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Bang/grinding noise when flaps start moving

Paul Eastham

Well Known Member
Hi all,
Curious if anyone else has seen something like this: when my (9A) flaps start to go down, there's a quick "bang" or very brief grinding noise, I'm not sure which. No noise throughout the rest of the motion, and if I return them to up and go back down again, the noise won't repeat until several minutes elapse in the up position (either on the ground or in flight).

Video with sound: https://photos.app.goo.gl/RiQ4yuGHUN3fxTR17

I figure it must be one of two things:

1. gear between flap motor and actuator is slipping briefly

2. flap weldment is "sticking" / not rotating freely in the bushings

For #1, I just had the whole thing apart to replace my flap motor (it was intermittent due to brush issues). The problem existed before the motor change, and got slightly worse / louder after the change. I didn't see any signs of wear on the pinion gear or the "rack" that it interfaces with.

For #2, I have tried spraying some LPS-2 at the bushing / weldment interface, no change.

I also wasn't able to reproduce the problem when the motor was out of the airplane (i.e. moving the flaps by hand), making me more suspicious for #1.

Can anyone think of a #3?
 
I've had the same sort of bang happen on flaps down. It is really intermittent and rare, so not something that always happens. I think mine might need some lube on the plastic blocks, although supposedly the HDMW plastic shouldn't need any. Maybe colder temps exacerbate this?
 
Screw too long? Might want to back out the screw at the junction of the wing root and skin aft upper wing skin. Sometimes the tip can drag intermittently on flap actuation?

If it is catching go to a shorter screw length.
 
fuselage hole not big enough?

...
Can anyone think of a #3?
Could it be that the hole in the fuselage is not big enough or in exactly the right place, and the assembly is catching on it, with a bang, then a little bit of grinding sound?
 
Flap Tape?

What kind of flap protection tape do you have? When I switched from the stock Van's tape (that yellows over time) to the very clear tape (that does not yellow), the flaps would stick to the wing skin, and make that bang as they released. I went back to the original tape, and no bang.

Merrill
 
What kind of flap protection tape do you have? When I switched from the stock Van's tape (that yellows over time) to the very clear tape (that does not yellow), the flaps would stick to the wing skin, and make that bang as they released. I went back to the original tape, and no bang.

Merrill

Same here

Larry
 
Hmm, interesting idea on the flap tape. I have the old stuff, but it is tucked under the wing skin, not on the flap itself. Is that where you guys had/have it?
 
Flap bang

Paul,

I had the exact thing happen in my RV-4. Traced it down to the opening in the floor pan that wasn't big enough. Enlarged the hole and the problem disappeared.
 
Tape is under

Ours has the tape under the main skin, I don't know if it is the old or new tape, but it did start to wear through and come off in a couple of places.

Can you do a test by spraying some lube e.g. silicone on the flap skin where it laps and see if the noise goes away?
 
Sounds like either the flap nose breaking away from the wing skin or a bushing catching the hole in the fuselage. I've had both. The bushings don't travel in a straight line so the hole has to be big enough that it doesn't look right. My flaps are a bit sticky to the wing skin, I break that on preflight with a bit of a downward push on the flap. Only happens after sitting for a while. I don't have any tape, but am thinking about adding some.
 
Mine did it when you went from up to down position. Rerigged the flaps and actuator, and now it does it from the down position to the up. I will keep flying.
And t does it with no load and load both.
 
I am almost hundred percent sure it is the type of flap tape used. The UHMW from Vans will not do it. It turns yellow if used on the flap so rather apply it under the wing skin as others have suggested
 
I’ve also had a soft “bang” upon lowering the flaps. I have been unable to replicate it on the ground. Only happens after flying for a while. Wont even do it in the run up area.

I am using the 3M paint protection tape on the flaps. Very happy with it otherwise. No discoloration in 5 years. Will try the teflon tape Steve suggested on the under side of the wing.
 
To close out the thread, the issue in my airplane turned out to be the plastic bushing blocks were apparently creating some resistance. Whether the bushings were causing the noise themselves, or the friction was causing the motor to audibly complain, I'm not quite sure, but some LPS-2 on the bushings (all 3 of them) mostly cleared it up.
 
I got that one day and it turned out to be the piano wire for the flap hinge. It had worked it's way outboard and the sharp end scraped against the aileron bracket. The wire would first flex as the flap went down and then pop straight making a sort of "bang" sound.


Pushed it back in and secured it
 
What kind of flap protection tape do you have? When I switched from the stock Van's tape (that yellows over time) to the very clear tape (that does not yellow), the flaps would stick to the wing skin, and make that bang as they released. I went back to the original tape, and no bang.

Merrill

A swipe with silicone spray solves that problem.
 
Sounds like the rod end bearing banging against the flap arm. There should be a safety wire from around the bolt to the flap motor tube to keep the flap drive screw from twisting in the flap motor drive tube. I find it difficult to get right since it's not easy to get at.
 
If it's linkage

When I suspect a rubbing linkage I'll coat any suspect area with a good wide sharpie pen. Any kind of contact will leave a witness mark in your sharpie ink
 
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