Payloads – Effects of Payload Configuration

The Operational Envelope

The influence of the Centre of Gravity (CG) position on aircraft performance, stability and manoeuvrability varies along the flight, depending on the phase of flight. The main safety issues related to an inappropriate position of the CG depend on whether the CG is forward or aft as developed hereafter.

Forward CG

• A “forward C.o.G.” means the C.o.G. is “in front” of the C.o.P. point
• In this configuration, the C.o.P. will create a downwards moment
• This can help stabilise the aircraft, if it’s within the operational envelope

Aircraft Manoeuvrability

A C.o.G. position that is too far forward induces such a big pitch-down moment that the aircraft manoeuvrability can no longer be guaranteed.

The more forward the C.o.G., the bigger the horizontal stabilizer and elevator deflections needed to give the aircraft a pitch-up attitude to compensate for the pitch-down moment. However, at some point of C.o.G. forward position, the horizontal stabilizer and elevator maximum deflections are reached, and the aircraft cannot be manoeuvred any more.

As an example for take-off, if the C.o.G. position is too far forward, the aircraft has such a “heavy nose” that the correct take-off rotation rate using the elevator becomes impossible to reach. The impact of an excessively forward C.o.G. position on aircraft manoeuvrability applies at all phases of flight. However, it is most noticeable at low speed due to the reduced effectiveness of the elevators.

Aircraft Performance

A C.o.G. exceeding the most forward C.o.G. position of the envelope is also the most penalizing situation in terms of aircraft performance.

• Aircraft performance reduces as distance between C.o.G. and C.o.P increases
• The additional moment create is not free (it costs drag, i.e. lost power)!

The take-off and landing performance is calculated based on the most forward C.o.G. position within the envelope. Therefore, if the C.o.G. position is even more forward, the actual aircraft performance will be lower than the calculated one.

Impact on aircraft structure at take-off

On the ground, the total weight of the aircraft is supported by both the nose and main gears, the further forward the C.o.G., the bigger the proportion of total weight is carried by the nose landing gear. At high weights (TOW), if the C.o.G. position exceeds the most forward C.o.G. position of the envelope, the aircraft structural limits of the nose landing gear can be reached with a consequent risk of damage.

A C.o.G. aft position brings the C.o.G. close to the C.o.P.

Exceeding the C.o.G. most aft position of the envelope can lead to a variety of safety issues.

Flight plans are highly dependent upon aircraft weight.

When an aircraft is heavier, this affects climb performance and flight level capability as well as available payload and range potential.

Weight is always one of most important flight planning considerations as it impacts overall performance/range so dramatically.

Indicated airspeed (IAS)

The airspeed read directly from the airspeed indicator on an aircraft, typically a pitot-static system.

IAS uses the difference between total (dynamic) pressure and static pressure, provided by the system, to either mechanically or electronically measure dynamic pressure.

Mach Number

It is the ratio of the speed of the aircraft to the speed of sound in the gas determines the magnitude of many of the compressibility effects.

Because of the importance of this speed ratio, aerodynami-cists have designated it with a special parameter called the Mach number in honour of Ernst Mach, a late 19th century physicist who studied gas dynamics.

The Mach number M allows us to define flight regimes in which compressibility effects vary…

… i.e. it takes into account airspeed, and the type of air you’re flying in!

Normal Operating Speed

• The level flight speed of an aircraft, at its design altitude, with the engine operating at no more than 75% of its rated horsepower

Maximum Operating Speed

• The VMO is the maximum permitted speed for the aircraft.
• This includes a safety margin, so pilots can reasonably fly near that speed.
• Exceeding it is not immediately dangerous, but as it reduces safety margins, is an incident that should be investigated.

Design Diving Speed

• The highest speed planned to be, or actually, achieved in testing.
• Demonstrated flight diving speed.

Other Design Speeds

There is typically also a “designed flap speed”

V1 is the critical engine failure recognition speed:

• Also known as “take-off decision speed”
• Take-off will continue At speed above this, even if:
  • An engine fails; or
  • Another problem occurs, such as a blown tire.

The V1 speed can be affected by many different factors, such as:

• Mass of the aircraft: More weight means more lift is required, therefore the V1 speed will increase
• Landing Area Slope: A sloping runway (downwards or upwards) will assist or oppose take-off, which in turn will determine the V1 speed
• Landing Breaking Coefficient: A factor relating the braking capabilities of the aircraft to the total weight (more payload means lower coefficient, meaning it is harder to stop)
• Pressure Altitude: Runways at higher altitudes will have a lower air pressure (i.e. a lower pressure altitude) which means the aircraft must go faster during take-off to generate the same amount of lift, which means V1 increases as pressure altitude decreases
• Temperature: Similar effect to Pressure Altitude, lower temperatures generate less lift, so a higher V1 is needed
• Wind Component: V1 is measured as a groundspeed, therefore, a headwind or tailwind will produce more or less airspeed, and thus a direct affect on the required groundspeed for takeoff
• Flap Position: Flaps can be used to generate more lift at slower speeds, thus a lower V1 speed is required if using flaps (at the cost of drag)

V2 is the speed at which the airplane will climb in the event of an engine failure. It is known as the take-off safety speed.

Officially the term flight number refers to the numeric part (up to four digits) of a flight code:

• For example, in the flight codes BA2490 and BA2491A, “2490” and “2491” are flight numbers.
• Even within the airline and airport industry, it is common to use the colloquial term rather than the official term.

Registered aircraft in Australia are assigned unique registration marks. The format includes:

• The nationality mark for an Australian aircraft ‘VH’ (established under the Chicago Convention)
• A hyphen
• Three characters

Note: There are special cases where dealer marks are assigned to individuals and organisations.

A completed load sheet contains weight and balance data pertaining to a particular flight.

This includes the weight of the aircraft, crew, pantry, fuel, passengers, baggage, cargo and mail. Where necessary, it contains details of the distribution of this load.