ARF MB339 Design Fix !!!!
 

Hi ok well went out this morning for a shake down flight and after I got the plane running went to pick it up and the wings moved up both of them well now I know what the factory black patches behind the wing mount plates were placed in for, they used a cheap light ply for the back main wing former well upon my hard landing yesterday this light ply former didnt break lose or crack but the bolts that hold the spar receptical cut threw the former and now they both are lose severly the front ones have a tiny bit of play, I take it when the rear ones moved the wings upward movement forced them up but the font former is heaver duty ply but not as good as ply here in the states. I will need to take engine out and inspect for futher info and I will have to figure out how to fix it my first thought is to drill out and place some metal inserts to let the stress of the bolt be distrubitedand with some ply behind for strenght I have had these type set ups with only one spar on my Byron planes and even after a crash they never rip loose or out, the former would break first! so If you have this kit beware of this problem I called electrodynamics and they just said ok thats it....

design flaw
 

Hey Jetset,

Its only a "design flaw" if you make a bad landing!! Thanks for the info though, its worth looking at before construction is finished.

Vin...

Flaw
 

Ok pulled engine and mounts they are a bit oversized but not as bad as I thought they would be so I will pull them all out and sand the back with 80 grit and epoxie them and lock tight the bolts this will be as about all you can do I do this on the byron spar mounts another note they use 2 bolts on the top of the mount as a hanger mount attach, others use in the center of the mount this helps into distrubute stress also the landing wasnt perfect im glad or I wouldnt of figured this problem out I only bent the 3/16 gear wire only damage, the plane is heavy,,, but I dead stick a A4 one time glide like car keys the resulting body slam broke one wing but wing mount braket didnt budge in this case they all lossend up but the black carbon patch helped out Im sure . I would recomend this mod use 30 minute epoxie for glue and blue or red locktight on all spar bolts!!!

All 339 Owners please read!
 

All 339 Owners, I just received the folowing email from Andy Low owner of ElectroDynamics. Please read and do as he advises. I will be doing this to my fuse tonight...

Email:
Hi Randy

I just got a call from a guy called **** complaining about a loose wing on
his airplane after a bounced landing. This guys flies DF and I think he may
be the one who's been posting his progress on RC Universe.

I am not on RC Universe, but I'd appreciate it if you could post the
following:

====
Engineering Notice for Cermark MB-339 ARF owners
------------------------------------------------

We have received a report from a customer that his wings worked loose after
a hard landing. We have examined the kit, and advise all owners to take the
following action:

The wings plug into 4 aluminum spar pockets and retained by setscrews. The
formers to which the spar pockets are bolted to are further reinforced by
carbon fiber plates. These have proven strong enough to tolerate a
cartwheel on landing.

The 4 aluminum wing spar pockets in the main formers are mounted on the
formers by two M3 socket head steel bolts and lock nuts each. The mounting
holes for these bolts are intentionally drilled slightly oversize to
accomodate final incidence adjustments at the factory.

It is possible that if the bolts work loose, the wings could rock slightly
in their mounts.

Please go over all the spar pocket bolts and make sure they are are very
tight. Also, wick thin CA behind each aluminum extrusion to make sure they
cannot move relative to the former. It may be a good idea to spread a
fillet of slow-curing epoxy (e.g. Aeropoxy) around the spar pockets as well
to ensure security.

We will be advising the factory to epoxy the spar pockets in place after
final adjustments on the next batch of airplanes.

Andy Low
ElectroDynamics, Inc.
Home of "No-Hassle" RC Products
http://www.electrodynam.com

Fix
 

Hi will fix today and try again in morning for the shake down flights!!

ARF MB339 Design Fix !!!!
 

Jettset,
Please keep us all updated on your progress. Are you aware of the stabilizer mod that Andy also recomends? If not I can send you a very detailed PDF that is also an update to the Manual for the 339's that Andy sells.

Randy

Mods
 

Hi I did my elevator mod a bit diffrent but did mod it here are the pictures for the wing attach mod I needed to cut 2 small hatches on the bottem in front of the former to get to the bolts to remove the plate to glue it!

mod pics
 

Here are the bolt holes seems my landing only lossened the bolts up no damage ?

more pics
 

Plates

Fixing
 

after gaining accsess to the front nuts and removing all 4 plates ill do both wings at once so I can set them properly , ill use 80 grit on the carbon patches allso epoxie, then bolt them up with locktite this will take about 15 minutes before I tighten them completely I need to make shure they are alined Im glad I made the engine mount rails removable or I would have some tight quarters to work in