A quick summary of the flight testing while it is fresh in my head.
CG
At the time of design the Emax motor was our smallest easy to source cheap motor with plenty of thrust for the mission, and many manufacturers of outrunners offer something in this size. I thought I'd be in the ballpark for CG with the battery under the hatch. (I have a long history of building or suggesting smaller motors that many people find too weak for the application, I didn't want to do that again.) Sadly, I was wildly off on the nose and tail moments for setting the CG. The tail moment is kind of short, and the tail is very light. We ended up with the servos near the wing bolt, and the 3S850 battery fully under the wing. Only the motor and tiny ESC were in the nose.
Flight One
First flight was with 2 weights under the tail to make a nose heavy CG. Figured it would be fine for a first flight. We launched her out for a glide test, and if all felt well, then Jarron would add in the throttle and go for a full flight. This worked out great and we were off flying. She rewarded with pretty good handling, but could use more rudder deflection, and was certainly nose heavy, also the motor brake was not enough to stop the prop. We flew around, caught some thermals, did some ugly loops and generally got a feel for her. Then landed. The CG was so nose heavy that you could pull full elevator at cruise and the nose pretty much stayed put. Yikes! Shocked she could thermal.
Flight Two
We dialed in a little more rudder via the transmitter, adjusted the ESC brake and added more tail weight moving the CG onto the spar cap. This CG proved to be much better and she now looped nicely. Still nose heavy, but now in the ballpark. We passed her around and got some time on the sticks. I caught a nice thermal that took her from close to the deck upto where I wanted out of the thermal in maybe 25 seconds. Not bad! Jarron caught a thermal that really got him up there.. He bailed after we reminded him he has lost two models by specking them out of vision. She was working really well. I like the rudder to be active enough to induce a barrel roll, and that she did.
Time to try to break it in flight. I put her in a big dive, and just as I was pulling out I thought I saw a little flutter from a wing tip. That's not good! I passed the plane off to Jarron and I broke out my camera thinking this is going to be interesting and asked him to do the same. The video and pictures tell the rest of this story. I did not think the wing would fail, but thought it was going to show some flutter. It broke very early into the dive. What the heck happened?
Post Wreck Investigation
First off - this is why we do destructive testing of our models. This model is not intended to dive straight down, but we know some of you might need to do that to get out of big thermal. We want to find any issues before they are put into production. We were shocked to see this wing fail so easily, not one of us had any concerns prior to it fluttering. It was strong and torsionally felt really good in our hands. All joints were tight and looked good.
After reviewing the structure it looks like the top sheeting was not fully glued to the shear web on this panel. Basically we had failed to fully glue this critical joint! We can actually move the sheeting up and down on the neighboring bays. So may not need any structural changes to fix this issue. Just better attention to the building steps. This is how we learn sometimes.
The fuselage held up great! The only breakage was the wing bolt bulkhead broke. This could easily be ground out and a new one installed if needed. Otherwise, just the tail ripped off when it hit the ground at speed. The tail only broke one support and the control horns. Overall very little damage.
Take Away
I'll be redesigning the fuselage, it will be 100% new. I'm waiting to hear back from a manufacturer, we will be designing around a smaller motor that weighs nearly 50% less, and will allow us to shrink the entire fuselage cross section. Don't worry, still a very affordable motor. I'll lengthen the tail moment and give some more thought to the canopy design as it was a bit of a pain to build. Should be a leaner and cleaner design and well lend itself to both electric and pure glider layouts much better.
A closer look at the wing:
