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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).

enter image description here
(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

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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.

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Not only is rotor bow not unique to this generation of engines, per @user71659's excellent answer, it's not even unique to jet engines. If you go back through the literature, you can find references to it on steam turbines, which have a very similar problem. Many steam turbines are kept rotating at 1 rpm or so while shutdown in order to keep the rotor temperature uniform and prevent bow. See for example. us patent US3919894A This was from 1975. I'm sure the problem goes decades before that, although I stopped searching there.

Edit: in response to your comment here are a few addition factors that I can think of beyond the existing answer

  • Temperatures. Every successive generation of jet engine has pushed compressor and turbine temperatures up in order to get better thermal efficiency. Running hotter means that there is more heat available upon shutdown to drive the natural convection processes that generate thermal bow. Expect this trend to continue in the future: the next generation of engines will be even worse in this regard

  • Turn times. There is a certain time constant to thermal bows. If the engine is shut down for 5 seconds, there's no bow. Natural convection hasn't had a chance to act yet. And if the engine is shut down for, say, 24 hours, the entire engine is completely at a uniform ambient temperature. Also no bow. Somewhere in the middle is a sweet spot where the bow is large. Usually it is in the vicinity of 1 or 2 hours. Long haul planes might have a turn time of 3 or 4 hours. They have passed the peak of the bow and it is starting to decrease as the engine cools off. But short hauls like the A320 and B737 usually have a turn time in the 1 to 2 hours range -- they are almost always starting at the worst case time.

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    $\begingroup$ I'd also add the drive for fuel economy and airport noise reductions means engines are being run with less of a warm-up and cool-down time. Single engine taxi is a great example. $\endgroup$ – user71659 Feb 21 at 3:07
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    $\begingroup$ Steam plants in power generation have electric 'turning gear' to keep the rotors turning at low speed while shutdown. These things are massive, but have HUGE thermal mass and (like gas turbine cores) very fine tolerance on tip clearance. Even marine Diesel above a certain scale has issues with differential heating if you allow the temperature to change too quickly (You risk cracking the crankcase). This is not just an issue in aero engine cores. $\endgroup$ – Dan Mills Mar 28 at 12:46

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