Wing loading

The fraction W/S, weight divided by wing area, is called the wing loading of the aeroplane. An increase in the wing area should reduce the value of W/S, and so also reduce the minimum velocity at which level flight is possible.

The objections to variable area are chiefly mechanical; the operating gear means extra weight and so, since W will increase, it by no means follows that W/S will actually decrease if the wing area is increased; also, once again, the extra sophisti­cation means more complication, more levers for the pilot to fiddle with, more chances of something going wrong. Some flap systems do, however, give an increase in wing area during landing as they extend beyond the trailing edge of the wing.

Apart from the question of altering the wing area during flight, the equa­tion W/S = CL. 2pV2 shows us that, other things being equal, the aeroplane with a low wing loading will have a lower minimum speed than one with a high wing loading. The so-called ‘light aeroplane’ may have a high wing loading, and therefore a high landing speed; in other words, it is not a ques­tion of weight, but of weight compared with wing area, that settles the minimum speed. The wing loading of a sailplane may be less than 100 N/m2, of a training aeroplane 300 to 1000 N/m2, of a fighter, bomber or airliner any­thing from 1500 up to 2000, 3000 or more N/m2. The modern tendency is to increase wing loading by reducing wing area and thus raising the maximum speed, and then using flaps to keep down the landing speed. The student is advised to work out the wing loading of existing aeroplanes and to compare the figures obtained with their landing speeds; in making this comparison, however, the student must be careful to notice the above phrase ‘other things being equal’, because the maximum lift coefficient of the aerofoil used also affects the result. An old example of high wing loading, very high for that time, was the 1977 N/m2 of the S.6b Schneider Trophy racing seaplane; the corresponding figure for fighters like the ‘Spitfire’ and ‘Hurricane’ at the begin­ning of the Second World War was 1187 N/m2, and for the German fighter, Messerschmitt 109, 1522 N/m2. Modern figures may be considerably higher than these; the Concorde was about 4800 N/m2.