Hello Everyone
I found this forum a couple of days ago and have been enjoying reading through many posts particular in the technical sections, it seems like a great forum.
There are a few questions regarding aircraft flight control systems that have been bugging me for a couple of years now, I was wondering if anyone could shed some light on this oh so mysterious aspect, that has eluded me for so long
Now I wasn't sure whether to post here, in the Tech Log or the Flight Testing forum. Perhaps if this is the wrong area it could be moved? .
My aviation interests lie mainly with military aircraft, I understand that most of the posts in this forum are directed to civil aviation, however my questions below could apply to both I guess.
1) An aircraft such as an F-16, F/A-18 or F-15 is in 1g level flight at 1000 feet and at a constant speed of 300kts. The pilot suddenly selects maximum afterburner. By virtue of FCS and CAS descriptions, I would have expected the aircraft’s nose to drop, as the AOA necessary for 1g level flight decreases as speed increases and for the aircraft to maintain 1000ft. In reality from what pilots of different aircraft have said, what happens is the aircraft actually remains out of trim while its accelerating and this condition exists until the aircraft has stopped accelerating. The phenomenon is greatest during rapid accelerations and decelerations, but tapers off as the magnitude of accels and decels diminishes. Now all the aircraft mentioned have control augmentation systems that are supposed to keep stick force per g constant. In the example I described above the pilot did not touch the control stick, but was commanding 'hands-off' 1g level flight by having the aircraft trimmed as such, only throttle movements were made. Considering the pilot did not touch the control stick, but the aircraft began to climb as it accelerated i.e.
g load was greater than 1g it diverted from constant stick force per g. Why does this happen? .
I can appreciate that there are regions in an aircraft’s flight envelope such as very slow speed flight, were there simply isn't enough dynamic pressure for response to meet command inputs, but I wouldn't expect this to be one of them.
2) Another situation, in which aircraft response is not consistent with stick force, is when lateral asymmetries arise. Some trapped fuel in a wing can cause this, aerodynamic imbalance or an asymmetric weapons load occurring as a weapon is dropped or fired. If an aircraft with two equal bombs (one per wing) was in level flight and trimmed for zero roll rates, I would have expected the flight control system to compensate when one bomb was dropped. Despite my preconception, the truth is that the plane will roll into the heavy wing, even though the pilot before weapons release had commanded zero roll rate and didn’t enter any subsequent roll rate commands after weapons release. Why is there no compensation? .
3) Rudder inputs command yaw rate but also have a natural tendency to induce roll. Again I would have thought the FCS/CAS would read this secondary roll as being uncommanded and apply lateral control surface movement to eliminate the event, this doesn’t happen, why is that? .
Thanks
Obi