Lift coefficient

Physical key figure
Surname	Lift coefficient, lift coefficient
Formula symbol	${\displaystyle c_{\mathrm {a} }}$
dimension	dimensionless
definition	${\displaystyle c_{\mathrm {a} }={\frac {F_{\mathrm {a} }}{q\,A}}.}$
${\displaystyle F_{\mathrm {a} }}$ Buoyancy ${\displaystyle q}$ Back pressure ${\displaystyle A}$ Reference area
scope of application	Dynamic lift

The lift coefficient or lift coefficient is a dimensionless coefficient for the dynamic lift of a body around which a fluid flows. It is an important parameter in the characterization of profiles in fluid mechanics . In the case of cars, the lift coefficient is one of six coefficients. B. be determined in the wind tunnel. In formulas, the abbreviation is usually chosen for the lift coefficient in German-speaking countries . In English texts it is often ( l for lift ) or . ${\displaystyle c_{\mathrm {a} }}$ ${\displaystyle c_{\mathrm {l} }}$ ${\displaystyle c_{\mathrm {z} }}$

The lift coefficient is a special form of the transverse drive coefficient or . A lift coefficient can be determined experimentally for all elongated bodies with all cross-sections against which fluids flow. ${\displaystyle c_{\mathrm {F} }}$ ${\displaystyle c_{\mathrm {q} }}$

Buoyancy coefficients are shown graphically depending on the angle of attack, for example to assess the transverse waves from icy overhead lines ( cable gallop ) or bridge routes (English: Galloping ; example: " Galloping Gertie "). ${\displaystyle \beta }$

The lift coefficient results from the lift force , normalized to the dynamic pressure and the area of the reference surface; The wing surface is selected as the reference surface for profiles and the front surface for vehicles : ${\displaystyle F_{\mathrm {a} }}$ ${\displaystyle q}$ ${\displaystyle A}$

${\displaystyle c_{\mathrm {a} }={\frac {F_{\mathrm {a} }}{q\,A}}.}$

The lift coefficient is like other aerodynamic coefficients, e.g. B. the drag coefficient , depending on the orientation of the body in the flow, expressed by the angle of attack. The relationship between the lift and drag coefficient as a function of the angle of attack is given by the polar diagram , which differs significantly for different profile shapes.

Reduction in the finite length wing

The information in a profile polar can be transferred directly to an infinitely long wing with this profile. For a finite wing, however, the influence of the wing tip must also be taken into account. Because at the extreme end of a wing, cross-currents reduce the pressure difference between the top and bottom further inside the wing, which means a smaller dynamic lift. The cross flow also causes the tip vortex .

The lift coefficient of a real wing is therefore smaller than indicated in the polar. The longer the wing relative to its depth (i.e., the greater its aspect ratio ), the closer the wing will come to the coefficient of an infinitely long wing. The lift coefficient of a finite wing with the aspect ratio can be calculated approximately from the lift coefficient of an infinitely long wing as follows : ${\displaystyle c_{\mathrm {a} ,{\text{endlich}}}}$ ${\displaystyle \Lambda }$ ${\displaystyle c_{\mathrm {a} ,{\text{unendlich}}}}$

${\displaystyle c_{\mathrm {a} ,{\text{endlich}}}={\frac {\Lambda }{\Lambda +2}}\,c_{\mathrm {a} ,{\text{unendlich}}}}$

Web links

• FMSG Alling: EWD, longitudinal stability and all that ( Memento from October 28, 2009 in the Internet Archive ); Basics part 1
• FMSG Alling: EWD, longitudinal stability and all that - Part 1, section "Replacement image for a wing", formula 1a

Individual evidence

1. ↑ Peter Kurzweil: The Vieweg unit lexicon: terms, formulas and constants from natural sciences, technology and medicine . 2nd ext. u. act. Edition. Springer, Braunschweig 2000, ISBN 978-3-322-83212-2 , doi : 10.1007 / 978-3-322-83211-5 .
2. ↑ ^{a b c} Robert Gasch, Klaus Knothe: Diskrete Systeme (=  structural dynamics . No. 1 ). 2nd Edition. Springer, Berlin / Heidelberg 2012, ISBN 978-3-540-88976-2 , pp. 13-16 , doi : 10.1007 / 978-3-540-88977-9 ( google.de [accessed December 20, 2018] limited preview).
3. ↑ Florian Ettlinger: Sailing with the advertising pillar - The ideal rotation speed for the Flettner rotor . (PDF) In: Young Science . No. 104, 2015, pp. 16–23.