So far in discussion of the Max 8 people have mentioned CFM LEAP cowling size and 737 tail design as possible factors in the need to develop a MCAS system to control tendency to pitch up at high AOA.

Since the tail has much more leverage than the wing (much longer lever arm), is it possible that the high bypass exhaust blast is affecting the tail at high AOA by pulling more downwash onto it by Coanda effect? At first I though the may be affecting the back of the wing, but then realized the 737 Max 8 is very close coupled (tail close to wing) and may be susceptible to stronger downwash effects.

I wanted to be sure before suggesting a larger horizontal stabilizer or considering lengthening the fuselage. Is it possible? Has it ever been seen before?

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    $\begingroup$ I haven't seen that mentioned in any of the multiple articles I have read in any of the aviation areas. $\endgroup$
    – CrossRoads
    Apr 8, 2019 at 1:59

1 Answer 1


I seriously doubt the engine exhaust hits the horizontal stabilizer except possibly when fully stalled. Otherwise, the wing downwash follows approximately the extended chordline and takes the engine with it safely below the horizontal stabilizer, which is significantly higher than the wing.

The one thing I've seen mentioned was change in the centre of gravity generally decreasing the longitudinal stability—which is most pronounced near stall, because the centre of pressure moves slightly forward with increasing angle of attack (until stall, where it moves abruptly aft all the way to midchord).

The other possible effect is that the cowling generates vortices—possibly with help of vortex generators needed to avoid stagnation in the gap between the wing and the engine—that go over the wing at high angle of attack and increase the suction peak near the leading edge and making the forward shift of centre of pressure more pronounced.

The only point regarding tail I've read is that it is not large enough to allow moving the allowable range of centre of gravity forward to improve the stability ‘naturally’.

  • $\begingroup$ "Naturally" or passively, unless the larger diameter airstream produced by the high by pass jet is strong and close enough to pull tail towards it like a vacuum aspirator. I wanted to check this before clamoring for a larger Hstab. If airflow under power checks out for all possible AOA (no surprises such as blanking the elevator) this may be an easy fix for them, as they have many orders. If not, perhaps an enhanced version of the 717 tail, if not the entire 717, may be needed. $\endgroup$ Apr 9, 2019 at 21:55
  • $\begingroup$ @RobertDiGiovanni, larger tail in itself would not fix anything. It would only allow moving the centre of gravity further forward, which would, but it would also create a bit more drag. Also designing new stabilizer, even if it is just slightly scaled up version of the previous, is a complex process that would take quite long. Redesigning as few parts as possible was the point of the upgrade. $\endgroup$
    – Jan Hudec
    Apr 10, 2019 at 5:09
  • $\begingroup$ In other words, cutting corners. Notice the larger tail rebalances the CG back to its original point as well as aerodynamicly stabilizing the aircraft. I would not worry about drag on a regional jet. A 717 tail/737 LEAP engine mount hybrid may be in the works. $$$, but it might save their reputation. We'll see. $\endgroup$ Apr 10, 2019 at 9:01

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