Pitching Up and Down in Cruise

Post any technical issues here. This forum gets priority from our staff.
new reply
Bazonka
Senior Airman
Posts: 106
Joined: 05 Jul 2015

Pitching Up and Down in Cruise

Post Bazonka »

Currently I am in Cruise on the FSX autopilot on HDG and ALT. FL190 with settings as mentioned in the shift2 panel - 29mp and 2200rpm.
She is constantly pitching Up and Down - From +50 fpm to -50 fpm.

As I use AS16 I already tried to switch off all winds, but this changed nothing on the pitch up and down.
Is there maybe a config file I could work on, for sensitivity or so?

Thanks!
Image

alan CXA651
Senior Master Sergeant
Posts: 2416
Joined: 15 Mar 2016

Re: Pitching Up and Down in Cruise

Post alan CXA651 »

Hi.
This as been reported and answered in a previous post by others , this aircraft does not have alt hold , the osc you report are very small and exceptable , i had it rock steady with both types of auto pilot , so look at your weights /alt/ and weather conditons , they all effect how she reacts also use the trim to fine tune the flight at your alt.
regards alan. 8)
Image
Image
Image
Image

User avatar
CAPFlyer
A2A Aviation Consultant
Posts: 2236
Joined: 03 Mar 2008
Location: Wichita Falls, Texas, USA

Re: Pitching Up and Down in Cruise

Post CAPFlyer »

Also make sure you're not too high for your weight. If you takeoff with a MTOW above 88,000 pounds, you shouldn't be flying above 14,000 feet. Any higher will result in oscillations beyond minor fluctuations due to FSX's default autopilot being linked to the VSI, which has a properly delayed response. However, these deviations shouldn't exceed 10 or 20 feet at most. If you're getting more, you're probably too heavy. Also, the "Shift-2" cruise settings are just a suggestion. I would open the manual to page 138 and use the power settings there. You'll get better performance on long flights that way.

BTW, just as quick little note - I checked a video of a C172SP with an autopilot to confirm my memory and it does indeed "hunt" a little like the Connie. This is because they use the VSI to detect changes and the VSI has "lag" (for lack of a better term) which means that the plane will make deviations before the autopilot can detect them and will constantly be adjusting.
Image

User avatar
AKar
A2A Mechanic
Posts: 4673
Joined: 26 May 2013

Re: Pitching Up and Down in Cruise

Post AKar »

Just for sake of discussion,

A typical simple, bolt-on autopilot of recent years could have characteristic stabilities of ±150 fpm in vertical speed, and ±50 ft in alt hold. To keep the AP smooth, and to avoid autopilot-induced oscillations, so to say, the error gets integrated over relatively long period of time before correction gets more noticeable. An error gets slowly eaten away instead.

This kind of autopilot as described above, in most cases, does not follow the VSI that is visible to the pilot, it is not an input to it. Typically it doesn't know about the altimeter either, having its own static pressure sensing. It is noteworthy that there could be a mismatch in between the autopilot's altitude or VS when compared to the airplane's indications; typically this is perfectly acceptable and within tens of feet or perhaps a hundred FPM on respective instrument.

A key component of vertical speed / altitude hold stability (in real world especially where these are not such clean indications they are in the simulators) is the vertical acceleration sensing the most if not all autopilots are capable of doing. The control loop could be described like this. If the airplane gets smoothly displaced from its altitude hold or vertical speed target, the autopilot will slowly integrate the sensed error over time, kind of becoming more and more concerned that it senses itself being out of position. It slowly commands a smooth return towards the target value. This cycle could take, say, ten seconds, or more even. In some units, one can "simulate" this by altering the autopilot's baro setting. It will start a very smooth correction. If, on the other hand, we get bumped off the target value by a change in power or by turbulence for instance, the autopilot generally senses the vertical acceleration (which a static-only VSI is incapable of detecting) and corrects almost immediately against this. Bumps and all that should get relatively promptly corrected for, in many cases without being much visible at altimeter/VSI at all. It is important to note that the VSI is often one of those instruments that mean nothing to the autopilot (together with the attitude indicator for instance). In FSX default it appears to be different, and it also is the case with many integrated cockpits. But yet, as these two principles work superimposed, the autopilot ought to hunt some in even slightly bumpy conditions, and the instruments are also hunting some, especially if flying near terrain on a windy day. In smooth air however, a constant hunting of an autopilot is unacceptable and is a sign of mis-tuned system or even an issue in control system, such as something in cable tensions.

-Esa

Bazonka
Senior Airman
Posts: 106
Joined: 05 Jul 2015

Re: Pitching Up and Down in Cruise

Post Bazonka »

Thank you guys very much - and a happy new year!
I think it was just overweight, but in the future I will operate via the sperry and get more deep into the info of you posts!

Thanks!
Image

trisho0
Technical Sergeant
Posts: 737
Joined: 31 Mar 2018

Re: Pitching Up and Down in Cruise

Post trisho0 »

alan CXA651 wrote:
31 Dec 2016, 08:43
Hi.
This as been reported and answered in a previous post by others , this aircraft does not have alt hold , the osc you report are very small and exceptable , i had it rock steady with both types of auto pilot , so look at your weights /alt/ and weather conditons , they all effect how she reacts also use the trim to fine tune the flight at your alt.
regards alan. 8)
Hello Alan, where can I read more about A2A Connie VSI oscillations (pitching up and down) in the A2A forum? I am experiencing for the first time with FSX this pitching going from 1000fpm to -1000fpm that never becomes stable on cruise.
Pat

alan CXA651
Senior Master Sergeant
Posts: 2416
Joined: 15 Mar 2016

Re: Pitching Up and Down in Cruise

Post alan CXA651 »

Hi Pat.
The Oscillations you discribe are more than minor , the ones mentioned in previous posts before are less than +/-50ft , what was your weather / alt / speed / weights and CG at time of oscillations and which autopilot was you using , i always use the sperry autopilot on the connie , which you need the aircraft to be trimmed correctly prior to engauging and on the sperry you need the trim pots for the ele/ail/rud response times setting as well to give smoothe inputs.
regards Alan. 8)
Image
Image
Image
Image

trisho0
Technical Sergeant
Posts: 737
Joined: 31 Mar 2018

Re: Pitching Up and Down in Cruise

Post trisho0 »

It was my mistake, sorry. I skipped the Payload and Fuel Management setup before take-off.
Pat

new reply

Return to “Lockheed Model 049 Tech Support”

Who is online

Users browsing this forum: No registered users and 1 guest