Trimming our airplanes is done in several stages. For basic trim we may hire Arnold Schwarzenegger to bend the flaps. For advanced measures we can ask Vitaly Klitchko to help us heave some lead into the tip weight box. For high end fine trim we use a more subtle method. It’s called Line III . A friend lead me to this software. Give it some numbers and it will calculate a point which will help us set the correct line rake.
We all know that our lines don’t run in a straight line from handle to airplane. Depending on model weight, speed, line length, thickness , and some more factors our lines are bent backwards, sometimes quite a lot. We’ve also seen that the lines are running almost straight close to the handle, but they are bent more close to the airplane. This is the result of the airspeed "felt" by the lines, and this is higher near the airplane, of
 
course ( we’ll need this appearance later ). If we would set the line guide exactly on the lateral axis ( which runs right through the Center of Gravity ; and, by the way, NOT on a line running through the bellcrank pivot point ! ), our airplane would immediately turn towards us with the well known results. We’ve learned to set the line guide somewhat AFT of this lateral axis line. From experience and for safety reasons we usually start with somewhere between 2 and 3 degrees ( sketch 1 shows the angle ).
For a first bench trim we hang our airplane at the leadouts. Using a protractor we can adjust the position of the fuselage until the desired angle is reached. Of course the fuselage side ( where we place the protractor ) has to be parallel to the longitudinal axis ! This set up is a good base to start with. Probably we will have to make corrections later, especially if we alter one or more of the above given numbers.
 
 
The software Line III allows us to set a precise trim right from the beginning and/or to check the correct setup later in the trimming process. It can easily be downloaded from www.stunthanger.com/temp/LineIIISetup.exe and needs 4 Kb space. After opening the program we can look for some information first. Clicking on “Index” will open a list of topics which will explain how to use the software. Before we start to work we’d better have all the numbers ready which are required for calculation. This includes the type and length of lines, inboard wing length, handle length, airplane weight ( including fuel !), wing area, and our ( actual or desired ) lap time. After we have typed in all these numbers we simply press
“Enter” - and the program will show all kinds of helpful data in the right half. It’s a pleasure to change one of these numbers and instantly see what effect this can have on the outcome. Under “what if . . .” we can simply insert another lap time and/or line length and get the correct answer. We also ( finally!) get the correct values for wing loading and ground speed. On an advanced level we can give atmospheric specifications and Line III will consider even these.
 
What concerns us most is the leadout position. It’s given in inches, behind CG ( which means behind that lateral axis. We should somehow mark this on the inboard wing tip ). Now the trick. Line III will calculate a dimension called “Line Projection” (D). After having marked the CG position on our left fuselage side, we measure this distance BACKWARDS from the CG and mark the end. I did this with a bright red piece of masking tape.  
Now comes the proof of the pudding. When flying our airplane we hold the handle as close to the eyes as possible; we should see the handle, the lines, and our airplane “in one picture”. As we look down the straight part of the lines this view direction should exactly hit the line projection point on our fuselage. If it does our trim is okay ( well, at least the line guide position ).
 
I’ve made this test with my Messerschmitt Me 109. It is not my best airplane but an acceptable flyer. It had already been trimmed to a stage where I cannot see much further improvement. Line III calculated the correct point, I placed the red tape, and went flying. It was a real shock : my line guide position is correct ! ( well, every dog has his day ).
While I don’t think that this program is indispensable, I thoroughly consider it a helpful tool. It surely helps to make a quick decision when for whatever reason we want to change line length or lap time. We can let Line III do the math in advance and draw some marks on the wing.
Imagine that professional look with those decent marks on your left wing tip !