NORMAL Law and auto-trim / stick required during Turns ?...

[jcomm 8]

I have re-installed FSX recently but on it's new SE re-incarnation, so I didn't install my old Aerosoft Airbus X.

I've been watching a few videos about the way this new series ( including the A318/19 ) model the FBW laws ( NORMAL law in particular ) and it appears that during turns, with AT active, and bank not greater than 30º, the auto-pitch trim function doesn't follow what is described as the real Airbus FBW behavior, namely, no need for "pitch" input to maintain altitude.

Even worst, in one of the videos, when changing speed through the FCU, during the turn the "DECELERATE" message is displayed in the PFD and the pitch down being aggravated ?

Can someone explain me if this is indeed the case with the latest patches, or if indeed I am wrong about my assumption that on initial bank to the PF has to pull the stick until the required bank angle is achieved ?

[Joshua Che. 1601]

I am not sure what you mean, it would be best if you take a video or a picture.

[jcomm 8]

Joshua,

I am referring to: FLIGHT LAW where

• Becomes active shortly after takeoff and remains active until shortly before touchdown.
• Sidestick deflection and load factor imposed on the aircraft are directly proportional, regardless of airspeed.
• With sidestick neutral and wings level, system maintains a 1 g load in pitch.
• No requirement to change pitch trim for changes in airspeed, configuration, or bank up to 33 degrees.
• At full aft/fwd sidestick deflection system maintains maximum load factor for flap position.
• Sidestick roll input commands a roll rate request.
• Roll rate is independent of airspeed.
• A given sidestick deflection always results in the same roll rate response.
• Turn coordination and yaw damping are computed by the ELACs and transmitted to the FACs.
• No rudder pedal feedback for the yaw damping and turn coordination functions.

during a turn, there i no need for pitch input provided we keep within 33º bank. As far as I recall, in the initial Airbus X I had to pull on the stick to avoid it starting a descent when the turn was established ?

If I reduced speed during that same turn it would, additionally, start an even stepper descent with the "DECELERATE" message being issued.

[kevinh 6]

Well, I don't know which videos you've seen and I don't have FSX:SE. However the current Airbus A318/319 product FBW behaves very well in turns and with reducing speed. No sign of the nose dropping or DECELERATE messages. Airbus X is an awful lot of updates ago. The current product is greatly improved.

When you initiate the turn you must be careful not to make any pitch inputs, otherwise the FBW will follow those of course. Maybe that's what you saw in the videos. If you exceed 33 deg thr nodr dros and you need to make a pitch input to hold it. If you release the roll input bank decreases to 33 deg, just as it should.

The only thing I would say is that in a steady level turn with no pitch or roll input and bank angle less than 33 deg, bank angle will slowly decrease over time. It should remain constant. But this is a minor criticism.

[Emi 5161]

I'm not sure which videos you have seen, but in none of my testflights I needed to give any elevatorinput to maintain my pitch during a normal bank. As far as I am aware (and as the real A320pilots who tested our product say) our Airbus works the same way the real one does.

I can just guess, but I know there are some simmers who do not fully understand how the Airbus works in normal law.

Some people think it would maintain altitude without sidestickmovement, but that is wrong. The Airbus will maintain attitude with the sidestick in the neutral position, but during a turn this of course requires to pull the stick back a bit to maintain altitude, since you need more lift during a turn, than during levelflight.

[kaktus018 7]

The FBW just keeps the pitch and bank angle and not the flight path!

When you de-/accelerate the same pitch causes less/more lift, meaning you start descending/climbing. When you de-/increase the bank angle the vertical component of your lift respectively in-/decreases. This means you have to adjust the pitch to maintain your altitude.

[november678x 1]

Ok so I'm confused, I'm not saying anyone is wrong but the video & info below shows otherwise. If you watch the video between 11:55 to 12:54, it shows that pitch is compensated to maintain level flight within 33 degrees. Although the video refers to an A330, the philosophy should be similar on the A320. So which way is it?

11:55 to 12:54

Page 130 - A320 Family Instructor Support

LATERAL CHARACTERISTICS

When acting laterally on the stick, the pilot orders and gets, most naturally, a ROLL RATE. Hence STICK FREE, he orders 0 roll rate; thus the current bank angle is maintained within ± 33°.

As a consequence, the A/C is LATERALLY STABLE and NO AILERON TRIM is required. But the lateral law is also a mixture of ROLL and YAW demand with: - Automatic Turn coordination, - Automatic Yaw damping and - Yaw damper initial response to a major aircraft assymetry.

During a Normal Turn (bank within ± 33°), in level flight: - move the stick laterally only since there is pitch compensation in turn, - the more you move the stick laterally, the greater the resulting roll rate (e.g. 15°/sec at max deflection) and - you dont need to use the rudder.

In case of Steep Turns (bank angle greater than 33°), you have to hold a lateral pressure on the stick to keep the bank and an aft pressure on the stick to keep level flight. Indeed spiral stability is reintroduced and pitch compensation is suppressed beyond 33°, since there is no operational reason to fly with such high bank angles for a lengthy period of time in normal circumstances

Thank you!

[kaktus018 7]

This has been discussed several times. Aerosoft claims they got this right. According to instructor support and other (less reliable) sources, they didn't. However, a lot of people (qualified or not) state that pitch is maintained, thus a roll rate has to be commanded for level flight in turns.

Aerosoft has test pilots, who I assume tested every aspect of the FBW. Still I don't know if it is correctly modeled.

Sorry for bringing that up again, but a clarifying statemant of a developer would be appreciated .

[Joshua Che. 1601]

According to instructor support and other (less reliable) sources,

That is your source and I have mine. Not discounting your source in anyway, but I wont trust strangers who claim things.

Don't forget that this is a desktop simulator. The FBW here has been flown by at least 7 bus pilots now and they find it rather accurate.

Lastly, this topic has been discussed to death before. Do a search and you will be able to find something.

[kevinh 6]

The FBW just keeps the pitch and bank angle and not the flight path!

When you de-/accelerate the same pitch causes less/more lift, meaning you start descending/climbing. When you de-/increase the bank angle the vertical component of your lift respectively in-/decreases. This means you have to adjust the pitch to maintain your altitude.

This is not completely true. The sidestick commands roll rate and vertical acceleration (g). So if you release it within the envelope protection limits it will maintain a zero roll rate (not quite the same as maintaining a bank angle) and 1 g vertical acceleration. This means if you apply a bank angle less than 33 deg, angle of attack will increase to maintain 1g vertical acceleration. This exactly compensates for the loss of vertical lift due to bank angle. In effect it maintains vertical flight path. The aircraft makes a coordinated turn as a result. It's only if you bank beyond 33 deg that you need to apply a pitch input to maintain pitch attitude.

Now I don't know what maths models Aerosoft have used in the FBW control laws, but subjectively it feels very good. I've flown A320 full flight sims and this is the best desktop simulation I've yet seen of the A320 FBW. By being very careful to only introduce a roll input I've seen the sim maintain vertical flightpath well. If you make a small pitch input inadvertently with the roll (which is easy enough with a typical joystick) then it does deviate vertically. The only problem I've noticed is that instead of maintaining bank angle it rolls slowly back towards wings level.