ORIGINAL: AGR413
I recently built and maidened my Nitro Planes Bobcat-50. This is a pusher configuration with a Tower Hobbies .75 turning a 12x6P (pusher) Zinger.
Now here is where the story gets interesting: After three weeks of either "honey I must work all weekend you'll need to watch the kids" or too much wind, finally Saturday the wife gets off a little bit early and hurries home.
Truck is all loaded, did I forget anything, oh well too bad, I make a flying trip (no pun intended) to the flying field at 3 pm.
(Now I broke this engine in by a process of leaning and richining and I only decided to fly it because the engine broke in to the point that it insisted that I lean the low end needle to get it to start reliably and it would get "up on the pipe" quite nicely
.)
Arrive at field 3:25 "come on son help me unload we don't have much time", get unloaded and I make one flight with my fun fly airplane to knock the dust off.
Fuel up the Bobcat, fuss with fine tuning the low end for 10-15 minutes, make a few dry runs...everything checks out, "I may really hafto do this".
Top off fuel tank....out on the runway...full power.....nose wheel turns and I give the Bobcat its first badge of honor a scraped wing tip.
Ciricle around "ok I know shes gonna go left a little on takeoff".....full throttle almost to peak and then it levels off...."crap too far to stop now or its ditch in the desert" I stay on the runway as long as possible pull back on the stick and she rises into the air reluctantly.
(At this point I thought there was no way I was not going to turn my ARF back into a balsa kit.)
Ok well the engine is running very very rich but it's running (this is a good thing I think to myself) and I begin to realize I don't have enough elevator to gain altitude very fast, swtich to high rates "ahh thats better". I and the Bobcat continue to fly at full throttle "feeling like it could fall from the air at any moment" and I get it trimmed out "whew that was scary" and just then I realize I hafto land a plane that might not make a go around.........
The Bobcat feels like it crawling around the sky just waiting for an excuse to commit suicide, but I steady my nerves make a few fly bys a couple of rolls and test out the stall parameters. (she would not turn at this speed without tip stalling). "holy batcrap catman" I think to myself.
Well on one particular turn comming back to the flight line I realize "hey I'm pretty low...this it it now or never.....LANDING!!!!!"
Well I got her down and gave her another battle scar you guessed it I scraped the other wing tip, ran off into the stick and the weeds, chipped up my prop etc etc.
The Bobcat lives to try again!
So before I do this again I need to figure out what the deal is and I want to hear your opinions.
I have run the tar out of this system on the ground and it makes full power, pick it up nose high, shake it around, upside down it still makes full power but it's crapping out and going rich in the air.
Top end is set slightly rich using the pinch test.
Only answer I can come up with is that the tower motor (in comparison with all my other two strokes) exhause outlet is pointed almost straight back (which in this case means directly into the on-comming airstream). So my theory is the air rushing into the exhaust is over-pressurizing the fuel tank causing the engine to run rich at flying speed???
My plan is to try a dubro exhaust deflector which will get the exhaust at least perpendicular to the on-comming air any thoughts?