I do not believe the Earth is flat as I have seen it curve during flight. I also have evidence of round earth due to HF having increased range as compared to UHV or VHF. I don't mean to be offensive, but am wondering why you guys believe what you do. 
May I ask you, how high have you flown? Typical airliners fly up to roughly 40k feet, correct?
I am a RE er and I can say that at that height, you cannot see the earth curve. You can see a round horizon, but that is really just a perspective effect, not a spherical arc.