ORIGINAL: pimmnz
IronEagle, the period you are referring to was when the tab ideas were developed. I think even some of the late WW1 airplanes were using adjustable tailplanes, certainly many had offset fins to counter yaw trim. Fokker pilots were known to carry a bit of U/C bungee to assist with elevator trim...So far as what the difference is between a balance and servo tab is, I think, and only think, that a balance tab is operated by the surface movement, whereas the servo tab is the pilot operated device, which then moves the surface. My own experience is that models, generally, have no need of the things other than reproduction on a scale model, as we have a rigid, servo operated system with a rigid trim system, in other words, the tab is physically unable to do what it was originally intended.
Evan, WB #12.
That's exactly the reason that
trim tabs (or the trim function on balance or servo tabs) are, at best, useless or, at worst, dangerous on models. I remember an article in RCM&E back when large models where rare and large servos didn't exist. A couple of modellers had built a large Lancaster and used the scale elevator trim tab to trim the model nose up by setting the tab trailing edge
up! It worked until the day they flew the model faster than before and 'the tail took over'. What really happened, obviousely, is that the trim tab did what it's meant to and drove the elevator down against the (weak) elevator servo causing the crash.
As for
balance and
servo tabs, as evidenced by the people posting here, it seems that they are very useful on models. Allowing larger and faster models (higher control loadings) achieving full control deflection with smaller, lighter servos and linkages.
I may have misunderstood your post but while I agree that on a model a
trim tab is 'physically unable to do what it was originally intended', there is no reason why a balance or servo tab can't.
My understanding of the difference between balance and servo matches yours.
BTW you are spot on with the WW1 bungy cord trim, we do that on some of the Vintage Aviator aircraft at Masterton. Not that we fly stsraight and level that much, but its handy on a long transit.
Cheers
Dave H