Why are the engines on the MAX and neo prone to bowing? The engines of the 737 MAX$^1$ and Airbus A320neo family$^2$ are [more] prone to bowing (shafts deforming if no debow is applied during start – as I understand it, this happens in a turnaround while the core is still hot).

The debow solution is cold motoring to harmonize the temperatures of the various engine parts. This in turn leads to much longer start durations (was upwards of 7 minutes on the A320neo until a dual-cooling button (shown below) was added to allow the motoring of one while the other was being started, which brought the delay down, but still takes longer than the ceo).


(YouTube)

Operational impact aside, why are those engines prone to bowing? The only other jet-liner I'm aware of that needed a debow procedure was Concorde.

My first thought was the higher overall pressure ratios (OPR) of the new engines, while plausible, Concorde's OPR on ground was nowhere near the levels of the new engines (15.5 vs. 40).

What design element/changes made those engines require the noticeable debowing procedure?



$^1$ http://www.b737.org.uk/737maxdiffs.htm
$^2$ http://aviationweek.com/commercial-aviation/new-pw-president-has-nothing-hide-gtf-starting-issue
Rotor bowing is not unique to this generation of engines. It was an issue with the early IAE V2500. I remember initially they would have somebody manually rotate the fan some time after shutdown. This engine still has a longer start motoring period over the comparable CFM.

The root cause is simply making the shafts thinner, increasing their flexibility. When natural differential cooling occurs, they bow. This is done in the pursuit of weight reduction, which is important as new engines are larger and heavier than before.

The PW1000G is a special case, which is where the majority of the issues are occurring. Because of the fan gearbox, the fan operates at a slower speed than the core. Thus, for the same amount of power, the core will spin faster with less torque. The thickness of the shafts are only dependent on torque and not speed, so Pratt and Whitney was able to thin the shafts significantly and cut weight. Unfortunately, they did not properly account for rotor bowing.