Tether Line Drag for Aeromodels : F2C Status
Despite the fact that line drag dominates in F2A, F2C, and related events line drags are increasing to slow the airplanes down. F2A has undergone a line length rules change, increasing in length by a factor of 10/9. F2C is being considered for a line thickness change. Were it possible, line length increase may have been the preferred way to go for F2C as it reduces the rotation rate more for the same increase in line drag. The added line drag is an adverse factor in low speed handling and flying in wind in either case. In any event venue requirements dominate here and line diameter is the presently advocated change (other moves such as restrictors, propeller diameter, and no change are in the bidding, too).
These changes will only make line drag more important than ever before. The aerodynamics of the aircraft will be less significant. Many feel that it would be better if the situation were reversed. Everyone feels they would like to find a way to reduce the line drag before anyone else.
The rules are written to make any reduction in line drag highly unlikely and so far they have worked well. I have been told (but only by two people) that one function or effect of line 'groupers' is to reduce drag. To my knowledge there is no evidence that this is true and, from the next section, you will see I think it is impossible.
Göran Olsson's spreadsheet analysis from 1997 estimates that 65% of the drag is in the lines. If line diameters are increased from .30 to .35 mm the total drag will go up by about 9% and the lines will represent 68% of the total drag.
In general Reynolds numbers at the wing tip for control line vary from 400 to over 4000 for the various competition events. For F2C at 55 m/s at the wire exit the Reynolds number for the proposed 0.35 m diameter is 1300. On a 'high and hot' day - 75% humidity, 30 C, and a pressure altitude of 1 kilometer it would be about 1000. Look at the drag coefficient graph Cylinder Drag and you will see that even an order of magnitude change in Reynolds number will not do much to the drag coefficient.
Not only is the majority of the aerodynamic drag in the lines, it is mostly in the portion of the lines close to the aircraft. in F2C 25% of the line drag is in the first 1.3 meters of line measured from the tip back toward the handle. So the first few meters of line, if the line diameter increase is effected, will have as much drag as the airplane.
With this in mind, several discussion threads on the net and questions to me have tried to look at the question of drag reduction one more time. What I think about it is in the next section.