PPRuNe Forums - View Single Post - B737 controlability-questions & surprises.
Old 3rd Feb 2019, 14:54
  #30 (permalink)  
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Join Date: Jun 2009
Location: florida
Age: 81
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Salute Centaurus and Jen from OZ!

I would not worry about thread drift, too much. There's another thread about the Flight Director and we still see many references to "children of the magenta line".

Although I haven't flown for decades, a blog/forum like this would have been invaluable 40 or 50 years ago. Lessons-learned are part and parcel of our profession, and many times a glimmer of recollection concerring some other pilot's problem and actions will save the day. II used to sit in the waiting line for a haircut, or actually in the chair, and go thru every bad thing I could imagine and what I could do. Many reactions/procedures were spelled out for us, but many were not. i also "flew" my next mission in my head. If something happened that I had not done in my 'perfect" mission, I noticed it very quickly.

The biggest difference between the controls in the 'bus and the Viper and Raptor and Lightning is using pressure versus the physical movement of the stick/wheel/yoke. Then there's the control laws that are dictated by the plane's mission. The Viper and 'bus are both a gee command for pitch. But the fighters lack corrections for pitch and roll attitudes, as you can imagine. And BTW, I flew one "light" that had a great control stick steering mode and was used frequently due to the "pitch up" problem ( the VooDoo).

The insidious effect of the 'bus control laws has to do with the attitude and trim corrections. At any attitude other than st-and-level, Hal cranks in both elevator, aileron and trim. And the nominal one gee "neutral" command is biased by pitch and roll. For example, at 30 degrees of pitch the gee command from Hal is about 0.87 gee. If it remained at one gee the plane would continue to increase pitch. This was easy to demo in the Viper because we didn't corret for pitch angle. In fact, at extremely high pitch this attribute helped us to enter a deep stall when we ran outt speed before the elevator could get the nose down. In the 'bus, we saw some of this in the AF fiasco. The stab kept trimming to allow a neutral stick, and the normal reduction in gee command wasn't there due to the control law reversion once air data was deemed unrealiable by Hal.

I really like Jen's comment about the servo tabs. In fact, they can even be used with full FBW systems. I flew one light with the things on the ailerons to enhance roll authority when carrying heavy munitions. A simple torque tube would command the tab to move when control pressure reached a certain value. Our system was mechanical, but with FBW actuators it still works the same. When the ciontrol surface actuator reaches a certain pressure or torque, the tab noves to reduce the pressure. Good ideas here most of the time.

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