Control inputs and stability
@Gray
We went thru this ( relaxed static stability and c.g. and....), and even posted charts and graphs and testimonials. I had to back off of some thoughts once some of the 'bus manuals were placed here for consideration. The 'bus is not nearly so "twitchy" as the jet I flew due to the c.g., and the company's goal of reducing drag due to down force by the rear stabs in cruise flight seems O.K. as long as you are FBW versus conventional flight control systems. I only brought up a discussion of the concept due to my own experience with "relaxed static stability" and the potential to reach a flight condition that is difficult to deal with.
There are several things about the 'bus flight control law implementation I do not like, but the basic concepts of the jet seem fair.
and for the 'bird observations:
- It is true that we tend to put in aft or forward stick when rolling right or left. Seems that some FBW systems with the small "side stick" are even canted to account for this. OTOH, I don't see this as a big deal for this accident.
- As 'bird has noted, rudder trim is available, but I can't find a note in the manual about the system "trimming" the rudder for zero yaw in the stable flight condition - no aileron input or pitch input, just flying st and level. With the laws that the jet uses, about the only thing I can see the rudder pedals for is nosewheel steering or countering roll when in a stall or close to a stall. This is due to basic swept wing aero.
- There is no roll trim on the beast, best I can determine reading all the manuals. This is unlike the FBW jet I flew, where roll command was identical in concept to the 'bus, but we carried ordnance and could have asymmetric loads that were a significant part of our gross weight. Our roll rate command was a bit more in magnitude, think max was about 280 deg per second at 16 or 17 pounds of pressure. Our trim max command was less than 280 deg/sec, hence, when my LEF folded up I had to hold constant pressure for about 15 minutes until getting back on the ground. Also had to trim rudder a bit.
- The pitch trim mechanization on the 'bus is gonna be a player in the final findings, and I'll place my bet now.
Why is that, Gums?
It's because you can't manually trim for a desired gee or attitude or AoA , even using the THS manual wheel. If you constantly hold even a slight bit of back pressure, the THS moves to help maintain your existing gee command and the elevators do the "fine" work. Need a 'bus driver to try this out in flight - jez kidding! Let go of the stick in Normal or Alt and the sucker re-trims the THS for a 1 gee baseline ( corrected for pitch attitude and bank angle, both to specified limits).
So my feeling is the constant back stick moved the THS to almost max, then got stuck there in Abnormal mode or was maintained there in Alt mode due to constant back stick gee command. No AoA limiter as we had in the Viper. In that sucker we hit 9 gees at 15 deg AoA and then if we continued to demand max pitch the gee command would be reduced to 1 gee at 25 deg AoA, even if we pulled 100 pounds on the stick. Our AoA limiter worked very well except at a combination of extremely high pitch attitudes and very slow speeds and little or no roll rate.
Gear down we added an increased AoA bias to the basic gee command, plus we increased pitch rate feedback. The result was a fair simulation of "normal" jet approach characteristics, in that we could trim for an airspeed/AoA versus a pure gee.
Finally, the THS can't help but be the primary driver at the edges of the envelope, and the elevators act like "trim tabs". It acts like the "all-moving" stabilizers you see on all the jet fighters.