500 watts is only 2/3 HP which is less than a sport 40 size engine. There is no way you will drive a 12x12 or 13x10 prop with that. You will have a hard time with a 9x7.
And before you repeat that 300-500 watts was your estimate remember that at 15 Amps you will need 33 volts (22 cells) to get 500 Watts. As has been said you instructor really needs to talk to someone in RC familar with electric planes or helicopters.