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Single servo ailerons

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Bob Cotsford16/08/2019 14:45:16
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RCPF - one servo or two has no effect on how differential acts, dif is dif. Only the modeller cares as two servos makes it easier to implement differential, either by offsetting the horns or by using two channels and implementing it at the transmitter.

Now how about twin servos through a Y lead (minimum 3 connectors) or twin channels (minimum 2 connectors)?

Incidentally, my WotsWot failure was with a Futaba 148, and while I controlled the roll I failed when it dropped into a spin/flick while trying to turn it on rudder. Ultimately pilot error - ambition exceed ability to quote Casey Stoner.

It turned out that the soldered joint between lead and pcb failed (fatigue due to insufficient support?)..

Nigel R16/08/2019 14:45:44
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Out of interest, what kind of servos were the failures on?

edit: question to Martin H

Edited By Nigel R on 16/08/2019 14:46:39

Bob Cotsford16/08/2019 14:56:31
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I still say the biggest benefits of two servos for the average club modeller are that the force acts nearer the centre of the control surface, and that this can be accomplished with less slop than using either pushrods and belcranks or curved snakes, as we used to do.

Also incidentally, I've only ever had two cheapy servos fail in flight, one on a flying wing elevon (early Corona) and the other on floaty glider's rudder (DYS). Both proved interesting.

Martin Harris16/08/2019 14:56:40
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Posted by Nigel R on 16/08/2019 14:45:44:

Out of interest, what kind of servos were the failures on?

edit: question to Martin H

Edited By Nigel R on 16/08/2019 14:46:39

1. Futaba S3001

2. Hitec HS225MG

3. Probably a Futaba S148 but I can't be bothered removing the hatch to check and it's irrelevant as it was a connection failure between the receiver and an extension lead.

4. Ask Dickw!

P.S. I've had several cheap servos fail (happily pre-flight) which came pre-installed in foam ARTF models.

Edited By Martin Harris on 16/08/2019 15:04:06

Nigel R16/08/2019 15:13:57
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Posted by Bob Cotsford on 16/08/2019 14:56:31:

I still say the biggest benefits of two servos for the average club modeller are that the force acts nearer the centre of the control surface, and that this can be accomplished with less slop than using either pushrods and belcranks or curved snakes, as we used to do.

 

A reduction of slop is a definite mechanical benefit.

edit - thanks Martin.

Edited By Nigel R on 16/08/2019 15:14:22

Geoff Sleath16/08/2019 15:35:38
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I've just bought 4 x Corona CS238MG metal gear/ball bearing servos for my Ryan ST project (which will have a servo/aileron btw). I've soak tested then for a few hours using a simple servo tester and they seem fine. They small (22 grams) and have quoted torque of 4gm.cm (higher than a 148) and I think will be perfectly adequate for an estimated 2.5 kg scale(ish) model.

One feature I like is the supply of a pair of brackets to allow side mounting. That won't affect this model but I used wing servos in that orientation in the past and it's always a bit of a fiddle - these brackets should make it a walk in the park (or a fly at the airfield!).

Single servo aileron operation was always (or very often) using Futaba 148 mounted in the middle of the wing. This meant using torque rods fitted down the trailing edge which don't give very slop free operation in my experience. Also I was always uncomfortable with the reduced glueing area for the t/e stock which sometimes even included the holes for the wing bolts. I admit I never had a failure but it just seemed a bit iffy.

Using small servos, one for each aileron, usually at least doubles the torque available for the control surfaces and provides very positive slop free operation. With a built-up wing it's just as easy (or even easier) to mount and connect the leads. The added advantage of being able to set differential easily and/or flaperons is a bonus.

I use a lot of exponential on the controls (20%+). I like the dead band between elevator and aileron controls (mode 2) because, with limited control over my right hand in particular, it helps a lot. I've never noticed the strange response of the mode; to my inputs; I just watch the model and adjust as required.

Geoff

Nigel R16/08/2019 15:54:48
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"This meant using torque rods fitted down the trailing edge which don't give very slop free operation in my experience. Also I was always uncomfortable with the reduced glueing area for the t/e stock which sometimes even included the holes for the wing bolts. "

There's always a way... Ditch the dihedral brace and instead glass the centre section; then add a ply speader plate under the bolts, which covers the TE stock and an equal distance forward thereof. Solid.

I've not personally found torque rods to be sloppy. Use med to med-hard solid stock for ailerons; drill very carefully for the rods; final fix with 30 min resin. They are my go to solution for standard sport model type strip ailerons.

If you use mid mounted control horns, my opinion is that they need as much attention to the mount point to avoid a slop, perhaps a pair of 1/32 ply squares on the surface or a dowell embedded into the aileron, etc. etc.

edit: Two micro or mini servos can live in the wing centre if you need flaperons, I think a photo was posted already on this thread of a setup like this.

As ever, YMMV.

 

Edited By Nigel R on 16/08/2019 15:56:33

PatMc16/08/2019 19:24:35
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When using flaperons, or any other mix involving dual tasking of the servos, the servo torque is reduced & slop increased.

Richard Wills 216/08/2019 19:43:59
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Posted by PatMc on 16/08/2019 19:24:35:

When using flaperons, or any other mix involving dual tasking of the servos, the servo torque is reduced & slop increased.

excuse my ignorance, but can you please explain?

PatMc16/08/2019 21:43:22
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Max angular movement of a typical servo output is 120 deg total. Let’s say we want aileron movement of 30 deg total.
When using the ailerons purely for the roll function, in order obtain best advantage of servo torque the whole 120 deg movement will be used resulting in a gear ratio between servo output & aileron horn will be 1 : 4 – i.e. the ail horn will be 4 x the servo o/p in length.

Let’s say we want to use the ailerons as flaperons by splitting the total servo movement 50 –50 to each function.
If the same size aileron horn is used then the servo output length must be doubled. The gear ratio now becomes 1 : 2. So the servo is being made to do the same work with half it’s movement.

Also the servo’s gear slop is a fixed angular movement. The linear movement at the servo o/p is proportional to the o/p length, By doubling the o/p length for flaperons the linear slop movement is also doubled.

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