I agree with Hal as far as he goes. He is describing an airplane designed for most efficient cruise flight and full scale aircraft usually have the incidence angle set to the maximum L/D for that reason.

However if the airplane being designed is a free flight model where maximum endurance is the goal, the best L/D incidence will result in the fuselage being aligned with the direction of flight when gliding at the flattest angle, but being a little nose high at the minimum sink rate speed. A little more incidence than best L/D will more nearly align the fuselage with the direction of flight in that condition, and give less overall drag and slightly more endurance.

Aircraft designed for aerobatic flight where inverted flight is a consideration require a different approach. Here the airfoil is symmetrical about the chord line and both wing and horizontal stabilizer are set at zero incidence. With this setup, the aircraft will fly basically the same upright and inverted. Of course, in level flight, the attitude will be nose up in both cases, but it will be the same.

Sport models that are used for general all around flying with some limited aerobatics often use a slightly cambered airfoil (sometimes erroneously referred to as “semi-symmetrical”). These are set at somewhat less than max L/D in order to minimize the nose high attitude in inverted flight.

In most R/C flying neither endurance, range, nor fuel efficiency are major design considerations. Rather incidence is usually set to provide attitudes appropriate to the maneuvers desired.