by Bill Draper    
Bill Draper has written this story many years ago for the British Aerobatics newspaper CLAPTRAP. Since this timeless treatise is too valuable to being lost, it is offered here for your information and enlightment. Thank you very much Bill and CLAPA for your permission to present it on this website for all control line aerobatic enthusiasts to enjoy and deepen their stunt know how.  
  Some of the queries raised in recent editions of the newssheet have prompted me to present my views on some of the more common methods of maintaining line tension. Some systems are only effective during certain manoeuvres or in certain attitudes of the aircraft, and are designed more to maintain line tension than to recover line tension once it is lost. Others are effective at all times, and will help to recover lost line tension.
I cannot cover all possibilities, but the items which are commonly used to control line tension are as listed, but not in any particular order.

 
                           
  A) Rudder offset (or wing lifting section fin)
B) Engine offset
C) Weight offset
D) Wing area offset
E) Flap Area offset
F) Flap differential movement
G) Fuselage side area, and centre of area position
H) CG position
I) Lead out position
K) Centrifugal force
( sketch 1 )
   
                           
  Some of these items are inter related and their effects cannot be considered in isolation. However, basically, those items which are effective during straight and level flight are always effective throughout the manoeuvres and can be considered as the primary sources of line tension. However, the one common denominator in all cases is maintaining sufficient airspeed.
  Rudder offset ( respectively lifting section fin )
Engine offset
Fuselage side area

               
  Considering these items as a group, they provide the basic source of tension, besides centrifugal force. In the event of losing tension such that the aircraft free flights, then controls neutralise, the effects of weight and lift offset are lost, and it is these 3 items only which will fly the plane outwards and recover tension before terra firma. However, an excess of rudder offset etc will cause the aircraft to fly too much crabwise and cause kicks in tight turns. Plenty of side area helps to hold out the plane, but the centre of area should ideally be positioned behind the CG and at the same height as the control lead out position.
For a total fin/rudder area of 6% wing area (3% fin, 3% rudder) no more than 10 degrees rudder offset should be necessary (or equivalent lifting fin), and 20 engine offset (3o max). A fuselage side area of 25% wing area will provide all the sideways lift necessary, with its centre of area central.

  CG position
Lead out position
                                                 
  The lead out centre line at the tip should be 10% to 20% mean chord behind the CG such that the outward forces of centrifugal force tend to yaw the plane to point outwards. The effects of a forward CG are to increase line tension with speed, but it does not help to recover line tension once it has been lost. However, a forward lead out position will reduce line tension in manoeuvres. Converting the wing plan into a simple parallel chord, including flaps, then the lead out centre line should be 30-40% back from the L.E. Keep the leadouts fairly close together ( within 1 ½ “) to avoid yaw changes when turning. For most aircraft, the CG will lie between 15% and 25% back for the parallel chord case.

Sketch 2 shows dimensions and locations on comparable rectangular or parallel chord wing. L= sweep back from CG to lead outs ( middle between both lead outs; M= lead out location relative to root chord: N= CG location relarive to mean chord; O= mean chord on trapezoid wing planform. Mean chord is wing area divided by span.

The fuselage side area behind the CG and lead out position will help reduce excessive line pull in fast downwind loops etc but will help to yaw the model outwards and hold line tension when flying crosswind on the upwind side in a strong wind.
Ideally, the height of the CG, fuselage centre of area, and line leadouts should be on the same level.
The drawing illustrates the effects of a low lead out position coupled with a high CG and centre of side area position. The effect would be poor line tension in outside manoeuvres or inverted which would then have to be trimmed out by trim tabs or twisting flaps ( sketch 3 ).  
   
Weight offset                    
The simple method of weight offset is the outer wingtip weight. The basic exercise is to move the centre of gravity and centre of inertia outboard of the centre of lift. The effect is to tilt the plane slightly such that a small percentage of the lift derived from the wing is directed away from the control handle. FIG 2 illustrates the effect in level high level flight and it is exactly the same both upright and inverted. The effect is, of course, shown greatly exaggerated.
  The inertia of the tip weight will tend to throw the tip outwards in loops, etc., again causing the direction of the lift to be partially away from the lines. Sketch 4 illustrates the effect, and it should be noted that at the top of the loop the lift is towards the centre of the loop and the outer tip is pulled upwards.

The amount of tip weight must be carefully chosen so as not to be excessive. Too much weight will cause the wing to drop and the plane to wobble in square pull outs. The amount required will depend upon the size of aircraft, type of line lead outs, wood selection, type of lines used, etc. Due to line lead outs, engine silencer, etc. the inner wing is likely to be the heavier. I try to select heavier wood for the outer tip and ribs to try to use the weight to provide additional strength.
To establish the weight required, weigh the actual lines to be used (60 feet pair of light laystrate weigh 1 oz). Support the aircraft by the engine prop shaft and the tailwheel (or centre of fuselage below fin), and add tip weight until the aircraft balances; then add half the line weight ( ½ oz for 60ft light laystrate). If, due to wood selection etc, the outer wing is already the heavier, prior to weighting, then weigh the amount required on the inner wing to provide balance. This will give the information as to the equivalent tip weight already built in, and this amount can then be subtracted from the weight required to counterbalance the weight of the lines.
The amount. of counterbalancing achieved by this method will just balance the aircraft for level shoulder height flight. In tight turns however, or in high level flight, the inertia or centrifugal “G” forces of the lines are only the equivalent of about one third of their normal weight, since only the outer end of the lines is travelling at aircraft speed. Also the centre of inertia of the lines is about two thirds of the line length from the handle, thus resulting in forces at the inboard wing tip of the equivalent of less than one quarter of the line weight.

Consequently we have sufficient over compensation in turns to achieve the desired effect of sketches 4 and 5 without causing wing tip wobble. Line drag will cause the plane to yaw inwards (hence fin offset etc) but will also help to reduce the effects of line inertia.
It is interesting to note that a 1 ounce weight at 60 mph in an 8 ft radius turn will exhibit a throw out force of almost 2 pounds.
                     

Wing offset ( or lift offset )

     
By building in more lift on the inboard wing by making it larger, we obtain a similar effect to that shown in sketches 4 and 5, whereby the CG is outboard of the centre of lift. At first sight this must seem better than adding weight, but it has certain drawbacks ( sketch 6 ).
Lift, drag, centrifugal and inertia effects all follow square laws, and are a function of the square of the speed. Whatever the manoeuvre the outer wingtip will be travelling faster than the inner wingtip, probably by about 8% dependant upon size and line length. Relative to the fuselage therefore, the centre of lift will move outwards.
Also, the centre of inertia of the wing will move outwards relative to the fuselage, thus moving the centre of inertia of the entire aircraft, although not as far as the centre of lift. Since the weight of the covered wing will probably be about 40% of the total aircraft, this means that for a 5 feet span aircraft on 62 1/2 ft actual lines, the centre of lift will move about 0,7”, but the centre of inertia about 0,3”. Therefore the fuselage should be moved outwards by 0,4” to re-establish the centre of lift on the centre of inertia during turns.
The foregoing has now provided us with an inner wing 0,8 inches longer than the outer wing, but has only compensated for changes in lift and inertia within the aircraft, and assumes that the aircraft was already balanced. No account has yet been taken of leadout wires, silencer, etc which would still require counterbalancing or additional inboard lift, together with line inertia. Even when using wing offset therefore it is general practice to use outer tip weight.      
                                   
The main drawback of the larger inboard wing is the increased drag tending to yaw the plane inwards ( sketch 7 ). While line tension is good, the increased lift on the inboard wing will help to maintain tension. If tension is lost however, together with control, then the inboard drag will tend to turn the plane inwards and will counteract the fin and engine offset with which we hope to recover the tension.
My personal view is that for a small weight penalty, the tip weight is equally as effective as the offset wing, its effects predictable, can more easily be adjusted for trimming, but has not the drag problem on slack lines. Since we already have over ¼ oz of line compensation in turns, an additional 1/4 oz is all that is likely to prove necessary to move the centre of inertia sufficiently.
Flap area offset

Flap differential movement

Again, both these systems are designed to provide more lift on the inboard wing. Designs which have a longer inboard wing usually have a corresponding longer inboard flap anyway. With symmetrical designs, the outer flap can have its outer tip fixed horizontal, or as a trim tab as required, whilst the inner flap used full length. This provides the extra inboard lift in turns, but does not have the drag penalty under slack line conditions that the larger inboard wing has. The inner flap should only have about 4 to 5% more area than the outer, to avoid the rocking tendency in turns.
The differential flap system requires the linkages to the flap horns to be separated. Again the inner flap movement is greater than the outer to provide more lift, but only by 2 to 3 degrees movement, and certainly not more than 5, to avoid the rocking situation,
I personally prefer a symmetrical wing with a longer inboard flap. As a final comment on line tension - when you stand the plane on its tail for a wingover, and the engine splutters and perhaps dies at the 45 degree angle point

                                               
  a) the wing tip weight is not effective
b) the larger inboard wing is not effective and is a hindrance
c) differential flap movement and area are not effective
d) as speed falls centrifugal force is not effective
e) as speed falls CG and lead out position are not effective
f) as speed falls, a forward CG will pull the nose inwards
g) Engine offset is not effective

Under these conditions, as you go over the top and down the other side, the plane is hanging up there on side area and fin offset.