moving the center of gravity

[ruwafl]

hello all! i have a 60 size big stik with a saito 100 swinging a 16x4w prop. i have had the plane balanced at 4 inches from the leading edge which is what the manual said it should be. however, when i try to do a roll with it the nose drops like a rock, and i am giving it up elevator at the beginning of the roll. am i right in assuming that if i move the center of gravity toward the tail, the manual says 3 3/4 to 4.5 inches from the leading edge, then that should help me out with that nose drop problem or am i thinking in reverse?

[Sparky Paul]

Are you sure you're not holding some up-elevator when you begin a roll?
Your c.g. is right in the middle of the correct location..
Moving it aft might help.

[downunder]

From a trim chart that I've got it says the way to check for a reasonable CG is to put the model in a 45 degree bank then neutralise all controls. If it continues in that bank for a moderate distance (which I'd take to be a few seconds) then it's fine. If the nose pitches up then you need to add nose weight (tail heavy) and if it pitches down then remove nose weight (nose heavy).

But before you make any drastic changes to the CG, make sure you're not still holding that up elevator you began with. In a slowish roll you'll need to feed in a little down elevator as it goes inverted. If it's a flat bottomed wing then it'll need much more down elevator for the inverted part than a symetrical or even a semi-symetrical section.

One other thing to check is if you've got enough down thrust on the engine. Get it straight and level at full throttle then quickly come back to idle. If the nose pitches up you've got too much downthrust and if the nose pitches down you don't have enough.

[ivanc]

I'd change the prop to a 15x6 or 14x6 (whichever is more appropriate for the Saito 100) and see how she rolls. My guess is that the plane is too slow with the 4-stroke and the 16x4W prop - this is a 3D prop while the plane is not.

Ivan

[ruwafl]

thanks for the replies guys, there is a guy at our field that i was just talking to and he is going to help me out with it. he thinks there may be some trim and coupling issues with the plane. of course, pilot inexperience is probably a factor also. but on the note about the prop, i am really liking that 16x4 prop on this combo. there was a reason for it also. reason being that with a smaller higher pitched prop i was turning more than 10k on the ground. i went with the 14x6 and now have it turning about 9100 on the ground. this way i have some play room for the engine to unload in the air without over revving. and i like the slower flight speeds with all the authority it is giving me. i can't actually hover the plane yet for more than a few seconds or so, but all i have to do is gas on her and she pulls strait on up vertical. this plane weighs in at 7.4 lbs. so i don't think it's to much for this motor, although i hope someone will chime in if it is. the motor responds instantly and there doesn't appear to be any spool up lag so i don't think it's over propped.

[ruwafl]

ivanc, i should have paid more attention to your post. you may have a point about it just going too slow. i guess i should try the roll with some more speed on. i have almost gotten accustomed to doing my limited repotoire of manuevers at a slow throttle. usually i am flying this plane at 1/2 throttle or less, so i guess a little more speed before going into the roll may help out some. i'll try it and see. depending on weather tomorrow, i may get to go to the field again and i will see what happens.