The 3D boys are spending gobs of money putting multiple high dollar servos on all control surfaces to handle the high loads. Why not use servo tabs to actuate these monster control surfaces???

Mechanizing the attachment to the tab from the servo is probably the reason.
It's simpler to cascade servos than finger out how to make such a thing operate.
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Or, it could be simply lack of confidence in the ability of a servo tab to perform at all the flight conditions a 3D plane encounters..
Lots of zero airspeed stuff, where having the suface fully deflected by the servo train can be depended on, rather than relying on low air pressure to move the surface the same amount.. or hold it there..
See a similar thread on boost tabs.
I've used boost tabs, but can't say they worked any better than no boost tab.

Paul
Yea the zero airspeed explanation makes sense so that's probably why they don't consider it. But look at the expense it would save. I know guys who have $1200 tied up in just servos. If it weren't for the extreme maneuvers i'll bet one standard servo hooked up to a boost tab could do the same job.

Is there a time delay in control response that would make the trim tab fine for cruising across the country at 1g and less than desireable in an aerobatic mode?

I doubt there's a time delay... this guy likes to use his...

Reynolds number effects might get into it ????????

I honestly have no idea, but something is nagging me at the back of wherever those things that nag you come from. The thing about being old (well 60 and not that old although badly beat up physically) and senile (ask my lovely wife) is that things seem to hover just out of mental grasp.

As far as loads go if there is sufficient dynamic pressure over the surface to get airplane response then the tab might also be working for you. The higher the q the better it works. In a low q condition all you are moving is the weight/inertia of the control surface, not a lot.

That being said, if I were making a big TOC machine I would build in trim tabs and stick a medium load servo on them. Unloading the main servos is not going to hurt anything that I can think of even if all the loads are not being removed. As a test you could start out with all four on rudder, then unlink one at a time until some lack of rudder response or precieved deflection rate is noted. As was noted you could end up with several hundred dollars of savings.

As I am no expert in aerodynamics, I can't offer any advice, but I can show you this...http://www.gsal.org/projects/BranExt...boost_tab.htm.

I found a great artical on tabs at this site http://naca.larc.nasa.gov/ but I am not sure exactly where it was on the site. I use the information to design tabs and balances for my latest .90 sized aerobatic plane. I havn't flown the plane yet so I dont know what they will do yet.

I tried them as an experiment on an old 1/4 scale Corby Starlet on the ailerons. Didn't change anything but put tabs on. 10% of aileron area. Had small size horn on the tab. As I moved in on the horn, one hole at a time. on the third hole in, it was as though you put the next size bigger servo in and increased the roll rate by 1/3. So they do work and not that big a deal to retrofit.