The Aerodynamics of Gliders

Transcription

1 The Aerodynamics of Gliders Your goal is to create an airplane that will sink at the slowest possible rate when launched from altitude. The lighter the plane is, the easier it will be to maximize the time aloft. The rules regulate the minimum weight, so keeping it as close to the minimum as possible will be advantageous. Gravity is our source of energy. Converting gravity to thrust will allow our airplane to fly. How this energy is managed will determine your time aloft. Aerodynamics will be used to help resist the force of gravity from pulling the plane down too quickly. For an airplane to create lift (and counter gravity), it must move forward (thrust). Air fills all of the space around airplane just as it does around us. As the plane moves forward, the wing will divide the sea of air, some will go over it and some will go under it. If the plane causes the air molecules to move out of their natural position, it will take some of the energy (called drag, or the resistance to thrust). So the goal is to move the least amount of air necessary to achieve the aerodynamic benefit of creating lift. Lift is nothing more than creating a lower air pressure on one side of an object than the other. To create an upward lift (to combat the gravity pulling downward), we must cause the air molecules to slightly move out of their natural position. If we can get some of these molecules to spread apart from each other, this will decrease the air pressure in that area. If we can get the air molecules to push together a little closer, this will increase the air pressure in that area. The wing is our devise that is used to accomplish this. As the wing moves forward, it divides the air. If the wing is tilted so that the front edge is higher than the rear edge, the air going under it will be pushed down as it slides along the under side of the wing. This air is packed together a little more tightly with the other air molecules beneath them. This creates a slightly higher air pressure (under the wing). The air above the wing will have to be pulled downward to fill what otherwise would be a void in the path of this tilted wing moving forward. As the air rushes downward to fill this void, it must slightly spread apart. Air molecules that are spread apart have a lower

2 air pressure than molecules that are packed together. So the difference in the air pressure around the wing (higher below and lower above) is lift. The act of moving these air molecules takes energy. So making lift is not free. The payment is in the form of drag, which is the opposing force to thrust (the forward movement). So our goal is to make only as much lift as necessary to resist being pulled down too quickly. It is advantageous to go only as fast as necessary to create enough lift to help resist the pull of gravity. This will minimize the number of air molecules that are disturbed, which will reserve the energy available for forward movement (thrust). The speed is affected by changing the balance (or Center of Gravity) to make the airplane tilt its nose downward causing it to fly faster or upward to fly slower. This can be accomplished by the position of the clay on the airplane. The optimum setup will be to add clay to bring the plane up to the legal minimum weight of 2 grams. Hopefully when all of this clay is on the very front of the nose, it will have the Center of Gravity (balance point) too far forw ard causing the plane s nose to drop and fly too fast. T his w ill allow you to pinch off a small portion of that clay and move it back on the fuselage stick to adjust the Center of Gravity to where the plane flys best. Repositioning this small piece of clay w ill allow very delicate adjustm ents in the plane s balance to let you discover its best speed (and angle) of flight. If the Center of Gravity is still too far to the rear with all of the ballast clay on the nose, you will have two choices. Add more clay to the nose to get the desired Center of Gravity (making your plane heavier than required), or reposition the wing rearward (maybe ½ to start) in order to allow som e of the original ballast clay to be moved rearward to experiment with different Center of Gravity settings. Changing the shape or the angle of the wing will affect the position of the air molecules as they move around it, creating higher or lower pressure areas. Wings are normally cambered (curved) over the top surface. This is an effective way to let the air molecules more gradually move into the spread apart (lower pressure) position that we are after. N ew ton s F irst Law of Physics states that a body at rest will remain at rest until acted upon by an external force. Even these little air molecules follow this rule. Our wing will force them out of their position. But the quicker and the further that we make them move, the more energy that it takes. The highest performance results in disturbing the air molecules the minimum amount necessary to get the desired results and to more gradually move them into a new position. Or in aerodynamic terms, minimize drag. For this airplane (since it is so light), a rather flat camber was chosen because only a very small difference in air pressure is necessary to achieve our goal. A more curved upper surface may be needed if it were necessary to carry more weight with the same wing area. This airplane is also utilizing the technique of allowing the horizontal stabilizer (at the tail) to act as a second wing. This divides the duty of lifting the weight of the plane over the greatest wing area possible. This will make the Center of Gravity very sensitive as it will have the greatest effect on determining the angle that the plane flys through the air (the Angle of Attack).

3 The Angle of Attack is the angle of the wings in relation to the air that the plane is moving through. This angle has a major effect on how many air molecules we disturb as the plane moves forward. The flatter the angle the easier it penetrates through the air and the less energy that it takes. However there must be a positive Angle of Attack (that means the front edge is higher than the rear edge) to achieve lift. So once again, it is important to make the minimum amount of lift necessary to meet our goal of achieving the slowest sink rate possible. This means that we want a very small Angle of Attack. Each of the two wings (the regular wing and the horizontal stabilizer at the tail) is slightly different. The angle that they are attached to the fuselage is called the Angle of Incidence. Because of the tapered cut on the top of the fuselage stick, the horizontal stabilizer is attached with a very slight upward angle (positive Angle of Incidence). The wing is mounted on posts that allow adjustment of its Angle of Incidence. The term Decalage is a comparison of the Angle of Incidence of the wing and the Angle of Incidence of the tail. The more the forward edge of the wing is raised (without changing the angle of the tail), the more positive the Decalage is. Generally, some positive Decalage is necessary to allow the wing to gain slightly more lift than the tail when the speed increases. This is called positive Dynamic Stability. This is what allows the airplane to recover from a nosedive by gradually raising the nose until it reaches stabilized flight. For dependability this is a good thing since a freeflight airplane has no pilot to control the angle of the tail to help the plane recover. However, the slowest sink rate will be achieved when both wings are flying at their most efficient Angle of Attack. This may not provide dynamic stability. So this is a very important area of experimentation to find a very slow sink rate, but still have some positive stability to help return it to stable flight. You will have to decide which is better for you, more or less Dynamic Stability. Your launch technique will be a large factor. If you are convinced that you can dependably launch your plane off of the balloon at the perfect Angle of Attack, you can get by with less stability (and less Decalage) to possibly get better performance. With every change in the Angle of Incidence of the wing to the fuselage, it may take a change in the Center of Gravity. There is a relationship where the greater the Decalage (set by moving the forward wing post up, or the rear post down) the further forward your Center of Gravity will have to be (set by moving the small piece of clay forward) or adding more clay to the nose. Generally, the farther to the rear the Center of Gravity is, the less the Decalage will be and the less stability you will have. A Stall occurs when the wing increases its Angle of Attack and the speed decreases so m uch that the w ing can t generate enough lift to keep the airplane flying. A S tall is actually the point w here the air m oving over the w ing becom es so turbulent that it can t create the low pressure necessary for lift. A Stall is one of the most important things to watch for in studying the glide. Remember, the higher the nose comes up (increased Angle of Attack), the slower the speed will be and these are exactly the conditions that lead to a Stall. When the wing stalls, the nose drops, which will now lead to increased speed and allow the wing to start flying again. If you have some dynamic stability adjusted in, it should recover to a stable flight or more likely, begin flying and gradually

4 bring its nose up enough to slow it down until it stalls again, where the cycle is repeated. Altitude is lost with each stall cycle and will hurt flight times. As mentioned before, if you are pushing the edge and have a very rear Center of Gravity (and a corresponding small degree of Decalage), you will have less stability and the airplane could drop much farther in a stall or possibly not recover at all and dive straight to the floor. To decrease the tendency to stall, either move the small piece of clay forward or move the forward wing post down (or the rear up). A tiny bit of buffeting (bounciness) that occurs just before a stall is generally OK and tells you that you are as slow as you can possibly be. This will probably be detected at a few places on its glide down (with very little loss in altitude) if you are near the optimum adjustment. When building the wings, the tips are raised to be higher that at the center. This is called Dihedral. The Dihedral allows the plane to be laterally stable (always keeping the wings level, top side up). If the airplane were to tilt to the left, the left wing would now generate more lift (being more parallel to the ground) than the right wing that would be pointed up at a greater angle, not allowing it to lift upward against gravity as well. This will cause the lift wing to rise while the right wing will come down until both wings are level. Another technique used in this airplane to enhance lateral stability is through the use of wing posts to raise the wing above the fuselage and allow the fuselage and ballast weight to act as a pendulum to keep it right side up. It is necessary to glide in a circle to stay within the boundaries and away from obstacles found in nearly all flying sites. Indoor modelers traditionally fly in a left circle though for your glider, either direction could work as well. Flying to the left could help to avoid collisions when flying with other modelers. There are two very effective ways to make you plane turn left and a blend of both seems to be the most effective. The Rudder can be slightly offset or curved with the rear edge to the left. However the other very low drag method is to tilt the Horizontal Stabilizer slightly to the right (the left side higher than the right). It looks rather unusual but allows the lift generated by the Horizontal Stabilizer to pull the tail slightly outward, causing the plane to turn left. More tilt, more turn. Generally, a larger circle will yield more efficiency (with lower overall drag), but any larger than about 30 can allow the plane to drift into obstacles by the end of the flight. A perfect launch will accelerate the airplane forward to its stable speed of flight at the Angle of Attack that it flys in stable flight. Faster, and the airplane could slightly nose up causing a stall which will cause the plane to drop downward as described above. Slower, and the plane will drop until it reaches its stable flying speed, but altitude and time aloft will be lost. In extreme situations where the Decalage is minimal and the Center of Gravity is aft, the plane may dive all the way to the ground and not recover. The best launch seems to be made by using a second launch string to rotate the balloon in the direction of flight, with the airplane resting on a cradle above the balloon to control the angle of the plane during launch.

5 Troubleshooting Problems If you have built your plane according to the directions and you find a peculiarity in the way that it flys, a close inspection should uncover the problem. It is very important that the two sides of the plane are balanced and symmetrical. When balancing the airplane s fuselage on the back of a knife blade, see if one side consistently drops down showing that it is heavier. If this is the case, either the heavy side will have to be lightened (once covered this is very difficult) or the light side needs to have weight added. A tiny speck of clay added to the very tip of the wing should bring it into balance. A more permanent fix is to add a little glue to the light wing tip until it balances. Check for any warps in either wing or in the horizontal stabilizer. One of the most puzzling situations can occur if the horizontal stabilizer is warped. Each wing should look like an exact duplicate of the other when looking from the front end. If one looks a little twisted it will definitely cause problems. A twist will cause the lift to change across that wing or stabilizer. Generally the effect on the flight is more pronounced when flying faster (speed dependant). If the twist is in a direction where the rear edge is lower, it will have increased lift on that side and will affect the turn (sometimes even making the plane turn the opposite direction than you planned) and the levelness of the wings in flight. A twist where the forward edge is lower will reduce the lift of that wing. Because of the leverage effect, the area near the tip is the most critical. Generally, the use of your fingertips to gently massage the effected spar into the proper shape will do the trick. In more severe cases, a small brush with acetone will soften up the glue joints to allow the pieces to gently be repositioned. In this case it would be a good idea to figure out a means to prop or weight the parts into the proper shape while the glue rehardens. Acetone (the major ingredient in fingernail polish remover that can also be used) is a very hot solvent and will readily melt plastic, however not the Mylar covering. Most often it is necessary to reapply the acetone several times to fully soften the model airplane cement joint because it evaporates so quickly. If your plane repeatedly stalls, either add clay to the nose (or move what you have forward) or slightly lower the front edge of the wing (either by moving the front wing post down or raising the rear). If it seems to fly too fast and hold its nose down, move a piece of the clay back or raise the front of the wing.