An article way back when in MAN discussed using 'offset' tabs to assist servos on the control surface. In case my technojargon is incorrect, The tab moves opposite of the intended direction of the surface. E.g., if the elevator goes up, the tab goes down. Then there is what is referred to as a 'boost' tab, which would go 'up' further than the 'up' elevator-or down as the case may be. Is either effective, and what factors determine which one is used? Building a Storch which has aileron tabs, but I don't know which type was on the original. Actually made a mock up from pics, but they don't show the whole surface sufficient to tell how it worked. This pic operates like an 'offset' tab, but I don't know which way the full size worked.

The monikers "boost tab" and "servo tab" are sometimes used to describe the same thing - a bit confusing. I wrote an article in MAN about both boost tabs and servo tabs about ten years ago. Boost tabs move passively in the opposite direction to that of the control surface. For example, when an elevator angles up, the boost tab angles down in relation to the elevator. This causes it to produce an upward aerodynamic force near the trailing edge of the elevator that assists the servo, or muscle power, in moving the elevator up. The boost tab normally does not move to a larger angle than the control surface deflection unless the tab is a bit undersize. I always adjust the linkage of the boost tab to produce less angular deflection than I expect to need, then gradually increase it until I get the control response that I want. Too much movement of a boost tab can result in the control surface overpowering the servo, and forcing the servo to restrain the movement of the control surface. Evidence of this is that the model just refuses to trim, since the control surface tends to flop up and down within the linkage slop. I find that boost tabs can reduce the force needed from a servo by up to about 85% before the system gets a bit 'twitchy', or non-linear.

The term "servo tab" normally applies to a tab that is controlled directly by the servo, with the control surface free-hinged, and mass balanced, to weathervane in the slipstream. Some large airliners use this system, which provides beautiful control feel and permits muscle force alone to control a very heavy airplane. Examples are the DC-9 and MD-80 ailerons and elevators, and DC-8 ailerons, which use servo tabs connected directly by cables to the control column, and boost tabs on the control surfaces. The combination probably provides a control boost multiplier of something in excess of 100 times.

I am not familiar with the term "offset tab".

This is the boost tab on my beater 1/4 scale Corby Starlett. Don't laugh, they are very affective, like you put in the next bigger, faster servo in it.

Your article may very well be the one I was referring to. I still have the mag somewhere in my stack. What I was calling an 'offset' tab is what you are describing as a boost tab, the tab moving in a direction opposite to the control surface. I guess, then, the question is whether there is some type of 'tab' that moves in the same direction as the control surface, but slightly more than the main surface-that's what I was calling a 'boost' tab. I'm trying to visualize the servo tab and how, or if, it would work on a model. Your reference to the airliners' use of both is confusing me, I can't visualize how that works. Maybe I need to dig out the article! Seems like that system would work well on model jets, e.g., those with all moving stabs that use mega powered servos, or double ganged, (got one of those U/C too- Su-27). I don't recall seeing any on models (jets), think there a good idea?

Flypaper: Can you post a close-up pic of your setup? I'd like to see exactly how you did it. Bill S.

The boost tab works this way..
Check out the T-6 aileron.
The L-1011 uses a similar situation on the horizontal... but it works in reverse.
What appears to be a normal elevator is in actuality a geared tab, which moves -more- than the horizontal surface moves and in the same direction, to alter the camber and therefore boost the control authority of the slab.

If there's a tab on the trailing edge that moves in the same direction as the control surface, then it's an "anti-servo" tab. You see this on the trailing edge of the stabilators on Piper aircraft.

A stabilator tends to have low control forces, so there's little 'feel'. The anti-servo tab moves into the airstream to oppose the movement of the stabilator, so the pilot feels more force on the controls. The tab also tends to make the stabilator move back to the desired "in trail" position, making it self-centering. By connecting the anti-servo tab to a control in the cockpit, you now can adjust the centering of the stabilator, and have an effective trim tab.

Change the size and amount of deflection of the anti-servo tab, and you can increase or decrease the control feel and the self-centering ability of the stabilator.

WS:
Don't know if my beater camera would take much closer without blurring. Piece of a servo arm glued to the ail. hinge spar and pushrod hole is close to it. Tab has a mini control horn at the leading edge. Started at the outer hole and moved in a hole at a time till the roll rate went up. Don't go any closer as it will tend to overdrive the tab and you'd have a hard time finding neutral Tab is 10% of the area of the aileron. Don't Know why they don't use these on these monster 3 D planes, instead of using two , three and four servos to drive a control surface. Tab has the mini nylon hinges to the ail surfaces. Not much actual load on it. Hope this helps.

Think I got it, Flypaper. Interesting concept, perhaps tabs are not that well known in modelling circles. I don't know of any kit built plane that incorporates them-maybe some warbirds. Think I'd like to experiment trying them on an all moving stab. Looks like the servo tab might be the best for this type of control surface.

I've used the "anti-servo tab" on a flying horizontal on an SAE Lifter... can't say it impressed or depressed me, it worked as expected.

I would like to hear some thoughts about using spades on the ailerons, instead of boost tabs. It is very common on full scalers like Decathlons. The spade looks like, well a spade, is hung horizontally under the aileron, forward of the hinge line, and rigidly mounted to it. Seems rather straightforward to me, plus there are no moving parts. It has the side benefit of giving the modeler a chance to counterbalance the aileron to help avoid flutter.

Anyway, I have been thinking about adding them to my next project, but have never seen them on a model. Hanger rash is my main concern.

Bedford

Spades only work if the ailerons are inter-connected, like they'd be on a full-size aircraft. Their purpose is to lower stick forces. If you're using separate servos to drive your ailerons, then the only purpose spades would have is for scale effect. They would not add anything aerodynamically to the model, except as counterweights. Then, you'd have to make them well out-of-scale to get enough weight to be effective...may as well use a streamlined weight.