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.

https://cimg1.ibsrv.net/gimg/pprune....873af8f111.png
https://cimg7.ibsrv.net/gimg/pprune....3aa1f36058.png

In normal law it's always in trim so why would you want feedback? If you want to fly at the edge of the envelope or below VLS, you'll have to apply continual back pressure on the stick, which is quite a lot of force to hold (or it is for my spaghetti noodle arms).

I think this thread is about the A220/C-series and their different control laws which apparently (I’m not on type) have speed stability baked in now? Maybe someone who is rated could comment more authoritatively?
So to comparing with the Airbus implementation it is somewhat dissimilar in that a) the ‘neutral point’ is variable, controlled by actuators and b) the force required to move the controls is also variable and is mostly speed related. Similar flight control laws but very different user interface?

I'll give it a go.
The side stick has springs, the force required to move the stick from centre increases linearly with angle of movement so with a little null of about 2 deg the force you need to apply increases from nothing up to 18lb at about 15deg from centre. This is known as the soft stop and is considered the normal maximum command. However, if you are brave and increase the force the stick will move a bit further. max physical deflection of the stick is 20 deg and that last 5 deg of movement required force from just under 30lb up to 35lb to hold it at the mechanical stop, this is known as the hard stop and is there so you can pull a bit harder and go over your 2.5g limit if you really have to. If no one told you that extra 5 deg of movement was there you probably would never know about it because the jump from 18lb to 30lb in force is completely unnatural and that is what is required to move it from 15 to 15.1deg. From there on it's linear again.

OK so what does moving the stick in pitch do? It commands a pitch rate. The rate is pretty much the same at all weights, CG, speed and configuration. (Pretty much but not exactly). So if you're in flight and you pull the stick back 5 deg you feel 8lb of spring force and the aircraft pitches up at x degrees per second. When you are at the pitch attitude of you choosing just release the stick. No force and it springs back to neutral. This is now commanding ZERO PITCH RATE. Sorry for the caps.
Simple so far. You pitched up and you start to climb because of all that basic principles of flight stuff.

Now if you did nothing else the aircraft will decelerate. This is where the speed stability part comes in.

As the speed starts to deviate from your previous trimmed speed this creates the same type of pitch rate command as you moving the stick. The further away from the trimmed speed (lets think of that as just like the neutral position of the side stick) the higher the commanded pitch rate. So as you decelerate the FBW starts commanding a nose down rate and the more you deviate the higher the rate. Exactly like moving the stick from the neutral position.

So with no further input from you and of course no change of thrust while any of this is happening the aircraft pitches nose down, at a slow rate initially but the rate increases as you get further from your trimmed speed (with a reasonable max limit) until you start accelerating again.

If you are acceleration you must be descending and the pitch rate reduces until you get back to your trimmed speed. Rate = 0. However you are descending and acceleration so the opposite happens. You go above your trimmed speed and the nose starts to come up. You can expect about 3 or 4 overshoots in the familiar phugoid oscillation.

The aircraft will end up flying roughly level at your trimmed speed so long as your altitude(and therefore thrust to drag) hasn't changed too much.

This is mimicking exactly what a positively stable aircraft would do if it deviates. But it's done through elevator inputs based on speed deviation rather than all those complicated AOA changes on your chuck glider.

Now if when you let go the stick earlier you didn't want the nose to go down you could always start pulling gently back on the sidestick again. So long as you apply an equal and opposite rate command to what the FBW is doing in trying to get back to trimmed speed the two will cancel out and the rate will be zero. But you will of course keep decelerating. As you do the FBW will be increasing it's command and you will have to increase yours. That means pulling further against an increasing spring force.
Put your other hand to use and increase the thrust and the aircraft will stop decelerating. The force required will stop increasing and now you can maintain your current force and the speed will not change. Hey presto you are flying out of trim and it feels like every other conventional aircraft. You have to apply a constant force equal and opposite to the difference between your actual speed and the trimmed speed that the FBW is desperately trying to get back to!

My arm is getting tired of holding this force so the little trim switch under my thumb comes into play. I start adding nose up trim. With every blip of that switch the trimmed speed (and it's graphical representation on the air speed tape) get closer to my actual speed. The FBW reduces it's rate command and I need to do the same by inching the stick back towards neutral.

This is hellish hard to do smoothly and the aircraft will pitch up and down as you work your way back into trim. Eventually the little bug will be on your current speed FBW will no longer be trying to put the nose down and the sidestick will be in neutral. You can now let go and grab the sick bag.

We fly pitch on the trim contrary to straight and level 101 to avoid this oscillation when retrimming.

Oh and while all this was happening the Stab was minding it's own business. It is only concerned with keeping the elevator somewhere near central. It looks at deflection and time and if it get's annoyed it quickly moves the stab to put the elevator in the middle of it's travel again.

Questions?