WING LOADING

The importance of weight relative to wing area is apparent from the above. The wing loading, often written W/S and expressed in kilogrammes per square metre (pounds or ounces per sq. ft.), is the easiest way of portraying this relationship. The weight of a model, neglecting small changes caused by fuel consumption, is constant during one flight. The speed at a given trim (angle of attack) will depend entirely on the wing loading. This may be shown by re-arranging the lift formula to bring L/S onto one side. (L *= W in level flight.) Dividing both sides of the equation by S gives:

W/S = L/S = YipV^Cb

For gliders, and descending power models, lift and weight are not quite equal (Lift = WCos a, see Fig. 1.4) but for normal angles of dive or climb less than ten degrees there is very little difference and the wing loading formula holds good. Adding weight increases forward speed, but requires more power to sustain flight (In a glider a more powerful upcurrent is then needed for soaring.)

2.3 WING CL AND SECTION q

The Cl of a whole model or whole wing should not be confused with the lift coefficient determined in a wind tunnel for an aerofoil section. The section lift coefficient is sometimes written ci, in lower case letters, or Cl, to distinguish it but this is not always done and confusion results. The Cl of a real wing or tailplane cannot as a rule be arrived at by a simple transfer of values from a tunnel test of ci. The various effects of cross flow and downwash on a real wing cause the section lift coefficient to vary from place to place across the span, even if the wing is nominally at the same geometric angle of attack to the line of flight. The Cl finally arrived at is approximately the average of all the local values.