Clunking noise under seat and loud creaking of wings in flight.

[Michael Price]

Maybe these symptoms are related? What should I check? Discus CS. Thank you! - Michael Price

[Morgan Hall]

Neither of those sound good.

My Duo has a creaky seat pan. The rubber surround definitely contributes to creaking noises and particularly in the back seat.

The seat pan is pretty easy to pull out and should give you clear access to all the control circuits.

When do you get the clunking noise?  Stick movement? Rudder movement? Just randomly?

How tight is the fit of your wing on the lift pins?  With the wings on and pinned, is there any fore-aft or up/down play on the lift pins? 

Have you checked the resonant frequency of the wing during annual inspections?  You should find the original resonant frequency of the wing in the paperwork from the factory.  I'm trying to remember what it is on our Discus A. I want to say around 120 cycles/min, but don't quote me on that.  It's been several months since we did the annual.  If you've never done this, it's pretty simple.  Rig the glider, move it off the dolly and with the wings level, stand at a wingtip and start bouncing the wing up and down until you find it's natural frequency. Then time it for 30 seconds while counting the cycles.  Double that number and compare to what the glider originally had.  A major structural flaw should change that value pretty significantly.  A few cycles/min aren't anything to be concerned about.

Those are at least points I would start with. 

A little bit of play can be resolved with the use of very thin shims on the lift pins.

Where are you located?  It would definitely be worth the piece of mind to have a good mechanic look at it. Load up the wings, listen for odd noises.

We sadly lost a pilot locally that had questions about the assembly of his glider. Please follow up on this.  If something doesn't seem right, it probably isn't.

Morgan

On Tue, Jun 26, 2018 at 7:16 PM Michael Price <mpr...@execsummits.org> wrote:

Maybe these symptoms are related? What should I check? Discus CS.  Thank you!  - Michael Price

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[Michael Price]

Thanks Morgan, I will do that resonancy check. On your question, I believe whenever there are forces on the cockpit but I will study it carefully on the next flight to see exactly what motions trigger the sounds (just bought it last Saturday so only flown it twice). Overall it is a low hour (590) CS in nearly factory condition externally/internally (must have mostly been stored inside) with no repair work ever done so very odd.

[Morgan Hall]

590hrs, I would be surprised if the lift pins had any significant wear or slop. 

The creaking might be coming from the seat pan or back panel. That can easily sound like it is coming from farther back since your head is at the mouth of a megaphone. 

Also check the shelf’s in front and behind the spar. Those could be sources of noise as well.

I don’t remember if the CS or what years were subject to the spar cap AD. My Duo was, 1999 model year, and I think wings built in the Czech Republic were the target of that AD for a certain period.  Definitely confirm that your glider is not affected or that the AD was complied with. The issue was that some wings didn’t have enough resin of the right viscosity when the wings were joined and that left the potential for voids. 

Clunking, might also (probably will) be the gear doors. The bungee get tired and when you get uncoordinated in flight, the door can get pulled open a little bit and then snaps back into place when you straighten up. 

This will sound like it is coming from underneath you and obviously won’t happen on the ground despite all the wiggling of yourself or the stick. 

Easily tested though. Before you drop the gear on the ground, just check the force required to open the doors. If it’s really easy, you need to replace the bungees. 

Not a difficult job. You can make it easier by purchasing 4 small quicklinks for chain at the hardware store when you buy some bungee material.  I use stainless safety wire to secure the bungee to the quick link. Just fold the bungee back on itself and wrap with safety wire and tighten it down. For the other side of the bungee, you need to first figure out the length. A wire tie can be used to temporarily hold the bungee while you get the length right. It doesn’t need to be super tight. Trial and error and you will get the right tension. Then add the safety wire to back up the wire tie. Make a second setup using the same length and install.  No more clunking gear door. 

Good luck. 

Morgan. 

On Wed, Jun 27, 2018 at 5:53 PM Michael Price <mpr...@execsummits.org> wrote:

Thanks Morgan, I will do that resonancy check. On your question, I believe whenever there are forces on the cockpit but I will study it carefully on the next  flight to see exactly what motions trigger the sounds (just bought it last Saturday so only flown it twice). Overall it is a low hour (590) CS in nearly factory condition externally/internally (must have mostly been stored inside) with no repair work ever done so very odd.

[Michael Price]

Extremely useful, thank you so much for your insights Morgan! That gives me a good To-Do list of likely items to check and resolve -- I will get to work on it. Thanks again Morgan and have a wonderful weekend -- hopefully AGL! - Michael Price

[Michael Price]

Morgan, for seat and back panel creaking is it usually just a matter of tightening the screws or does one sometimes need to creatively insert some rubber/foam? On the gear doors does any medium bungee from say Walmart suffice or anything special? - Michael

[Morgan Hall]

I just sort of learned to live with the seat back creaking.  One thing that might work is to wax the area where they overlap.  Might slip a little easier and creak less, but I haven't tried that.  Mostly in the Duo it's the back seat and if I'm not flying from there, it isn't an issue for me.

Any medium size bungee will work ok.  Probably 5/16" diameter or so.  Noticed mine are getting a little soft on the Duo.  Time to create some new ones I suppose.

The glider section at WalMart or Ace should have exactly the bungee you need.

On Sat, Jul 7, 2018 at 7:44 PM Michael Price <mpr...@execsummits.org> wrote:

Morgan, for seat and back panel creaking is it usually just a matter of tightening the screws or does one sometimes need to creatively insert some rubber/foam? On the gear doors does any medium bungee from say Walmart suffice or anything special?  - Michael

[Michael Price]

Getting ready for the next season (exciting)! I still have that loud clunking sound which occurs when the pitch quickly changes in/out of thermals or if I just push/pull the stick. It is quite intimidating passing through the seat pan into me. Ruled out the wheel covers (good tension), cannot detect any excess wing play with the pin in, fooled with the shocks but I think they have been eliminated, thought it was wheel play due to a reversed washer but that did not resolve, shelves are now nice and tight, padded the water release pivot which could theoretically rattle, will do that resonance check suggested just in case of any structural issues, what else should I check? Running out of ideas...

[Morgan Hall]

Not a good thing when you have unidentifiable mystery noises.

Do you feel the clunk in the stick?  Like it is related to the stick movement rather than the pitch change?  

Is it always reproducible?  Can you go from a 70kt cruise and pitch up 15 degrees and get the clunk?  Smooth air reproducible?  

What about the landing gear when it is stowed?  Since the gear door are apparently tight, is the landing gear itself OK?  Take the bungee off the gear doors and see if you can move the gear around when it is stowed.  You really shouldn't be able to move it much.  I can't recall the exact setup with the gear up, but it should be pretty snug.  Maybe there is a missing stopper or something that lets the gear float up/down?  Although I'd think you would only experience that with a negative G loading, not just a pitch change.  

How much play do you have in the horizontal stabilizer?  A little bit is normal.  The pins on the elevator pivot will eventually wear down and create additional play.  I've replaced mine on the Duo, but could not ever feel anything in flight.  But I'm just grasping at straws with you at this point.

Another flight test would be to try to reproduce the issue with the gear down.  

The only clunking I have ever experienced  was the gear door.

Definitely need to get to the bottom of this. 

Have you sat in the glider on the cradle with the gear up and had someone pull the gear door open an inch or so and then let it go? I know you think they are tight, but it would be good to know if that feels/sounds like what you've been experiencing.

Give me a call if you want to talk through it at a higher bandwidth than typing.

Morgan

8054409315

  

On Thu, Mar 21, 2019 at 7:47 PM Michael Price <mpr...@execsummits.org> wrote:

Getting ready for the next season (exciting)!  I still have that loud clunking sound which  occurs when the pitch quickly changes in/out of thermals or if I just push/pull the stick. It is quite intimidating passing through the seat pan into me. Ruled out the wheel covers (good tension), cannot detect any excess wing play with the pin in, fooled with the shocks but I think they have been eliminated, thought it was wheel play due to a reversed washer but that did not resolve, shelves are now nice and tight, padded the water release pivot which could theoretically rattle, will do that resonance check suggested just in case of any structural issues, what else should I check?  Running out of ideas...