What condition is required for an aeroplane with a CG location behind the centre of pressure to achieve straight and level flight?

For an airplane with its center of gravity (CG) located behind the center of pressure (CP), achieving straight and level flight necessitates upwards horizontal tail loading. This upward loading is critical because it counteracts the nose-down tendency caused by the position of the CG behind the CP.

When the CG is aft of the CP, the aerodynamic forces acting on the aircraft create a moment that tends to pitch the nose downward. To maintain stable flight, the horizontal stabilizer, or tail, must exert a force that pushes the tail downwards, effectively generating a moment that counters the nose-down tendency. This downward force from the tail requires that the horizontal tail experiences an upwards load.

In summary, upwards horizontal tail loading provides the necessary balance and stability for maintaining straight and level flight when the CG is located behind the CP. This condition is essential in ensuring that the aircraft remains controllable and stable during flight.