Hey Ryan,

The "why" is:

Better resolution
More torque to the control
Perfectly linear control throw
Less hinge and servo wear
Less cost
Increase in reliability
Ease of maintenance

The real question becomes "why not?" Honestly, for those who have tried the system, the only reason they ever go back to linkages is for CG issues. That is, some kits are tail heavy to begin with, so placing servos in the tail may compound the problem.

Stek79, if you are still around - your argument is absolutely right. It does, however, ignore the issue of deflections due to loads. We are not dealing with rigid materials, but rather flimsy balsa, plastics, and rubber grommets. Your linkages will have more resolution than direct drive...on the work bench. Add some load to the control surface, though, and direct drive will beat it every time. Here are some of the "extra" loads a linkage creates:

The push-pull loads created by a linkage act on the servo arm, hinge, and horn.
A ball link adds a twisting force to the servo arm.
The angles of a linkage at full throw cause odd loads to the control, and reduce the linkage efficiency most when the most torque is needed, i.e. when it is deflected the farthest.

A linkage can only add play to a setup - play that is not there with direct drive. Essentially, direct drive allows you 3D throws with better than pattern resolution.

John

John
Are you still making those stick planes?
Shaun

Nice setup clean n neat.
However.
If surface moves 60 degrees and servo moves 120 degrees -- the servo power delivered to surface is doubles and slop shown at surface from servo gears is halved .
That's basic eng

If you have all the power you want with 1-1 then go for it -It's your plane and your hobby -
It does however, double required work of the servo.

Also IF you consider the "linkage" being a small pulley (or gear) driving a twice as large pulley( gear) attached to the surface-- then you get an entirely linear setup - with no changing angles- the same as direct hookup but with double the power at the movable surface

I've looked over this set up a few times over the past few months and keep coming back to what Dick said. You are loosing servo power with this set up and increasing the slop at neutral. The power issue would have to be overcome with additional servos, thus taking away any cost benefit from foregoing the linkage in a standard set up. Slop at neutral is a huge issue for precision flying and even a larger one for flutter concerns.

That being said I applaud the thinking "outside the box". What you might want to look into is having a manufacturer make a special servo that only has 45 degree throws (each direction) BUT have new gears and/or motors to double the torque at expense of servo speed. Think about it this way as in reference to Dicks numbers above. A current servo (in a standard linkage set-up) needs to be .12 seconds as it MUST rotate fast because it must travel twice as far of a distance to move the surface the appropriate deflection at a desired speed (i.e. 120 servo travel for 60 degrees of surface travel....the output shaft travels .12 secs but the actual surface only travels at .24 sec). If you can have a manufacturer build a servo that has additional step down gears (which would result in a slower servo......all else being equal) you could in essence double the torque at the expense of halving the output shaft speed (which would still render the same control surface in this direct drive system).

.........Mark

Hey Dick, Mark,

We are talking about two different issues. There is the resolution issue, and then there is the torque issue. Let's look at each one:

1) Torque. You are correct. If you halve the motion with a linkage, you are doubling the torque. Of course, no machine is 100% efficient, so you actually lose a little torque in the conversion. BUT, torque is not an issue with the servos we have today. If you need more torque, get a stronger servo. If you want to use a weak servo, then you MUST use a linkage to increase the torque. My view (as a trained engineer) is why add an unecessary linkage just to increase mechanical advantage when your servo gears already do that for you? All you have do is choose the right one for the job! As for working the servos harder...I have servos with over 1,000 flights on them. They can't be too overworked with that longevity.

2) Resolution. As I stated earlier in posts, you are correct in saying that reducing the throw will reduce the servo "gear" slop seen at the control. Servo slop is only a fraction of the total play in a set-up, though. The rubber grommets alone that you are using to mount your servos are giving you more deflection than normal gear slop. Then you still have to add servo arm twist and bend, connection play, rod deformation, and hinge play from your linkage loads. Your linkage can only increase all these other factors. Direct drive eliminates them! Flutter suppression is greatly increased when you have nothing to deflect. And, with two servos on a control surface, you can completely eliminate gear slop too. Adding a linkage to slightly reduce gear slop, at the expense of adding at least triple the other slop, just does not make sense to me. Does it to anyone?

The only linkage we have that is even close to linear is a well engineered pull-pull system - because it is the "pulley" you mentioned. It has limits in control travel, though, is VERY hard on the servo bearings, and is rarely installed properly without binding. Then you have cable stretch to deal with. ALL rod style linkages are not even close to a pulley, and very far from linear. They are only linear if you look at them over a very short range of motion. To add insult to injury, they have a different divergence curve for each direction of travel - not good for an aileron or rudder. It is not possible to have more linearity than a direct drive.

There is really little need to build a servo with reduced throw. If you need less, then just software it where you need it. Personally, I haven't built a plane in 5 years that uses less than 50 degrees of control throw. That's the whole beautyof this system...60 degrees of throw with pattern precision.

But, hey guys, flying one of my planes speaks for itself. If you ever make it to Fort Worth, give me a call and we'll go flying!

John

Hi John, Still not with you buddy on the "today's servos.........torque is not an issue". Physics is physics. Aerodynamic law pertains whether the surface is driven directly or with linkage. If one gains (for arguments sake) double the power by a 2 to 1 linkage advantage then your system will need twice the amount of servos to deflect the same surface the same amount of deflection. This is like telling an engineering professor that you can lift a 50 ton beam with a 25 ton jack, your rational is that the jack travels 12" but you only need to go 6". I'll give you the "no machine is 100%" but by no means will two ballinks rob 50% power from the servo.

Would you advise a client to use only two 8611a's on a 52" 7 x 4.5" tapered aileron delivering 35 degree deflections with your system?

As for the gromets producing more slop than the gear lash, can't walk down that road with you either. You will have some motion but not more than the moving of the servo arm. Plus the most play the gromets will render would be at high deflection (i.e. 3D rates) where resolution is not much of an issue. At neutral we'd see little play and most of it is caused by motor vibration. How do you supress motor vibration on the direct drive system with out something that absorbs the vibration like the gromets?

Just like today's servos produce more power than yesterday's, today's servo arms produce less flex than those of yesterday as well. There are double arms that have no flex.

Again, it's a cool process and I"m glad you've got tons of proven flights on yours. I'd be happy to fly with you if I get down your way some time!

Take care..........Mark

We run something similiar to this in our Giant Scale Racers.
We use a tube in tube.

Hey Mark,

Naw, I think most of the guys are using 3 - 8611's on that size surface these days, even with a reduction linkage. If you were flying pattern only, I'd have no problem recommending just a couple JR's - because at the reduced deflections of pattern, the air loads are also reduced. I'd recommend only 2 servos if you're using Hitec 5955's, even if you do 3D. For each direct drive mount you save about 2 ounces on average. With that weight savings (4 oz) you can install a third servo (2.2oz) for good measure, still come out lighter than 2 servos with linkages, and put the $30+ you save on the linkages toward the cost of that extra servo - that I assure you is just for your piece of mind anyway.

We have found that whatever number and type of servos you used with linkages, that's the most you need with direct drive. My guess is you need extra torque when using a reduction linkage to overcome issues not related to air loads - like engine resonance and flutter suppression.

I have placed the drive on 39% extras and 53% Ultimates with no issues and no "extra" servos. You either have enough servo torque or you don't. Servo gearing is a more efficient method of increasing mechanical advantage than a ball linkage. This is because gears always have an optimum interface with each other, and linkages are only optimum at their center point. I have one Ultimate with a single 1800in/oz rudder servo...you can find the right servo for your application. In the '70's we were stuck with 26in/oz servos that absolutely depended on reduction linkages. That is no longer the case. I agree with you completely when you say if you use a weak servo you need the reduction linkage. I just disagree that you need to use a weak servo so you need twice as many of them. In other words, if you want to move your 50 ton beam, use a 50 ton jack - instead of a weak one that has to be rigged. Or, from another view, with a traditional setup you are adding a linkage that weighs almost as much as another servo just to increase the torque!

Next time you idle your plane take note of the amount of deflection your ailerons make as they shake. It's difficult to make an arguement that only servo gear lash accounts for the amount of aileron deflection you get when the aileron resonates to engine vibration. The more the aileron deflects when shaking, the larger the forces on your servo gears - at an exponential rate. That vibration does not stop when you are airborne, or when the surface is centered. Those are normal loads the control is continuously subject to, and they do cause deflections in the linkage. Direct drive reduces the resonant deflections of the aileron, thereby reducing the shock loads to the servo gears. This fact alone allows you to use a servo with less torque than you would need with a 1:1 linkage.

I have used soft engine mounts for more than 25 years, even on my sport planes. They protect the whole airframe from engine vibration, not just the servos. That's how I get away without using the servo grommets and still get much better than average servo life. Full scale planes also only isolate the engine, since that is the source of the vibration. They do not use any compliant rubber in the controls, mainly because it invites flutter - which is usually fatal. I feel that sometimes in this hobby we choose to reinvent the wheel. The boys with the money have worked out a lot of our problems for us - we just need to follow their lead.

Still, I have friends who are using the drive with no soft mounts - to save weight - and have no problems with servo failure either. I would have guessed they would have problems, but they don't. I think that says a lot about the quality of the system.

Julius, have you got any pictures of the system? I'm always fascinated by what the racers do, as they seem to evolve their own way of building...

John

Just an ubiased opinion, I thought the same way many of you guys did about it not being enough torque with fewer servos and had 3 8611s (not As) on my 39% ailerons and 2 on each elevator. The plane flew great so I never messed with it.

Then on my new yak, I went down to two aileron servos and can't tell the difference (Not quiet as large as the one you had in your example (mine are 6.5" to 4.75" taper)
I initially set it up with 2 on each elevator, but the plane came out tail heavy even with a pull pull on the rudder (pretty dense sheeting) and I was reluctant to pull 2 elevator servos (1 on each side), but to keep from putting 1.5#s of noseweight in the front, I decieded to give it a try. I knew I wouldn't loose the plane, I was just concerned if a regular 8611 would have enough tq to make it snap the same and not get blowback in any 3d. I was pleasantly suprised to find that I didn't lose any resolution or "needed torque" in the removal of the servos.

I'm in the same boat you guys are and just want what works the best in my plane. I fly unlimited and if I was losing any advantage in resolution or blowback, I would change it in a heartbeat, but it works fine.
I just keep reminding myself that we were flying 3d and precision with 2 - 3 8411s on their ailerons and 2 8411s on the elevators not that long ago with no problem. I think trying to use one 8411 on an elevator DD might be pushing it, but an 8611 has twice the tq and a 8611A has 2.5 times the tq.

Just thought I'd share actual real world feedback with you guys.

John

John,

I read with interest post #12 where you mention;

" I've got a trick to negate the servo gear slop too...if you are running dual servos per control. It envolves intentionally offsetting the neutral points to "take up the slack" in the gears. Be very careful, though, because you can fry a servo in short order if you offset to the point that the servos fight each other. This is an advanced technique I would NOT recommend until your flying is good enough to notice the gear slop. "


I've been doing this on a 35% and most recently on a 40% wondering if the servo gods were looking unfavorably on me or
something.

How is it working for you?
Do you re-offset frequently and when necessary with no problems?

Since you are an engineer, doesn't moving the sub trim throw of the geometry-resolution-linearity issue in some way?

It's definitely a neat way to beat the system if you will.

Take care_bob