Why do my needles wiggle?

Ok all you sherlocks out there. A question.

I noticed that at certain times, the needles on my altimeter, VSI, and ASI oscilate a bit. Sometimes more than others and at different altitudes. This leads me to believe that somehow the static system is involved.

One elimination - the static port system are the round aluminum disks that are riveted in and the lines are hard nylon with nyloseal connectors. Not the Vans rivets Port locations are the same as Van's.

Another elimination (maybe) Static system checked and certified to 20,000 feet with no leaks. Also the AP holds altitude very nicely.

Another elimination (maybe) No excessive vibration and smoother than other planes I have been in.

Could it be prop wash?

Webb
What is the frequency of the wiggle? Are they vibrating with engine? Do they change with RPM either in amplitude or frequency? Do they match what your butt is feeling? Do they change with YAW?

These answers will help us narrow it down.

same thing in mine...

my altimeter and vsi needlese both wiggle...it's in harmony with what my butt is feeling. I have the regulation van's pop rivet static ports and have had my pitot-static checked out as well...it checked out fine. I bought my instruments from Vans. I have flown other RV's whose vsi and altimeter are rock steady. I've given up on trying to chase it down...the needles move just enough to notice, its more of a distraction than a real problem

It seems that the only thing in common...

...with all of these instruments is the Static System.

But, is this three separate steam gauges, or a common EFIS system? Or, is there a separate EAFS system used as a backup (or for whatever reason)?

Is the encoder also part of this static system?

Do you have static ports on both sides of the aircraft? If so, can you plug one of them up and do the symptoms persist.

And, do you have a moisture trap installed or an "Alternate Source" valve installed. Could it be that drops of moisture / condensation are in the static lines and causing some atmospheric pressure burbles??

If you were to purposefully block the static line before it feeds any of the instruments, do the needles still dance?

Finally, could the VSI instrument have a case leak? I believe that it is supposed to have a calibrated orifice that allows the static pressure to leak at a specified rate...or something like that.

It is not unusal to have needles to vibrate esp on inexpensive gauges it has nothing to do with the static system.

Norman, I'm with you. I declare it a non-problem.

I spent a ton of time trying to get rid of an airspeed wiggle in a previous airplane. A half dozen different fences at the static port got the indication quite accurate but the wiggle never left.

It is most curious that it can be rock solid in one airplane and not in another. Makes me wonder if it is an anomaly with how the tubing is routed. Maybe the system needs an accumulator like a hydraulic system. The wiggle does almost seem like mini pressure stalls.

Good questions

Kahuna said:

Webb
What is the frequency of the wiggle? Are they vibrating with engine? Do they change with RPM either in amplitude or frequency? Do they match what your butt is feeling? Do they change with YAW?

These answers will help us narrow it down.

Click to expand...

Good questions. Next time up, I'm going to try some experiments. Tape over one port. Slip it hard and check the yaw. Write down the RPM vs amount of wiggle. Check the altitude vs the power settings. Also will do the butt check.

All three have the wiggle at about the same amount of amplitude. I am beginning to wonder if it's harmonics between the two ports.

The fact that 3 instruments wiggle makes me wonder what is going on....esp since the static system didn't leak when getting tested. Quite frankly, its more of an anoyance.

Static Instruments Wiggle

Webb,

Did you ever find a solution. I have the same problem. Airspeed, VSI and Altimeter all wiggle. I have standard double Van's rivet ports and the pressure check was ok.

Mine too VSI

I don't have any trouble determining my rate of altitude change and it is always the same undampened movement vibration - not a problem. Probably not a good idea to fixate on it.

Bob Axsom

Unstable static source

Every static source is going to be subject to some variation. Inserting a static sump bottle will smooth out the variations somewhat. Inserting such an accumulator (static bottle) will slow the responsiveness of the altmeter and VSI somewhat. These are pretty common on GA planes. Spruce sells one for $11.

http://www.aircraftspruce.com/catalog/inpages/pitotstatic.php -

If you are wiggling mechanical gages, your mechanical mechanisms are seeing additional wear. The accumulator bottle might pay for itself in extending the life of the mechanical instruments.

- Roger

Static bottle

I'm going to try the static bottle to see if it has some dampening effect.

Since my first post, I tried taping one side.....no effect. Slipping didn't make a difference either.

I also have replaced the experimental VSI and altimeter with yellow tagged TSO'ed units and it is better. For those that weren't able to tell, all my flight instruments are steam gauges. It is also worse down low and at lower speeds. Almost no movement at altitude (10-12k) and cruise speed.

Gut is telling me that the experimental ASI may be the cause of it since it is getting the static line first.

I guess I'm lucky in the whole scheme of things if this is all I have to complain about after 442 hours on the Hobbs.

I recently swapped out my altimeter for a new Falcon from Spruce when my old one failed in testing. The needle on it bounces around in turbulence much more than my old altimeter, which I don't like, but will accept for the price of it.

Gauges Bouncing

I had the same static line problem. My ASI vibrated about +/- 3 mph and it was inaccurate. So, I bought a new ASI from Spruce which was more accurate, but the bouncing got worse, so I sent it back. I called the vendor and they responded that they never heard of this problem before and their instrument should dampen a small amount of vibration.

You can start to find the cause by temporarily clamping off the static line while in flight, if the needles stop bouncing than its in the static line. What is the diameter and what kind of tubing is used in your static system, and how is it mounted in the fuselage? Its a long run from the static ports to the panel, and if the tubing starts flopping around in flight it will cause the gauges to bounce. Sound waves at certain frequencies love to resonate thru a tube just like a trombone or organ. I switched from 1/4" i.d. vinyl to 1/8" silicone rubber tubing for pitot and static lines, and use nipple connectors for tees and on all my gauges. I tie wrapped them to the fuselage rails. It is easy to install and modify and it is very stable.

It "ain't" the ASI

Put a TSO'ed ASI in and still got bounce when low and slow.

Agree with the statement that what the buttocks feel is what the gauges feel.

Haven't tried the static bottle yet but that may be next.

It's not a problem, just annoying. Up high, smooth and fast, it is still.

The recap:
Real static ports (2) instead of rivets
Hard nylon static line (tied down)
Taping one port - no change
Slipping - no change
Prop is balanced
All connections checked for leaks
Static/Pitot check OK

Wonder - Is there any way to dampen a ASI needle?