Proposal submitted by: The Swedish Model Flying Federation
Sporting Code
| Section | Part | Class | Paragraph |
|---|---|---|---|
| 4c | 4 | F2D | 4.4.5 |
4.4.5. Characteristics of Combat Model
.
.
Engines with glow plug ignition shall have their exhaust port(s)
connected to silencer(s) (one or two), consisting of a simple chamber
with one circular cross-section outlet opposing the exhaust port
connection. The total volume of the exhaust system shall exceed 12,5
cm3 (in the case of two exhausts, each silencer shall have a
minimum volume of 6,0 cm3 ). The total area of the outlet(s)
is not to exceed 50,3 mm2, corresponding to 8 mm diameter for
one outlet and 5,65 mm for two outlets. The total length of the exhaust
system from exhaust port(s) (cylinder) to outlet(s) not to exceed 15 cm.
4.4.5. Characteristics of Combat Model
.
.
Engines with glow plug ignition shall have their exhaust port(s)
connected to silencer(s) (one or two), consisting of a simple chamber
with one circular cross-section outlet opposing the exhaust port
connection. The total volume of the exhaust system shall exceed 12,5
cm3 (in the case of two exhausts, each silencer shall have a
minimum volume of 6 cm3). The outlet(s) should have an area
restriction not to exceed 50,3 mm2, corresponding to 8 mm
diameter for one outlet and 5,65 mm for two outlets. The restriction
should be located no more than 12 mm from the rear end of the outlet, and
be verifiable from the outside. The total length of the exhaust system
from exhaust port(s) (cylinder) to the rearmost point of the outlet(s)
not to exceed 15 cm.
The F2D silencer rule was introduced fairly recently, and the learning process is still not finished. Lately there has been a controversy regarding the outlet, where some have interpreted the rule text as requiring the area restriction to occur at the very end of the system. This has been in response to silencer designs with the outlet designed as an expansion cone, where the 8 (or 5,65) mm restriction is some distance from the rear end. Such a design apparenty improves flow, for a slight increase in power. At the same time, such a long outlet duct will reduce noise!
For the recent world champs the subcommittee settled the issue by stating that a distance of 5 mm from the restriction to the rear end was allowed. This led to that a large part of existing silencers had to have their outlets cut off, which likely increases noise, contrary to the intent of the rule!
There needs to be no other limit on the distance from the end to the restriction other than to allow verification. With the distance limited to 12 mm, the competitor is able to verify his silencer using no tools other than a standard caliper.