Hi, I want to shoot a question past some of the aerodynamic wizards here to get some opinions on possible causes of erratic behavior in one of my aircraft. My main question is, if an engine is starting to separate from the firewall in flight, could this cause the aircraft to pitch in different directions erraticly? Namely down and to the left to be precise. Here's the dope on the story. Its an approx 45" span Fokker DVI bipe. I've flown it many times before and it always behaves beautifully. Anyway, on this particular day I took off from my strip, climbed up about 100 to 200 feet and was just tooling around a bit. Then the wierdness occured. Every once a while the nose would drop/dip sharply and quickly, but then stop and be level again. (Ok, I know what your thinking "servo glitch" but hear me out) So, I'm thinking hmmmm. not good, but not deadly either so lets keep cruising and see what happens. Just in case I thought, lets do this observational/diagnostic cruising over near the end of the strip so I can land in a hurry if things go south rapidly. As most of you know who have had anything to do with aircraft, this can happen quickly.....and did. I turned, then I flew parralel past the strip to my ingress end and was preparing to turn back when the nose not only dipped sharply, it flat out dove! I threw in full up elevator. She leveled out. I still had about 120 feet in alt. so I started the turn back into the strip again. She turned to face the strip and then dove severely yet again as I approached! Again I hit full up elevator, she leveled off but maintained a severe right bank, I fed in full rightrudder but she just flew straight with this crazy right bank thing going on, almost like a held knife edge. At this point she was 25 feet out fromt he strip and about 30 feet up dropping fast. I startred to chop the throttle and she slowed. There is deep hay before you reach the strip so I thought this might save some damage. She smacked into the deep grass roughly end over ending, but the damage was minimal. Four bent and separated cabane struts that need repair, amazingly enough that was about it!! But the clincher was the engine was barely hanging in place. When I removed the cowl it simply fell off the plane! Now this could have been crash induced, but I'm thinking it may hav been the culprit all along. I would like to know if anyone else thinks this is valid? It was only held on with wood screws anyway......sooo...... guess its time to install some blind nuts. I'm thinking they mey have worked loose and caused this whole thing. I have found the engine mount loose before and retightened them more than once. I'm going to do a range check this week with the current battery charge and see if I get any glitches, but right now I'm thinking the problem may have entirely been due to this engine mount getting worse and worse and finally pulling loose. Anybody out there think this is realistic? TIA for your replies!!

Loose engine very likey. Or perhaps your cabanes were getting loose as well? If you look at the screw holes and cabane joints you should be able to see if the damage is recent or if it occured over time. I'll bet the engine was bouncing around in there moving the thrust line round by 4 or 5 degrees or more. Stuff like that could cause it. Did you not notice any flopping around of the engine during starting it?

Also next time when something like this happens the FIRST time it's wiser and safer for all persons and property to throttle back and land pronto and give the model a very close inspection. If you'd done that it's likely you would have avoided even the minimal damage. Folks have even avoided a total crash by landing to find loose hinges and stuff that would have caused a complete write off if they had stayed in the air to "test" it.

Hmmmm, when you were trying to nurse your plane back to the runway, were you at idle? If so, did it seem the engine would have the power to yank the plane as erratically as it did? The one missing clue of this being a loose engine is the erratic behavior being related to how much throttle you had. In other words, at a constant throttle, I would expect the behavior to remain somewhat constant, and then when you change throttle, the plane would change its behavior. A loose engine, however, can change its throttle on its own- did you hear the engine throttling up or down on its own?

And, yes, my first guess is radio. Did you immediately check battery levels and test the radio components?

In any case, I agree with Bruce in that you should ALWAYS immediately land a plane that shows any erratic signs. There is no reason to believe that the degree of erratic behavior will remain constant. I hope you can get your bird back in the air soon!

Thats a good point Mulligan, I really wasn't paying close attention to the idle speed sound of the engine, being so wrapped up of course in saving the planes bacon! But I believe I do recall minute pulses, when the plane did its minor dips, but after the plane did its severe banking drop I was still at full throttle and saw no change in power until I chopped throttle to slow the descent into the high grass. But, this does not mean it wasn't a loose engine, because the engine can only move a very small amount forward as it will run into the cowl. In addition, it is mounted inverted so if it was causing the plane to yaw like it was it would actually be more inclined to hold the throttle open due to my servo location. But, As you suggest, I will do a range check. She's almost ready to fly again even as we speak, I'm just waiting for a new backplate for the OS 25. LA motor. The needle valve assembly cracked off in the crash ( one other clue that tells me loose motor!) from bashing into the firewall on impact. Once that arrives I will install it, re-attach the top wing and we'll be off again. I already replaced the wood screws that were in the engine mount with blind nuts. I cant say how much this has improved the security of the engine mount. I've gotta say how impressed I am with the resillience of this little plane. Not only does it have scale appearance, but its tough as nails, and comes up smiling from damage that would bury a traditional stick built bird. I've used composite construction in the design that blends old and new construction techniques and materials, the result is a very light, very strong plane thats also cheap and easy to build. You can also make it as scale as you want. I'm satisfied with stand off, cause I like flying more than being real anal about the details, but you could if you wanted to and it would look great. Anyway, thanks to BMatthews and Mulligan for your help. I may post some pics later.