First of all, I would like to thank you for the answers.
At the moment I am a private pilot and self-studying for the CPL knowledge from a set of books. To be honest, I do have basic theory in aviation otherwise I won't be able to pass my theory exam and also the oral part of flight exam. I was an oversea student and I have now graduated and staying in my home country, therefore it is not a convenience way for me to contact an experienced flight instructor.
Even though I have gained much knowledge from my studies before, I love to clarify any questions I have in mind when studying. I do know induced drag is a by-product of lift, and wingtip voctices are formed because of the spanwise flow, but I want to know deeper to get a clear concept and even sometimes the books may giving wrong information, so I really appreicate everyone who helped or helping me here. The forum here is the best way for me to get assistance from professionals. While I was studying in Australia, I ran to my flying academy everytime I got a question even I don't have a car.
Anyway, let's get back to my question. What writing from the book is, the wingtip vortices are generated because the upper surface of airfoil has a relatively lower pressure than atmospheric pressure, so it moves towards the fuselage. On the other hand, the air at the lower surface has a relatively higher pressure than the atmosphere, therefore they move towards the wingtip. Because these two airflow are moving in opposite direction and meet up at the trailing edge, vortices are formed.
While a normal cambered airfoil at a small angle of attack, such as 4 degree, both surfaces have a relatively lower pressure than the atmospheric pressure i.e. both airflow would tend to move towards the fuselage, since they are moving in the same direction, how could wingtip vortices form?
Lift is produced by the pressure difference between upper and lower surface, so induced drag is also generated. However in this case, wingtip vortices do not seem to be exist.
So, my two questions are:
1) Are wingtip vortices exist in the situation mentioned above i.e. 4 degree AoA.
2) If vortices were not formed, where was the induced drag coming from?
Thanks again, and I really do appreciate from the bottom of my heart.
Issac.