What is the purpose of variable stator vanes in the power turbine part of a free shaft gas turbine?

Question

both M1 Abrams tank and T-80 tanks have free shaft gas turbine engines and both of their engines have variable stator vanes in their power turbine part. It says in the picture below that these variable stators exist to prevent engine damage during gearshifts. How could variable stators prevent engine damage during shifts? By the way, PCA talked about in the picture is the automatic fuel injection system that regulates fuel injection during gear changes.

There is no clutch mechanism in the traditional sense in the transmission of the T-80 tank. Its role is played by the above-mentioned PCA. When changing gears, the PCA pedal is moved to a stop. In this case, the nozzle blades rotate 70-80° from the position corresponding to the maximum tractive power of the turbine, which actually leads to its stop (zero power position). Moreover, it is possible to move the voter lever only when the PCA pedal is depressed, since in the initial position it mechanically and electrically blocks the hydraulic servo gearshift. Thus, the guaranteed protection of the turbine rotor from misuse is ensured. By the way, the absence of such a mechanism in the GTE-ZT engine of the object 167-T led to the destruction of the turbine blades during the tests.

Also a picture of the AGT-1500 from M1 Abrams where it also shows the variable power turbine stator blades:

Answer 1

When you take a turboshaft engine and subject it to large and sudden load changes, that it doesn't have to cope with in an aircraft, like during gear ratio changes in a tank, you get very sudden reductions or increases in power turbine RPM as the torque demand fluctuates.

The airflow having a lot of inertia, sudden changes like that create large changes in local angle of attack (AOA) of the power turbine blades themselves. The blades become unloaded from a reduction in AOA, or conversely they become too loaded from a sudden increase in AOA and can aerodynamically stall.

These changes in airflow at the power turbine effects the upstream flow, disrupting it, and when you disrupt the airflow in the gas generator section (the compressor/burner/HP-LP turbines), you can get compressor stalling, leading to surging of the gas generator, leading to loss of control of the flame in the burner can (which is contained by creating a boundary of cooler air around the flame using vent holes), leading to too much heat leaving the burner can, leading to overtemps in the HP/LP/power turbines. And thus the engine gets damaged.

So the variable stator vanes are to compensate for sudden flow changes from sudden changes in torque load on the turbine caused by having to deliver power though a gearbox with different gear ratios, by redirecting the incoming flow to keep the angle of attack on the power turbine blades within an ideal range.

If you built a tank with a Continuously Variable Transmission, where you can set a constant engine RPM and only vary it gradually, you'd be closer to an aircraft turboshaft/turboprop environment and you could probably dispense with the variable stators for the power turbine.