C G effects please
 

Hi can somebody attempt to explain to me the effects of moving the c g forward or backwards and the way a model flys and theflight charteristics of this ie stability 3d capaability etc thanks colin

RE: C G effects please
 

There is an optimum CG point where the load on the horizontal stab is zero. This usually happens with the balance point at about 30% of the mean aerodynamim chord. If the CG is farther forward, the plane flies more stably: Less prone to stall, but more sluggish in aerobatics. If the CG is too far aft, the plane will be unstable, to the point possibly of being unflyable (try throwing an arrow backwards...same idea)

Here is one way to test:
Balance per the plans.
Take off and trim the plane for level flight at something less than full throttle. Get some altitude. Place the airplane in a shallow dive.
As the plane increases speed one of three things will happen.
(1) The airplane continues in a straight line as the speed increases. This is good.
(2) The airplane pulls out of the dive all by itself. This means the balance point is too far forward. The Elevator was trimmed to lift the nose a bit in level flight, and as your airspeed increased in the dive, the elevator became more effective and lifted the nose.
(3) The dive steepens all by itself. Balance point too far aft. Reverse of (2) , in this case the elev is trimmed to lift the tail in level flight and forces the nose DOWN with increasing speed.

Here's another way:
Take off and Trim for level flight.
Roll inverted.
If the plane wants a lot of forward stick to maintain altitude, the balance point is too far forward. If the plane tends to climb by itself when inverted, Balnce point is too far aft. Try for a balance point where the plane flies the same inverted as upright.

RE: C G effects please
 

Thanks for the reply, I have a funtana 90 with a saito 100 it is so pitch sensitive and you have to keep trimming it for attitude the C.G is per the manual it is really pitch sensitive I mean really sensitive so would it help moving the C.G backwards and balancing or leave the C.G and make it nose heavy...this has been an ongoing problem with this model I have the 40 version also and no flight problems????so will moving the C.G aft improve stability??? Thanks again Colin.

RE: C G effects please
 

Go forward with the balance point. Possibly dial in more expo if you have a computer radio, or reduce the control throws a bit.

RE: C G effects please
 

Here's a little more information that might help clarify. The stability of the plane in flight is determined by the relationship between the center of Gravity and the Aerodynamic Center (Center of Lift). The lift component will always act perpendicular to the wings at the aerodynamic center and gravity will always act perpendicular to the ground at the center of gravity. Typically, the aerodyamic center is between 25-33 percent of the average chord of the wing. The plane will always tend to turn up or down around the center of Gravity. If the Center of Lift is coincident with the center of Gravity, no matter what direction the plane is flying in, there will be no differential moment developed between the two, and the plane will simply continue in whatever direction it is pointed (theoretically optimum for pattern flying). If the Center of gravity is directly below the aerodynamic center (as in a properly balanced high wing trainer), when the plane is flying straight and level, there is no angular moment developed between the two. If the plane points up,or down the relationship between the aerodynamic center and the center of gravity will develop a righting moment between the two, causing the plane to want to return to straight and level flight. For low wing planes, exactly the opposite is true, which makes them a little less stable, unless there is some dihedral put into the wing to raise the aerodynamic center closer to or above the center of gravity.

When it gets interesting is when the center of gravity is either in front of (nose heavy) or behind (tail heavy) the aerodynamic center. In the nose heavy condition, during straight and level flight there is a moment developed causing the nose to drop, and this is offset by elevator. If the direction of airflow over the wing is changed (up or down turbulence), the relationship between the CG and AC tends to cause a righting moment pointing the nose of the aircraft into the air flow, making the plane stable in the air flow again. In the tail heavy condition, a change in the airflow due to turbulence causes the moment arm to push the plane perpendicular to the air flow, making the plane VERY unstable in turbulence. Hence the expression "Nose heavy planes fly poorly, tail heavy planes fly once".

Do a google search on "Aerodymanic Center" and you'll get some very interesting graphics explaining the effects.

Brad

RE: C G effects please
 

WOW thanks for the explainaton and thanks for the time to write it, so basically what I need to do is move the C.G forward and balance/add weight there and it will be less sensitive to attitude changes but less happy doing moves like hovering and the such??? tahks again Colin.

RE: C G effects please
 

Make small adjustments in your CG at a time until the plane flys the way you like it. It sounds like the plane may be slightly tail heavy.

Good Luck!