Here is one take I’ve read:
“It has already been well established that the new Max version of the 737 has greater tendencies for the nose to climb higher under heavy thrust conditions than previous models. There appear to be several factors that may contribute to this. When this plane first came out back in ‘69 or so, the point where the thrust from the engine met the surrounding environment (push point as I call it) was well behind the wing, and vertically very close to the wing’s rotation axis with the long skinny JT-8 engine. When the larger diameter CFM 56 engines were later installed with 61” diameter fans, this “push point” was moved near the front of the wing, but it appears this was not enough to cause serious handling problems.
Because the engines of the new Max version now have a fan diameter of 69 1/2”, and are 21” longer, (124” vs 103”) the engine and push points have been moved forward yet again. I recently overlaid 2D CAD drawings for both planes, and discovered that the exit point for the thrust from the bypass shroud has been moved forward by 22”, and the exit point for thrust generated by the LP turbine has been moved a whopping 29” ahead of the old locations. The leading edge of the nacelle is now 13 1/2” inches ahead of the old location. The axial center line of both engines is roughly the same, with the additional diameter being divided equally above and below the old location.
So from a purely mathematical perspective, as long as the plane is flying relatively flat at high speed, this horizontal shift shouldn’t have much effect, but now that both the LP and bypass thrust points are much further forward than before, and the LP/bypass ratio has changed, the effects of this could be fairly dramatic under certain conditions.An example would be if the plane is oriented at a pitch of say 45 degrees, but flying at a low enough speed that it’s trajectory is still roughly parallel to the ground plane, the downward projecting thrust from the engine would be would be at least somewhat deflected up and back and into the front of the wing as it traverses across the stationary air. This would in theory at least create additional lift at the front of the wing, inducing more pitch rotation forces.
Using the same two CAD models but this time oriented with a 45 degree pitch, some interesting things started to appear. The first is that the top edge of the bypass shroud outlet is now horizontally or slightly above the leading edge of the wing, (18” higher than the older model), and exits very close to the wing’s leading edge at this angle. How much effect this has on lift I’m not sure, but I was very intrigued by a photo of the A 320 NEO that showed where Airbus added long “ear” shaped extensions to the top (and bottom) of both the bypass shroud and LP exit point to deflect thrust away from the wing. (Boeing has a smaller one on the LP section only).
Another part of the equation that I noticed relates to the nacelles themselves in relationship to the axis of rotation. With the same models pitched at 45 degrees, the bottom of the nacelles are so much higher than the rotation axis of the wing root that they would act as lift devices for sure, and to some degree air brakes as well depending on severity of pitch. I measured the distance from the bottom front of the LEAP nacelle to the approximate centeroid of the wing root and found it to be 113” above, and 180” in front of the root center. The older version had measurements of 106” and 168” respectively, along with less surface area at the front of the nacelle where this potential leverage would be the greatest.
At first I didn’t think too much of it as the nacelles are rounded rather than flat planes, but the projected surface area of the nacelle is roughly 60% of the horizontal stabilizer, so there would be enough surface area to affect the dynamics of the plane. Regardless of whether my theories are correct or not, it is well known that this plane is not as aerodynamically stable as previous versions, as the implementation of MCAS was from what I’ve read, was apparently required for certification. Whether the plane is unstable enough that it truly requires MCAS to stay in the air, or whether it was only implemented to help the pilots deal with a few handling quirks under certain conditions, that I can’t say. If my theory on thrust deflection is correct, then a relatively easy fix for this plane’s stability issues may be to modify the bypass shroud for more lineal control of the thrust direction. If not correct, then there’s likely no fix to the stability issues other than re designing the entire wing root to accommodate longer landing gear, which is not likely to happen.” ~ Greg Miller
That is generally consistent with the explanation I provided. Thanks.
It’s interesting that the author you cited found the nacelles have a projected frontal area 60% of the horizontal stabilizer. The nacelle lift at high AoA is now farther forward of the aircraft CG, creating the upward pitching moment.
“Whether the plane is unstable enough that it truly requires MCAS to stay in the air, or whether it was only implemented to help the pilots deal with a few handling quirks under certain conditions, that I can’t say.”
Dude must live in a cave with no internet ...
Should this aircraft have been required to get a new type certificate?
I wonder if it would help balance things out if they put a 3’ fuselage plug in in front of the wing. The added cabin space (which they’d probably squeeze a couple of rows of seats in) might require an additional exit point (although the added exit designed into the MAX 10 might take care of that). The 10 is already 14 longer than the 8s that have been crashing, so perhaps it is proportioned a little better. (Or maybe a little worse - the extra exits are in the back).