Hi all

I purchased a ARF mb339 from those guys in Canada. I had a professional builder do all the final assembly and set up. Last Thursday we took it to the flying field for its maiden flight. The first flight was un eventful except that the turbine flamed out. We landed the jet and made some minor ajustments and flew again for a second time. I have to say that the Mb339 flew incredable it has such a sweat glide path with no bad habits that we noticed. It truely is a trainer like jet that responds sweatly in the air.

Unfortunatly when the jet was making its last pass before landing a loud flutering noise was heard and than the port side stab ripped off causing the MB339 to crash into the runway. The jet is a complete loss.

The good news is no one was injured and it looks like they can repair the turbine. However when we inspected the port side stabalizer we noticed that the round solid spar did not penetrate into the leading edge of the stabalizer, it was shoved into the fuse. when they assembled it in china the improperly built the stab. The only thing holding the stab on was a dab of glue. Additionally they glued paint surface to paint surface. Another problem with the ARF is the landing gear formers are to weak and need to be renforced befor installing the gear.

When we measured the spar that is used in construction of the stab we noticed it was only long enough to penetrate about a 1/4 inch into the leading edge at best anyway.

I thought at first I was the only unlucky one to have this problem, but when I called Bob Wilcox to have my turbine repaired he told me I was the 3rd person who had called him with this same problem. So 2 other MB339s have crashed with the stab flying off in flight.

I am not knocking the kit I have ordered another one and will make all the necessary modifications on the next one. I just want to make people aware of this defect so they have an opportunity to repair this defect before they go flying.

This part seem to be a leading edge attachment rather than spar. Never the less it's important. It seem to me it would be better limit how far it would slip into one side when the two were
put together.
Darryl Usher

? Hi Did your builder glue the stab on without sanding the paint off on both the elevator and the fuse?, or did he just glue it on?, it should of never came off if it was installed first by doing this. I think this is what has been happening . Mine survived a flame out crash on take off! allso you need to put a spar accross on the inside ,did you take pictures

Apparently they are selling alot of these kits(I have one back order now), Maybe everyone that has one flying can contribute the modifications that they have done to help prevent any future mishaps. I know I would appriciate any helpful suggestions.

thanks David

I have photos but the file is too large to import. If you want a picture, go ahead and email me and I will send it to you direct.

Nosedive

My email address is [email protected]

I would also recommend reinforcing the fuse with some carbon fiber, I noticed the wing has no carbon fiber in it ( sense the wing was busted in two) Just balsa and some kind of monochrome outer skin, could use a little strengthing as well.

Nosedive

I emailed you four pictures, let me know if you got them. I have been having trouble with my internet provider regarding outgoing email, some folks can not get my emails but I get theirs.

Regards
Vern

I have never gotten an error message. Ever sense @home went bankrupt last fall and ATT had to scramble to replace their service,I have had this problem. I sometimes send the email too my work email and then forward the email to on in order to complete the transmission of the email.

About 30% of the folks i email too don't get them ever. Did you get mine with photos?

Nosedive

I have now sent them one at a time.

NOSEDIVE, Please send me a copy of the pictures.

Also, some of you might be pushing this plane too hard. If you have an engine putting out more the 12lbs of thrust, I would use the speed limitter to say 150 MPH. From what I can tell on my plane (the one I got from Paul (JETTSET99)) there might NOT be a spar there to begin with, hence the need to put one there and keep the speed down a bit.


Turbulence
[email protected]

None of the crashes involved excessive speed. They were caused by a defect in the construction of the stab, not too much speed. We never gave it more than 3/4 power and 95% of the time it was 1/2. The stab fell off on the finnal approach before landing.

The jet is more of an over sized maverick trainer than anything else. Also mine weighed 21 pounds with fuel I would not put a 12 lb thrust turbine in this jet.

Did you fly yours yet? I bet you find it very safe with a P80 in it for power.

Let me know how your's flys.

Regards
Vern

I would like to see the pics also

thanks
David

[email protected]

Folks,
For those of you have not assemble the kit.
Take little time to modify the stab.
I open up my stab to inspect the work. There is no spar go across the stab. The stab possible can fail when flying fast.
Here how I would do to reinforcement the stab. Make 2 spars out of of 1/8" liteply. Cut little slot to insert the spar. Clue with CA or expoxy. Make sure the spar just firmly fit. Not to wide or to narrow. You want the spar to touch both side of the skin.
If you guys already assemble kit. Cut the slot from the bottom to put int the spars. Fill up with aeropoxy and sand it level. Cover with mono coat or painted over to hide the work. Do the same to the fin.
I'll post some pictures of the mod very soon. I stongly recomended everyone to this.
Also strenthen the main former with carbon fiber. Ther gear box also need to reinforcement.
This is very nice flying trainer. It just need some modification to handle the stress.


Also keep this bird under 150mph. My fly with 12lb thrust engine and got the smoke system onboard. My can fly over 150mph if I really pushing it.
This plane is not bullet proof. Put in the speed limiter to kept it from flying over 150mph.

I'll kept you guys posted.
Tam

Nosedive,
I have to know, what did your professional builder do to make your airplane weigh 21 lbs. There is absolutely no unused weight in mine and it weighs 15lbs plus fuel (64oz.) What does yours weigh TAM, including the smoke set-up? The only thing that was done for balance was to use a six cell car battery pack in the nose. The CG is about 1 in aft of the hatch line, using the formula to calculate the location (ask Eric or Ron at GPA Hobbies for that one). Eric flew mine at Fentress a few weeks ago, and it flew GREAT!! Unless you have a "Need for Speed", a Ram 500 is perfect for this airplane

Tam built the jet and with 74oz of fuel,it weighed in at about 21 pounds with a P80 in it. Tam is building me another one, he is a distributor for this kit now.

I am a glutton for punishment

To: [email protected] and [email protected]

I sent you a picture, let me know if you recieved by leaving me a message on this thread.

Thanks
Nosedive

Nosedive,
Can you resend the pics again,
Thanks David
[email protected]

Nosedive. Can you re-send? I did not get anything.

[email protected]

Thanks
Turbulence

Nosedive: Can you get a real ISP

What ISP would you reccomend?

Im not sure what is around you but im sure there are many. ATT appears not to be the best choice, nor will the ao hells and all the funny board types.

AOL is the only high speed internet provider where I live. DSL is not avaliable.

Im
'm stuck

Tomorrow I'll post pictures of the stab modifcation. I start some work on the stab this evening. It take me about 1 hrs to do all the neccesary reinforcement on the stab. I built my stab so strong that it can be removeable. I had Hitec digital wing servos on each stab for elevator. The servos are internal. The only thing are sticking out are the metal arm and linkage. I'll do the same to the fin.
Stay tune for photos update.
Brg,
Tam
www.tamjets.com

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.


---
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!
*************************

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

----------------

Hi Andy,
I did some test on the wing also. Me and my helper try to break the wing. We put at least 70 pound of pressure on the wing too see it break. The wing is not even hurt at all.
I will post some photos tonight on my website for everyone to view it.
Check out how I reinforcement the stab. It only take little time to do it.
About $6 on material.
Now the stab are so strong. The only think left for me to worry is land it safe.
Tam
www.tamjets.com