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View Poll Results: Have you experienced Stress related damage in your PA Extra MX ARF from normal use
Yes! I had serious Stress related cracks or damage! 7 20.59%
Yes, but they were minor and easly repaired. 4 11.76%
NO! Never happened to me. 23 67.65%
Voters: 34. You may not vote on this poll

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Old May 04, 2011, 10:48 AM
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Poll
Precision Aerobatics Extra MX Owners Poll

This poll is for current or former owners of the PA Extra MX.

I have been very impressed with the PA products I own, and am thinking of purchasing their Extra MX ARF. However, a couple of guys in my club have warned me that they have experienced stress fractures in their Extra MX air frame just aft of the cockpit. These cracks appear after normal 3d flight usage, and are not crash related in origin. After searching the web, I have found a small number of others reporting the same.

In order to judge how common this problem I am hoping all you Extra MX owners out there can help me find out by responding to this poll.

Thanks
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Old May 04, 2011, 01:25 PM
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Seems to be a very capable but very light plane. A friend was flying one the other day in a fair amount of wind when a furious wing flutter occurred that shook the plane so violently that both aileron servos were stripped out. He did a fantastic job of getting the plane back down...with both ailerons just dropping down loose after landing.

Similar planes of this size weight about 10 ounces more. Even though PA uses a generous amount of carbon fiber, I really think that there isn't quite enough rigidity in the fuselage around the wing area. The plane withstood the flutter but was not ridgid enough to stop the flutter to begin with.
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Old May 04, 2011, 04:19 PM
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Super strong airplane... mine is over 2 years old and has received all type of full throttle blenders.. knife edge spins, harrier roll landings.... never had a single problem with mine!

Extra MX new vid by Aerofundan (6 min 13 sec)
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Old May 05, 2011, 06:59 PM
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That was my MX that Ke Spin referred to as to flutter. I believe that the flutter was caused by not using strong enough servos. It was very turbulent nasty air that day and i feel that the bouncing started the flutter. I only got a dozen or so flights on the plane before receiver failure put it in but I doubt anyone could stress it any harder than I did before the thump happened. One blender in particular was very violent and the plane stayed together. I liked it enough that I bought another.
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Old May 05, 2011, 08:31 PM
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Quote:
Originally Posted by ircfly View Post
That was my MX that Ke Spin referred to as to flutter. I believe that the flutter was caused by not using strong enough servos. ...I liked it enough that I bought another.
Tim,

Hope thats what it was. Can't wait for you to get the next one together and wring it out a little. We'll see what happens
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Old May 06, 2011, 09:03 PM
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New mx is ready right now
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Old May 06, 2011, 10:32 PM
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Flying after the meeting tomorrow..weather permitting?? You are coming aren't you??

Mike and I went out tonight locally...just before the HAIL. The VFO needs more authority and I flew your favorite plane of mine.
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Old May 07, 2011, 06:21 AM
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The MX is a very strong airplane and I proved it last weekend. While trying to make a couple of trim adjustments while flying, I managed to somehow move the on/off switch to the off position on my DX7 . My MX hit the ground (field with unplowed corn stalks from last year) nose-in very hard from an altitude of 70 feet. The damage...broken prop, canopy cracked in several places, three small broken pieces of balsa in the fuse and a couple of other small cracks in the fuse balsa. The repairs took about an hour. I'm telling you guys, the damage was very minimal. My RC buddies who witnessed the crash couldn't believe the crash didn't result in a "rekit." One of my buddies was so sold on the durability of the PA airplanes, ordered the Addiction X that evening . Anyway, the FiberFusion technology really works. Oh, and I have flown the MX since my repairs and it flies just like it did prior to the crash .
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Old May 07, 2011, 07:29 AM
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I typically "upsize" servos on planes including my MX. I fly mine with HS-225 servos. While not the best they have plenty of torque for this size plane. Getting any type of flutter is a killer so always try and avoid that. IMHO trying to keep a plane light using marginal servos is not a good idea.

With that said I have an Extra MX, AdditionX and Ultimate AMR. I just looked over these planes yesterday as the weather was fowl and couldn't find any problems with any of them. I do a lot of high stress maneuvers with these planes too. So while I have my own set of beefs with the AMR (hard to get battery in with the cross spar in the way) and the AX (fuse so narrow, it's hard to get to the wing nuts), build quality isn't one of them.

In my experience, stress fractures and similar problems are usually caused by less then ideal landing or transport issues. These planes aren't ply built like GP or Hanger 9 planes. When on the ground they need to be treated with "kid gloves" so to speak.

Or in other words they are a bit more fragile then other manufactures planes on the ground but are very light AND strong in the air. I buy my planes for their performance in the air, not on the ground...
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Old May 10, 2011, 01:04 PM
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Thanks to all for their input so far. Great information!

For those of you with damage, how about a description of it's location? And how you believe it came about?
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Old May 10, 2011, 02:15 PM
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Checked my MX over last night and it is fine, no cracks and I am pretty rough on my birds.
JC
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Old Aug 11, 2011, 08:04 PM
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+1 to what Cayars mentioned. My only damage was from a few rather hard landings that knocked the landing gear mounting plate out of whack. Some epoxy and strategically placed carbon strips have it as good as new. The only other damage came the other day when I was picking it up. I grabbed the fuse by the wing opening and heard the balsa go "crack". I guess that part's fragile. That said, my son (65lbs) halfway stepped on one of the wings while it was in a wingbag (he got half ground and half aileron), and it was a-okay. Overall, it has its tough and weak spots, but is a fantastic plane. If I could only have two planes, they'd be this one and my E-Flite Beaver. If you haven't gotten one yet Trac, you should!
Josh
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Old Jul 02, 2015, 03:03 PM
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PA will not stand behind their product

I am having a problem with support from PA. Here is an email I sent to Precision Aerobatics about a problem I had with the motor mount on my new Extra MX:

"Two days ago I was doing some testing to prepare for the maiden flight and an anomaly occurred. As I slowly spooled up the motor to full throttle, the motor suddenly departed from the aircraft and flew straight ahead several feet. Upon examination, it appears the motor box failed.

I followed the installation instructions very carefully. I "painted" the entire motor box with 30-minute epoxy, paying careful attention to every seam and joint. I also applied the glass fiber reinforcement to the back of the motor plate as instructed. Since it took me several weeks to complete the build, and because I epoxied the motor box first, the epoxy had several weeks to fully cure.

Although the motor was not the Thrust 50, I used a nearly identical Turnigy 4260 motor with 500kv. Considering the size and kv are within 1% of the Thrust 50 specs, it should produce similar power, perhaps even a little less since it uses cheap magnets.

Although the propellor was not a Vox propellor, I heeded the advice not to use an APC prop and chose a 15x7 wood propellor. The propellor was carefully balanced using a magnetic balancer and there were no vibrations in the airframe prior to failure. Note that the motor was only under full throttle for 3 seconds before failure and that the airframe had never been flown.

The batteries I was using are two-year old cheap Turnigy 2200mah 3s lipos rated at 20c (2x in series for 6s operation). These are old and tired with hundreds of flights, and the low 20c rating means that they were not able to give the motor full power.

Because I was doing testing, I happened to be looking at the power meter at the time of the incident. It was reading 41amps at full throttle which is well below the rated power capability of the Extra MX.

I have included some pictures. Note that the motor box is not twisted as would occur if a motor spooled up too quickly and caused a torque related failure. The wood simply separated.

It is my opinion that this must have been a factory defect. Therefore, I am requesting some replacement parts under warranty. "

Shaun at PA responded by suggesting my choice of motor and prop was the culprit. (I doubt the motor/prop were the problem as discussed above.) He also pointed out that the "composite plates" were not epoxied onto the sides of the motor mount. For this I do take some responsibility as the motor box did fail in that area. HOWEVER, upon re-reading the manual to see what step I missed, I discovered that the manual makes no mention of installing the composite plates whatsoever!

I pointed this out to Shaun and he dismissed it by saying it was represented in a photo in the manual. That is the equivalent of saying "Find Waldo" without actually saying I needed to look for Waldo! Yes, looking at the manual I can see the composite plate installed on one of the photos. But it was such a subtle change on a black and white photo with no wording or reference to the plates that any normal person would have missed it. In addition, the plates were packaged with the tail wheel assembly. How could I have known what they were for? Does a person need to be psychic in order to build a plane from PA?

My beef with PA is that they lay blame on the builder rather than addressing the core issue...the motor mount on the Extra MX has a KNOWN weakness and needs to be redesigned at a factory level. As evidence of this weakness, PA has issued three separate patches to try to strengthen the motor mount. It is the equivalent of using a band aid as a quick fix. Even Shaun admitted breaking several motor mounts himself.

I feel that PA should have given me a free motor mount since the manual did not mention the installation of the composite plates. They have refused. So I am taking this issue to the flying community. What do you think PA should have done? Was I out of line asking for a replacement part under warranty?
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Old Jul 02, 2015, 03:28 PM
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If you have parts left after you finished the build, there should be alarm bells going off.....you are lucky it failed on the ground
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Old Jul 03, 2015, 09:27 AM
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I have put my MX through the wringer and it keeps coming back for more. I just recovered it and ca'd all the joints. It flys great! I would not hesitate to purchase one.
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Last edited by Willie T; Jul 03, 2015 at 04:20 PM.
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