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Centre Of Gravity & Centre of Pressure

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Jeffrey Omwela

on 25 May 2011

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Transcript of Centre Of Gravity & Centre of Pressure

Centre Of Gravity
&
Center Of Pressure
The center-of-gravity (CG) is the point at which an aircraft would balance if it were possible to suspend it at that point.

It is the mass center of the aircraft, or the theoretical point at which the entire weight of the aircraft is assumed to be concentrated.
Its distance from the reference datum is determined by dividing the total moment by the total weight of the aircraft.

The center-of-gravity point affects the stability of the aircraft.

To ensure the aircraft is safe to fly, the center-of-gravity must fall within specified limits established by the manufacturer.
CG forward of forward limit
A forward CG may occur when a heavy pilot and passenger take off without baggage or proper ballast located aft of the rotor mast.
This situation becomes worse if the fuel tanks are located aft of the propellor because as fuel burns the weight located aft of the rotor mast becomes less.
CG aft of aft limit
Without proper ballast in the cockpit, exceeding the aft CG may occur when:

A lightweight pilot takes off solo with a full load of fuel located aft of the rotor mast.

A lightweight pilot takes off with maximum baggage allowed in a baggage compartment located aft of the rotor mast.

A lightweight pilot takes off with a combination of baggage and substantial fuel where both are aft of the rotor mast.
Lateral Balance
In fixed-wing aircraft, lateral balance is often much less critical than fore-aft balance, simply because most mass in the aircraft is located very close to its center.

An exception is fuel, which may be loaded into the wings, but since fuel loads are usually symmetrical about the axis of the aircraft, lateral balance is not usually affected.
The lateral center of gravity may become important if the fuel is not loaded evenly into tanks on both sides of the aircraft, or (in the case of small aircraft) when passengers are predominantly on one side of the aircraft (such as a pilot flying alone in a small aircraft).

Small lateral deviations of CG that are within limits may cause an annoying roll tendency that pilots must compensate for, but they are not dangerous as long as the CG remains within limits for the duration of the flight.
Center-of-gravity limits
CG limits are specified longitudinal (forward and aft) and/or lateral (left and right) limits within which the aircraft's center of gravity must be located during flight.

The CG limits are indicated in the airplane flight manual.

The area between the limits is called the CG range of the aircraft.
Reference datum
The reference datum is a reference plane that allows accurate, and uniform, measurements to any point on the aircraft.

The location of the reference datum is established by the manufacturer and is defined in the aircraft flight manual.

The horizontal reference datum is an imaginary vertical plane or point, arbitrarily fixed somewhere along the longitudinal axis of the aircraft, from which all horizontal distances are measured for weight and balance purposes.
There is no fixed rule for its location, and it may be located forward of the nose of the aircraft.
Arm
The arm is the chordwise (fore-and-aft) distance from the datum to any point within the aircraft.
Moment
The moment is a measure of force that results from an object’s weight acting through an arc that is centered on the zero point of the reference datum distance.
Moment is also referred to as the tendency of an object to rotate or pivot about a point (the zero point of the datum, in this case).

The further an object is from this point, the greater the force it exerts.

Moment is calculated by multiplying the weight of an object by its arm.
Factors Of CG
Center of pressure
The center of pressure is the point on a body where the total sum of a pressure field acts, causing a force and no moment about that point.
The resultant force and center of pressure location produce equivalent force and moment on the body as the original pressure field.
The center of pressure of an aircraft is the point where all of the aerodynamic pressure field may be represented by a single force vector with no moment.

A similar idea is the aerodynamic center which is the point on an airfoil where the pitching moment produced by the aerodynamic forces is constant with angle of attack.
The aerodynamic center plays an important role in analysis of the longitudinal static stability of aircraft and other flying vehicles.

It is desirable that when the pitch angle and angle of attack of an aircraft are disturbed (by, for example turbulence) that the aircraft returns to its original trimmed pitch angle and angle of attack without a pilot or autopilot changing the control surface deflection.
Questions?
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