My Kougar took a rather hard landing this past Saturday but apparently it will also survive. I came close to just ordering another Kougar kit and stripping this air frame down of all parts that are good and reusable. But after getting it home and going over everything inside and out, I think it's going to be ok. I actually got it all repaired within a few hours on Sunday (yesterday). The Kougar is a 40 sized a/c but ever since the first Kougar I've built and flown, I've always put in .61's and ran stock exhaust, and recently .65AX with a Mac's tuned pipe system in my newest Kougar. I've only flown her on the pipe a few times for just a few moments at a time because it's really a hand full at that throttle. I usually keep her right at about 50% throttle. She's got telemetry sensors and I'm using the FASST system so I'm seeing 92mph on the display at 50% throttle.
I think my issue is, I may not have completely built her up to handle that type of speed, pressure, power, etc. The one and only flight for this specific day (Saturday) kicked off well. I discovered from previous flights that I had to adjust the length of the nose wheel assembly due to the fact that the plane had a very slight nose-up stance and would lift off prematurely while on it's roll-out. I was able to get her up but was right about at the threshold to having a wing stall. So I removed the slight nose-up stance and the plane had a really nice roll-out last Saturday. I was flying her like I stole her and she was performing awesome. However, about six minutes into the expected eight minute flight, I lost roughly 95% of the up elevator ability. The minutes leading up to this issue, I had her doing high speed passes roughly 75 to 100 feet off the ground and I'd put her nose up to about 300 feet, do a 180 roll and then have the plane do an inside loop and head downward while pulling it up with roughly 75 feet of alt when she levels out. She flew like a dream, and this is also why I really wanted to save her instead of my initial idea of stripping her down and starting with a new Kougar build.
So at about the six minute mark into this flight, I started noticing that she was not that responsive when pulling up on the elevator stick. I knew something was not right so I got her into a final approach pattern. I throttled back a little and she lost alt very fast, even though I was pulling up on the elevator stick. If I gave her half throttle she'd start gaining alt but the speed was too much to try and land. So my final decision was made where I got up some good speed and some alt and then cut the throttle and turned her back towards the runway in a final approach pattern. She lost incredible alt in very little time when I cut the throttle. I hit the throttle once more during the final approach where she just maintained her current alt but again gaining tons of airspeed. I cut the throttle for the last time and tried to glide her into the tall grass at the end of the runway. She lost so much alt, I'm sure if I had set her down on the paved runway she'd a been a loss. Instead I hit the tall grass which most likely really broke a lot of the speed and fall. Only one blade on the prop was sheared completely off at the spinner which leads me to believe the forward motion of the plane caused it and not the engine spinning. I've got a plastic spinner on her and that didn't get damaged in the landing. The clunk in the tank got thrown through the vent plumbing loop so It wouldn't move freely. There was a crack in the cowling near the spinner. And the last bit was a stress crack in the very center top of the wing running along the seam where both wing halves are glued together.That was all the damage I was able to see from the outside of the plane up on inspecting her once I got it back to the table at the club.
After removing the wing from the fuse, I inspected the internal structure of the bird. The cause of this crash was the elevator servo lost three of the four wood screws holding it to the hardwood cross beams. The servo is held into place from forward to aft. The two servo screws aft were both missing, but the hardwood support beam was still attached at both the starboard and port sides of the fuse. The one screw left holding the servo in place was located forward of the servo, however, the hardwood support beam had broke free on the starboard side of where it had been glued to the inside sides of the fuse. So basically the way that the servo was still attached, I could only get about 5% elevator action, since the servo was connected forward with one wood screw to a hardwood support beam that had broken free on one end and no wood screws holding it in place towards the aft. Basically the servo had so much play, sort of like a vehicle transmission with a seriously bad/broken transmission mount. It's apparent that at the speeds I fly my Kougar, the air pressure creates a tremendous downward force against the top side of the elevator when pulling up on the stick, which in turn creates an enormous amount of stress at the aft portion of the servo, where the two screws hold the servo to the hard wood support beam. These wood screws were completely pulled out and the front hard wood support beam then broke free on one side. Inspection of the elevator revealed absolutely no damage to the control surface outside the plane. So I'm convinced that the missing screws and the hard wood support beam was the results of the in flight stress.
With this type of speed, power, pressure, etc. this is the type of structural integrity jet guys prepare for when building and maintaining their jets.
Lessons learned for sure, in the area of construction. Wondering if I should now use screws to secure the hard wood support beams to the sides of the fuse instead of just gluing them in place. Wondering if I should now use machine bolts and nuts to hold the servos to the hard wood support beams. Also wondering if I were to position the servo a different direction that this issue may have not occurred. After getting her home and thoroughly inspecting her inside and out, getting the clunk back to where it needed to be, mending the hairline fracture in the top of the wing and remonokoting that area, putting on a new prop on and mending the crack in the plastic cowling, reconnecting the hard wood support beam with a bit more strength, and picking up some machine bolts and matching washers and nuts after work today from the local hobby shop. I think she's going to fly again.
Either way it was a really good lessons learned and if I do build another Kougar I'm sure I will incorporate a lot of this lessons learned into the new build. I'm just lucky no one was hurt, no property got damaged, and I didn't lose the entire aircraft.
Last edited by SushiHunter; 07-21-2014 at 02:10 PM.