Not all wing airfoils have greater curves on top. There are airfoils that are equally curved on the top and bottom. This is adjusted for by the angle of incidence, that is the angle at which the wing is mounted to the fuselage relative to the direction of thrust (power). These high performance wings reduce drag. As for an upside down airplane, it will usually be flying with the nose high so that the angle of incidence will effectively cause the upper surface to have the “curvature” to sustain lift. In real life most general utility aircraft cannot be flown upside down. Aircraft that can be flown upside down are designed for aerobatics and will almost always have eliptical airfoils as well as be structurally build and powered for aerobatic flight. You may want to read a book on fluid dynamics, physics of flight. and be well versed in calculus (sp).
Swept wings at high angles are only effective in the trans-sonic and super-sonic regions of flight. Think F-14. Delta shape at high mach numbers, straight wings at low Mach numbers.
Have you ever seen a Twin Otter do mach two at 60,000 feet? The wing design depends on what the airplane is being designed to do. If you want an airplane that will take off and land at relatively slow speeds and in short distances, such as the Otter or Beaver, then a straight high lift wing design is required. In the simplest of terms the swept wing reduces drag. Drag slows the airplane down and/or takes more power to overcome the drag.
Again, in very simple terms, the delta wing is intended to compensate for the fact that the aircrafts center of gravity and the fact that the lift moves back as the speed of the aircraft increases. This is especially true when the speeds are in excess of Mach 2. The Otter has its center of gravity about 25% back from the leading edge of the wing. With the airplane being lifted around this point it is balanced, that is it is neither tail heavy nor nose heavy. For a “low and slow” airplane this is ideal. When the speed is increased the physics/aerodynamics change. The swept wing and the delta wing address the changes. Again, these comments are in the simplest of terms. If you are realy interested in how this all works Purdue University has an outstanding Aeronautical Engineering program.
Let me throw something else at you. When I attended Embry-Riddle Aeronautical University, where I got my degree, this is the topic that generated the biggest discussions.
A 60,000 pound aircraft, flying at 35,000 feet, traveling at .85 mach on a standard day, weighs how much? In order to take off, thrust (power = speed) must over come drag and lift must over come gravity (weight). In order to maintain flight the aircraft has to generate lift and thrust.
Some say the aircraft weight is balanced so it is actually weightless, while some say the aircraft will still maintain it current weight and the forces of thrust and lift keep it from falling. Your comments.
Ah, Berny, I think you just opened Pandora’s box. Should we start with the standard atmospheric pressure of 29.92 inches of mercury at sea leval at 59 degrees F. (“standard day”)? As the altitude increases the forces of gravity lessens until the force of gravity becomes zero, that is outer space. Because of the change in the force of gravity the air becomes less dense as you ascend. Less dense air means the force of lift created is less at higher altitudes so you either increase the angle of attack, or increase the speed, or both, in order to stay aloft.
Does the aircraft become weightless? Only if it breaks away from the force of gravity, that is it goes into outer space. As that is not generally possible the aircraft is not weightless. The aircraft has reached the balance point of creating enough lift, by using thrust, to overcome the force of gravity. Can we assume that the altitude in your example is pressure altitude and not AGL? As the air becomes less dense it creates less drag, but it also creats less lift, so the balance is relatively constant.
So, back to the original question, that is what is the best wing design. The best wing design is the one that allows the airplane to do what it was designed to do. Putting a swept wing on a J-3 Piper Cub would not improve its performance!
All explanations of lift that depend on the idea that the air moving over the top of the wing must speed up to meet the very molecules that it parted from at the leading edge are flat wrong. There is absolutely no reason that airflow components must match at leading and trailing edge, any more than the water molecules in a river flowing around an irregularly shaped island must meet their mates molecule by molecule at the downstream end of the island. This is a commonly held, oft-perpetuated explanation that holds neither air nor water…
Indeed, I’ve been guilty of perpetuating it myself, in an article I did some years ago for Air & Space Smithsonian on the genesis of swept-wing design. Mea culpa.
Oh, and rangerj, there is no place in the universe where gravity is actually “zero.” Weak, weaker and weakest maybe, but never zero.
I’d say that the aircraft would weigh 60,000 lbs. -(minus) weight in fuel consumption to takeoff, get to FL 35,000 ft. @ .85 mach. (Even less after it’s return to the ramp and then reweighed)…I Think?
You are perfectly correct sir. I’ll have to be more careful when trying to simplify my explanations of things that are complex. The error was unintended, but an error none the less. Thank you for the correction. We do not want to mislead or misinform our young enthusiast. One of the great things about the hobby is the learning.
You are on the right path. The longer an aircraft flies, the less its weight becomes. Jet fuel weights approximately 6.45 pounds per gallon (PPG). As an aircraft burns off fuel, then less power (Thrust) is needed for it to maintain altitude and speed. The less weight also means more lift as gravity (Weight) has decreased. It is a balance between lift and thrust that keeps an aircraft in the air. If an aircraft does an inflight refueling and takes on 2,000 gallons of fuel, it has just increased its weight by 12,900 pounds. It will take more power (Thrust) and lift to overcome that added weight to maintain altitude and speed.
Weight is always playing a role in how fast and high an aircraft can fly. An example is our F-4E aircraft in South East Asia on a B-52 escort mission. Fully loaded with eight missiles, three full external tanks and one ECM pod, it was impossible for them to keep up with the B-52. In order for them to match speed and altitude, they had to jettison the 600 gallon centerline tank to reduce weight and drag. Power setting was always around 95% RPM until they burned off enough fuel. Coming off of a tanker with a full weapons load and three gas bags, the F-4 had to strain to get above 30,000 feet MSL. The BUFF usually flew at around 35,000 to 40,000 feet MSL. They had to time their arrival when the escort was light enough to keep up with them at their altitude and speed.
So the answer is, an aircraft is not weightless. It is a combination of lift and thrust that keeps it in the air. Decrease lift and thrust, the aircraft looses altitude. Increase lift and thrust, the aircraft will gain altitude. By working the forces of lift and thrust together the aircraft will maintain speed and altitude. Simple, my dear Watson.
rangerj, where did you get your calculations for standard day? I checked my notes from Embry-Riddle and from ground school. I had a standard day at 50 degrees F, 29.90 HG @ MSL. The calculations could work out to be the same though. Just wondering.
I think it has to do with boundary layers. Another good thing to check out is the “area rule.” Basically, air gets really compressed on the forward surfaces when a plane is going trans-sonic. Swept wings help reduce that effect.
