Question

Show how each of the following changes to a wing affects the lift curve. Explain the effects on the zero-lift AOA, lift-curve slope, and the maximum cl value. State a few high-lift device examples which cause this effect.

(a) Increasing camber

(b) Increasing chord length

(c) Boundary Layer Control Devices (Looking for slats and slots as the examples)

Answer 1

(a) by increasing camber

* if we increase the camber its zero lift angle will also increase.e

example ; consider a symmetrical airfoil its camber is zero so zerolift angle of attack is zero but for a positively camberd aerofoil have negative value of zerolift angle of attack is there.

* increase in camber will not affect lift curve slope because aspect ratio (AR) remains constant .lift curve slope of wing is a function of aspect ratio .aspect ratio is varying by varying wing span or wing area.

* increase in camber will incerase the cl max but it will reduce the stall angle of attack

(b) increase in chord length

* will not vary the zerolift angle of attack.it is independent of angle of attack

* increase in chord length will decrease the lift curve slope

reason : lift curve slope is directly proprtiional to aspect ratio

           A R = (wing span)^2/wing area

as the chord length increase the wing area will increse threfore the wing AR decreses so lift curve slope decreases

*chord length is independent of maximum cl

(C) Boundary Layer Control Devices

* leading edge slot is boundary layer control device it will not vary the zero angle of attack and lift curve slope

but some boundary layer control device like leading edge slats,trailing edge flaps (fowler flap) will vary the zero lift angle of attck by varying wing camber and also vary the lift curve slope by wing effective wing area.

all these boundary layer control devices re energises the boundary layer folws on upper side there by

delays the separation and improves the stall angle of attack.

* boundary layer control devices all ways increses the maximum cl