BALANCE WITH A TAIL DOWNLOAD

In figures 12.3 to 9, a number of different ways of achieving trim balance are illustrated. In the most orthodox case (Fig. 12.3) the centre of gravity of the aircraft is located at the aerodynamic centre of the mainplane. (The allowance for wing sweep and planform, as always, must be made.) Since the wing in this case has some camber, there will be a nose down pitching moment, so even though the weight and lift are exactly opposed and create no pitching tendency, the tailplane must carry a download to balance out the wing’s inherent moment If the wing were truly of symmetrical section there would be no wing pitching moment and the tailplane would not need to carry any load, but the greater the camber, the greater the tail load at a given airspeed.

Since the tailplane lies behind the main wing, it will be working in the region affected by the vortex-induced downwash of the wing. This may be several degrees, depending on the wing’s planform and the wing lift coefficient, Cl, at which the model is trimmed to fly. Since Cl is high at low flight speeds, the downwash may be several degrees. (See the approximate formula in para 5.10). If the model is trimmed for fast flight, Cl will be lower and there will be less downwash. But in every case the tail must be set at just the angle required, relative to the airflow in its location, to provide exactly the downforce needed to bring the total of all the pitching moments to zero.

Neither balanced nor stable

L Balance provided by a second wing in tandem

1 downwash

The most usual arrangement of forces for balance with a tailplane. The tailplane is rigged at a slight negative angle of attack relative to the air in its neighbourhood. The tail ‘lift’ is downwards. The additional load must be carried by the wing, and some additional vortex drag, of both tail and wing results.

|L —Lt = W] This is also a safe, stable layout.

To achieve a required new trim the whole tailplane may be moved to the new position, or, more often, it may be provided with a hinged portion, the elevator, which changes both the camber and the effective angle of attack of the tail to produce the same effect A different trim setting is required for qvery different flight condition. If the control is insufficiently powerful, some attitudes may be impossible to hold.

With the centre of gravity in this position, there will always be a download on the tail. It will be rigged at a more negative aerodynamic angle than the mainplane: this difference is often termed ‘decalage’ or ‘longitudinal dihedral’. In a vertical dive the lift is zero and the weight acts vertically, so there is no pitching moment arising from these two forces. The wing camber is still present, however, and its direction is such that it would turn the model through the vertical into a bunt unless balanced by the tailplane. Thus, although the elevator is moved down to produce the dive, and held down to keep the dive going, the tail load is still down. (This is discussed further below.)