The 11lb limit comes from the FAI limit for pattern of 2x2M max size & 5Kg max weight
Quite a few years ago there was also a 10cc (60) engine limit. There were calls at the time to have an unrestricted engine capacity limit to lower the cost of pattern as the 10cc engines were becoming quite specialised for pattern & therefore
expensive. If the engine size was unrestricted then cheaper but higher capacity engines could be used. Well engine capacity did become unrestricted, so did that lower the cost ? - NO it had the opposite effect and we got larger capacity
Specialised F3A engines that were even more expensive !!
IMO if the weight limit were increased then a similar effect would occur and cost would increase - I can imagine, for example, if a weight limit of 6Kg (13.2 lb) was allowed then we could get super “bloated” BiPlanes with full 2M wingspans and exotic/super expensive power systems to drag them around the sky....
Steve
SAB, I see now that historically pattern guys that wanted more power in a plane that had a 2M wingspan, while flying glow fuel meant heaver and more complex engines (the YS 185-CDI Four-Stroke comes to mind) to produce that needed torque and thrust, and keeping the weight at 5000g kept the horsepower wars from escalation into more and more complex IC engines.
But now I think we have the opposite problem, with Electric power the inverse is true, the less expensive power plants (or Motors) that are built to produce around 2500 Watts tend to be heavier than the expensive F3A type motors that are on the market. Batteries have the same inverse relationship, the cheaper the battery, the more it tends to weigh with the same capacity. (debatable I guess but it seems that way to me) and if you want to scratch build a 2M size plane out of Balsa and Ply, you will have a tough time keeping it under the weight of 5000g with same running gear that is recommend for top performing composite ships.
If that is the answer, that back in the Glow Fuel Days the displacement of the engines was a concern, and keeping the weight below 5000g was the goal to discourage bigger and bigger power plants, I can see that. I think the rule is archaic and should be amended to reduce the cost and simplify the construction of a pattern planes that are more and more using Electric power.
R3d