. Hopefully the flex of the aileron will absorb that.
Old 12-25-2007, 09:48 AM
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Default RE: Direct Drive Servos

The biggest problem I see, is for non-3D type planes. Servo travel is way too much for required smaller control movements. You can reduce this by reducing your end points but then you loose quite a bit of resolution. In a typical control horn set-up you can gain mechanical advantage and maintain resolution if needed. Of course you have all that other stuff Joe mentioned to deal with.
Old 12-26-2007, 01:34 PM
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Default RE: Direct Drive Servos

I have been running dierct drive on the ailerons on a Zoom Zoom 4D foamy for about 3 years. Biggest probelm is that you do not have the cushion of the linkage in the event of a crunch. You will now get the full brunt of a hard landing on the servo gearset rather than a possible linkage pop off... On mine I imbedded the servo at the root and put a piece of tubing in the aileron. I then CA'ed a short metal rod onto the servo arm that lined up with the inserted tube. I then removed the pins from 5 Klett style hinges and use a single .035" rod to make a full length hinge pin. When the plane hits hard it pops the CA'ed rod off the servo arm. I simply pull the full legth hinge pin and slide the aileron off, re glue the rod on the servo arm and reassemble...

Rick

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