Hacker News Reader

Comment by HankB99 | Hacker News Reader

I'm unclear on how moving the engine up causes application of power to cause the attitude to tend to go nose up. I would expect the opposite to happen. I think something else must have changed such as the center line of the engine relative to the center line of the plane. Or did moving the engine forward at the same time cause this effect?

View on HN

The new engine is more powerful, and at high angles of attack the nacelles also generate lift. The added power, along with the greater displacement from the center of lift/drag causes the plane to pitch up more easily, and the lift from the nacelle causes the controls to get lighter as you approach stall speed.

The FAA requires the controls to get heavier as you approach a stall. MCAS was primarily designed to address this flaw, which is why the added lift from the nacelles is an issue.

I don't think it does. Other sources I've read suggest the larger nacelle introduces a pitch up during certain flap configurations and at a high angle of attack. It's not the engine thrust that's the problem, but the drag due to the large and further forward nacelle. That's the justification for the MCAS software.

https://theaircurrent.com/aviation-safety/what-is-the-boeing...

It's both. The engine thrust center is also farther below the center of pressure of the airframe. And it can apply a lot more thrust than the original engines.

The point about lift from the engine nacelles was that it acts as positive feedback: if you are already pitched too high, it acts to worsen the problem. And, because it is farther forward, it has a greater lever arm to act.

Thanks, Those factors make sense.

I think it is true to say that all low-slung engines produce a pitch-up moment that increases with power, which is a desirable trait, in moderation (for one thing, if the power fails, pitching up would be more dangerous than pitching down.) Also, all engine nacelles produce a small amount of lift when at an angle to the incident air. It's just that the 737 MAX has become so different from the original 737 that this has finally become a problem - the bigger engines further forward were the last straw (and it is the lift, specifically, that is the problem, hence a solution that takes angle of attack as its input.)

Forward, where the wing is higher off the ground. The rest is like a skateboard suddenly accelerating, which will tend to pitch you nose up (and on the ground).

They moved it forward so they could move it up, but the centerline of the engine was still under the original engine's centerline.

Reply on news.ycombinator.com