Say I have two 8 pound planes that are the same and one balances with the battery near the C of G but because of wood weight and other variations the second plane which weighs the same needs the 5 ounce battery moved back near the tail to balance at the same C of G.

My question is am I going to notice a difference in flying characteristics cause I have more mass closer to the tail in the second plane than in the first?

I have heard it is better to have aileron servos close to the fuselage cause it keeps th mass closer to the C of G What about the added mass close to the tail even though the C of G and the weight of both planes are the same?

Thanks,

WRK

That's an interesting question. I look forward to some guys smarter than me answering. For the time being I would say in certain maneuvers it would make some small differences. For example in a flat spin or lomcevak the extra tail weight should equal momentum.

David

i have thought about this before and culd not come to a conclusion.. i bet the aerodynamics guys would put this to rest.

If you move weight from near the cg to near the tail and nose of the fuse, the weight of the plane does not change and if you do it a particular way the cg can remain the same. This would require moving most of the weight forward and the rest aft to maintain constant cg. What then does change is the moment of inertia about the pitch axis (and the yaw axis). Just as acceleration equals force/ mass, angular acceleration equals torque/ moment of inertia. This means for the same elevator deflection and speed, a plane with the weight far from the cg (fore and aft) will rotate in pitch more slowly than one with the weight more concentrated near the cg. The plane will be sluggish in pitch , slower to respond and slower to stop rotating.
The same applies with weight toward the wing tips. This increases the moment of inertia on the roll axis making the plane more sluggish in roll.

If you place a heavy weight (think of the term heavy as being relative to what you're flying) and you do 3D or extreme aerobatic maneuvers you risk the chance of the weight producing momentum in the tail and carrying it through placing a heavy load on your elevators or rudder. Clear enough???? In other words the plane will be harder to fly "precisely" and take more to recover. Now go back and re-read the above message. That's what he said, just shorter.

i understand.. after he mentioned the thing about having weight far out towards the wing tips causing sluggish rolls. just apply that idea to the plane "rolling" end over end.. or as it is called, pitching.

having the weight farther out just makes more work for the control surface.

if the mass is far from cg
it has more weigh to move (inertia) and more weigh to stop (momentum)
thus slurggish response
thx

I agree. If you want a plane which waterfalls very fast, get one with a very short nose so the engine will be very close to the CG.

Most guys already realize that putting the servos out on the wing dampens the rolls. This is good and bad. The pattern guys don't like over-rotations caused by the wing roll inertia.

Now that 3D is here, guys are starting to realize the pitch axis is also a factor as is the yaw, so it pays to get all the weight near the CG.

This translates into a design which has very light structures, and centrally mounted components.

CENTRALLY MOUNTED COMPONENTS?
I SEE SERVOS AT THE BACK AND MIDDLE OF WINGS
FOR 3D U NEED THE MOMENTUM TO WHIP THE TAIL AROUND ESP WATERFALL
SLURGGISHNESS WORKS BOTH WAYS HENCE MORE STABLE DURING SLOW SPEEDS FLYING
THX

Well, you need to state your flight priorities. Guys usually prize the ability to start and stop a motion instantly more than "stability" (or sluggishness as you say) in an aerobatic airplane.

The Pro bros had a discussion regarding this topic a while back comparing and contrasting the virtues of the Jeff Williams "Sledge" vs the Paul Swanson "Mojo". The Sledge puts all the gear in the wing. The Mojo puts it on the tail. The sledge starts and stops the tail whipping type moves noticeably quicker than the mojo, but the sledge makes you route pull pull cables through the fuselage, which isn't all that enjoyable during construction. The Mojo has easy control link hookups and only the servo wires need to be routed in the fuselage. If I remember correctly, the general consensus was that it was worth the slight trade to go for the ease of assembly.