Ive been agonising over the relative math required to make a trimtab effective in all conditions.
I'd appreciate your opinions on my theories.
It seems to me that I'm dealing with three interconnected levers which then drive a fourth.,..the rudder.
So I started from the view, 'What force is required of a tab to cause the rudder to change its angle of attack.' Answer. 'Don't know.' Then I went on to how far the tab had to move to cause the main rudder to move. Answer. Don't know.
But my experience with aeroplanes suggests it doesn't take that much effort from an aeleron to lift/drop a wing, especially if the lift/drop is minor. Obviously, major inputs ( a significant shift in attitude) require greater forces.
So allow me to address the input forces. The first is the vane. The size of this beast 'must' determine an initial input force. That force, in turn, would be determined by the wind-speed...wouldn't it?
Sure, there are some minor vaguries in this, in as much as a hot wind will have less substance than a cold wind, but how much diff is problematic.
Then there's the arm which is turned by the vane which, in turn, turns the tab. The length of this arm has a force relative to the pressure exerted by the vane.
For example: Let's say the vane applies 10kgs pressure to the arm...The pressure, may I add has to be a quantum of pressure along the length of the vane. But for the moment let's say at a certain wind-speed it delivers 10kgs to the arm.
This arm is, say, 200mm long, whereas the vane is 1000mm long. That says, unless I'm way off the mark, the vane is delivering a force of 5-to-1 (50kgs) to the arm which is connected to the tab.
The tab, as mentioned, is fixed in both length and width...although it can me made less.
And so, is a tab which is 1/10th the size of the main rudder enough to cause th main rudder to turn? Is 1/8th, or 1/6th the requirement?
So here's the theory.
I figure on making a tab of about 1/5th the area of the rudder. I can hack this down as the experiment
However, the main input force (the vane) will be a simple sail, captured between two horizontal arms, and on a simple roller-reefing system. Ergo, I will be able to increase or decrease the presented area of the vane, according to conditions.
Next is the relationship between the lever-arm (attached to the vane) and the levered arm (attached to the tab's axles). I figure that making this a multi-choice arrangement then will enable me to fiddle with settings.
For example, I may find the tab is too large in all conditions and that over-steering is an issue. No probs. I'll take to it with a saw and lop of some area.
But, because I can fiddle with the pressure applied by the vane, via foller reefing, and fiddle with the relative lever arm connected to the tab arm, I should be able, over time, to discover a whole bunch of ideal arrangements.
For example, I may find, in 30 knots on the beam, with my particular boat, that I need 2/3rds vane sail and a 6-to-1 lever. Or half vane and a 4-to-1 lever.
Then I might discover that hard on the wind, in 10 knots, I need full vane and 8-to-1 lever.
The beauty of the design I'm working on is, apart from the size of the tab, I can adjust the two other major parameters in seconds, which is really good for instant gratification when it comes to experimenting.
As asserted. Your opinions would be most welcome.