Adjustable leadout location

[Keith Spriggs]

If you are building a plane that was designed before adjustable leadouts became common and the location is shown for the fixed leadouts, how do you determine where to put the adjustable leadouts?

If that is not confusing to you please explain it to me.

Another way of putting it would be if the leadouts were made adjustable should the bulk of the adjustment be to the back or to the front.

[Paul Smith]

That's an interesting question, especailly if you're designing a new airplane.

Basically, the front leadout will need to be someplace between the CG and 4 degrees behind the CG.  Since you don't actually know where the CG is when you install the guide, it's something of a mystery.  Maybe that's why they make 'em adjustable.

Also, the longer the wingspan, the bigger 4 degrees becomes (when measured in inches of travel).

I hardly ever use adjustable leadouts, but it sure would be a shame to put one in and still miss the range.  The best bet is to copy a similar existing airplane.

[RC Storick]

This is a good question! I always use the TLAR method. For hose who are confused as to what TLAR is. It is That Looks About Right Method.

[Bob Reeves]

Unless the design varies far from normal numbers the leadouts will end up some where around 1/2 to 1 1/4 inches behind the CG. CG should/will/might end up 15 to 25% of the wings cord. These numbers should give you enough information to locate the adjustable gizzy so you have the bases covered.

[Trostle]

For a new design, I assume the initial CG will be at 25% of the MAC.  I have an estimate of the model weight and the initial line length.  I estimate the model speed (around 55 MPH for most stunt ships in the 500 to 700+ square inch sizes) and will know the line diameter given the engine I plan to use.  I then go to the Netzeband nomographs in his "Control-Line Aerodynamics Made Painless" article in the Jul/Aug 66 issue of American Modeler to determine the position of the leadouts with respect to that assumed initial CG position.  My adjustable leadout system then is centered on that location.

After flight trimming, I have found that trimmed leadout position usually is within 1/4 inch or less from the calculated position relative to the trimmed CG position.  (This means that as your trimmed CG position changes, so will your optimum leadout position.)

Something that works almost as well and you do not need the nomographs or the fancy computer programs that some might recommend also comes from that same Netzeband article.   Bill worked out the "typical" line rake for each of 16 categories of control line models including the speed classes with monoline; several carrier categories based on weight, three lines and line sizes; various racing events; various sizes of "sport planes"; and various sizes of stunt ships.

For stunt ships, the following is from the Netzeband table and will get you in the right ball park for your initial line position at the tips.  The angles given below are measured as the rake back from the CG toward the tip.

For 35-40 powered stunt ships using two 60 foot, .015 lines, the chart shows a range of 2.5 degrees for a 36 ounce ship to 1.5 degrees for a 56 ounce ship.

For 45 - 60 powered stunt ships using two  70 foot, .018 lines, the chart shows a range of 3.25 degrees for a 44 ounce ship to 2.0 degrees for a 70 ounce ship.

You should be able to extrapolate between those ranges given your estimated model weight and assumed line length.  (For example, more weight, less rake; longer lines, more rake.)

For the record, the angles shown above can be translated to the rake in inches back of the CG for each 10 inches of span of the inboard wing. (Distances shown are approximate but close enough  - within + or - .03 inches - for the purpose of this exercise to locate the leadouts)

1.5 degrees, use  0.27 inches per 10 inches of span
2.0 degrees, use 0.35 inches per 10 inches of span
2.5 degrees, use 0.43 inches per 10 inches of span
3.25 degrees, use 0,57 inches per 10 inches of span

(The larger displacement motors - those beyond .65 displacement - had hot yet been invented for stunt when these tables were made.) 

Hope this makes sense.

Keith Trostle

[Keith Spriggs]

Thanks to everyone for their interesting and very informative replies. I believe I have a pretty good handle (no pun intended) on it now. I think I have the magazine with the Netzeband article in it. I will have to look it up.

[Bob Reeves]

Thanks to everyone for their interesting and very informative replies. I believe I have a pretty good handle (no pun intended) on it now. I think I have the magazine with the Netzeband article in it. I will have to look it up.

If you had rather take the easy way and you haven't done it yet just download and install LineIII from the downloads section, poke in some numbers and it does all the hard calculations for you. The formulas used in LineIII were the result of a joint work by Pete Soule and Netzeband. If nothing else the included PDF files (written by Pete Soule) are interesting reading.