# What are Uses of thin airfoil and thick airfoil?

I have heard that thin airfoils are used for fast fyling and thick airfoil a are used for more lift. Is it true? Can you explain specific uses of thick and thin airfoils ?

## 2 Answers

Thick, cambered airfoils such as the Gottingen, Clarke Y and Davis are the most efficient airfoils up to speeds where Mach number becomes critical.

Lift is determined by the lift formula:

$$Lift$$ = 1/2 × Coefficient of Lift × Air density × Wing Area × Velocity$$^2$$

Coefficient of Lift can be further broken down into Angle of Attack and Airfoil type.

So a smart designer picks the most efficient airfoil and the least amount of area required to lift a given weight.

Now time to choose. By virtue of Bernoulli, the highest lift to drag ratios are achieved by accelerating air over the top of the wing. This is where thick wings shine, much to the amazement of aircraft designers 100 years ago. Thick wings also have gentle stalling characteristics, helping make the Fokker D.VII a real hit in 1918. The thickness also enabled internal cantilever construction instead of bracing wires, paving the way for higher speeds until...

Higher airflow velocity above the wing approached trans sonic speeds, which lead to an increase in drag. This forced designers to give up the top lift and use bottom lift, where the airflow is slower. That lead to the "supercritical" wing for high subsonic cruise. Super critical wings feature a very "flat" upper surface. Note airliners drasticly increase camber for slower phases of flight with retractable slats and flaps.

Supersonic aircraft wings are thin to avoid drag, but can create lift with a swept leading edge, much like a paper air plane. This type of lift is called vortex lift. As seen from the formula above, Velocity is squared. This means that at high speeds, the wing need not be as efficient to generate the desired lift, as seen in designs such as the Concorde.

Of course, with very little weight, like a paper airplane, it works too.

This is true for most cases, because thin airfoils normally produce less drag and less lift than thick ones at the same angle of attack. But they are also exceptions.

Thick airfoils, to be specific Supercritical airfoil, are commonly used by commercial airliners, but not only because they could produce less drag. Airlinears fly at speed which shock wave could emerge at the upper surface of the wing, which produces additional drag. The solution to this is to make the upper surface of the airfoil flat, but at the same time lift was reduced. In order to produce enough lift, the lower surface of the airfoil have to be complex, which guarantees a thick airfoil. These airfoils have high lift-drag ratio, which is crucial to airliners.

Thin airfoils are commonly used by supersonic aircrafts. Experiment shows that airfoli which of 6 % relative thichness produces almost twice drag than a airfoil of 4% relative thickness. At the same time, thickness have negligible effect on lift for thin airfoils in supersonic speeds.