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Bill Alderson

Lear 35A Phugoid Oscilation at FL430

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Hi,

 

I have version 2.3 and noticed that, while, on autopilot and at level cruise  at FL430, the aircraft begins a phugoid oscilation about 1000 feet up and down on the VSI.  This happens after after flying for about an hour or so and when the tip tanks are just about empty.  The only passengers are the pilot and copilot and their luggage.  Has anyone noticed anything similar?

 

 

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I hit the Alt/Sel and it set it self right again but I can't figure out what started it in the first place.

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Yes. I have seen that also at FL 410. Hit the ALT/SEL and it recaptured.

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Hi,

 

could you tell me what the aircraft does if you just let it oscillate without hitting ALT/SEL to have the oscillation sorted out?

Will it become worse so that without intervention the aircraft finally goes out of control?

 

I'm asking because this could be a dealbreaker when doing longer flights during which I might want to get up from my chair and leave the aircraft cruising unattended for some time.

(Yes, I know that is not realistic, and as a pilot I should always be in the cockpit, but you know what I mean, so save me the lecturing :-) ...)

 

Thanks

 

btw: I've seen these oscillations on other payware bizjets as well. Seems to be a general issue with flying high (>= FL410) and fast.

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Hi,

 

could you tell me what the aircraft does if you just let it oscillate without hitting ALT/SEL to have the oscillation sorted out?

Will it become worse so that without intervention the aircraft finally goes out of control?

 

I'm asking because this could be a dealbreaker when doing longer flights during which I might want to get up from my chair and leave the aircraft cruising unattended for some time.

(Yes, I know that is not realistic, and as a pilot I should always be in the cockpit, but you know what I mean, so save me the lecturing :-) ...)

 

Thanks

 

btw: I've seen these oscillations on other payware bizjets as well. Seems to be a general issue with flying high (>= FL410) and fast.

 

Well I actually let it go for awhile before hitting the ALT/SEL button the second time and all it does is continue to oscillate between 1000 and -1000 on the VSI.  It doesn't seem to get worse than that but I got the impression that if I were to let it go, it would have probably continued but I could have probably finished the flight

 

BTW, I have been flying this aircraft pretty much since it came out and this is a first for me.  IOW, I have a feeling this is something that came about as result of a fix and more than likely easily fixed (these guys upgrade this thing about, seemingly, every 10 minutes or so).  Don't let it stop you from purchasing this otherwise fine aircraft.  It is worth it! 

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Hi,

 

thanks for your reply. I bought the aircraft yesterday but due to work I could test it only today.

First test flights are quite promising. Definitely looks like a keeper, especially considering the fact that the developers are continously fixing and enhancing the Lear.

Best Lear I've seen in FSX yet.

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(these guys upgrade this thing about, seemingly, every 10 minutes or so)

This is a little off topic, but how do you tell which version you have? I purchased it mid-March from SimMarket so I'm assuming I have the latest version, but it would nice to know for sure one way or the other.

 

First test flights are quite promising. Definitely looks like a keeper, especially considering the fact that the developers are continously fixing and enhancing the Lear.

Best Lear I've seen in FSX yet.

If you don't have the F1 GTN 750 I highly recommend it. FlySimware integrated it perfectly and IMO the combination makes for absolutely the best biz jet available in FSX/P3D, hands down.

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This is a little off topic, but how do you tell which version you have? I purchased it mid-March from SimMarket so I'm assuming I have the latest version, but it would nice to know for sure one way or the other.

 

The version number is part of the actual .exe of the installer for the aircraft, the latest is "Flysimware_LEARJET_35A_FSX--P3D_v2.3.exe"

 

And to stay on topic, I've had no problem flying her at FL410 for extended flights using P3D v2.4, however that was with an earlier version v2.1, will try with v2.3 and see if anything has changed?

 

Cheers

Martin

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If you don't have the F1 GTN 750 I highly recommend it. FlySimware integrated it perfectly and IMO the combination makes for absolutely the best biz jet available in FSX/P3D, hands down.

 

I do fly this beauty with the GTN750 and the RealityXP WXR gauge in FSX.

Switched to the freeware ASN based WXR gauge though. Its VC integration may not be as good as the RealityXP one, but it probably gives a more accurate weather depiction.

 

And I can tell you: I flew the Lear for almost the whole night since I had installed it. . High addiction risk!

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The version number is part of the actual .exe of the installer for the aircraft, the latest is "Flysimware_LEARJET_35A_FSX--P3D_v2.3.exe"

 

And to stay on topic, I've had no problem flying her at FL410 for extended flights using P3D v2.4, however that was with an earlier version v2.1, will try with v2.3 and see if anything has changed?

 

Cheers

Martin

 

I forgot to mention (I keep doing this) that I'm flying the Lear in FSX.  Sorry.  :fool:

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Well, after now around 50h on the Lear, the phugoid oscillation hit me as well on my first longer flight (>1.5h).

 

FL450, cruising at M0.73 with 1000lbs/h fuel flow, pretty light aircraft (2 pilots, 2 pax, 100lbs baggage), at about 2.5h into a 3h flight, about 2000lbs fuel remaining, wing tip and fuselage tanks empty.

 

That was some pretty violent stuff, going up and down at 1.500-2.000fpm. It would not sort itself out by just pressing ALT SEL again.

Disengage ALT HOLD, establish a sensible climb/descend rate and re-engage ALT SEL and SPD or V/S mode was required.

 

Quite a nasty surprise when you're away from the chair for some time and find the aircraft in this state upon returning.

 

Don't know what would have happened without intervention since I did not want to potentially screw the whole flight, but these oscillations should definitely be looked into.

 

I use ASN, FSX SP2, FSUIPC on a Win10 machine.

Flysimware Lear v2.4

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A friend I fly with also experienced the above problem recently, and I have had some problems with the AP as well in Nav mode. When Nav was engaged with the CDI off about 1 dot (5 degrees) , the AP did not command a heading change to intercept the course, and another time the AP actually turned the a/c in the wrong direction for 30 or 40 seconds or so, and then finally recovered and turned the a/c back in the right direction. Most of the time the AP seems to work as expected, but there apparently still seems to be some bugs that show up. I've also noticed if you engage VS before ALTSEL, you can get strange behavior. It can be very challenging trying to track down these anomalies.

 

I think it would be helpful if there was a document the specified exactly how each AP mode should be entered. This would reduce the occurrences of stragne AP behavior because a user entered a button sequence unanticipated by the software developer.

Al

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Yes, it is also my experience that for proper autopilot operation, you need to stick to a defined button press sequence to make it work properly, e. g. exactly what you observed. ALT SEL must be engaged before selecting SPD or V/S. Once you do it that way, it proved to be quite reliable.

What I am missing though is the functionality on the SPD button to change from IAS to Mach hold in a climb. Currently it says IAS/Mach on the button and it seems to do IAS hold only since climbing in the higher flight levels requires constant manual pitch adjustments to stay on the desired Mach number.

 

As for lateral guidance, the autopilot has not let me down so far. It has always caught the right track in NAV/GPS mode, even when 1-2 dots off or a sharp turn was commanded. I have yet to experience turns in the wrong direction.

I use the F1 GTN 750, btw.

What is a bit annoying though is that turns are generally slow at a low bank angle, which takes the aircraft a couple of wobbles to e. g. settle on a localizer, even when intercepted within a 30 degree window and a slow aircraft at 180-200 kts IAS. Also, tighter turns required in some departure or arrival procedures cannot be flown without massive waypoint overshooting, even when keeping the speed at what is required according to the procedures.

 

I suspect that these pitch oscillations are triggered by some sudden change in weather (pressure) which makes the AP overreact at high speeds and flight levels and hence induce pitch oscillation by trying to keep the selected flight level.

I have not found any kind of pitch oscillation in general, the aircraft flies pretty stable on AP otherwise.

Maybe autopilot response to pressure change induced small altitude changes at high speeds and levels needs to be softened out in general.

 

I really do hope that we will soon see more fixes and tweaks from the developers for this fine aircraft. Haven't been flying anything else since I got it.

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I forgot to mention (I keep doing this) that I'm flying the Lear in FSX.  Sorry.  :fool:

 

 

I suspect that these pitch oscillations are triggered by some sudden change in weather (pressure) which makes the AP overreact at high speeds and flight levels and hence induce pitch oscillation by trying to keep the selected flight level.

I have not found any kind of pitch oscillation in general, the aircraft flies pretty stable on AP otherwise.

Maybe autopilot response to pressure change induced small altitude changes at high speeds and levels needs to be softened out in general.

 

I really do hope that we will soon see more fixes and tweaks from the developers for this fine aircraft. Haven't been flying anything else since I got it.

 

I get the same thing using v2.6 and in P3D; flying at FL430, at about mach .780 and using AS16 and ASCA.  There were active thunderstorms in the area and I suspect the oscillations  may be a result of sudden changes in barometric pressure due to the storms.

 

Just an update in case anyone is interested.  :wink:

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I get the same thing using v2.6 and in P3D; flying at FL430, at about mach .780 and using AS16 and ASCA.  There were active thunderstorms in the area and I suspect the oscillations  may be a result of sudden changes in barometric pressure due to the storms.

 

Just an update in case anyone is interested.  :wink:

 

Seems to be a common issue for other guys too:

 

http://www.fsdeveloper.com/forum/threads/pitch-oscillation.437205/

 

I played around with the parameters given in the thread (mainly pitch stability) and have the unwanted oscillations reduced to a level so that I trust the Lear to fly about an hour or so without me being in the seat and holding a hand at all time.

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Seems to be a common issue for other guys too:

 

http://www.fsdeveloper.com/forum/threads/pitch-oscillation.437205/

 

I played around with the parameters given in the thread (mainly pitch stability) and have the unwanted oscillations reduced to a level so that I trust the Lear to fly about an hour or so without me being in the seat and holding a hand at all time.

 

 

Interesting.  Takes me back to my old aircraft building days (waaaay back in the day).  

 

I might give it a try.

 

Thanks for the link.

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