Now I understand...
allbee said:
...If this force is aft the nose of the airframe WILL be heavy, if this point is forward the tail WILL be heavy...
Now I understand why we are not communicating...
If the balance point is towards the rear, I call that tail heavy and you call it nose heavy...and if the balance point is towards the nose, I call that nose heavy and you call that tail heavy.
Not where I come from...!!!
Balancing an RC airplane or balancing a full size airplane locates the point at which all the gravitational forces acting on the airplane (or any object) can be supported by a single upward force. So if the airplane balances toward the nose, there is more mass toward the nose and we call that NOSE HEAVY, NOT TAIL HEAVY!!! The weight of the airplane is a downward force acting through the center of gravity. To support the airplane, a single upward force equal and opposite can be exerted at that point...or when the airplane is flying, the sum of all forces generated by the airframe (wing, horizontal stab, fuselage, etc.) moving through the air oppose the weight of the airplane. When that sum of forces is greater than the weight of the airplane, it rises; when the sum equals the weight, the airplane flies level; when the sum is less, the airplane descends.
The balance point you are finding on the ground can be thought of as the point through which the weight is exerted DOWNWARD in a static situation. When the airplane starts flying, there is a net lift vector and it is not at the same point as the weight vector. For the airplane to fly straight and level in unaccelerated flight, the Lift Moments must balance. Does the Center of Pressure of a wing change with airspeed, angle of attack, flaps extended, etc.? Yes, in both location and magnitude, that's why the resulting forces are called vectors. The location and magnitude of the force acting through the CG (weight vector) changes in flight with fuel burn, baggage thrown out, bombs dropped, etc.
This is getting very deep. For an airplane in flight there is a lift vector and corresponding moment for the wing AND a lift vector and corresponding moment (force times distance) for the horizontal stabilizer. For a conventional airplane with the tail in the rear, the lift vector for the horizontal stabilizer is downward and balances the lift vector of the wing with a downward force (moments are equal for straight and level flight). For a canard airplane, with the horizontal stabilizer in front, the lift vector for the horizontal stabilizer is UP so it balances the lift moment of the wing with an upward force (and moment). Draw a side view cartoon of an airplane with the CG ahead of the lift vector of the wing for these two types of airplanes and it will be easier to understand.
As the CG is moved forward or back, controllability and stability are changed, and can reach the point of instability.
The book I referred to previously (Aeronautics for Naval Aviators) has some interesting reading about controllability in ground effect, at touchdown and during rollout. Too long to quote here.
(Oh, no, I'm beginning to feel like George.
I think I'll go flying.)
Very interesting, indeed!
"A puzzling thread you have begun," says YODA.
Happy flying!
Don
P.S. to Steve Allbee, I read your post again. CG is not a force but the location of a force, and the total weight of an airplane acts DOWNWARD through the location of the CG. It is not an upward force on the wings as you have stated. The CG is the center of GRAVITY, the point at which total weight of the aircraft acts DOWNWARD and is balanced with LIFT, an UPWARD force. All the forces on an airplane in level flight can be reduced to four forces,
lift,
weight,
thrust and
drag. That was in the basics I studied for my Airman's Test, and it's still taught today. As the airplane maneuvers, these forces are constantly changing, some more than others, but for the pilot to remain in control, the forces and their corresponding moments must be balanced.