A346 Handling Frequencies

[jalbert1]

A question from an OLP.
The 346 is a long and bendy machine and as such has had its control laws tweaked but I'm confused by references to "handling frequencies" and "natural frequencies" and a less than complete understanding of the extra accelerometers incorporated in this aircraft so if anyone could post an explaination or link to help out I'd be most grateful.
Best Wishes.

[ICT_SLB]

Jalbert,
My guess is that rather than accelerometers, there will be additional filtering of the IRS signals to prevent the aircraft following the vibrations set up in the airframe. A similar situation was encontered on the CRJ-900 which required a notch filter in the Pitch Rate term to remove unwanted movement. It will be interesting to see what's required for the CRJ-1000 - only 5m longer!

[gr8shandini]

I'm not familiar with the term "handling frequencies." A natural frequency, however, is simply the frequency at which a structure responds to a force. For example, when someone jumps off a diving board, you can watch the end of the board flex up and down at maybe 3 or 4 Hz.

Now if you were to try to shake the board using a constant force starting slowly then speeding up, you'd find that there wouldn't be a whole lot of response until you get close to that natural frequency. At this point, the deflection of the tip of the board maybe several times what it would be if you just held a constant force on it. But if you keep going faster, it'll die down again. Keep going faster still and you might find another natural frequency where the board wants to bend in an S shape rather than just deflect at the tip, etc, etc.

This is important because, as ICT mentioned, autopilot systems and especially fly by wire control systems might try to respond to a disruption such as turbulence by rapidly commanding control inputs. On an airplane with a lot of flex, that's like shaking that diving board. So they put in notch filters that reduce the amount that the autopilot is allowed to move the controls at frequencies near the natural modes. But as these modes are known through analysis and ground testing, accelerometers are not usually required.

However, some larger aircraft might use wingtip and/or tail mounted accels to measure fatigue cycles or even try to minimize wing flex. A good example of that is the C-5 which has an active lift distribution system that keeps the wings from flapping too much.

[Tester07]

My understanding of this is that accelerometers were indeed installed in the wingtips, but to feed back into the flight control laws to prevent the dutch roll in the cruise from which the aircraft suffered when it first came into service. So it may be that it is the natural frequency of the dutch roll which is referred to here.

[engineer07]

The A340-600 has accelerometers installed in the outer engine pylons. They are used by the flight control laws (through small aileron defelections) to damp a vertical bending mode of the wing (long and bendy !) which couples with a lateral mode of the forward fuselage (also long and bendy !). In testing and route proving this lateral mode in the forward fuselage was found to be very uncomfortable (the human body does not like lateral 'g'). The problem is only really noticed in turbulence. Since we put the First Class people in the forward part of the long bendy fuselage, comfort is quite important ! Incidentally the Boeing 777-300 also suffers from the same problem - lateral 'g' in the forward cabin in tubulence - don't know if the reason is the same.