Originally Posted by
MechEngr
There's no drastic pitch-up. How does this get started and why is it repeated? Comparing the plan form to the NG and the differences are difficult to make out.Slight changes in configuration tend not to make drastic changes in performance.
The fact that engine drag can induce a pitching moment is well known. Below is a quote from the book
An Introduction to the Longitudinal Static Stability of Low-Speed Aircraft by F.G. Irving, written in 1966:
"... effects induced by the power plant.
If the resultant drag force does not act through the c.g., it produces a pitching moment."
(www: sciencedirect.com/book/9781483200194/an-introduction-to-the-longitudinal-static-stability-of-low-speed-aircraft, from chapter 11 abstract)
Regarding plan form comparison of the 737 NG to the MAX:
The engine drag (NG vs MAX) can be estimated by calculating the drag for a cylinder inclined at 15 degrees AoA with the dimensions of the CFM 56-7b (737 NG) compared to the LEAP-1B (737 MAX). It will indicate a
40 % increase in drag at 200 m/s and 15 degrees AoA for the MAX. The magnitude is in the order of 10's of kN per engine, and unlike the thrust pitch coupling effect mentioned, the drag has the potential to be much more severe as it does not act along the axis of the engine. Consider that drag increases with AoA, as more surface area is exposed, but interestingly due to the high position of the MAX engines, it is likely that the moment arm of the drag will also increase with AoA! I haven't really seen this second point being discussed, but personally I think this is a key that could explain why MCAS was so aggressive.
To summarize: It is clear that the Max engines produce a significantly increased drag compared to the NG, and it is clear that this causes a pitching moment that can increase non-linearly with AoA. How bad is it? It will depend on the exact relation between the aircraft CG and the nature of the engine drag. Only Boeing can answer this question, and the fact that they have declined to do so for about 1 year tells me that it is probably pretty bad...