Is there any wingtip vortex (by induced drag) at low angle of attack?
Hi,
As I know, induced drag is an unavoidable product of lift and is directly related to the angle of attack. To produce lift, the static pressure on the upper surface of the wing is lower than that of surrounding atmosphere. The static pressure on the upper surface tends to move in towards the fuselage, creating a spanwise flow. The static pressure on the lower surface is higher than atmospheric, and this creates a spanwise movement of air towards the wingtip. The spanwise flows on upper and lower surfaces are on opposite directions, and vortices form when where the two flows meet on the trailing edge of the aerofoil. The vortices takes energy out of the airflow around the aerofil and so increases drag. Induced drag decrease as the speed increases because the angle of attack is reduced, but it sill exists (according to the drag curve).
However, at small angle of attack, about 4 degree, there are static-pressure reductions over both the upper and lower surfaces, with the lift force generated by the pressure differential. If both upper and lower surface static pressure are lower then atmospheric pressure, how could wingtip vortices / induced drag form as shown in the drag curve?
Thank you so much.
Regards,
Issac