AEROBATIC MODELS

It is undesirable to camber the wing, or tailplane, of any ‘pure’ aerobatic model. For inverted flight, it is important that control response and model behaviour in all respects are the same as when flying normally. A symmetrical profile is necessary. Such models may, indeed, be fully symmetrical about the thrust line, except for the undercarriage. (Indeed, with an undercarriage on both sides, inverted ‘touch and go‘ landings would be possible.) Aerobatic sailplanes usually require some camber to permit soaring when conditions are weak. Camber flaps, carefully designed and acting also as ailerons, should be employed, to allow camber to be adjusted to suit conditions, and or inverted soaring.

7.11 CAMBER AND CENTRE OF PRESSURE

There are two equally valid ways of describing the forces which are generated by a wing in flight. The older and more traditional method dates back to the time of sailing ships and was employed by the first scientific research workers who used wind tunnels to investigate the behaviour of wing profiles. When a test wing was mounted in the wind tunnel, the lift was measured as a force at right angles to the airflow and the drag as a component of force directly downstream. There was also an additional force tending to twist the wing round to a different angle of attack from that chosen by the technician. This force, tending either to pitch the wing to a higher angle or to a lower angle of attack, was measured as a pitching moment but its direction and strength seemed to vary from one wind tunnel to another. It was soon realised that the point at which the test wing was suspended, whether at the leading edge, or at the mid chord point, or somewhere else, was responsible for these apparent variations. It was as if the point of action of the lift force moved back and forth relative to the wing chord and when looked at in this way, it became possible to plot the position of this apparent point, in terms of percentages of the chord from the leading edge.

Now termed the centre of pressure, as the sailing masters had termed it, all wind tunnel engineers reported similar results in terms of centre of pressure movements. As the angle of attack was reduced, the c. p. seemed to move aft, and as the angle was increased, it moved forward. It never came further forward, however, than about 25% of the chord. At the stall, as the flow separated, the centre of pressure moved rapidly towards the 50% :hord position. Symmetrical wing profiles did not fit into this pattern very well, since they seemed to have centres of pressure practically fixed at one point at all angles below the stall. There was also a difficulty when the wing section was turned to its angle of zero lift [f movement of the lift action point was causing the pitching moment when zero lift was produced, the pitching moment ought also to be zero. This was not so. At zero lift all cambered wing profiles have a marked nose down pitching moment.

• It is important to remember that the centre of pressure movement was always a result jf calculation, using the basic information from the tunnel apparatus, which gave three listinct forces: lift, drag and pitching moment measured at one point on the wing. The :entre of pressure was an abstract, theoretical point, for there was no way the measuring apparatus could be moved back and forward in the tunnel to track its supposed movement. Arithmetically dividing the measured lift force by the pitching force produced a length for the supposed moment

At moderately low angles of attack, corresponding to a fast aeroplane flying at Maximum airspeed, calculation and plotting of the centre of pressure showed it had moved far to the rear, so far that it was no longer within the wing chord at all but must be xmsidered as lying somewhere beyond the trailing edge. The idea that the lift generated by the wing was taking effect somewhere behind the surface causing it created difficulties For the imagination (See Figure 10.5). The lift, after all, supports the aircraft and to suppose it to have its effect somewhere behind the main supporting component was strange. The calculations produced the extraordinary conclusion that the centre of pressure at zero lift (i. e. corresponding to an aeroplane in a vertical dive), must lie an infinite distance behind the wing.

Providing it is remembered that the centre of pressure is an abstraction, this rather old method of describing the wing forces remains quite valid and some modellers still use it Nonetheless, it can cause confusion because it is often quite wrongly assumed that the :entre of pressure cannot move beyond the trailing edge, or that it stops somewhere before reaching the trailing edge. This impression is reinforced by the older textbooks of aircraft Engineering, which describe methods of calculating the loads on a wing for two conditions: centre of pressure forward’ and ‘centre of pressure back’. In these respectable ancient texts, ‘centre of pressure back’ corresponded to the loads expected when the aircraft was flying at its normal maximum permitted airspeed and the aerodynamic fact that the c. p. would move further aft if, for instance the aircraft was in a steep dive, was not always mentioned.