Hi guys

What are you your views on this idea, for's and against please.

I am trying to build a scale jet without the normal servo and pushrod hanging out the bottom of the flying surface to the aileron, rudder and elevator. The full size has no suitable scale actuating rods to use as camouflage and I do not want external horns if possible.

Well after some head scratching I was looking at the Voltz and hitec wing type servos 3+kg. I wondered if I could put the servo on the inboard end of the surface and connect directly to a suitable servo arm ie no linkage fix the arm to the surface with the centre line of the servo output in line with the hinge line.

Plus points as far as I could see

no slop other than in the servo itself, no linkages to build more slop in, hidden surface actuation.

Downside

may be more vulnerable to servo damage even with metal geared servo from ground handling, not sure how the torque would workout ie would I get the 3kg?

There must be some downside I have not thought of, or someone would have done it.

So your views would be appreciated

thanks Ian

If you can get a servo that close to the moveable surface, can't you use a regular torque rod system?

I suppose your approach could work, but you will loose a lot of the servo's torque.

THe servo is design to rotate 90 degrees total, give or take. I would figure on a scale jet, you are not going to want more than 10 or 12 degrees in each direction (and even that sounds like a lot). So that would be 20 -24 degrees total.

So the effective torque is say (24/90) times less, only 26% of the torque you would get if you could set up the linkage to use all of the servo's throw.


PS your slop will be a lot worse too.

I am not so sure Mr Matt!!. The torque is rated at oz.inches hence if the rating is 40 oz.inches that means you will get 40 oz of torque at 1 inch servo arm length (correct me if I am wrong).

Ths slop should be LESS, all things being equal.

So, Ian's idea would work if he has an inch of servo length and he will get the full torque. Also, Digital servos (assuming Ian is using them) have higher HOLDING torq and develop their full torq at smaller movements than linear ones.

Ian, if you can get the geometry right, use the smallest servo arm you can to achieve your desired deflection and I think it should work. But place the servo (use metal gear ones) in the middle of the surface if you can and run 5 cells!!

Some of the pattern guys were using this set up on the rudder. From what I saw it worked great.

I'm sure this will work but you will have to use the TX ATV or endpoint adjustment to decrease your throw ( unless of course you want 45 deg. either side !) This is less than optimal use of the servo. The slop in the servo will result in quite a noticeable amount at the control surface and your resolution will suffer a bit. Check out the RADS system for a good hornless way of doing things.

http://www.proptwisters.org/rds2/

thanks guys for the good comments

More positive than I expected, I thought I was missing something fundamental

Yes I would use metal geared digitals. I was thinking about the arm fixed at the end of the surface, but understand the point of being in the centre to reduce surface flex.

The rads system I have heard has play, I cannot see how a direct servo to surface has more play than arm to pushrod to control horn normal set-up or the rads set-up

Any other views gratefully received

Ian

The RADS setup I use are rock solid ! You have to build the pocket with a slight interference sliding fit.
The play won't necessarily be worse but with a conventional horned system where you use a longer control horn than servo arm you can diminish the amount of play at the back of the surface as well as increase mechanical advantage. The RADS system allows you to increase mechanical advantage by changing the bend in the wire.

Marc

The direct servo setup will be HORRIBLE from a servo resolution standpoint. You're going to have to limit motion to around 30% or less I'm guessing. Not an efficient use of the available servo power.

Hi Doug

Your correct thinking about it, the resolution will be an issue, I guess the pattern guys on the big stuff use it on rudder only where more movement is needed.

I will have to have another look at the rads approach and try a set to see how it works for myself rather than others views about the play. Like most things with this hobby the better the build the better the flying

Thanks for your views

Ian

Hi Ehab,

Yes indeed the torque of the servo does not change. The torque available to move the SURFACE, however, definitely changes based on the linkage geometry (remember the Bobcat elevator thread ?!?).

The system that Ian is describing is basically the same as a system with a very long servo arm, attached to a very short horn on the surface. Of course then you need set the ATV to 25% to get the correct throw!!

Now we all know good practice is just the opposite....short servo arm, long control horn on the surface and ATVs set up to 120%....just the opposite of Ian's case.

As a rule, you want a linkage that provides for the maximum movement of the servo for the associated movement of the surface. We all sort of know intuitively that setting up a surface and then setting the ATV down to 25% is not a good idea.....this is the reason.

And yes the slop will increase in Ian's case as well. Just as a normal servo linkage increases torque, it decreases the effect of most kinds of slop, when slop is measured at the trailing edge of the surface.

As for an RDS, this is a better choice, as an RDS allows for better geometry (ie you get the torque mulitplication of the "standard" servo linkage, but slop is much tougher to control in an RDS installation as well).

Hi Mr Matt:

Thx for the explaination. I was assuming that Ian will use the longest horn and the shortest servo arm. I agree with all of your comments...This time we agree to agree