Aircraft Trimming -A320

[Boyington]

The A320 FCOM states that in normal cruise around M.77 in straight flight with the AP engaged and fuel in the wing tanks distributed symmetrically the rudder trim should stay between 1 degree right and 2.3 degree left.
Why is it different for left and right?

[PilotJames]

Anyone have any ideas?
It reads strangely from the translation into English.
I understand the rudder varies 1.5 degrees left and right but the indication is different between the left and right. Not sure why the indication is different?

[vilas]

May be due to fin offset.

[Lantirn]

May be due to the fact that engines turn clockwise as seen from the cockpit. Slightly more thrust arm from the left side.

[Amadis of Gaul]

May be due to the fact that engines turn clockwise as seen from the cockpit. Slightly more thrust arm from the left side.

So, p-factor?

[Lantirn]

Sort of...Seriously I dont know if it really applies. But i can not find another explanation

[PilotJames]

The FCOM basically says that the actual trim is from 1.5 left to 1.5 right but says the indication is different between left and right. At least that is how I read the translation. Have a read in the FCOM and see what you think.
I have no idea why the indication would be different.

[Chris Scott]

Quote from Lantirn:
"May be due to the fact that engines turn clockwise as seen from the cockpit..."

Errr... that would be true if the cockpit was behind the engines!

Both CFM and IAE engines are right-hand tractors, so rotation is anticlockwise as seen from in front (i.e., looking aft).

[Lantirn]

Yep you are right, I wanted to say when looking in the direction of travel! The correct term is anti clockwise sure.

Regarding the FCOM and the trim, what it says there is that the aircraft when flying should have (theoretically) zero rudder trim input. With this in mind, when it is trimmed, it should be at zero. Because the average value in cruise is about 0,5R and 0,8L, there is a small 0,2L residual

Now the true rudder trim deflection should stay between +/- 1,5 degrees, otherwise maintenance has to work again :)

So the acceptable values are between 2,3L (0,8+1,5) and 1R (1,5-0,5)

Those values has nothing to do with the indication. The point is that there may be a difference from the real trim value by some defined amount

This amount shows that for some reason, the aircraft is trimmed more to the left, always

[ExV238]

In fact, assuming symmetrical fuel loading etc, the rudder is neutral. The left indication bias is due to thermal settling of the structure and trim actuation mechanism at low static air temperature.

[Lantirn]

ExV238,

Thanks for the feedback

[PilotJames]

In fact, assuming symmetrical fuel loading etc, the rudder is neutral. The left indication bias is due to thermal settling of the structure and trim actuation mechanism at low static air temperature.


Bloody hell, how did you find that out?
Doesn't it say that the actual rudder position is 1.5 degrees left and right. I thought the bias is in the indication.

[ExV238]

Yes, in a perfect world (fuel balanced, straight airframe, etc) the rudder is neutral. The left bias is in the trim indication, for the reason above.

[PilotJames]

Yes, in a perfect world (fuel balanced, straight airframe, etc) the rudder is neutral. The left bias is in the trim indication, for the reason above.

That's great, do you mind me asking how you found that out. Just purely out of interest. An engineer?
Why is the permanent offset of indication more to the left than the right?

[Owain Glyndwr]

ExV238 just beat me to it!
The bias is one way because the mechanism is always colder in cruise than when the system was set up in the hangar

[Chris Scott]

Hello ExV238 and Owain,

Just to clear up a point raised earlier in the thread, and given that in an untwisted airframe the rudder is neutral: is the fin laterally symmetrical and mounted precisely fore-and-aft, or does the fact that the two large fans are not handed result in any yaw effect that requires compensation?

Chris

[PilotJames]

Just to see how this all fits together, there is a permanent offset of indication to the left because of the cold SAT.
If the average offset is 0.5R and 0.8 left how does that fit in the with 1.5 left and right maximum real deflection and the corresponding maximum value of rudder trim 1R and 2.3L.
Surely max indicated trim would be 1.5L+0.8L=2.3L (that makes sense and that's what's written in the FCOM)
But it doesn't make sense for the right 1.5R+0.5R=2?? (They say 1R)
How does that work?

[FlightDetent]

Auto-spellcheck in play? Anyway the're jets, mounted symmetrical, under the wing, exhaust path well below the tail structure... What physical reason do you see for any yawing force?

[Owain Glyndwr]

Chris

Good question but it never came up in my career.
Yes, the fin is laterally symmetrical and mounted symmetrically.
Like FlightDetent I can't see any reason for a yawing force if the engines are being operated symmetrically

PilotJames

I haven't read the FCOM so this may be complete nonsense, but if the average offset were between 0.5 and 0.8 left (which is more sensible than 0.5R to 0.8L if it is really a one-way temperature effect) then the 1R/2.3L would also make sense wouldn't it?

[PilotJames]

Chris

Good question but it never came up in my career.
Yes, the fin is laterally symmetrical and mounted symmetrically.
Like FlightDetent I can't see any reason for a yawing force if the engines are being operated symmetrically

PilotJames

I haven't read the FCOM so this may be complete nonsense, but if the average offset were between 0.5 and 0.8 left (which is more sensible than 0.5R to 0.8L if it is really a one-way temperature effect) then the 1R/2.3L would also make sense wouldn't it?

That would make perfect sense. It says (average 0.5 right, 0.8 left).
Maybe it's left (for the right side average left 0.5 and for the left average left 0.8)
Haha ;) very confusing

[Chris Scott]

Thanks Owain.

Quote from Flight Detent:
"Anyway the're jets, mounted symmetrical, under the wing, exhaust path well below the tail structure... What physical reason do you see for any yawing force?"

None, actually. I was referring the experts to Lantirn's suggestion in post #4.

Certainly never noticed any thrust-induced yaw on the A320 or A310. In fact even the (propellor-driven) DC-3/C47 didn't seem to exhibit any, despite its higher thrust line, and I've always assumed that the effects of the two rotating slipstreams on the fin more-or-less cancelled each other out. That also applied to the high-winged, high-thrust-line Dart-Herald.

As for turbojets and turbofans, my understanding is that any slipstream rotation is negligible, as you suggest. But I sometimes wonder about big fans with short fan cowlings, such as the early JT-9s.

[FlightDetent]

Well I saw no reason for that "more thrust arm from left". And thus asked your opinion of it :)

[clark y]

ExV238,
I vaguely remember back in the late 80s being involved in the modification of a B767-200 rudder actuator system for very similar reasons. We were told that in flight the rudder would physically settle a bit offset.

[PilotJames]

ExV238,
I vaguely remember back in the late 80s being involved in the modification of a B767-200 rudder actuator system for very similar reasons. We were told that in flight the rudder would physically settle a bit offset.

This is offset on the indication of rudder trim only. Not the actual rudder position