Originally Posted by
riff_raff
FB-
That's a great point about twins being gearbox torque limited under most operating conditions (AEO). While engine performance will typically be adversely affected by lower pressure and/or higher temperature atmospheric conditions, the performance of the gearbox remains largely unaffected.
Consider the
example of the EC145 twin. Each of its Arriel 1E2 engines has a T-O rating of 738shp and an MCP rating of 692shp at SL/ISA. The MRGB has a (AEO) T-O rating of 1040shp and an MCP rating of 848shp. You can see that the engines have plenty of power margin above the MRGB limit at SL/ISA, 42% at T-O and 63% at MCP. I don't have specific performance data for the engine, but it would require a reduction in power of 218shp per engine at T-O and 268shp per engine at MCP before matching the MRGB power limits (AEO).
With a twin you will always have oversized engines to be able to fly away on one engine. No point making the Transmission larger (and heavier) as that is not the design goal for a category A helicopter. The twin is there for redundancy and not heavy weight lifting.