Suggestions to Preston Briggs
This site was last updated on February 15, 2008.
Combat is flown, in either an organized fashion or just for fun, in many countries. If you fly for fun, pretty much any planes will do, though it's more fun if they fly about the same speed. For organized competitions, the planes and engines are somewhat regulated.
In many countries, several different classes of competition are popular. However, there's an international class called F2D (and sometimes FAI combat) that is popular across Europe, with some participation in the US and other countries. The F2D rules are set by the FAI (the international aeromodelling organization) and the World Championships are flown with these rules. Other classes are more-or-less controlled by local modeling organizations.
For F2D, engines are limited to 2.5 cc (.15 cubic inches). Fuel and venturi restrictions keep speeds down to about 80 mph. Rules are very complicated, including negative "ground-time" points, and many different offenses for which penalties are assessed. In addition, two airplanes are allowed per pilot per match. The spare airplane may be used when the first one is damaged. A typical model might weight 14 ounces (with engine). The engines produce around 1 bhp at 30,000 rpm.
Here's a copy of the slides Mark Rudner used for a talk, introducing F2D to the Charles River RC club (they are in PDF format, so you'll need a viewer).
In the US, three rulebook events are flown: Fast, Slow and 1/2A. Fast and Slow utilize .36 cubic inch (about 5.7 cc) motors. The differences between the two events center around airplane and motor restrictions. For Fast, no restrictions are made on models or fuel systems; but for Slow, airplanes must have 5-inch nose moments and 24-inch overall lengths. In addition, only suction fuel systems are allowed. In Fast, taking the whole streamer is considered a "kill" - an outright win. In Slow, cutting the string counts as just another 100 points. Speeds runs about 120 mph for Fast and 100 mph for Slow. A Fast plane might weight around 22 ounces, with an engine that produces 2 bhp at 24,000 rpm.
1/2A is flown using .049 motors and 35-foot .012" lines (as opposed to 60-foot .018" lines), but is otherwise identical to Fast Combat.
Other countries have other local events with other rules. In England, for example, there is an event built around the Oliver Tiger (a famous diesel engine from the days of yore). On the other hand, Australian open combat allows engines up to .40 cubic inches, which is getting serious.
In addition to the official events, many areas have locally popular events, especially speed-limit combat. Usually, these events will allow any plane and any engine, but will impose a top speed (typically 75 or 80 mph) to keep things manageable for less experienced fliers. Lots of people will fly old Fast planes with smaller engines.
Another for-fun variation is to fly a form of nostalgia combat, limiting the planes and engines to varieties used in "the good, old days." There seem to be classes of nostalgia events in England, Canada, and Australia, and it's often proposed in the US.