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RV10 elevator oscillation at cruise

C

Chippster1

Active Member
Tell what you all think
I have noticed lately an oscillation of the elevator and stick movement about 3x/ sec at cruise speeds, about 165 KTAS.
Increasing the speed or decreasing it makes it disappear, which makes me think it’s a resonance and not a flutter, otherwise it would get worse with more speed.
Upon landing, I noticed a small but noticeable play in the right outboard elevator hinge near the rod bearing. Of course I will immediately correct this discrepancy by replacing the bracket and possibly the rod bearing.
My question to everyone here is- Do you think this play is what’s causing the elevator to bounce up and down in cruise?

Please tell me what you think

Ted
N498EC
 
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Fix it; I would also check that the moment of the elevator is within spec. Instructions on how to do this came with the kit.
 
A guy on the bonanza forum reported a similar squawk with his Baron. Said the horizontal was moving an inch in each direction and he could feel it in the yoke. Turned out to be a worn trim tab hinge.

Also, although flutter is associated with high speed I’ve heard of a Vtail that fluttered in the landing flair and tore up the tail. Flutter is weird and unpredictable. I agree about resonance being speed related, we would get it with the carbon fiber props on the formula one racers and once you went thru that transition rpm it was smooth again. Scary at that resonance rpm though!
 
Be careful with “balanced”
RV10 elevators are not balanced, they have a balance limit - which is very generous.
37.5 in-lb TE heavy.
I doubt this has anything to do with it unless there has been a gross misinterpretation of the plans
But I have seen an example where someone has indeed put an extra 3-4lb of lead on their RV10 elevator counterbalances to “balance” them so this may be worth investigating.
How many hours on the airframe?
 
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Elev

I’ve had the same problem at cruise speeds . It was more when I had my lighter composite prop , less with the Hartzell, I could hit a little down trim and it goes away. I tried more weights, did nothing , readjusted both trim tabs just made it worse so I removed the extra weights, put the trim tab back to print specs and that helped allot.
Now it all depends of what my CG is with passengers, baggage , fuel that will trigger it.
Going to do very small trim tab adjustments again, I thinking that’s why Vans has you adjust them on an offset from each in the first place.
 
Chippster1 said:
Tell what you all think
I have noticed lately an oscillation of the elevator and stick movement about 3x/ sec at cruise speeds, about 165 KTAS.
Increasing the speed or decreasing it makes it disappear, which makes me think it’s a resonance and not a flutter, otherwise it would get worse with more speed.
Upon landing, I noticed a small but noticeable play in the right outboard elevator hinge near the rod bearing. Of course I will immediately correct this discrepancy by replacing the bracket and possibly the rod bearing.
My question to everyone here is- Do you think this play is what’s causing the elevator to bounce up and down in cruise?

Please tell me what you think

Ted
N498EC
Click to expand...

Yes, IMHO a loose elevator hinge bracket is likely to cause something like this if only seen at higher speeds. I suspect it would depend on how the h stab incidence was set. Closer to neutral in cruise would be more likely to exhibit this, or at least so it seems to a non aero engineer (me). I have to wonder how much care was taken on setting up the hinges at build time. If there was any binding from misalignment, that could have stressed some rivets and possibly caused them to loosen. I would also check the other side. It was my understanding that rigidity in the bearings is key to maintaining the speed for onset of flutter and not having that reduces the speed at which flutter occurs, so I would address this ASAP. Again not an Eng, so take for what it is worth.

I didn't know any better when I built my 6 and did not take great care is picking spacer for the aileron hinges. I was close, but foolishly allowed the bolt to slightly close up a gap. At 300 hours, one of the aileron bearings wore out due to the axial load from this mistake. Those particular bearings are designed ONLY for radial load, so not comparable in your case, as rod bearing can handle some axial load. Just an example of how novices can create problems like this.
 
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elevator

Do you get the oscillation when hand flying or with the A/P on?

If only with the A/P on, do you have auto trim?

Just trying to see if your issue matches several others (including mine).

I get a very small oscillation, at cruise speed, when the A/P is on and I have auto trim. It does NOT do it when hand flying. It eventually damps out with the A/P on.

Elevators were checked within balance limits, hinges were aligned with a laser and have no binding, no slop in the pushrod system.

Have messed with the pitch trim gain and timing in the A/P but I can't quite get rid of it...just a bit annoying.
 
rocketman1988 said:
Do you get the oscillation when hand flying or with the A/P on?

If only with the A/P on, do you have auto trim?

Just trying to see if your issue matches several others (including mine).

I get a very small oscillation, at cruise speed, when the A/P is on and I have auto trim. It does NOT do it when hand flying. It eventually damps out with the A/P on.

Elevators were checked within balance limits, hinges were aligned with a laser and have no binding, no slop in the pushrod system.

Have messed with the pitch trim gain and timing in the A/P but I can't quite get rid of it...just a bit annoying.
Click to expand...

I do not get what you describe on my 10. I have to wonder if there are some harmonics in play that vary from plane to plane. If you look at the latest SB about problems in the trim tab hinge area, it would seem that some of these elevators are getting enough stress to crack the skin and others do not. Maybe it is unique variances in the way that each builder sets up the balance between the two tabs. I did not follow the plans, as I felt they were not optimal, creating a one up one down scenario as certain points in their range.
 
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rocketman1988 said:
Do you get the oscillation when hand flying or with the A/P on?

If only with the A/P on, do you have auto trim?

Just trying to see if your issue matches several others (including mine).

I get a very small oscillation, at cruise speed, when the A/P is on and I have auto trim. It does NOT do it when hand flying. It eventually damps out with the A/P on.

Elevators were checked within balance limits, hinges were aligned with a laser and have no binding, no slop in the pushrod system.

Have messed with the pitch trim gain and timing in the A/P but I can't quite get rid of it...just a bit annoying.
Click to expand...
This aircraft is now ten years old and this is a relatively new phenomenon, and it only seems to occur on occasion, and cannot figure out what the variables are.
Weight? Balance? Air temp?
I did not build the airplane, but it has been looked at by very competent AP who only works on RVs, so I doubt it is a build error.
I am awaiting the new parts from Vans, which are now delayed due to their financial crisis.
Until the play in the elevator hinge is corrected, it will be safely grounded, then re evaluate the phenomena after repair
 
Elevator Oscillations

I had serious elevator oscillations happen in the Embraer 135 aircraft, which were later traced to an outboard elevator hinge bolt migrating out of place.
These showed up at fairly high speed with the A/P on and appeared to be triggered by the anti-ice hot air flow.
Taking a firm grip on the control wheel stopped them.
The fix, large area washers on each end of the bolts to prevent the insert bushing from sliding sideways.
 
Be careful, flutter will cause damage; so I'd say you don't have flutter. Also, as someone else described it can happen at another speed (or set of conditions) and that might be the condition that will result in flutter.
 
Secure the elevator so there is no movement. Check your control stick and see if you have excessive play. If there is play, tighten the through bolt for the control stick. If there still is, check all push rods. The prop can cause a pulse on the elevator at certain power settings. If there is play in the system, that pulse with be felt.
 
I had play in my trim tabs and found that the majority of it was cause by slop in the clevis pin used to attach the elevator tab to the rod end. I replaced the clevis pin with and AN3 bolt, using a castellated nut and cotter key. This tightened up the slop in the trim tabs significantly.
 
Related issue: I don’t have oscillation but there is a bit of slop in the stick in the fore-aft direction but none in lateral. I think this is related to a bushing in the control system below front seats. Anyone else notice this? Fix?
 
bob888 said:
Related issue: I don’t have oscillation but there is a bit of slop in the stick in the fore-aft direction but none in lateral. I think this is related to a bushing in the control system below front seats. Anyone else notice this? Fix?
Click to expand...

I think another factor here is the stick into the weldment. The stick can move inside the weldment and pivot on the bolt. I’ve often thought a vertical bolt would be better. Maybe both
The bushing is definitely an issue as well, more so the weldment sliding fore and aft on the bushing.
The side by side RVs have the same issue. It’s important to not have an excessively long bushing for the weldment to slide axially along.
I eventually stopped noticing this on my RV7 and gave up trying to fix it. On the 10 I know it’s there but don’t give it much thought.
The RV14 has completely done away with that bushing for the roll axis. Sealed bearings all around now. Much better.
 
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Pitch oscillating with autopilot on.

I had a similar issue recently and Garmin said to check the flight control system for any play. I found the pitch trim servo motor was a little loose allowing some small play in the trim tab. I had to go into the tail cone on my back, pull the cotter key and tighten the mounting bolt for the elevator trim servo.

This sure took almost all of the pitch oscillating out when the autopilot was engaged.
 
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