Actually, what is the purpose of Wing's Fineness ratio? Is it thickness divided by chord?

Question

or chord divided by thickness? Some reference said length divided by width???

Answer 1

It is chord divided by thickness. And if you look at the cross section of the wing this is also the length divided by the width, hence the confusion. [The "other" length by width ratio (span/mean chord) is the "Aspect Ratio].

Fineness describes the overall shape of the streamlined body. Fat or skinny. Fat ones tend to have greater lift at slower speeds and correspondingly high drags. Skinny ones have high fineness ratios and tends to be more effeicient at high speeds. The fineness ratio helps you make the choice while designing your aircraft.

Answer 2

Lift. The greater the thickness compared to the cord length the larger the distance that the air over the upper surface of the wing must travel compared to that over the lower surface. Bernoulli's principal states that if you accelerate a fluid you reduce the pressure within that fluid. In order for the air over the upper surface to meet the air from the lower surface at the trailing edge it must travel at a higher speed because it has farther to travel. So the air pressure over the upper surface is reduced relative to the air under the wing creating lift. The thicker the wing, the greater the difference between the upper surface and the lower surface. Of course the penalty with a thicker wing is more drag so you need to decide how important the increased lift is. If you want to haul a lot and don't mind going slow, a thick wing is a good way to do it.

Answer 3

Finess Ratio