In general, there are four aerodynamic forces that act on the paper aircraft while it is in flight:
• Thrust, which keeps the plane moving forward;
• Aerodynamic lift, acting on horizontal surface areas that lifts the plane upward;
• Gravity, which counteracts lift and pulls the plane downward; and
• Air drag, which counteracts thrust and reduces the plane's forward speed.
Altogether, the aerodynamic forces co-interact, creating turbulence that amplifies small changes in the surface of the paper aircraft. Modifications can be made to most paper airplanes by bending, curving or making small cuts in the trailing edges of wings and in the airplane's tail, if it has one.
Main article: Roll, pitch, and yaw The most common adjustments, modelled after glider aircraft, are ailerons, elevators, and rudders. http://howthingsfly.si.edu/flight-dynamics/roll-pitch-and-yaw http://www.av8n.com/how/htm/4forces.html Critical Re The Reynolds number range of the paper model aircraft is reasonably wide:
• 2,000–12,000 for Origami aircraft
• 4,000–16,900 for Compound Origami (involving adhesives and aerodynamic refinements)
• 9,000–39,000 for Profile Performance (White Wings, Paper Pilot, et al.)
• 19,200–56,000 for Scale Performance (White Wings, Paper Pilot, et al.)
• 22,000–93,000 for Scale Models (complex structures)
These ranges are indicative. As noted above the mass: density ratio of paper prevents performance from reaching those of Balsa models in terms of expressions of power to weight, but for models with wingspans of between 250 mm and 1,200 mm, the Critical Re is very similar to balsa model gliders of similar dimensions.