Su27 longitudinal FBW in real life

[LJQCN101]

First I'll state the sources.

 

1. 中国航空工业总公司第601所 (AVIC No.601, Shenyang Aircraft Design Institute): http://www.doc88.com/p-9139796730763.html

 

Quote

起落状态仅有俯仰速率反馈，空中飞行状态包括俯仰速率和法向过载反馈两者；两种控制律均为有差控制。由于有差系统不能实现中性速度稳定性，加减速飞行时杆力和杆位移变化很大，系统采取了随速压增加“自动向前推杆”的开环补偿环节

 

Translation: SDU in Takeoff-Landing Mode only has pitch-rate feedback, while Flight Mode includes both pitch-rate and Nz feedback. Both are systems with steady-state error. Those systems cannot achieve neutral speed stability, and can cause a great variation of stick force and stick position when changing speeds, so an open-loop compensation was added to "automatically push the stick forward" as dynamic pressure increases.

 

 

2. 中国飞行试验研究院 (China Flight Test Research Institute, by test pilot Xu): https://www.doc88.com/p-3857487861687.html

 

Quote

飞行马赫数在小于0.85的范围内变化时，飞机具有自动配平功能，飞行员几乎不用使用调校去配平杆力。

 

Translation: When flying below 0.85 Mach, the aircraft has an auto-trim function, and the pilot seldom has the need to re-trim the aircraft.

 

The paper from Shenyang 601 institute states that such a system has steady-state error, which is expected since the system doesn't use an integrator to eliminate errors between command and feedback.

 

Therefore an open-loop compensation law is added to reduce the amount of forward trimming as airspeed increases, as contrary to DCS implementations. (But it does not completely achieves neutral speed stability, since the system does not have an integrator.)

Edited February 3, 2021 by LJQCN101
update info

[Fox One]

The Chinese probably have modified the original flight control system to their liking. Below is a diagram from Su-27SK flight manual showing the necessary stick position in horizontal flight. I'm not saying the simulator corresponds perfectly to this diagram, it certainly does not. However the simulator can be considered generally correct compared with the diagram: in subsonic flight if you increase speed you must push the stick forward more and more to maintain horizontal flight.