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"resonance" rattle when taxiing at idle RPM

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ve0kog

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My 9A has a persistent slight "high frequency" rattle/vibration when taxiing at idle RPM. It seems to be coming from the middle or aft of the airplane but it's difficult to point the exact location. All panels appear secure. The rattle disappears with a just slight increase in RPM (900-1000). My concern is that this vibration can eventually loosen something important, even with all the lock and jam nuts tight. The previous owner/builder says it's been there for a while and he tried to find it but unsuccessful.

Is there some art or science to help narrow down the source of it?
 
The best I can tell the vibration is coming somewhere from behind the baggage panel . May be around the Dynon AP pitch servo area in the tail cone. Can't think of how to narrow this down or how to address. 🤷‍♂️
 
ve0kog said:
The best I can tell the vibration is coming somewhere from behind the baggage panel . May be around the Dynon AP pitch servo area in the tail cone. Can't think of how to narrow this down or how to address. 🤷‍♂️
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You sure it is not the brake pads rattling? Very common at low taxi speeds. If it goes away when you tap the brakes that is an indicator.
 
lr172 said:
You sure it is not the brake pads rattling? Very common at low taxi speeds. If it goes away when you tap the brakes that is an indicator.
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I have this and always thought it was the tailwheel chains because when I hit either pedal it stopped but never figured out how I could hear the chains from way back there. Turns out it's my brakes. Thanks.
 
Outside of brakes, there is the vibration problem with the landing gear themselves (happens on some and not on others). Much is written here on many threads as well as in Vans instructions on how to limit this, by putting stiffeners on both sides of the rod gear to dampen out the vibration.

Mine is rather minor and wasn't always present. When it does show up, it could be stopped by tapping the brakes, so I started off by placing springs on my brake master cylinders in order to keep them open when not in use to prevent this possibility. But in my case, that didn't eliminate the shimmy in the gear.

Given the few times it occurs on my 9 and that it's very minimal when present, I haven't went down the path of the dampening stiffeners and just tap the brakes
 
lr172 said:
You sure it is not the brake pads rattling? Very common at low taxi speeds. If it goes away when you tap the brakes that is an indicator.
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No vibration at 1000 RPM and dragging brakes. I can't recall if it vibrates at idle while stationary and holding brake. Will check next time i make it to the airport. (O-320)
 
Some 6, 7, & 9 airframes experience ‘oil canning’ in the belly & lower side panels in the first bay behind the luggage bulkhead causing all kinds of noise.
My solution is to install angle stiffeners (similar to rudder stiffeners) in the areas that are flexing. I glue the stiffener to the panel with Proseal & pop rivet the ends to the bulkheads.
 
bjdecker said:
adding to this -- move the controls to the stops, hold them there, and see if the rattle goes away...
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Walt said:
Change with elev stick movement?
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Controls full deflection does not have any effect on the rattle. Tried different variations of elevator, rudder, aileron movement and no change noted.

lr172 said:
You sure it is not the brake pads rattling? Very common at low taxi speeds. If it goes away when you tap the brakes that is an indicator.
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holding brakes or taxi with feet off the pedals have no effect.

Two Wing aviation sent me a nice dynamic balancer kit rental and I'm within 0.04 IPS, but no change in idle vibration.

1773081064955.png


a head head-scratcher!
 
Well shoot...

Time to get radical -- Remove the baggage compartment top & bottom bulkhead covers, remove the triangular shaped covers over the flap motor and torque tube covers, remove the baggage compartment forward covers over the flap actuator rods, remove the mid cabin cover just forward of the flap assembly brace, the seat pans.

Put the plane in chocks, tie the tail to something immobile, start up the airplane, set the RPM where you hear the rattle, then start look/listening for the source(s)
 
ve0kog said:
Controls full deflection does not have any effect on the rattle. Tried different variations of elevator, rudder, aileron movement and no change noted.


holding brakes or taxi with feet off the pedals have no effect.

Two Wing aviation sent me a nice dynamic balancer kit rental and I'm within 0.04 IPS, but no change in idle vibration.

View attachment 111958


a head head-scratcher!
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Rattles are very tough to find. Only recommendation is to haveca helper listen to find the source while you manipulate controls for maximum effect.
 
ve0kog said:
My 9A has a persistent slight "high frequency" rattle/vibration when taxiing at idle RPM. It seems to be coming from the middle or aft of the airplane but it's difficult to point the exact location. All panels appear secure. The rattle disappears with a just slight increase in RPM (900-1000). My concern is that this vibration can eventually loosen something important, even with all the lock and jam nuts tight. The previous owner/builder says it's been there for a while and he tried to find it but unsuccessful.

Is there some art or science to help narrow down the source of it?
Click to expand...
Find it?
 
bjdecker said:
Put the plane in chocks, tie the tail to something immobile, start up the airplane, set the RPM where you hear the rattle, then start look/listening for the source(s)
Click to expand...
Did just that today and a friend RV pilot helped locate the source of the rattle. It is the Dynon pitch servo. At idle RPM the servo shaft moves a few mm along longitudinal axis, the arm's retaining assembly hits against the servo body with a loud bang, and the entire servo and push rod resonates and shakes pretty violently. It transfers vibration to the elevator pushrod. All torque crosschecks are still intact and bolts and nuts secure.

I made a couple of videos. I don't know if the servo shaft supposed to have this much play. Planning to call Dynon support on Monday. If anyobody ran into anything like this please let me know.



1780112180531.png
 
ve0kog said:
Did just that today and a friend RV pilot helped locate the source of the rattle. It is the Dynon pitch servo. At idle RPM the servo shaft moves a few mm along longitudinal axis, the arm's retaining assembly hits against the servo body with a loud bang, and the entire servo and push rod resonates and shakes pretty violently. It transfers vibration to the elevator pushrod. All torque crosschecks are still intact and bolts and nuts secure.

I made a couple of videos. I don't know if the servo shaft supposed to have this much play. Planning to call Dynon support on Monday. If anyobody ran into anything like this please let me know.



View attachment 119063
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Good work -- and -- wow! That's quite the rattle --- Yes, I think a phone call to Dynon is in order.
 
The amount of play in the shaft coming out of the gear box seems out of spec according to Dynon. But that might not be the root cause. They suggested prop balancing (already done) and optionally installing a brace to the stringer running along the side of the fuselage. I'm going to try swapping the servo first, if the replacement has noticeably less play.

also shipped pictures and the installation manual to claude design and it seems to have an opinion 🤷‍♂️ :

The servo bolts to the Dynon pitch bracket (P/N 100868-000) at the gearbox face only. The right-angle gearbox and the heavy stepper motor then hang unsupported off the end of that single mounting face. With the motor's mass on the end of a stiff-but-springy lever, the whole assembly behaves like a tuning fork.

1780447923518.png
 
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