How exactly does the apparent speed-stability of the Airbus A220 work?
 

The Airbus A220/Bombardier C-series utilizes the C*U fly-by-wire flight control law, which is similar to the ones used in the 777/787.

Now, I don't understand much about the C*U flight control laws, but I understand that it has apparent speed-stability, which makes it behave like a conventional aircraft.

My question is: With a side stick that has no force-feedback, how do the pilots "feel" the plane? I am sorry if this sounds a bit dumb, but shouldn't the stick be stiffer/softer at different speeds? How does the plane achieve that with a side stick that has no force feedback?

Very valid question IMO. I was surprised how little feedback the 320 gives. I was asked to keep the speed up as long as possible in the descent but to cross fix XXX at 12000/210. Planned descend at 320, put a fix at XXX-10/12000' to have a level deceleration. Decided to hand fly to get a feel. Zero difference in feel between 320 and 210 kts, almost zero input to keep the level flight while decelerating from 320 to 210. This was on the Airbus 320, no clue how the A220 does anything, but I have not heard of sidesticks that have force feedback.

The stick does provide feedback. There’s a speed cue on the IAS tape, which, in manual flight, is set by the trim button. Set the IAS, if you’re at a lower IAS the nose will drop or the AT will add thrust, if engaged. If on takeoff, at CLB thrust, you have to keep trimming down or the pitch keeps going up, up, up. One minor complaint, is the “trim” actually speed bug, button doesn’t move fast enough to keep it in trim, so you have to keep the pitch at the desired angle with forward side stick pressure or it’s increasing pitch. This from the 7500 which is pretty much identical flight controls.

You don't need force feedback to have 'feel'. Imagine you're flying level and you reduce thrust. The airplane will pitch down and start descending to maintain airspeed. If you want to maintain altitude, you have to pull back on the stick. If you don't adjust trim, you'll have to keep pulling back. The feel comes from the fact that you're constantly pulling.

And the angular offset on your control input, i.e. the force you feel is equal and opposite to the command generated by the difference between trim speed and actual speed.

but isn't that a very inefficient process? For example, in a C152, I would trim it out until I feel the pressure on my yoke is gone, and then refer outside to see my aircraft's attitude. So in the sidestick, they won't feel that force, they can only refer outside and trim accordingly (ie release the stick, see that the aircraft is still pitching up, trim again), whereas in my case, I can simply do it once by trimming until I have 0 pressure on my yoke.

On a light aircraft like a C152, pitch trim adjusts the neutral ('stick-free') position of the elevators. So you hold the yoke/stick in a constant position and trim until the stick-free position matches the position you're holding it in.

Large transport airplanes use trimmable horizontal stabilizers. [Note: The rest of this paragraph applies mainly to non-FBW airplanes.] Changing trim has no effect on the elevators. Instead, it adds to the effect of the elevators. So let's say you're pushing forward on the yoke. When you apply nose-down trim, the plane starts to pitch down. You have to release some of the forward pressure on the yoke if you want to maintain the pitch. So you make a series of pitch inputs while gradually reducing pressure on the yoke. When you've finished trimming, the yoke (and elevators) are back in their original positions; their neutral position doesn't change.

It may be somewhat inefficient, but this is how it's always worked on planes with trimmable horizontal stabilizers. I don't think the difference is really all that significant. All of this is harder to explain than it is to do.

Thanks u so much for the explanation, but I still don't understand 1 thing. So let's say in a FBW aircraft (one that uses the C*U FBW law or maybe an Airbus in direct law). Wouldn't this process be kinda tricky? Because in a side stick, the range of motions/wrist movements are much smaller, and without a proper pressure feedback, you'd kinda have to take a guess on when the aircraft moves to a neutral position, no? I would expect this would be significantly easier with a yoke.

There is a spring force in the side stick, so you can feel that you are at a stick position that is not neutral. The main difference is that this spring force does not change as speed changes.

From what I understand, the difference between the two implementations is if you are out of trim on a 777/787, you will need an amount of force on the controls proportional to how far from the trim reference speed you are. On the 220/C you’re pulling/pushing against a constant spring rate?

Imo, they should maybe apply a small clicking noise every time the trim is set to the current held attitude so that the pilot would understand that the trim is aligned and release the trim switch. That'd make things a lot easier, and he wouldn't have "feel" for the pressure on the control column, or have to look at the speed cue below.

If the trim switch is continuously held, It'd stop for like 1 second once the trim aligns with the current attitude and then give a satisfying click noise, and if the pilot still doesn't release the button and is holding up/down at the trim switch, it'd then again continue to trim up/down.

You mean current held speed.

Anyway I'll go one step further, and say they should make a button that sets the trim to that.

Why are you trying to solve a problem that doesn’t exist for an aircraft you don’t fly using a FBW system you don’t understand?

Well….., that’s kinda one of the reasons why I’m here in this forum, and not in some design engineer’s office at Parker Aerospace. Maybe in the future hopefully

very close to re-inventing the wheel. Atp, you may as well go one step further and create a system that just automatically trims out the aircraft for u to the new flight path vector, and holds that vector for u, name it C*

Your first sentence describes how the aircraft behaves. Your second sentence describes how it's physically implemented. Both are true for both types of aircraft. The Boeings have springs, too.

The pilot still has to release the stick. The click would always happen at the exact moment the stick reaches neutral, so it wouldn't convey any information.

I’m pretty sure the Boeing controls, like the autothrottle, are back-driven, not spring loaded. You can pull up a live page that shows the force being applied in each axis.

On the Airbus, with the sidestick in the neutral position, there will be no force on the controls irrespective of speed, attitude, etc.? On the Boeing FBW, if you are out of trim with the controls centred, you will feel a force. It’s also how the envelope protection works in manual flight: you will feel an increasing resistance to your commands (as you would go outside normal limits) but can override this with greater force.

Under normal conditions, the force is applied by a (in some cases mechanical) computer, overriding the springs. Under severe failure conditions, the force might be from springs only.

The 777 and 787 use springs and cam-roller mechanisms to provide pitch feel. At higher airspeeds, the feel and centering mechanism increases the forces by increasing the preload on the springs. The backdrive actuators are active only when the autopilot is engaged or when flight envelope protections are active. The springs are never overridden.

It's possible to measure the column force regardless of how the feel and centering is accomplished. The force transducers are in series between the controls and the feel and centering mechanisms. Control forces are required FDR parameters since 2009 (IIRC). That means that 767s and 737s have to measure it as well. Even aircraft from the 60s have force transducers; they're needed for the control wheel steering mode of the autopilot.

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