VARIABLE CAMBER

Models which are required to perform efficiently over a wide range of airspeeds present great difficulties. This applies particularly to cross-country and multi-task radio controlled sailplanes. When soaring, they must be trimmed for a high Cl and for low drag should have a strongly cambered wing. At speed, a low-camber or even a symmetrical profile is required. As noted previously, a high aspect ratio is the chief means of achieving a low sinking speed, but profile drag is not negligible. At speed, profile drag is most important. No one value of camber can be ideal for all flight conditions. If a simple wing is used, the camber should be on the low side, for high speed, relying on high a. r. for the soaring flight The aerofoils used should be chosen to give low drag coefficients over wide range of angles of attack. Preferably a laminar flow aerofoil with a wide low-drag-range or ‘bucket’ should be used (Chapter 9). Even better, such a profile combined with a variable camber wing allows the drag to be reduced in all conditions. Plain flaps widen the speed range as shown in Fig. 7.9. Most modem full-sized sailplanes combine flaps with wide – drag-range aerofoils. In thermals or hill lift, the flaps are depressed, shifting the drag curve to the right and the lift curve left Between upcurrents, to achieve good penetration, the flaps are raised, usually beyond the neutral position, to shift the drag and lift curves in the opposite directions. The pilot constantly adjusts the flaps as the airspeed is changed. With a well-balanced design, the attitude of the fuselage to the airflow hardly changes, in fact one well-known high performance sailplane could be trimmed by means of a spirit level – at any speed with appropriate flap setting, the attitude remained exactly the same. The advantage of this was that the fuselage presented the same aspect to the airflow at all speeds, and thus parasite drag was a minimum for the particular shape used. (See also 11.4). With less refined design, the fuselage changes its angle of attack somewhat, and produces more drag, with different flap settings.

For powered duration models the advantage of variable camber is also clear. The low 3l required for the high speed climb is achievable by trimming the tailplane, to hold the ring at a low angle of attack, but with a high camber, this produces much too much profile lrag. It is better to reduce the wing camber for the climb, cutting profile drag at speed. Піеп for the glide, the low drag and high Cl required may be achieved by increasing :amber, lowering wing flaps after the motor cuts. Re-trimming the tail will also probably ye necessary.