HOW LARGE A STATIC MARGIN?

Depending on the methods used to calculate the position of the neutral point and the various allowances made for tailplane efficiency, fuselage effects, etc, estimates of the. size of the static margin required for adequate, but not excessive, stability vary a good deal. However, when the static margin is worked out, it is usually expressed as a decimal fraction of the mean wing chord. It is then usually found that a satisfactory s. m. comes out less than 0.2 mean chords, for radio controlled power models. Much greater figures than this suggest too much stability for satisfactory control response and much less begins to approach ‘twitchiness’ which may nevertheless suit an aerobatic model. For most free flight models, higher margins are required as a rule. If only wing and tail areas and moments are taken into account, the static margin should be on the generous side to allow for the other factors. As already noted, the degree of stability any particular pilot requires depends to a large extent on the kind of model and flying done, and the centre of gravity position can be adjusted to suit personal preference.

12.15 SPECIAL PROBLEMS. DEEP STALLING

The T-tail configuration has several aerodynamic advantages in normal flight attitudes, but it may in some circumstances lead to a trouble known as deep stalling. In jet airliners, an example is the ВАС 111 prototype which crashed, killing all on board, in 1963. The aircraft lost most of its forward speed and descended in a flat attitude. Model aircraft with ‘tip up tail’ dethermalisers are placed in this deep stalled condition deliberately to bring them down, but some T-tailed models may do the same thing when the modeller does not intend it, and the controls may be incapable of returning the model to normal flight The cause is quite complex. As the main wing approaches the stall, the wake becomes broader and at the same time the tailplane, because the nose of the model is rising, comes down into the wake and loses efficiency. If the centre of gravity is rather too far back, this also contributes to the undesirable nose-up pitch. When the main wing is stalled, the wake tends to strike the whole tailplane, whereas a low mounted tail will be out of the wake and will be more efficient than usual. Once in the deep stalled condition, the model may be unable to get out of it because the airflow over the ftiselage, or engine nacelles mounted at the rear, causes the formation of strong rotating vortices similar to those at the tips of a lifting wing. Given a bad combination of circumstances, the downwash caused by these vortices may strike the high tail and keep it down in spite of the pilot’s efforts to restore forward flying speed. The problem is baffling unless the modeller understands the cause, since an aircraft that flies perfectly well most of the time may without warning fall out of the sky and pancake, with fuselage more or less horizontal and hardly any forward velocity. After repairs, the same model may fly satisfactorily again for some time without ‘deep stalling’. On the other hand, in gusty weather or in aerobatics, the trouble may strike at any moment (the ВАС 111 prototype that crashed had completed many hours of successful test flying before the accident) The cure may be simply to return to an orthodox low tailplane configuration, but the tailplane will then probably need to be enlarged to cope with normal flight stability requirements. Other possible modifications that might be effective include increasing tailplane span with or without an increase of area, or adding dihedral to the tailplane, both with the object of getting some of the tail area out of the downwash from the fuselage vortices. Carrying the tail still higher would have the same effect but might be impossible for structural reasons. Moving the centre of gravity forward and re-trimming may also help and will in any case improve stability in normal flight, so reducing the danger of stalling in the first place. The fuselage may be modified in an effort to reduce the strength of the downwash. A broad fuselage is more likely to give trouble than a slender one, and engine pods or nacelles have a bad effect in some positions, especially just ahead of the tail unit Either lengthening or shortening of the fuselage may change the relationship of tailplane to vortices enough to solve the problem. Once the model is deep stalled, none of the controls except possibly wing flaps have much effect. The elevator tends to be useless and may even be forced upwards against the stops. The ailerons on a ‘super stalled’ wing are totally ineffective, and the redder is not powerful enough to roll the model out of its horizontal position. It might be possible to yaw the model and the fuselage vortices might then clear one side of the tailplane. The application of engine power is usually not enough to restore the situation.

Wing flaps, however, may give a sufficiently powerful nose-down pitching moment to overcome the tail downwash effect On the other hand, air brakes or spoilers may create more vortices or a more turbulent wing wake and make things worse. A model fitted with a tail parachute can be saved from the deep stall; the ’chute when deployed slows the model still more, the whole thing then hangs nose-down from the supporting parachute, and after a few seconds normal flight may be resumed with the parachute jettisoned.