Plane is unstable

[mynameisbob]

My plane is the Fruit bat design by laser cut planes and it wobbles in the air. I tried to change the center of balance to make it fly better but nothing worked. Does anyone know what my problem is?
Thanks.

[coachchuckaahs]

Either something flexing in the breeze (wing, tail), or rubber is not centered on the prop shaft, or possibly prop is not balanced.

Coach Chuck

[bjt4888]

Bob,

Review the items Coach Chuck gave you.

The team I am coaching has built a couple of these airplanes and we opted to replace the fuselage with 15" of 3/16"x3/8" of 6.5 lb cu ft balsa (1.7 grams) and replace the tailboom with a 6" 3/16"x1/8" taper to 1/8"x1/8" 5.5 lb cu ft balsa tailboom (.25 grams)(total 21" fuselage and tailboom). Constructed weight of the entire airplane (covered with Ray Harlan Ultrafilm) with these replacement parts was 5.8 grams allowing 1.2 grams of clay ballast that could be attached near the nose so that the nose moment arm (nose to wing leading edge distance) could be set to about 4.25". This nose moment arm length and the resultant "longish" tail moment arm (distance from the wing TE to the horizontal stabilizer LE) make the design very stable with the center of gravity set to 1.75" forward of the wing TE and with wing incidence set to about 1/4". Thanks to Coach Chuck for these setting guidelines. We also use a custom bent wire rear hook to help keep the rubber motor a little farther from the motor stick. We found that this modified version of the airplane required about 2.5 degrees of tailboom offset and about 1/8" of left wing washin. It flies pretty well. 2:15 on the first testing day with 18.5" loop of .056" (.0374 g/in) rubber wound to 2,780 turns with 75 backoff turns and a launch torque of 0.42 inch ounces.

The kit parts build a fine airplane that should fly for about 1:30+ in a 25 ft ceiling gym, but as the fuselage and tailboom are a little on the short and heavy side (which makes them tough, maybe a good thing for new model builders) the center of gravity has to be located quite far forward and the wing incidence has to be quite high in order to achieve stable flight during the early part of the climb. Unfortunately, these settings would make the airplane prone to stalling, wallowing and rolling towards the middle and end of the flight.

This is not the best design airplane for the event, but it is very good for the level of effort required for construction. For someone new to the event and new to construction of this kind of thing, I definitely recommend this kit, modified or not.

Good luck and good flying,

Brian T.

[bjt4888]

Sorry, forgot to mention; replaced the red propeller in the kit with the small symmetrical Ikara purchased from Freedom Flight Models and cut to the 14 cm requirement (and the Ikara prop bearing hanger and prop bearing, of course); no pitch adjustment; stock pitch about 6".

Also, 2.5 degrees of stabilizer tilt to help make it turn 25 ft circles.

Brian T.

[mynameisbob]

Thanks chuck and Brian! I just got back from testing my plane and found that the center of gravity was off center with the addition of wings being different weights. It seems to be working fine now.
Thanks,
Bob