Thickness reserve

green = thickness reserve  ; blue = max. Profile thickness; red = tread depth  ; Profile nose each left ${\displaystyle x_{\text{d}}}$ ${\displaystyle t}$

In aerodynamics, the thickness reserve is the distance between the wing nose and the point of maximum thickness of a profile . ${\displaystyle x_{\text{d}}}$

The relative thickness reserve is the ratio of the thickness reserve to the profile depth . It is often expressed as a percentage: ${\displaystyle x_{\text{d'}}}$ ${\displaystyle t}$

${\displaystyle x_{\text{d'}}={\frac {x_{\text{d}}}{t}}}$

The thickness reserve corresponds roughly to the location of the maximum flow velocity and the lowest static pressure . Further back, the static pressure increases again and the speed decreases ( Bernoulli equation ). With a correspondingly high pressure increase, the boundary layer can become detached with a change from laminar to turbulent flow .

The flow velocity on the profile influences the wall shear stress , which forms the shear stress resistance of the body in flow , which in turn, together with the pressure resistance, makes up the entire flow resistance of a body. The thickness reserve thus influences the running lengths of laminar and turbulent flow and significantly the drag coefficient of the wing profile. ${\displaystyle c_{\text{w}}}$

Usual wings have a thickness reserve of about 25-30%. In gliders are often found laminar profiles that have a large reserve of nearly 50%. These profiles generate less air resistance, but require a particularly smooth and clean surface.

literature

• Ernst Götsch: Aircraft technology. Motorbuchverlag, Stuttgart 2003, ISBN 3-613-02006-8