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Guest Nick Bartolotta

Rudder issues in x-wind landings

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Hi folks,In a previous post, a user discussed their issues with the PSS 757's rudder in crosswind landings. They said (http://forums.avsim.net/dcboard.php?az=show_topic&forum=224&topic_id=19903&mode=full):2) Rudder useI noticed that during crosswing landing, close to flare, when I use the rudder, instead of making the aircraft "crab", it makes it slides to the opposite side. Let us say I am landing to Rwy 36. Wind is coming from 20. I need to move the rudder to the right and face 360+5 = 05 heading. The AC does that, but a the same time, it slides towards heading 350.I did not notice that on other addons. It is the same bahaviour with the 777.I have the Yaw damper on. Does it influence?Unfortunately, the post got down the wrong track and was locked. I am curious though if anyone else is noticing this, because I have, along with that original poster.I saw in another post by doing a forum search, another user has had this problem:http://forums.avsim.net/dcboard.php?az=sho...18950&mode=fullI have had both issues on landing, and believe they are related. Coming in on a crosswind landing with the yaw damper off, I will crab into the wind, and only find my aircraft is moving slightly the other way, and on landing will sometimes find myself skidding to the very edge of the runway. The only way to stop skidding is to put the nose gear down and steer with it. It seems trying to give more rudder in the opposite direction of the skidding only makes it worse.Thanks!

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Thank you for giving a 2nd exposure to this issue.As far as I remember, PSS support replied to me that they would bring the problem to the developper. So fra, no news of whether it has been really considered and eventually corrected.RegardsGhiom Viguie

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Hi Nick,This is an issue that I don't think will be very difficult to fix. I too have noticed the issue. It's a simple matter of changing a sign in the stability derivatives section in the .air file. In essense, for things like yaw and roll, - makes it go one way, + makes it go the other. In this case one needs to change a - to a + and vice versa on the appropriate fields. ;-)

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Thank you for this trick. Awesome if it works.Would it be possible though to give more detail on the procedure to do the change. Maybe not in the forum, unless PSS officials the change?My e-mail address is ghiomkl@hotmail.comThank you againGhiom

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I'd also be very interested in more details on this fix. If the PSS moderators are uncomfortable with it being on their forums, which is understandable, then would you mind emailing it to me also? My email is nick.bartolotta@gmail.com

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StevenVery interesting - message on its way.John Rooum

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Yeah, very interesting indeed!I think I was confusing myself with something else entirely gentlemen. I retract my claim in my first reply that I've noticed this tendency too, because I was thiniing of something else. The slip behavior of the aircraft seems correct to me. With the yaw damper off, I've tested this aircraft in a 90 degree crosswind at 30kts on the runway centerline and I found the behavior of the aircraft to be correct.Are you guys sure you're entirely versed on crosswind landing techniques? Let me describe the technique I'm using.The first segment of the approach was flown in the Crab method, with the wings level and the nose of the aircraft offset and pointing into the direction of the wind. The runway was 26R at KONT. Actual runway heading is 256. Wind was 346 at 30kts. The actual heading I was flying was something along the lines of 265-269. About 5-6nm from the runway, I switch to the sideslip method. Here is where I think your confusion exists:At this point, the nose of the aircraft is actually turned out of the wind. We are using opposite rudder in the direction of the wind (Downwind rudder) which means I am depressing the left rudder pedal. This yaws the aircraft to the left. However, slip occurs in the opposite direction of this (this is correct). Because of wind dihedral and roll tendency of the rudder, we need to use opposite aileron to the rudder to hold the steady heading. At this point the nose pointing 256, the left wing is up, the right wing is down into the wind. Touchdown occurs first on the right main landing gear and then on the left.Ghiom, the technique you're using is called De-Crab I believe, where you fly the aircraft with the nose into the wind but just prior to touchdown you straighten it out. So, if runway heading is 360, you're flying a heading of about 005 to counter the wind drift. Just prior to landing, you use left rudder to straighten back out to 360 to get a wings-level, correct-attitude for landing. In this situation you'd actually be slipping very slightly to the right, into the direction of the wind, so your drift would be countered as well.I've checked the numbers in the .air file and they all seem to be fine (as far as YAW is concerned of course!) All the signs are pointing in the right direction.Would you like some screenshots?

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Thanks for this great explanation. I am no real airliner pilot so I am not familiar with the 2nd technique you describe. I may try.However, the problem I witness when de-crabbing (just prior to touchdown) is a slip to the left (down the wind), therefore kicking me out of the runway, on the left of it.You are saying that the sliding is expected to be to the right?Maybe the problem lays here?Can you please confirm again the .air file is correct from this point of view.By the way, the PSS-757 is a very good "descender" and "lander". Hopefully, PSS will soon make it a good take-off-er and climber, by fixing the de-rated climb issue and imporving the engines thrust at higher altitudes.Thank you againRegardsGhiom

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If your rudder is deflecting to the right (right pedal down) then of course you will be slipping to the left, this is physics of flight. The example you gave was of a runway on heading 360 with winds from heading 020. If you were flying a true Crab/De-Crab landing, your nose is oriented towards the wind during flight (let's say heading 005). In this case your wings are level, no rudder is required.You're holding this attitude all the way until the last 20 or 30 feet of the landing. Once you start hearing those callouts, you're going to applying LEFT rudder (not RIGHT) because you want to align the body of your aircraft perfectly with the runway surface to avoid loading down your landing gear with sideforces. At no point in this situation should you be using RIGHT rudder if the wind is from the right. This movement is so quick and fluid that the amount of drift you receive from the wind at this point isn't enough to skew your course because you touch down a second or two later. Do you understand the concept better now or would you like some additional screenshots of it?

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This is what I am doing.Maybe I am doing it too early, at least early enough to be deported to the left...Thanks anyway.Ghiom

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It doesn't sound like this is what you're doing. You described to me using starboard rudder, which is the opposite direction you need to be going in.One last thing to consider is that the rudder is not giving you enough control because the damper's on. You might be getting very little yaw and in that short time it's actually the wind that's causing you to drift to the left. Try one of these approaches with the yaw damper off. Whenever I'm hand-flying an approach such as this I turn off the yaw damper so I have all the rudder authority available to me.

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Hi Shane,I was involved in testing the 777/757 series through the updates over the last year or so and worked with Rob Young on this aspect of the FDE. We also looked at the technique of switching off the yaw dampers and, at one point, Rob tried automatically switching off the 777 dampers at about 250 ft on the approach. I am not sure whether this stayed in the final 777 version but I reckoned we had both aircraft responding correctly up to 35 kts crosswind. However, I am a pilot and know very little about the magic of FDE programming so it was good to see it confirmed by an independent expert.BTW which version of the Tornado F3 has your FDE? That was my last full time aircraft and was also fun in a strong crosswind!RegardsJohn Rooum

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I can confirm the yaw damper seems to be disconnected at 250ft. I noticed this issue and sumitted a thread. Never got any answer so far. I am glad you explain to me now the situation I face. By the way, I could not find any yaw damper switch on the 777. Where is it?Thank you and regards.Ghiom

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Hi Shane,I am a real world private pilot and understand this procedure for landing as I get a chance to preform crosswind landings a lot at my hometown airport. :) I will try setting up your situation however and take some screenshots to see if I can show what I mean a little more clearly.Thanks!-Nick

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