PPRuNe Forums - View Single Post - AF 447 Thread No. 9
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Old 18th Jul 2012, 15:36
  #544 (permalink)  
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Join Date: Jun 2009
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I guess we have to define terms, and "flight path" is one term that may be causing us to debate a few things about the control laws and the natural aero characteristics of the 'bus.

A very good explanation of some things, Retired. But I expand...

Courtesy of some of the pilots here, I have a decent set of the manuals. And from what I see, the FBW system is strongly biased for a one gee flight, and not an "attitude hold" mode as we had/have in many jets with autopilots. It also appears to have little AoA bias, but only some "limits" to stay outta the stall regime. Body rates and gains are contributing inputs, as was also the case in my primitive system. The positive longitudinal stability most of us were/are used to is not based on AoA in this system. Let go of the stick and the sucker shoots for one gee corrected for pitch attitude. If that ain't the way the thing works or is supposed to work, please correct me.

I iterate this point because it's exactly the way our primitive system worked. The biggest difference was we could set the desired trimmed gee, and our system wasn't trying for one gee all the time, nor did it correct for pitch attitude. We had zero static stability "feel" for AoA as most of us are/were used to. Only reason the nose went down when relaxing pressure on the stick was to achieve the trimmed gee ( nominally one gee according our trim wheel, which allowed about 3.5 positive and a bit over one gee negative). If we trimmed full back, and let go, the thing would do a beautiful 3.5 gee loop, and the gee would decrease as we hit the AoA limit until we got to one gee or a bit less. AoA would be pegged at 27 degrees. Coming down the back side, and speed building, the thing would get back to the 3.5 gee trim command and finish the loop.

I bring all this to the table to help folks understand the difference between FBW systems and the conventional control systems we flew that were based on a trimmed AoA. Sure, we could build a great FBW system that was strongly biased for AoA, just like the "old days". In fact, our system did that with gear down. We biased the gee command with AoA to give us the same old feeling we had in the days of yore ( no auto throttle in the Viper, unlike the Hornet, so we could trim for an AoA and use throttle for vertical velocity). In a FBW system you can overcome all kindsa nasty aero characteristics until the confusers go to la la land. The 'bus seems to be a very benign, stable platform. So even in "direct law" it can be flown by humans. Ours was not stable until above 0.95 M or so, and if the control surfaces went to neutral due to computer failure, the thing went nose down ( 22 negative gees on one of our first computer shutdowns due to power failure).

We could build a FBW system that only used the electrons to command control surface position, just like the old days except the hydraulics didn't use a valve at the base of the control wheel/yoke/stick. We could even build one that used electrons to power the actuators for each control surface.

But we haven't done that. We use the neat computers to reduce workload and overcome aero characteristics of the platform. So we see a lot of "autopilot" functions embedded in the system. Those are the things that bother me. Those are the things that gradually destroy basic airmanship and understanding how our jet flies.

'nuf philosophy for now.
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