Roll control mechanisation, CJR50

[Genghis the Engineer]

I was sat for 90 minutes last week on a CRJ50, and being who I am, and having run out of reading material, I found myself watching the wings.

The aircraft has ailerons covering roughly the outboard 1/3 semi-span, and then two spoilers at 2/3 chord(ish) one within the inboard 1/3 semi-span, and one roughly in the middle.

The aircraft as it rolled was sometimes using aileron, and sometimes using the inboard spoiler - but I couldn't for the life of me work out which and when; it may have had something to do with airspeed, but didn't seem to have much to do with gear or flap settings.

So, purely for my interest and edification, can anybody explain what decides when the CRJ uses aileron, or spoiler, or both?

G

[Mad (Flt) Scientist]

I believe what you saw was the effect of autopilot or pilot command.

With pilot wheel input, the ailerons of course move. After a small deadband about zero, the spoilerons move to assist roll control.

When the AP is engaged, it controls the ailerons alone (I'd have to check how it feeds back to the wheel to see if it has spoileron control).

The other possibility is maybe it was the deadband?

[Mad (Flt) Scientist]

OK, I actually checked, the autopilot servo should be back-driving the wheel, and since the SECU is using wheel position to drive the spoilerons that means that the autopilot does have indirect control of the spoilerons. So my guess that maybe the autopilot servo was an aileron only device was wrong. Cap'n marvellous is right and the spoileron-to-aileron gearing should be identical whether AP is on or not (although in practice, being driven from a different point in the circuit there will be small differences, but you'd not see them I think).

So I'm confused who Genghis saw one device only move, unless it was the flight spoiler, but then why would the a/c roll?

[Mad (Flt) Scientist]

What happened to the previous post by cap marvel? It seems to have magically moved inside the thread, which is odd.