keep the geometry such that they are quite INeffective
let the servos do the work- safer setup
on the full scale the pilots legs had to fight airloads

Ian, those big tabs on the Vimy are simply servo tabs to assist the pilot moving the rudders. The air loads on this, and similar aircraft, are high and pretty tiring on the pilot so any assistance is appreciated.
There are three types of 'tab', balance, which assists the movement of the surface, anti-balance, which resists the surface movement, and trim, which the pilot can control or are ground
adjustable, and bias the whole system and should be considered as a flight control. Balance tabs have been discussed here 'ad-nauseum', anti balance tabs are used on some aircraft to provide an airspeed dependant 'feedback' to the pilot, particularly when the designer got it a bit wrong and the controls are a bit light and there is a possibility of the pilot exceeding the structural limits by applying large control movements at high speeds. The method of operation is also varied, but that discussion belongs to another time.
Evan, WB #12.

The three types taught to me were:

TRIM. Ground or flight adjustable. Don't move when the control surface does.

BALANCE. (anti balance, servo, or boost) Moves when the control surface does to assist or resist the control input (may also be trimmable).

SPRING. Pilot doesn't control the surface at all, but 'flies' the tab which in turn moves the control surface (the spring gives required feel). Also may be trimmable

All three are for use on aircraft with unpowered controls ie 'stick free'.

Our models are 'stick fixed' ie the servo locks the control surface to the tx commanded position. Therefore, in a model, TRIM tabs aren't a good idea (they will backload the servo). BALANCE tabs will work (and very successfully). SPRING tabs also work, but I can't say I've seen very many examples.

Sorry Evan, I know you know all this.

Dave H

Yep Dave, I was simply trying to differentiate the tabs by Function, rather than how they are operated.
Evan, WB #12.

I have experimented with boost tabs and believe they were effective. My experiment was on a 22 pound pattern airplane.

Boost tabs are easy to make, easy to modify or change, and easy to revert if you conclude they are ineffective. The experiment suggested boost tabs are effective. From an engineering perspective the problem suggests that lighter forces can be used to control larger surfaces.

I attempted to measure the servo amp loads and forces before and after but ran out of patience or tenacity before definitive conclusions were reached. The in air subjective flight testing suggested that the flight loads were lower and servos were more responsive.



BillS

I reached the same conclusions as you did. It was especially notable on the rudder on a large 1/4 scale plane; even with a weak servo, I had good control on rudder intensive maneuvers with the boost tab and virtually no control prior to installing the boost tab. I did find that I could overdue the boost tab though, if I exceeded 10% of rudder area for the boost tab, it would tend to hunt (over power the servo). On all my tests, 8 to 10% seemed to be an optimum size for the boost tabs.

Did they work in the real world?

How many were there? 12,571 Corsairs was it?