There are 4 main forces that act on a paper airplane (or a real airplane for that matter) while it is flying.
These are the forces:
- Lift
- Gravity
- Thrust
- Drag
Lift is the force that keeps the airplane in the air. Without lift the plane would not fly. Lift can be a very complicated force to explain, but here are two basic models to give an intuitive understanding.
1. Bernoulli's Principle (named after Swiss Physicist Daniel Bernoulli)
If you ever look closely at the wings of an airplane from the side, you will notice that they are not flat. The wing has a curved shape to it. This shape is called an airfoil. Airfoils are specially designed to produce lift.
To understand how Bernoulli's principle causes lift,
- we must first understand that air usually presses equally on all sides of an object. Suppose that as the plane flies forward, the approaching air splits up when it hits the leading (front) edge of the wing and rejoins at the trailing (back) edge of the wing. The airfoil shape causes the air to go farther over the top of the wing than under the bottom, both in the same amount if time. This means the air on top of the wing must move faster.
- When air speeds up, its pressure gets lower. Since the air pressure on top of the wing is lower than the air pressure on the bottom of the wing, the wing produces lift! This phenomenon is called Bernoulli's principle.
2. Newtonian Explanation
The famous scientist Sir Isaac Newton stated in his famous third law that ,"For every action, there is an equal and opposite reaction."
- Newtonian lift largely depends on the tilt of the wing or "angle of attack".
- If the leading edge of the wing is pointing upward, the bottom surface is deflecting oncoming air downward.
- When this air bounces off the bottom surface of the wing (action), it pushes the wing upward (reaction)...or produces lift.
Gravity is a force that we are all familiar with. It's what causes any object you throw into the air to come back to the ground. Gravity is also what keeps us on the ground. Without gravity, we would all float away into space! With airplanes, gravity works against lift by pulling the airplane toward the ground.
Thrust is the force that causes the plane to move forward through the air.
- In a real airplane, this is produced by the turning propellers or jet engine.
- With a paper airplane, the thrust is produced when you throw the plane into the air. Without thrust, planes could not produce lift.
Drag is the force that tries to slow the airplane down.
- Drag is produced when air flowing over the plane causes friction. When the plane is flying, it must push oncoming air out of the way. As this air is pushed around the plane, it bumps into other air molecules. Air close to the surface of the airplane also wants to try to stick to it. All of this causes friction. Have you ever ridden your bike on a windy day? The wind hitting you in the face that makes it hard to keep moving is drag.
Gravity and Drag work against it.
We can't do anything to change gravity, but we can try to minimize drag and increase lift and thrust. This will make a paper airplane fly well.
REVIEW:
[as you review these notes make sure you understand the vocabulary terms used.]
THRUST:
- Thrust is the force which moves an
aircraft through the air.
Thrust is used to overcome the drag of an airplane, and to overcome the weight of a rocket.
Thrust is generated by the engines of the aircraft through some kind of propulsion system.
Thrust is a mechanical force, so the propulsion system must be in physical contact with a working fluid to produce thrust. Since thrust is a force, it is a vector quantity having both a magnitude and a direction.
Remember:
A vector quantity is a quantity that is fully described by both magnitude and direction
Mechanical force is an energy that requires a medium for it to travel. When this force is applied on an object, it can cause it to bend, scratch it, or break the object. The force that opposes mechanical energy is called friction energy.
Drag is the aerodynamic force that opposes an aircraft's motion through the air.
Drag is generated by every part of the airplane (even the engines!). How is drag generated?
Drag is a mechanical force.
It is generated by the interaction and contact of a solid body with a fluid (liquid or gas). It is not generated by a force field, in the sense of a gravitational field or an electromagnetic field, where one object can affect another object without being in physical contact.
For drag to be generated, the solid body must be in contact with the fluid. If there is no fluid, there is no drag.
Drag is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid. If there is no motion, there is no drag. It makes no difference whether the object moves through a static fluid or whether the fluid moves past a static solid object.
Drag is a force and is therefore a vector quantity having both a magnitude and a direction. Drag acts in a direction that is opposite to the motion of the aircraft.
Lift is the force that directly opposes the weight of an airplane and holds the airplane in the air.
Lift is generated by every part of the airplane, but most of the lift on a normal airliner is generated by the wings.
Lift is a mechanical aerodynamic force produced by the motion of the airplane through the air.
Because lift is a force, it is a vector quantity, having both a magnitude and a direction associated with it.
Lift acts through the center of pressure of the object and is directed perpendicular to the flow direction. There are several factors which affect the magnitude of lift.
NO FLUID, NO LIFT
Lift is a mechanical force. It is generated by the interaction and contact of a solid body with a fluid (liquid or gas). It is not generated by a force field, in the sense of a gravitational field, or an electromagnetic field, where one object can affect another object without being in physical contact. For lift to be generated, the solid body must be in contact with the fluid: no fluid, no lift. The Space Shuttle does not stay in space because of lift from its wings but because of orbital mechanics related to its speed. Space is nearly a vacuum. Without air, there is no lift generated by the wings.
WEIGHT:
Weight is the force generated by the gravitational attraction of the earth on the airplane.
We are more familiar with weight than with the other forces acting on an airplane, because each of us have our own weight which we can measure every morning on the bathroom scale.
We know when one thing is heavy and when another thing is light. But weight, the gravitational force, is fundamentally different from the aerodynamic forces, lift and drag.
Aerodynamic forces are mechanical forces and the airplane has to be in physical contact with the the air which generates the force.
The gravitational force is a field force; the source of the force does not have to be in physical contact with the object to generate a pull on the object.
Weight is a force, and a force is a vector quantity having both a magnitude and a direction associated with it.
- Example: an airplane, weight is always directed towards the center of the earth. The magnitude of this force depends on the mass of all of the parts of the airplane itself, plus the amount of fuel, plus any payload on board (people, baggage, freight, ...).
Flying involves two major problems:
- overcoming the weight of an object by some opposing force
- controlling the object in flight.
Beginner Planes: [click on link]
You will make the following three (3) paper planes. Watch the short video clip on each type of plane - [click on highlighted link];
watch carefully how to fold each plane; then use the template given to you in class to carefully & precisely fold your planes.
I will not provide instruction on folding your planes. This is part of your experiment to carefully read, watch & follow instructions
*** You must work independently. You will be scored on this skill.
Do not work as a team on this project.
Read all information in this post & experiment paperwork for detail.
Highlight important points for bringing forward in class discussion.
You may build one of the INTERMEDIATE PLANES for extra points.
SAFETY:
NEVER throw a paper airplane at another person, animal, or object that could be damaged if you hit it. Paper planes can have sharp edges and points that can injure someone if you are not careful. Keep in mind that paper planes can curve or change direction after they are launched, so make sure your flying area is clear. When flying outdoors, never fly your plane near moving cars or run into the street after your plane.Folding Technique - Folding technique is very important for successful flights. Make each of the folds carefully and accurately according to the instructions. Creases should be made by applying pressure to the fold with the edge of your thumbnail. This is best achieved by holding your thumbnail on the fold, applying pressure, and pulling your thumb along the fold line toward you. This will produce clean, crisp folds that will allow for accurate paper planes. If you make a mistake on a fold that you cannot correct, don’t be discouraged! Just print another template.
Line Types - There are two main types of lines referenced by the instructions: fold lines and cut lines. Fold lines are dashed and cut lines are dotted.
Model Adjustments - No matter what anyone tells you, EVERY paper airplane needs fine-tuning to achieve its best performance. There are several things you should keep in mind while making adjustments to your planes.
Dihedral - Dihedral is a slight upward tilt of the wing tips with respect to the fuselage or body of the airplane. This produces a slight V-shape to the wings when viewed from the front of the plane. Dihedral provides aerodynamic stability to your models by making them want to self-center during flight. Paper airplanes have no intelligent flight controls after they leave your hand, so the plane needs to be naturally stable or else it will crash. All designs on this site perform better when some dihedral is added to the wings.
Elevator - Elevator is the aeronautical term for the hinged flap at the tail section of a plane that causes it to either climb (gain altitude) or dive (lose altitude). In paper airplanes these flaps are generally located on the trailing edge of the wings themselves, since there is rarely a separate tail. They are formed by making parallel cuts about 1 inch apart. This produces a small flap that can be folded slightly up or down. Tilting the elevator flaps up will cause the plane to climb. Tilting them down will make the plane want to dive. If you find that your models are heading nose-down toward the ground shortly after launch, you may need to add some up elevator. Likewise, if they are looping-up too quickly or stalling, you may need to add some down elevator. Adding slightly more elevator to one wing than the other will cause the plane to either turn to the right or left.
- All planes folded and in class ready for flying DUE: 12/09
ELEPHANT & FEATHER WITHOUT AIR RESISTANCE
But
if the elephant weighs more and experiences a greater downwards pull of
gravity compared to the feather, why then does it hit the ground at the
same time as the feather? 
