[Submitted to the CIAM C/L Technical Meeting, March, '98. (As an informational note, not a proposal)]

Changing Line Diameter in F2C Team Racing?

An estimate of the effects of changing from 0.30 mm to 0.40 mm:

Basis Power Kept Range Kept

Line Diameter

0.30 mm

0.40 mm

Time for 10 laps

17.50 s

18.32 s

18.74 s

True Airspeed

208.4 km/h

198.7 km/h

194.2 km/h

Engine Power

616.8 W

616.8 W

577.5 W

Laps per Tank

33

31.5

33

RPM

25000

25000

24000

Propeller Pitch

165.5 mm

158.9 mm

161.4 mm

Line Rake (model weight 350 g)

4.77

5.79

5.81

Line Guide - CG (inner span 340 mm)

28.4 mm

34.5 mm

34.6 mm

Line Weight

17.6 g

31.2 g

Tip Weight for Dynamic Balance (2/3)

11.7 g

20.8 g

The calculations are taken from my Excel spreadsheet summarising the important engineering data for a Team Racer. As a basis quite a fast model of today is taken, reaching a time of 17.5 seconds for ten laps. Refer to the spreadsheet for an overview of the assumptions made. Unfortunately the space here does not allow an in depth description. The spreadsheet can be downloaded from the above link.

The "Power Kept" column gives the result of a change to 0.40 mm line diameter while engine power stays the same. The ten laps time is changed to 18.32 seconds. At the same time, however, the range is reduced to 31.5 laps. In order to keep the range to 33 laps, engine power has to be reduced further.

The "Range Kept" column is based on keeping the engine efficiency and reducing power to keep the range. The time for ten laps goes up to 18.74 seconds. The engine efficiency is likely to increase a bit if output power is reduced, and somewhat faster times might be achieved.

The effects on propeller pitch, assuming the given RPMs, line rake, line guide position and tip weight are given. The means for adjusting the power and range are the usual ones known to all F2C competitors, such as intake area and propeller dimensions.

There are also other implications of an increase in line diameter. One of them is an increased sensitivity to wind. This is, however, a side effect of most schemes to reduce speed.

Conclusion: Changing the line diameter from 0.30 mm to 0.40 mm is an effective means to reduce speeds with only minor resulting consequential changes in models, engines and propellers.

Note that I'm not pushing for a diameter change, but just presenting the consequences.

Göran Olsson, March, 1998 SWE-1362