PPRuNe Forums - View Single Post - MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
Old 10th Jul 2019, 00:07
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john_tullamarine
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A sideline observation.

Reading though some of the MAX thread(s) comments, there are numerous references to the offset thrust line's causing a pitching moment change with thrust changes but a general dismissing of the normal lift effects attributed to the nacelle inlet (or the similar normal prop disc force on high-powered prop aircraft).

I wonder if some folks have missed the point of references to nacelle lift ? A useful diagram at (lower) p14 of this lecture presentation

https://people.clarkson.edu/~pmarzoc...0/AE-430-4.pdf

may clarify the concept.

It follows from a consideration of the graphics that the problem can be significant at high AoA and high thrust settings, particularly if the engine (specifically, the nacelle inlet or prop position) is stuck well out in front of the CG for other reasons.

The momentum/force change associated with the airflow direction change increases and can provide a very noticeable decrease in longitudinal static stability. This can be of sufficient magnitude that some aircraft require add-on stability augmentation (cf the typical SAS systems in some piston to turboprop design conversions). I am aware of one such design conversion in which the effect will cause the static stability to become quite the reverse in the missed approach situation with an inoperative SAS and full thrust during the miss. One usually sees a power restriction in the event of a U/S SAS to keep the problem at bay ..

Without being privy to the backroom Boeing details, I suggest that the MCAS concern relates more to the normal force effects than the offset thrust line couple problem. One can find plenty of net articles on the topic should you wish to read up further.
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