There are lots of subtle reasons for using belts as the primary means of transmission in light helicopters – here’s a few.
If you consider the R22 installation, the nature of the rigidity in-plane of the blades means that teeter-rotors convert the Coriolis forces due to flapping into a RRPM response that can create lareg loads in the transmission, though its small in piloting terms. This response is absorbed by the rotor, transmission and engine inertia and other resistive forces in the system.
With a stiff transmission such as a chain or gears, these loads would create large fatigue loads in all of the major drive-train components.
Furthermore, the non-continuous power delivery of the 4-cyclinder engine at relatively low speed would feed large oscillating loads back up the drive train to the rotor - again fatigue problems.
Incorporating a 'soft/flexible' member in the transmission will help reduce the transient shock loads fed both ways by the visco-elastic deformation of the belts. To the benefit of transmission and engine and rotor life.
Big turbine helicopters use turbine engines that have continuous combustion and hence smooth power delivery, though flexible members are sometimes still incorporated to reduce shock loads in the system - not belts though!
Further advantages of belts over chains are as follows:
The load bearing elements in belts are physically bigger than chains for the same torque and hence cracks/splits will be physically bigger and easier to spot before they are failure-critical. Belts have a relatively benign failure mode in comparison to a sudden catastrophic failure that would occur with a chain.
Belts don't need lubricating or oil changes and they are out in the open and so easy to inspect.
Belts don't need encasing to allow lubrication - additional weight & maintenance.
With the belt system you get an lightweight, reliable clutch mechanism almost for free! (in weight terms)
Belts are physically robust - tolerant to dust ingestion & the presence of various contaminants.
LIGHT LIGHT LIGHT!!!
Very easy to design-in redundancy with little addition of weight. Multiple belts on a single pully. Though locating the two belts right next to each other and in the open is perhaps not quite so prudent!
Hope this helps
Cran
