All,
Andy Low of Electrodynamics has asked me to post the following information regarding the MB 339. This email is in response to my concerns of the 339 stab failures I have heard about in this thread.
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I saw those posts on MB-339 failures, and understand your concerns. So I took a new stab from an unbuilt airplane and load-tested it.
I clamped the aluminum spar on the bench vice and hung weights on the stab at the mid-span point. I was able to hang a 10lb. portable air compressor (with integrated 12V battery) on the stab with no trouble. I then hung two 1 gallon fuel jugs full of fuel on the stab, and the aluminum spar rod started to bend, but the wood structure held fine.
I calculate that at 200mph, and 45 degrees of up-elevator, each elevator can apply approx. 7.6lb of force on the stab for a total of 15.2lb. for both stabs. As this is a 17lb airplane, that amount of force approx. 30in from the CG will be able to flip the airplane in an instant Lomcevak _AT 200MPH_(!), which is clearly more than any jet is going to do - the turbine will depart the airplane before the stabs break! (By the way, for the servo to actually be able to do this, I calculate the required servo torque [yes, TORQUE, NOT holding force] to be 245.4 oz-in!)
My tests and calculations show that the stab structure is strong enough for the purpose.
BUT, this is a stressed-skin structure, the strength comes from the
fiberglass-on-balsa top and bottom skins. The stabs skins are quite strong, but excessive pressure (e.g., pressing a thumb HARD on the stab surface) applied to them can cause cracking or delamination. If the skin should be damaged, or the fiberglass delaminated, the structural strength is compromised. We are cognizant of this, and will be revising the internal structure of the stabs to make it more resistant to skin damage.
****** IMPORTANT! *******
We DO NOT recommend cutting into the skins to add internal structure. But if you choose to do so, please ensure that the skin integrity is completely restored by proper glasswork well-bonded to the skin!
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Also, as noted on our Supplementary Instructions (included in all kits shipped from ElectroDynamics and Cermark), we detail a couple of procedures for strengthening the stab-fuselage joint, among other suggestions. Please make sure you follow these instructions!
If you have not built your airplane, but on examination of your stabs, you find structural cracks or delaminated glasswork, please return them to your dealer for a free replacement. In case you have already installed your stabs and damaged them, they can be cut free of the fuselage and replaced.
We have all replacement parts for these airplanes now just arrived.
As for the anti-rotation pins, these stabs are built with TWO locating pins each instead of the required one. These pins are buried in a substantial balsa block in the stab, not merely at the root rib. These pins and the aluminum spar are additionally surface-grooved to help in glue adhesion. IF the stabs are properly glued in place on the fuse, we think it is impossible
that the stabs can rotate or flex causing flutter at reasonable model airspeeds (under 200mph). However, improper gluing can cause the stabs to pull out of the locating pins. Please ensure that the gluing surfaces are well-cleaned and scuffed before gluing.
We do need to emphasize, however, that the servo linkages on a jet airplane must be built to the highest standard. We recommend 4-40 linkage hardware and tight, VERY slop-free linkages. Any tendency to flutter due to sloppy
linkages will be aerodynamically amplified at high speed and lead ultimately to flying surface (e.g. stab) failure.
Thanks!
Andy Low
ElectroDynamics, Inc.
Home of "No-Hassle" RC Products
http://www.electrodynam.com
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