Sign in to follow this  
Guest Vangelis

NOTAM : That ol' rudder problem (why and how)

Recommended Posts

Gents,There have been reports of wierd behaviour of the aircraft when rudder is applied. The reports were valid under a specific set of circumstances. The problem has been identified and fixed for an upcoming (within a fee days) mini-update. Here follows a small technical "briefing" for this issue.In practice this anomalous behaviour required the following :(1a) Significant amount of rudder(1b) Aileron the other way (i.e cross-controls)(1c) Sustained application of cross controls for over 5 seconds (i.e.combination of (1)+(2))Behaviour :(2a) If attitude (pitch) was maintained above a very narrow FS defined limit zone the plane accelerated unreasonably.(2b) If attitude was below a certain (minute) threshold the plane decelerated rapidly.Reasons for non-identification of this issue :(3a) Application of quick corrections to maintain attitude (primarily pitch and secondarily bank) the plane behaved normal, and/or(3b) Quick release of cross-controls(3c) This behaviour becomes apparent through **sustained** application of a fundamentally wrong procedure to fly an airliner class jet (as opposed to a single engine prop trainer). Rudder is primarily used at about 50ft to align the aircrat heading with runway and that's that ! (Whoever disagrees earns a free ticket for row 23 in a empty plane for a trully unforgettable special flight -- barf bag(s) come also absolutely free)Reason of occurence :MSFS goes crazy and calculates a huge negative or positive drag ! Negative is correct (see attachment)Remarks :(4a) An essential characteristic of 737's i.e. rudder induced considerable and sharp rolling motion is moderated (MSFS cannot take all of it).(4b) Problem is fixed(4c) An easy platform to cut teeth on cross controls in an airliner class jet (never said that, see item (3c) above !).Many Regards,Vangelis Vaos===================================== Evangelos M. Vaos Precision Manuals Development Group www.precisionmanuals.com=====================================

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

Van,Just one comment regarding the sustained cross-control situation being wrong in a transport category aircraft.In a x-wind situation, when you start to kick the crab away, simultaneous application of opposite aileron is always needed to counter the rolling momentum initiated by the rudder. It is not uncommon to stay in that crossed-controls situation for 5 seconds or more. Even in the most perfect situation (with a firm, non-greaser landing) you would hold the controls crossed for 3-4 seconds, at minimum. Thus, I feel it is well within reason to address this issue, just as you have done :).Terops. forward slips are not forbidden in a 737NG. Not thought of as one of the "best" procedures to resolve a hot and high -situation, but commonly used anyway. That is when you definitely have the controls crossed for 5 seconds or more.

Share this post


Link to post
Share on other sites

Tero-You might be misundertanding the specific scenario: Vangelis is referring to FULL control deflection for sustained periods.... i.e. control inputs clearly beyond the normal operating range for the aircraft....Imagine what conditions would require full application of rudder in close proximity to the ground.... I can't think of any that would require that it be HELD at full deflection- but that is the region where Vangelis discovered we were sending data to the FS flight model that it could not interpret correctly.... and the result was as described...

Share this post


Link to post
Share on other sites

Robert-I haven't noticed this issue personally, but after reading the threads, how is all this related to those having problems manually landing in a crosswind? I doubt they were using full rudder deflection. PMDG's post seems to imply that this is an issue ONLY with full rudder deflection and a cross control situation. So, I'm confused as to how that would fit with the manual crosswind landing issue.Think I'll try one to see how it goes on my system.Thanks,

Share this post


Link to post
Share on other sites

Robert,A very good point. I misunderstood it.The only thing that comes into mind would be a sudden engine failure just before decrabbing the aicraft in a strong crosswind :). Not very likely though... hehehcheersTero

Share this post


Link to post
Share on other sites

As soon as I load the PMDG737 the rudder seems to be a problem. Before engine start if I apply rudder it will move in the correct direction and then immediately flip back to neutral. This is without any other control inputs.I can get a temporary fix by unplugging the rudder pedals (CH) and immediately plugging it back in. Great product apart from this rudder problem. Can't wait for the fix and just hoping it cures my problem.

Share this post


Link to post
Share on other sites

No Mike,This is probably not an NG problem. Not even an FS probrem. Probably the infamous WinXP "USB gets suddenly de-calibrated" problem. I have the same pedals. Usually no trouble. Most times mis-aligned to the left. Sometimes go absolutely nuts. By un-plugging and re-plugging in the USB you "wake" the port up. This probably is your (and used to be mine) user specific setup problem. The auto system is disconnected from the rudder on the ground.By the way try the CH control manager from the CH site. Worked wonders with both the yoke and the pedals (never used and no need to use FSUIPC for calibration after that).Regards,Vangelis===================================== Evangelos M. Vaos Precision Manuals Develpment Group www.precisionmanuals.com=====================================

Share this post


Link to post
Share on other sites

Also check to be sure your hydraulic power is on. I get that when there is no hyraulic power on (I don't have CH Pedals).Murrell

Share this post


Link to post
Share on other sites

Also check your USB ports in device manager - turn power management off!ScottYBTL

Share this post


Link to post
Share on other sites

Vangelis,good to know you identified the problem! Now, just for the sake of curiosity, could you ekaborate a little more on how you plan to solve the problem? You say this has it's origin somewhere in MSFS's AIR file intrincacies... and the attached file produced using AFSD really shows the undefined values for the drag entries, but... how are you going to fix it? Will it be done by reducing rudder authority? Will it be somewhere in the max deflection of the rudder/aileron? Or... will you give a touch to the stability derivates?Any enlightnement really appreciated!!!!

Share this post


Link to post
Share on other sites

Looking forward to the "fix"I think I speak for many customers by hoping that it will be offered for download on more than one server this time to avoid the "SU2" fun we all had a few weeks back:-)RegardsAdam MoorePMDG Boeing 737NG Registered User

Share this post


Link to post
Share on other sites

JC,Haha -- trade secrets. Hardly. AFSD is correct. Herve has done a wonderful job. Yes everything is in section #1101 of air files -- nothing to do with aircraft.cfg files and all that. All I can say is that it was pretty simple in the end. Just cut down the yaw-induced roll (but not the speed of yawing motion). One of the last 4 parameters in this section is pretty critical. You will open the air file and you'll figure it out no doubt...By the way saw a hacked air file for this plane...Oh well !Anyhow, the real big surpise is that FS really screws up this one, no ? Could not believe negative drag. Sometimes I really miss Fly! in this department. Stability derivatives are **derived/post-calculated** quantities not primary design variables. Instead of stability derivatives and all that rubbish one could do proper aircraft design based on lift/drag/moment curves for wing elements. In the present case given that most air file parameters are "good" guesses as to what they do and the absence of any documentation, FDE people have a really nice wonderful time.Regards,Vangelis

Share this post


Link to post
Share on other sites

Vangelis,thanks for the prompt and informative reply!!!I am sure the modifications will feel good and contribute to make this aircraft an even better product!!!

Share this post


Link to post
Share on other sites

Hello everyone, I was just about to open a similar topic but reading your comments became a little reluctant.The way I experience the problem may be perhaps slightly different;on final approach to LOWI, below 50 feet I tried to get allined with the runway using rudder input but noticed massive adverse yaw which caused the aircraft to be out of control and due to low altitude I finally opted for the missed approach procedure.I'm no expert but the reaction I've experienced, I presume and forgive my ignorance in regard if I'm wrong that this is unrealistic behaviour.Are the above problems somehow related to the problem I've experienced?PS. Thanks to PMDG for considering a fix for the above in their next mini-patch, that's what I call customer service!!!Cheers,David

Share this post


Link to post
Share on other sites

>Looking forward to the "fix">>I think I speak for many customers by hoping that it will be>offered for download on more than one server this time to>avoid the "SU2" fun we all had a few weeks back:-)>>Regards>>Adam Moore>PMDG Boeing 737NG Registered UserAdam,I hardly think we will experience this kind of trouble this go ;) Mr. Randazzo and Lefteris have babied the server, and now it works just fine. It will handle the load of the mini(and when I say mini I mean mini) patch :)

Share this post


Link to post
Share on other sites

I'm also using the CH Pro Pedals USB and a CH Flight Sim Yoke. Had the very same problem and eventually solved it permanently by unassigning the rudder trim functionality within MSFS under Assignments which at least in my case appearently was the cause of the erratic behaviour of the rudder.Cheers,

Share this post


Link to post
Share on other sites

Hi PMDG team,Very simular to David's experience, today I too had the problem with rudder cross control. Flying from LEMD to BIKF, I was told to land on Keflavic's runway 11 when the winds were 020/24 gusting to 36. I should have opted to ignor the landing runway and select runway 02. But due to a limitation of the work we are doing on Radar Contact, I did not have the option for this runway.The PMDG737 autopilot stayed "glued" on the GS and localizer all the way down to 500 feet, with enough crab that I was looking at the runway on approach from the edge of the co-pilots windscreen. Knowing about the rudder problem is what prevented me from kicking off the AP at my usual 1500 foot mark. Well at 500 feet when I did take over, it was all I could do to keep the left wing down and the nose pointed at the runway with the rudder. Just little rudder inputs made me have wild swings, pulling all inputs from the rudder in those high winds I am sure is what caused the rapid swinging, but I did manage to plunk it down somewhat "hard" to keep from going off the runway. The roll out was not conforting to the pax as we whip back and forth on the runway.I hope the upcoming mini-patch will address this kind of cross-wind problem. I do want to thank the team for a just outstanding patch is with SU2, it is nothing short of stupendous, just great work team.Thanks again for all the hard workBob Johnson

Share this post


Link to post
Share on other sites

Bob,Thanks for the feedback. Yeah problem is that is has now become too easy :)But you were right in guiding the a/c to the runway the way you did even if unintentionally. Exactly that : small rudder inputs at pretty low alt (say 50 ft?). There are people back there not boxes and cattle.Regards,Vangelis

Share this post


Link to post
Share on other sites

Vangelis,ever since I bought PMDG's 737NG I can't stop landing at my preferred portuguese airport - Madeira LPMA.I have had the chance to land and takeoff from there a few times in jumpseats on the cockpits of 737s and Buses. I recall a particular situation on a TAP 737-300, very near to their retirement from the Airline of Portugal. I was in the cockpit during the approach and landing, and we landed on rw 05, with a crosswind component very near to the rw limits due to terrain closure and various "windtunel" effects.I was pretty much involved in following each of the captain (PF) and first officer moves, and for the very first time in my life I had the chance to "register" in my mind how it can be done in real life, wich is exactly what I immitate with MSFS + 737NG.The captain mantained the crab into the wind bellow 50 feet, and just a second or so before the main wheels touched the rw he applied plenty of opposite rudder at a high rate, while turning the yoke towards upwind. The first officer pushed the control column all the way ahead and mantained it there strongly with the yoke allways fully deflected upwind until reversers were disengaged!!!I will never forget about that. I use that very same technique, and I am sure that no other MSFS aircraft will replicate it's effects as well as PMDG's 737NGs with the upcoming patch ;-)

Share this post


Link to post
Share on other sites

Hi Guys,Today was even more interesting than yesterday. Flying from BIKF to KORD, with a full load of fuel. The approach into KORD was really a challenge. First there is snow and lots of it. Next the winds are 320/24 gusting to 30, with at the time 5 mile visibility, celing of 1200 feet. Landing on runway 32L, had just enough cross wind to make the final flare really fun with small rudder inputs. Put her down nice and soft, but she was drifting cross the runway. I hope my technique was OK, I lined up left of centerline and when I landed I was too the right of it. Once I touched down, I pulled her back to the center line with the rudder and we went right down the runway in blowing snow. Could not see worth a poop (grin). Landing at Ohare in the snow with Ultimate Traffic's, AI traffic at 90% what a site, planes everywhere. Having flown there many times, it really did look real. There were at least 30 planes lined up for takeoff and the gates were full at the international terminal where I had to park. What a fun day!!!!!!!!!!Bob

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this