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F2C Prospects

Analysis and Prospects

Line drag at the Reynolds numbers that models experience is primarily due to the alternating vorticies shed from the lines. This phenomenon is called the Von Karman vortex street and the vorticies absorb about 80% of the energy given up to drag. The frequency of vortex shedding for an F2C is near 30 KHz at the time at racing speed and lower as it gets closer to the handle. When the engine stops one can sometimes hear the lines `sing', the same phenomenon that gives rise to the tones emitted by an Æolean harp.

In systems in which one line trails the other - even out to 8 diameters - there is a very large reduction in drag. When in tandem both the leading wire and the trailing wire have reduced drag, but the drag of the trailing wire is something like 1/3 that of the leading wire. In these cases the drag of two wires is less that the drag of one wire alone.

This is the idea behind having the leadouts as close together as possible. However, the configuration of one line trailing the other is not stable. The lines between the handle and the entry to the aircraft wing have significant curvature. For F2C the maximum departure of the lines from a straight line is about 15cm (6 in.), and this figure is directly proportional to the ratio of line drag to line tension. If the rear line has half the drag of the front line it will not have as much curvature. Being more nearly straight it will be in front. The differences in curvature will produce line separations that are on the order of 100 diameters, far too much for any drag reduction.

The amount of drag reduction can result in a drag difference of 3:1 from leading to trailing wire. Hard to predict and eliminated with only a millimeter or two vertical movement of the lines.

Asymmetrical wings are, however, a way to produce significant reduction, and the thicker the lines the easier it is. For two 0.35 mm diameter lines a 12% thick wing section that around 7 mm thick has no more drag than the wires. Removing 1 cm from the outer wing and putting it on the inner wing places it in an area of slightly lower dynamic pressure and covers lines `for free'. Making a thinner section and extending it as far inboard as possible is still better.

Flutter is always a potential problem, the solution being to keep the center of mass well ahead of the structural center of torsion. This is why the Boeing jet transports pioneered the now standard jet engines slung on extended pylons. A mass balance at the tip - a wire attached to the wing and extending forward with a weight can be used as well as an additional tail or structural modifications to place the center of torsion further forward.

Rule lawyers may have an opinion on how extreme asymmetry might be treated, but at present it seems the only way forward to me.

The wing section need not have a sharp trailing edge, a small, squared off trailing edge - perhaps as much as 2% of the chord in thickness may give a little benefit at low Reynolds numbers and small tubulators aft of the 50% chord line may also make small improvements. The nose of the airfoil should be rather sharply curved. The NACA four digit series (e.g. NACA "0012") are VERY old designs, the thickness profiles dating to 1930 or so, and have too large a nose radius.