Combining gearbox

[PJ]

> Think normal car differential - with the power input at the wheel
> shafts and power output at the "input shaft".
>

Considering the 'sync' portion of the question:

The problem here is to provide for load sharing without a tendency of
the two engines to 'hunt.' In the service environment, it isn't
practical to expect turbine engines to maintain a constant output
throughout their service life and 'synchronization' becomes an expensive
and difficult task. One engine is always weaker than the other.

In a typical (ex: S-61) arrangement, both engines drive a common input
shaft via over-running clutches (to disconnect a failed engine's power
turbine or spool from the system). So both engine turbines run at the
same rpm (Nf).

The fuel control on each engine operates via a flyball or electronic
governor along a curve (sometimes called a "droop line") tailored so
that (in a single engine installation) Nf is allowed to decrease with
increasing kw. load. For the T58 variants the governor allowed Nf to
drop about 10% from idle to max power output.

So, with the aircraft at minimum load on the ground, the Nr might be
105%. Lifting up into a hover, the Nr would drop downward to perhaps
98%. The pilot, or an electronic governor, would increase the power
output to restore 100% Nr. Pilot or electronics would then trim the
load share to achieve the same torque load on both engines. (Matching
torques improves the service life of the combining gearbox.)

For a pair of typically unmatched T-58s, there will be slight
differences gas generator speeds (Ng) and turbine inlet temperatures
(T5) for a given torque load.

This same system also worked well in the four engine installation in
Bell's X-22. (Four engines connected to four ducted fans via common
shafting.)

--
PJ