THE ALL-MOVING TAILPLANE

The all-moving tailplane, or ‘pendulum’ elevator, is sometimes thought to have aerodynamic advantages over the orthodox hinged elevator and fixed tailplane. Its effectiveness or sensitivity is greater for each degree of deflection. This may be established by comparing die change in ci of a symmetrical profile deflected ten degrees with that of an elevator deflected the same amount, i. e. in Figure 13.1, by comparing the lift curve of the basic profile with the curve for 10 degrees elevator. Changing the angle of attack of the symmetrical section ten degrees takes the curve up to ci about 1.1, compared with a ten degree elevator effect of about 0.5 or 0.6. However, this same ten degree angle of deflection increases the profile drag of the ‘pendulum’ elevator by more than twice, since it moves out of the ‘low drag bucket’. The hinged flap changes the camber and so shifts the drag curve favourably, for small angles of deflection. Since the pendulum elevator is more effective, it can achieve the same result by moving through a small angle. If, for example, the symmetrical profile is shifted to an angle of attack about 5 degrees, it will be as effective as a hinged flap at 10 degrees. Even this small movement takes the symmetrical profile out of the low drag range, which is quite narrow on the thin aerofoils normally used for tailplanes. However, this increase of (hag lasts only while the control is effecting a change of attitude. Once settled down in a new flight trim, as discussed in Chapter 12, the tailplane load will depend on the centre of gravity position and the static margin. Usually the load will be downwards, on a stable model. Then the hinged elevator – tailplane combination, with elevator down, is cambered the wrong way. This may take the tail out of the low drag range, whereas an all-moving, symmetrical tailplane may remain within the ‘bucket’. Better still, perhaps, an all moving tail with negative camber should produce less profile drag, on average. Such effects are small for normal aircraft since tail deflections required for trim are not large.

By careful siting of the pivot point, the symmetrical pendulum elevator may be made to throw no loads at all on the servo. Since symmetrical profiles have no pitching moment about their quarter chord point, the pivot may be sited there and the servo then has only the function of overcoming friction forces and holding the elevator in position. Full-sized sailplanes which have pendulum elevators usually have swept back tails combined with a very slight camber in order to give the pilot some aerodynamic feel in the control column. Alternatively, counter-balance tabs may be fitted. For models these are entirely superfluous and should not be imitated, except of course for exact scale types. ‘Overbalancing’ the all moving elevator, by pivoting it aft of the aerodynamic centre, is a

somewhat risky matter, though it is sometimes done. The elevator may even be used to help drive coupled flaps, reducing the combined control loads. This requires special attention to pivot bearings and push rod stiffness.

A further consideration with the all-moving tail is the difficulty of preventing gaps and aerodynamic traps where the tailplane joins the fuselage or fin. If the elevator is pivoted on the side of the fuselage or fin a gap at the root is almost inevitable. This source of parasite drag is very diflictul to seal. If a‘T’ tail is used, the all-moving tailplane may be built in one piece but then the problem of mounting it neatly on the top of the fin arises. Very few installations are as tidy, from the aerodynamic point of view, as fixed tailplane and simple hinged elevator may be. It should go without saying that the elevator hinge line on an orthodox layout should be well designed to conform with the aerofoil, and sealed against leakages from bottom surface to top or vice versa.