Warning: SLF query re engine sync

[Chris Scott]

I also think that NSEU has hit the nail on the head. The primary requirement must be to match the thrust, not the RPM.

However, in the case of what is sometimes loosely referred to as an "N1 engine", such as GE and CFM, the thrust is considered to be a function of N1. EPR is not used. As the fan typically generates about 5/6 of the thrust, it's not a bad way of achieving the requirement, unless the fan of one of the engines is more worn than the other(s). The A/THR will choose the N1 it thinks appropriate, and order the FADEC of each engine to deliver it. So the N1s should be fairly well synchronised.

In the case of "EPR engines", such as RR, IAE and (previously) P&W, the A/THR must order all the FADECs to give the same EPR. If one of the engines is more worn than its neighbour(s), it will need a higher N1 (fan) RPM to achieve the ordered EPR.

Perhaps poorjohn will confirm that he is talking about RR engines?


Quote from Dan Winterland:
One type I flew had a 'Syncrotac' which had three little wheelts for engines 2,3 and 4. If all wheels were staionary, these three engines were sychronised with engine 1.

We had those on the HS114 (De Havilland Heron), to synchronise the four Gypsy Queen engines (each 250HP). Some years later, I was amused to find the identical instrument on the VC10, for the N2 spool of its 21000lb-thrust Conways. Being merely the copilot, the grizzled FEs didn't like me fiddling with the throttles in the climb and cruise (they had their own set), and were not impressed when I told them why I knew what I was doing.