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A320driver

Engine/Autothrottle Surge Issue

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Have been dealing with a very annoying problem with several of my addon aircraft for quite some time. During level flight with autothrottle engaged, the power "surges" up and down. This happens in all types of weather and at different weights and speeds. Have posted about this is all the affected aircraft forums with almost zero response. Have done alot of searching and could find several references to this issue with no solutions. The closest thing I encountered was a reference to suppressing pressure changes in FSUIPC. Tried that with no change. I am growing quite weary of this and hope that someone can shed a little light on the problem. Thanks in advance for any help,Joe

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Had AS6.5 installed. Removed it from my system. Still there. Cleared all weather in flight sim and the problem still persists. I am about ready to walk away from the whole flight sim over this, for me, very long standing issue. It just makes trying to do anything very unrealistic. I am sure this is not an aircraft specific problem, as it shows itself with many of my addon planes.Regards,Joe

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FS does not correct model wind shifts and pressure changes. what you experience is 'pare for the FS-course.'limiting how fast FS changes weather conditions using FSUIPC and/or ActiveSky is the only option.--

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" what you experience is 'pare for the FS-course.'"So let me see if I have got this right, there is no "fix" for this issue? I have tried every weather related smoothing item I can find in both AS 6.5 and FSUIPC and nothing helps. Perhaps you could point out which changes will affect this issue? Maybe I have overlooked something.Thanks in advance,Joe

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i have FSUIPC to allow only the following with wind and pressure changes -- 1 degree of wind/sec- 1 knot of wind/sec- .01 of pressure/seci still have 'surges'. have had these in FS7, 8 and 9.--

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Hi Joe,This most probably hasn't got anything to do with weather (addon's), but with inaccurate FDEs of some aircraft.Even the default B747 has this problem in some flight configurations.You cannot solve this yourselves (unless you know what to change in the .air file), but you can strongly reduce the effect by adapting a parameter in the aircraft.cfg file.In the Autopilot section of the aircraft.cfg, look formax_throttle_rate=Usually it has a value of 0.1; reducing the value to eg. 0.003 - 0.005 will considerably reduce this "surge" effect.Downside is that the A/T now reacts much slower in cases where a fast change in A/T IS required. But that's only in very specific cases.Regards, Rob

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Hi,A long shot:Did you calibrate your joystick correctly and have you set the null-zones?Maybe your joystick-throttle lever contantly is producing noise, causing AT to jump up and down.Also when having AT engaged, put your throttle either all the way up or down

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Hi Egbert,Just so you know: a "jittering" throttle axis doesn't have any effect on the Autothrottle, since (if autothrottle is On) this axis is completely ignored by FS.That is, at least for panels that use the standard FS Autopilot functions.Groetjes, Rob

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Hi Rob,Well it looks like you nailed it. When I made the adjustment you suggested the autothrottle leveled out. The only issue left is it runs back and forth over the commanded speed. I am sure that is due to the slower AT response. Would you happen to know what needs adjusting in the airfile to further correct this? Thought maybe I could make the required changes thru "AirEd". Thanks alot for your input...been a big help.Regards,Joe

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It's hard to answer with so little detail given...which aircraft, on what routes, at what altitudes etc.ActiveSky 6.5 has two options, "Force Constant Route Aloft Temp" and "Force Constant Route Aloft Pressure." I would use both to lock the temp and pressure, and then see if problem persists. Rather than guessing if this behavior is related to wx changes (press or temp in particular), you can watch those parameters using FSLook or FSInterrogate (free tools from the FSUIPC SDK) and see if they are changing. OAT, in particular, when it changes will drive a change in mach number, causing the A/T to adjust to get speed back to target mach. If temp is constantly moving around the A/T will be chasing a moving speed target. Both FS and AS can have issues with missing wx stations along a route, especially in oceanic airspace, and if you have bogus temp values from bad/missing wx data, the temp might be changing constantly over a wide range.If wx is not the issue, I would not use the max_throttle_rate param as an adjustment except as a last resort...a better solution is to try experimentally increasing the fuel_flow_gain parameter in the TurbineEngineData section of the aircraft.cfg file...a larger value makes the engine spool up/down faster. This can significantly reduce or stop A/T speed overshoots by damping out lag induced oscillations, thus improving A/T feedback performance.RegardsBob ScottATP IMEL Gulfstream II-III-IV-VSantiago de Chile

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>If wx is not the issue, I would not use the max_throttle_rate>param as an adjustment except as a last resort...a better>solution is to try experimentally increasing the>fuel_flow_gain parameter in the TurbineEngineData section of>the aircraft.cfg file...a larger value makes the engine spool>up/down faster. This can significantly reduce or stop A/T>speed overshoots by damping out lag induced oscillations, thus>improving A/T feedback performance.>>Regards>>Bob Scott>ATP IMEL Gulfstream II-III-IV-V>Santiago de ChileHi Bob,Indeed, a much better tip.I tried that for a few aircraft that had this issue, and it solved the problem perfectly as far as I can see.Regards, Rob

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One of the biggest offenders regarding this issue is my 2006 Maddog MD-82. The fuel_flow_gain was 0.003. Have increased this parameter however, the issue is still there, though somewhat less. This problem does not really show itself during the cruise, but much more so at lower altitudes (while being vectored to finals or during periods of level flight during the climb phase). As I stated before weather does not seem to make a difference as this problem is always present no matter how weather is handled. Maybe someone could offer a little more fine tuning of this parameter to better fix this?As always, thanks to allJoe

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This is a known problem specific to the Maddog. The developer acknowledges it...but unfortunately has done nothing to address it. http://www.flythemaddog.com/forum/index.php?showtopic=4428The issue might be related to how FS models winds or IAS, but it is caused by the way the Maddog panel interacts with FS. I have many other add-ons that do not display this behavior despite running on the same FS platform. The Maddog's A/T feedback is poorly programmed, no two ways about it.Bob ScottATP IMEL Gulfstream II-III-IV-VSantiago de Chile

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I just HAD to chime in here. I, too, have the same problem with the autothrottles cycling up/down as explained by A320Driver. I can confirm that weather has absolutley nothing to do with what we're experiencing. In my tests, I started FS without starting AS6 (or any other external FS programs) and selected "Clear skies (clears all weather)" in FS9's Weather menu. The aircraft I'm experiencing the problem with is also an addon (a Gulfstream G-V). As mentioned by A320Driver, the problem does not occur in the cruise at high altitudes and high speeds, but rather when the aircraft is at low altitude and low speed - usually :)With the G-V, the problem occurs when the aircraft is lightly loaded and I'm flying with little or no drag imposed (gear up, no flaps). I'm guessing the auto-throttle occillations have something to do with the RPM (N1 in this case) to maintain the speed selected in the speed window. At 10000 MSL and 52000 lbs gross weight, ~41% N1 is required to maintain the 210 kts selected in the speed window, and the occillations occur. When I increase the gross weight of the aircraft to 75000 lbs, the angle of attack increases slightly, N1 increases to ~47% and the throttle occillations cease to exist. So it appears to have something to do with low RPMs, but I'm not sure. Note that the occillations are even more pronounced at altitudes below 5000 MSL.As recommended, I adjusted the fuel_flow_gain. This resulted in the same occillations as before only at a faster rate.So it's back to the .cfg file I go to try adjusting some other parms. If I come up with anything conclusive, I'll post it here.

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After much trial and error, I think I have finally come up with the correct fuel_flow_gain setting for the Maddog 2006. It was originallyset at 0.003. I changed it to 0.6 and so far, thru all the testing I have done it seems to have done away with the problem. I have applied this same setting to the CLS DC-10-30F (with which I have endured the same aggravating issue) but with much less success. Will continue to test both aircraft to see how it all comes out. If anyone has any ideas on the CLS DC10 please let me know. Regards,Joe

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Which G-V add-on are you using?There can be multiple causes of A/T instability. The PID controller normally used for automatic control systems has to be flexible enough to accomodate the varied airspeed response that occurs as a function of throttle movement at different weights, altitudes etc. What works at altitude when the jet responds slowly to thrust changes may not work at all down low.The RPM change per unit of throttle travel can be (is normally) quite a bit different at the low end of the range, so A/T response can be problematic down there, because a small change in throttle setting can produce a relatively large change in RPM. But most jets don't fly with the RPM stabilized so low (including the G-V). Sounds like this could be fixed by some adjustment of the thrust factor vs throttle pos curves in the FDE (table 1506).Bob ScottATP IMEL Gulfstream II-III-IV-VSantiago de Chile

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Hi Bob. The G-V (actually the G550) addon I'm using is sort of a kludge-job made up of the visual model of the AI G550 found at www.htaimodels.com (a rather nice looking model imho) and the fde and .cfg files from Mike Stone's G-V. Both of these aircraft can be found here on avsim. I've tweaked the .cfg and .air files so that she replicates (as closely as I could get) the performance of the real G550 according to specs found at www.smartcockpit.com and Gulfstream's website.I found out something interesting during the testing I did since my last post. I noticed that when the G550 engines had to work harder (thereby producing higher N1 values) the occillations stopped. So I took a ride in my Eaglesoft Ciation X and noticed that with a lightly loaded aircraft flying at 10000' and 210 kts, she was producing ~66% N1 with rock solid autothrottles. My G550 however was producing only 41% N1 (as you may recall from my last post) with occillating A/Ts. I wanted a way to increase N1 at low speeds in my G550. So I increased the induced_drag_scalar to a whopping 6.0, took the lightly loaded G550 back up to 10000' (she barely got off the ground!) and, what do you know, N1 was up around 65% and the A/T occillations were gone. Don't know what N1 value the real G550 produces under these conditions, but i'm guessing it's more than 41%. Naturally, the problem with setting such a high induced drag value is that at her MTOW of 91000 lbs, she can't get off the ground.So the question now is how do I increase N1 without setting the induced_drag_scalar so ridiculously high? I think I'll take a peek at those thrust factor and throttle pos curves you mentioned.

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----------------------------------------------------------------If wx is not the issue, I would not use the max_throttle_rate param as an adjustmentexcept as a last resort...a better solution is to try experimentally increasing the fuel_flow_gain parameter in the TurbineEngineData section of the aircraft.cfg file...a larger value makes the engine spool up/down faster. This can significantly reduce or stop A/Tspeed overshoots by damping out lag induced oscillations, thus improving A/T feedbackperformance.----------------------------------------------------------------Great tip mate, thanks will check it out. The 50N 737s are notorious for this problem!

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PROBLEM SOLVED! My thanks to Bob (w6kd) and Joe (A320Driver) for the tip on setting the fuel_flow_gain parameter in the aircraft.cfg file to a value of 0.60. I had previously adjusted it from 0.002 to 0.006 with no success, but this new value works like magic. Thanks again guys!Jeff

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