Weight and Aircraft Momentum

What limits an aircraft’s structural weight?

The main force generated to balance the aircraft’s gross weight is the lift force, and if the lift cannot equal the aircraft’s weight, then the aircraft cannot maintain level flight. Therefore, the aircraft weight is directly restricted by the lift capabilities of the aircraft.

Note: The lift force generated is limited by the size (design) of the wing, the attainable airspeed (airspeed is limited by the power available from the engine/propeller), and the air density.

What are the effects of excessive aircraft weight?

If the limiting weight of an aircraft is exceeded, the following effects are experienced:

1. Performance is reduced.

a. Takeoff and landing distance is increased.

b. Rate of climb and ceiling height are reduced.

c. Range and endurance will be reduced.

d. Maximum speed is reduced.

2. Stalling speed is increased.

3. Maneuverability is reduced.

4. Wear on tires and brakes is increased.

5. Structural safety margins are reduced.

Center of gravity

The center of gravity (C of G, CG) is the point through which the total weight of a body will act.
Describe a component arm. The definition of a component arm is the distance from the datum to the point at which the weight of a component acts (center of gravity point). By convention, an arm aft of the datum, which gives a nose-up moment, is positive, and an arm forward of the datum, which gives a nose-down moment, is negative. Therefore, for a constant weight, the longer the arm, the greater is the moment.

Center of gravity moment

The moment is the turning effect/force of a weight around the datum. It is the product of the weight multiplied by the arm:

Moment = weight X arm

How is the pitching moment of the lift-weight couple balanced?

When the pitching moment of the lift-weight couple is not balanced perfectly, extra forces are provided by the horizontal tailplane to center the aircraft’s pitching moment.

Note: Lift forward of weight has a nose-up pitching moment, which is counterbalanced by the downward deflection of the horizontal tailplane, which creates a nose-down counterpitch. Therefore, lift aft of weight requires the opposite balance. The tailplane force has a turning moment in the pitching plane (nose up or nose down) about the lateral axis at the center of gravity point. Its effectiveness depends on its size and the length of its moment arm from the center of gravity.

Center of gravity range

The center of gravity range relates to the furthest forward and aft center of gravity positions along the aircraft’s longitudinal axis, inside which the aircraft is permitted to fly. This is so because the horizontal tailplane can generate a sufficient lift force to balance the aircraft’s lift-weight moment couple so that it remains longitudinally stable and retains a manageable pitch control. (Center of gravity range or envelope is listed in the aircraft’s flight manual, and accordance is mandatory.)

What are the reasons/effects of keeping a center of gravity inside its limits?

The forward position of the center of gravity is limited to

1. Ensure that the aircraft is not too nose heavy so that the horizontal tailplane has a sufficient turning moment available to overcome its natural longitudinal stability.

2. Ensure that the aircraft’s pitch control (rotation and flare) is not compromised, with high stick forces (tailplane turning moment), by restricting the aircraft’s tailplane arm forward center of gravity limit. [Remember, tailplane moment (stick force) = arm X weight.] Note that this is particularly important at low speeds (i.e., takeoff and landing), when the elevator control surface is less effective.

3. Ensure a minimum horizontal tailplane deflection, which produces a minimal download airforce on the tailplane and is required to balance the lift-weight pitching moment. Therefore, the stabilizer and/or the elevator is kept streamlined to the relative airflow, which results in

a. Minimal drag. Therefore, performance is maintained.

b. Elevator range being maintained. Therefore, the aircraft’s pitch maneuverability is maintained.

The aft position of the center of gravity is limited to

1. Ensure that the aircraft is not too tail heavy so that the horizontal tailplane has a sufficient turning moment available to make the aircraft longitudinally stable.

2. Ensure that enough pitch control stick forces (tailplane turning moment) are adequately felt through the control column by guaranteeing the aircraft’s tailplane arm to an aft center of gravity limit. [Remember, moment (stick force) = arm X weight.]

3. Ensure a minimum horizontal tailplane deflection, which produces a minimal upload airforce on the tailplane and is required to balance the lift-weight pitching moment. Therefore, the stabilizer and/or the elevator is kept streamlined to the relative airflow, which results in

a. Minimal drag. Therefore, performance is maintained.

b. Elevator range being maintained. Therefore, the aircraft’s pitch maneuverability is maintained.

What are the effects of a center of gravity outside its limits (range)?

If the center of gravity is outside its forward limit, the aircraft will be nose heavy, and the horizontal tailplane will have a long moment arm (tailpipe to center of gravity point) that results in the following:

1. Longitudinal stability is increased because the aircraft is nose heavy.

2. The aircraft’s pitch control (rotation and flare) is reduced or compromised because it experiences high stick forces due to the aircraft’s long tailplane moment arm. [Remember, tailplane moment (stick force) = arm X weight.]

3. A large balancing download is necessary from the horizontal tailplane by deflecting the elevator or stabilizer. This results in

a. An increased wing angle of attack resulting in higher induced drag, which reduces the aircraft’s overall performance and range.

b. Increased stalling speed due to the balancing download on the horizontal tailplane, which increases the aircraft’s effective weight.

c. Also, if the elevator is required for balance trim, less elevator is available for pitch control, and therefore, the maneuverability of the aircraft to rotate at takeoff or to flare on landing is reduced.

d. In-flight minimum speeds are also restricted due to the lack of elevator available to obtain the necessary high angles of attack required at low speeds.

Generally, the aircraft is heavy and less responsive to handle in flight and requires larger and heavier control forces for takeoff and landing.

If the center of gravity is outside its aft limit, the aircraft will be tail heavy, and the horizontal tailplane will have a short moment arm (tailplane to center of gravity point) that results in the following:

1. The aircraft is longitudinally unstable because it is too tail heavy for the horizontal tailplane turning moment to balance.

2. The aircraft’s pitch control (rotation and flare) is increased (more responsive) because it experiences light stick forces due to the aircraft’s short tailplane arm. [Remember, tailplane moment (stick force) = arm X weight.] This lends itself to the possibility of overstressing the aircraft by applying excessive g forces.

3. A large balancing upload is necessary from the horizontal tailplane by deflecting the elevator or stabilizer. This results in

a. A decreased wing angle of attack, resulting in lower induced drag, which increases the aircraft’s overall performance and range.

b. A lower stalling speed due to the balancing upload on the horizontal tailplane, which decreases the aircraft’s effective weight.

c. Also, if the elevator is required for balance trim, less elevator is available for pitch control, and therefore, the maneuverability of the aircraft to recover from a pitch-up stall attitude is reduced.

d. In-flight maximum speeds are also restricted due to the lack of elevator available to obtain the necessary low angles of attack required at high speeds.

Generally, the aircraft is effectively lighter and more responsive to handle in flight and requires smaller and lighter control forces for takeoff and landing.

If you were loading an aircraft to obtain maximum range, would you load it with a forward or aft center of gravity (forward or aft cargo hold)?

An aft center of gravity position/hold loading, for aircraft (especially jet/swept wing) with a nose-up en route attitude will allow it to achieve its maximum possible range. An aft center of gravity position, normally is accomplished by using the aft cargo hold, which gives the aircraft its nose-up en route attitude naturally; therefore, the stabilizer can remain streamlined to the airflow and produce no relevant drag (e.g., aft center of gravity = 6° nose-up attitude = 0° elevator/stabilizer deflection). Thus the aircraft can be operated at its optimal thrust setting to obtain its maximum range without having to use excessive engine thrust to compensate for drag. Note: It is beneficial even when the center of gravity is aft of its optimal position because the stabilizer would produce a greater lift force (to produce a downward pitching moment of the nose to gain its en route attitude). This is beneficial to the aircraft’s overall performance because it increases the aircraft’s overall lift capabilities, whereas a forward center of gravity has a detrimental effect on the aircraft’s performance. However, a few aircraft with a nose-down en route attitude would require a forward center of gravity position.

How does a forward center of gravity affect the stall speed, and why?

A center of gravity forward of the center of pressure will cause a higher stall speed. This is so because a forward center of gravity would cause a natural nose-down attitude below the required en route cruise attitude for best performance. Therefore, a downward force is induced by the stabilizer to obtain the aircraft’s required attitude. However, this downward force is in effect a weight and thereby increases the aircraft’s overall effective weight. Weight is a factor of the stall speed, and the heavier the aircraft, the higher is the aircraft’s stall speed.

Why does a jet aircraft have a large center of gravity range?

A jet aircraft needs a large center of gravity range because its center of gravity position can change dramatically with a large change in its weight during a flight.  Therefore, to accommodate a large center of gravity movement, the aircraft has to have a powerful horizontal tailplane to balance the large lift-weight pitching moments so that the aircraft remains longitudinally stable and retains its pitch controllability.

What causes center of gravity movement?

The center of gravity is the point through which weight acts. Therefore, movement of the center of gravity is due to a change in weight. The distribution of the aircraft’s weight can change for three reasons and thus cause the center of gravity position to move. The three reasons for a change in the aircraft’s weight are

1. Fuel burn. The most common reason for center of gravity movement on a swept-wing aircraft is its decrease in weight as fuel is used in flight. It should be remembered that because of the sweep, the wing and the fuel tanks housed inside cover a distance along the aircraft’s longitudinal axis. Therefore, as fuel/weight is reduced progressively along this axis, the weight-distribution pattern changes across the aircraft’s length.

2. Passenger movement.

3. High speeds.

Note: This is so because the greater the speed, the greater is the lift created. To maintain a straight and level attitude, the aircraft adopts a more nose-down profile, which is accomplished by creating lift at the tailplane. This lift on the tailplane effectively reduces the weight of the tailplane section of the aircraft.

Be the first to comment on "Weight and Aircraft Momentum"

Leave a comment

Your email address will not be published.


*