Boeing 747
Weight (W)= 800,000lb
Wing area(s) = 7800 ft²
Therefore each square foot of the wing must carry
800,000lb.
7800 ft²
This amounts to a pressure difference of
102.6 lb
ft²
Or 0.713psi
How a wing creates lift.
A wing increases the speed of the airflow over its upper surface so that pressure in this area is reduced. This is accomplished by making the upper wing surface curved – called the camber. The distance from front to back along the curved surface is greater than along the curved surface is greater than along the straight lower one. Because the molecules flowing along the curve have farther to travel than the ones beneath, they increase their speed and become spaced farther apart in order to resume their former position when they leave the wing at the trailing edge. This faster moving air exerts less pressure, which means that a partial vacuum is created above the wing – suction. (This relates to Bernoulli’s principle.) The now higher pressure beneath pushes the wing upward into the vacuum, creating a lifting force. This lift acts through a point about 1/3 of the distance between the leading and trailing edges of a wing, the point of maximum camber.
A German solved a serious early problem of flight (not enough lift) by seriously studying birds, not for their wings flapping but for their gliding flight. Otto Lilienthal understood that that the lifting force produced by the curvature of feathers was improved by the fact that bird wings are longer than they are wide. Air resistance relative to the lifting force was less. Critically he noticed that birds make slight adjustments in the positions and angles of their wings and tails to balance themselves in gliding flight. He realized that piloting therefore meant balancing the various forces at work, as he demanded by the fluctuations in air currents encountered by flying.
On Lilenthal’s gliding models he used weight shifting for balance. However this method of maintaining equilibrium (balanced flight) has limitations. Rather than initiating guidance to the aircraft in a positive manner (moving the whole craft) weight shifting is always compensatory, the pilot reacting to the aircrafts movement in any direction. If a misjudgment is made in the position of the pilot’s body or unexpected gusts occur, the machine glider can become dangerously unstable. Weight shifting also limits the size of aircraft that can be successfully balanced to rather small and lightweight machines.
Applying Aerodynamic Principles.
To work aircraft wings must alter air pressure. They do this in two ways. First, as they move forward they slice the surrounding air into two layers, one above and one beneath the wings. Both layers are made up of the same number of molecules. If the wing has a curved upper surface, the molecules moving across the top surface have farther to travel than the ones underneath. As they try to maintain their position in relation to the rest of the air molecules, they become spaced farther apart and their speed increases so that when they reach the back edge of the wing, they again match their position with the lower molecules. According to Bernoulli’s principle, the faster moving and more widely spaced molecules exert less pressure down-ward than the slower moving and more closely spaced lower molecules do upward, creating a pressure differential. The reduced pressure above the wings creates suctions much like a vacuum cleaner does. The air underneath pushes the wing into the area of reduced pressure and the aircraft is buoyed up as it moves forward, counteracting gravity.
(See figure 1)
Second, if the leading edge of the wings is raised slightly, allowing air molecules to strike the slanted lower surface, the amount of lift generated can be increased. The slanting is called the angle of attack. (See figure 3) However if this angle is too great lift stops because air no longer flows smoothly over the upper surface disrupting the suction and the wing stalls. (See figure 4). If you are in a glider and this happens it is bad news.
Drag And Thrust. (See figure 1)
Lift is only possible by forward motion. As a glider moves forward air molecules are pushed aside causing a certain amount of resistance. On the one hand this resistance turns into the pressure that makes lift possible, on the other hand, it becomes drag, which slows a glider down. The resistance of air molecules being disturbed by forward motion is called pressure drag.
There are three kinds of drag: pressure drag, induced drag, and frictional drag. These combine to make up the overall drag acting on an aircraft in flight. The bigger the frontal area of a glider the greater the pressure drag. Induced drag is when air slips around the wings creating a vortex. Air does this because it always flows from an area of high pressure to an area of low pressure. Frictional drag is created when air flows over any surface of the Glider.
As a glider has no engine there is effectively no thrust. The only way to gain speed is to lose altitude and the only way to gain altitude is to lose speed. However this equation isn’t perfect otherwise the glider would be able to manage infinite flight.
Materials.
There are currently around 10,000 glider pilots within the United Kingdom, with pilots, male and female, teenagers each weighing in at slightly different amounts. This means a Glider has to be flexible in the amount of weight it can take. Most gliders are designed to accommodate pilots under 6' 3'' tall and under 16 stone.
After the war ended in 1945 aviators turned their attention away from combat gliders and once again could turn to sport flying. Before the war the Germans (now leaders in Gliding technology) had experimented with lightweight alloy structures, slender fuselages with narrow oval shaped cross-sections, small canopies, cantilevered (no bracing) high aspect ratio wings, and larger rudders.
The Aspect Ratio Of Glider Wings.
The longer a wing in relation to the distance from its leading to trailing edge (higher aspect ratio) the greater its efficiency, having lower induced drag. The aspect ratio of glider wings is steadily increasing to try and improve glide ratios. The highest today is over 40:1. (Most are around 20). Correspondingly wingspans too have increased. Most are between 48 and 54 feet. Greater spans make handling on the ground and in the air difficult.
There is also a relationship between aspect ratio and a wings thickness as there is only so much strength that can be built into a wing of a given thickness. Early gliders all had struts to support the wings. Since the Second World War, with the use of stronger materials wings have been internally supported (cantilevered). In wings of high aspect ratio torsional strength requires careful and critical engineering.
High aspect ratio wings is one feature that distinguishes gliders from other aircraft.
New world-wide research into materials and methods resulted in a widespread change in how gliders were built after the war. Great improvement could be achieved by using different airfoil cross-sectional shapes and by reducing waviness in wings, using unbroken polished surfaces, adding rounded fairings, and removing even small gaps and air leaks. Manufacturers used a variety of materials in one machine – wood steel, aluminum, fabric and fiberglass.
Warplanes had been made mainly of metal, steel and alloy frames with lightweight riveted aluminum skins, yielding good performance. This is what the Schweizer Company used in their gliders. It proved to be ideal the right weight for Gliders. The Schweizer’s produced just the kind of gliders that would be attractive to the growing club market. Most Glider pilots in America (or even Canada) have received their pilot training in a Schweizer glider.
It soon became apparent that the added performance of fiberglass ships was advantageous to pilots competing at national and international level and the composite construction techniques introduced by Glasflűgel (a German company) were copied by other manufacturers. Those manufacturers who did not adopt fiberglass went out of business.
In composite construction the frame is eliminated. Thin layers of strong fibres are molded into the gliders shape infused with resin, and cured. This skin is strong, seamless and smooth. Composite construction is also compatible with CAD (computer aided design), which allows for very precise aerodynamic measurements to be made. Jim Marske used this method for developing a new ultra efficient glider, the Genesis. It is a hybrid design combining concepts from flying wing and regular aircraft.
Bibliography.
Fabulous Paper Gliders – Norman Schmidt
Google Image Search – For the Glider Picture.
