what is form drag in aerodynamics?

Form Drag. This is the portion of the resistance which is due to the fact

that when a viscous fluid flows past a body, the pressure on the forward-facing

part is on average higher than that on the rearward-facing portion. The

extreme example of this type of resistance is a flat plate placed at right angles

to the wind. The resistance is very large and almost entirely due to the pressure

difference between the front and rear faces, the skin friction being negligible

in comparison Experiments show that not only is the pressure in front of the plate greater

than the atmospheric pressure, but that the pressure behind is less than that of

the atmosphere, causing a kind of ‘sucking’ effect on the plate.

It is essential that form drag should be reduced to a minimum in all those

parts of the aeroplane which are exposed to the air. This can be done by so

shaping them that the flow of air past them is as smooth as possible, and much

experimental work has been carried out with this in view. The results show the

enormous advantage to be gained by the streamlining of all exposed parts; in

fact, the figures obtained are so remarkable that they are difficult to believe

without a practical demonstration. At a conservative estimate it can be said that

a round tube has not much more than half the resistance of a flat plate, while

if the tube is converted into the best possible streamline shape the resistance will

be only one-tenth that of the round tube or one-twentieth that of the flat plate

The streamline shapes which have given the least resistance at subsonic

speeds have had a fineness ratio – i.e. a/b – of between 3 and 4

and the maximum value of b should be about one-third of the way back from

the nose. These dimensions, however, may vary considerably without

increasing the resistance to any great extent.