Two things: first, friction between the outside of the plane and the air it's passing through. Second, turbulence in the air will drag on the plane.
There are two forces that causes an airplane to be airborne. They are Thrust and Lift. The other two forces resists the airplane's flight. They are Gravity and Drag.
winglet
An airplane accelerates due to the thrust generated by its engines. As the engines produce forward thrust, the aircraft gains speed. The thrust must overcome drag forces acting on the airplane to achieve acceleration.
Drag is a force that acts opposite to the direction of movement of an airplane. Drag is mainly caused by air. The other forces that act on the plane while moving in the air are thrust, lift etc..
The drag of an airplane is the air resistance caused by the plane flying through air. Similar to when you pull your hand through water and feel resistance.
Flaps would increase drag increasing your speed will increase it even more
drag
the force created by air resistance
Drag effects paper airplane just as it affects anything else that moves. It is either parasitic or induced on paper airplanes. Drag may reduce a paper airplanes speed and/or range.
The force of an airplane that opposes thrust is called drag. Drag is the resistance force that acts opposite to the direction of motion of the aircraft and is caused by the interaction of the aircraft with the surrounding air.
Because non-smooth surfaces cause drag, which slows down the airplane, causes excess noise, and increases fuel consumption.
Lift is an aerodynamic force that results from the air passing over the curvature of the wing. It causes a reaction. A high pressure on the bottom of the wing and a lower pressure on top of the wing. As a result, high pressure wants to go to low pressure and so the wind is lifted into the air. Drag is the force opposite to thrust. It is created by the airplane as it moves through the air. The faster the airplane, the more the drag.