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tu144d
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Posts: 217
Joined: Wed Apr 24, 2002 11:39 pm

Stick Force Gradient explanation with regards to MCAS potentially not being needed for 737MAX

Mon Jan 11, 2021 4:32 pm

I read the following article this morning and this was linked by another user in the Civil Aviation thread.

https://theaircurrent.com/aircraft-deve ... ax-at-all/

It goes on to discuss how the 737 may not have needed the MCAS as the "slightly softer feel" in aircraft control for stall recovery as it pitches up more abruptly than the NG could have been addressed through pilot training and Boeing could've sought a waiver in "slightly different" handling characteristics akin to the common type rating and different handling between the 757 and 767.

In the article it states that the MAX's stick forces are reduced at higher alphas requiring a pushdown and this results in "lighter stick forces". I am trying to understand how this makes sense as I've forgotten a lot of static stability stuff. From my understanding, as the aircraft stalls and pitches up a much larger elevator deflection and thus control stick "force" would be required to get it back to trimmed unstalled flight. As well, I would imagine the engine nacelle would contribute a non negligible pitch up moment as well thus steepening the stick force gradient. I'm failing to see how the Stick Force Gradient would be reduced and forces would be lighter unless the article is referring to the fact that the control authority is lost at these high than normal AOAs and thus deflection of the elevators results in very little change in pitching moment and airspeed. This doesn't make sense though because then MCAS which moves the tabs would also not produce any noticeable aerodynamic pitching moment if authority was lost. All explanations appreciated. Thanks.
 
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Florianopolis
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Re: Stick Force Gradient explanation with regards to MCAS potentially not being needed for 737MAX

Mon Jan 11, 2021 11:41 pm

tu144d wrote:
I'm failing to see how the Stick Force Gradient would be reduced and forces would be lighter unless the article is referring to the fact that the control authority is lost at these high than normal AOAs and thus deflection of the elevators results in very little change in pitching moment and airspeed. This doesn't make sense though because then MCAS which moves the tabs would also not produce any noticeable aerodynamic pitching moment if authority was lost. All explanations appreciated.


I'm not 100% sure I follow you, but I'll take a couple stabs that might help. There are scenarios when a horizontal stabilizer becomes completely ineffective at influencing the airplane's pitch, in particular T-tail airplanes "deep stalled" at very high angles of attack. But I don't think that's what is happening on the MAX.

If you're using "tabs" referring to pitch trim, I'm thinking you might be imagining a Cessna 172 stalling. Swept wings don't behave the same way. Swept wings stall at the tips first, and as the stall develops, the center of lift moves forward continuously, which results in a pitch-up moment. see video: http://www.youtube.com/watch?v=T2y3DX8hZAY&t=2m20s

If I'm correctly reading the Boeing e-mails in that article, they're saying to each other that as you pulled on the column to stall the MAX, you would notice you were pulling less and less hard until at the point the airplane stalled, you were hardly pulling at all. That's an undesirable (and uncertifiable) characteristic. They didn't say they lost control authority or the ability to control the pitch of the airplane. But the force on the control column to take the airplane through the stall was very light, because of all the other aerodynamic factors contributing to pitch-up (center of lift moving forward, and the engine pylons also contribute to pitch-up at high angles of attack, as they become big aero surfaces in front of the CG).
 
tu144d
Topic Author
Posts: 217
Joined: Wed Apr 24, 2002 11:39 pm

Re: Stick Force Gradient explanation with regards to MCAS potentially not being needed for 737MAX

Tue Jan 12, 2021 12:53 pm

Thanks. Nevermind, I had a complete brain fart when I wrote that. I was thinking in terms of once you are at stall, forces are stronger to push down (which of course would not be desirable either), and not in terms of less stick force is required to get into a stall with all the other aero surfaces contributing to a positve Cm,alpha.

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