Originally Posted by
Field In Sight
I would simplify it further.
Originally Posted by
hans brinker
Pretty much what FIS said
Ok, your answers seem to be valid, and especially useful for Real Airbus Pilots. I stay away from this discussion, I have no interest to interfere with advice for active pilots.
My personal interest is to try to understand the system behaviour. The lateral aspect of roll control got confirmed by you by and large. Thank you, guys.
Now the more difficult part: Vertical Control.
Airbus FBW seems to incorporate a mix of Pitch Rate and g loads control. Pitch is predominant at low speeds, g load control at high speed. The A320 Veq "equal Speed" or changeover speed is not a hard changeover, but the speed where the influence of both factors are mixed 50/50 (at least that's what I picked up)
I ask myself it this is an intended mix or just the result of physical basics.
How I understand it:
Pitch Control means sidestick controls the pitch rate for changing the pitch.
Sidestick at zero most probably means the pitch angle is constant. I'm not sure if all influences are countered. Certainly Speed change, Power Change. Turbulence? Shouldn't have a great influence anyway.
Push/Pull of the side stick will command the speed of pitch change, so the pitch rate.
This is predominant at low speeds.
g load or load factor control means sidestick controls the vertical acceleration of the plane and will adjust the pitch to get the result.
Sidestick at zero most probably means the current vertical speed is constant. If plane is in level flight, it remains at current altitude; V/S=0. In Climb or Descent the V/S remains unchanged = unaccelerated. So the reference of g load as a measure for acceleration. sidestick neutral means 1g load, or 0g deviation, no vertical acceleration.
Push/Pull of the side stick will command a deviation from 1g. Like 1.2g, that will exert an upward vertical acceleration on the plane, V/S builds up until sidestick is neutralized again.
If this will also resist to uncommanded external influences (symmetric vertical turbulence, vertical airstream), then it sounds as a dream. Again it would prevent a large deviation from assigned altitude, and reduce "bumby rides" and sudden drops. with some significant delay I assume, cause the pitch will take time to adjust lift
G Load is predominant at high speeds, I learned. Where it really could help in turbulent air (within limits)
But why not at low speeds? On Final approach is is also the main interest to remain on the flight path (GS). Pitch Rate control doesn't do that directly.
What is the intention? Or, as I asked, is it simply physical (aerodynamic) laws and no intention?
The basis of my question is this formula from user
CVividasku :
... but the normal law in pitch is not an Nz law. It's a C* law, where C*=Nz+Veq*q/g
Where Veq is a speed around 210-220 kt depending on airbus models, V is IAS, q is pitch rate (it's the letter usually used in France to describe an airplane pitch rate), Nz is indeed the load factor....
Me trying to use known words, pilot commands
C* = Load factor + Veq*Pitch Rate/g
I don't really understand it. Is it a zero sum on sidestick neutral? What changes when the sidestick is deflected?
The predominance of one or the other, he expains (modified to my terms):
At very high speed, you require little pitch (rate) to have a big Load factor, your C* law is closer to an load factor law. At lower speeds, the term around Pitch Rate is more noticeable.
(...)
Imagine you're asking for more lift. You're asking for C* = 0.2g. The airplane has pitch rate=0, so it translates at first with load factor = 0.2. It starts pitching up. You're having some positive pitch rate that will appear. This term will damp the load factor demand.
Conversely, if you're stable in level flight, and decide to decelerate. You're asking 0g (of variation), so when reducing speed, at first the airplane will have to have some positive pitch rate. Since you still want a null sum, the load factor will become slightly negative. It will damp the pitch rate, the pitch rate will stabilize at 0 again, but with a very shallow descent.
So it's not really trajectory stable.
I'm confused and hope for somebody smart to enlighten me (or some of us)
Again, it's academic and I accept that Real Pilots don't need to know,