<< Page 2 Page 3: Page 4 >>   Weight As far as these forces go, weight is the no-brainer. It never changes in direction, and only decreases when fuel, cargo, weapons or parachuters are used/dropped/shot out. These changes, although trivial to understand, can have severe effects on flight. As we will see, weight causes a component of drag. This means that if an airplane becomes lighter, it can fly faster and be more fuel efficient (because less thrust is required to balance this smaller drag from less weight). The wings would also have to generate less lift, so the plane can fly higher (where drag is even smaller, so fuel efficiency goes up even more) and land and take-off at slower speeds (which is safer and requires smaller runways). But if the wings can still generate as much lift as before, the plane can climb faster and turn tighter. Also, if a given airplane is made lighter, more cargo/people will fit in it before it reaches the original maximum takeoff weight. As you can see, low weight has all kinds of advantages. All other things being equal, a lighter airplane will -use less fuel -have a faster maximum speed -have higher cruise and max altitudes -have a longer range -have lower take-off and landing speeds and distances -climb faster -turn more tightly -carry more cargo -have a longer endurance (time in the air) (And obviously, a plane with too much weight will not get off the ground at all) Given how speed, altitude, efficiency and handling change as weight changes, you can imagine that the way an airliner flies changes A LOT from the beginning of the flight to the end, because most airliners lose about one third of their weight by burning fuel between takeoff and landing. (Most modern airliners can’t even land with full tanks because they would be too heavy and would damage the landing gear – they must burn or dump fuel first). The Rutan Voyager, the first airplane to fly all the way around the world without refueling, lost over 72% of its weight during the flight – it is essentially a flying fuel tank. Much of the history of aviation is a battle against weight. This is especially true at the dawn of aviation. Engines had to be small and light – but this meant that they didn’t produce much thrust, so early airplanes were slow, so the wings didn’t generate a lot of lift, so weight had to be very small. Wing surfaces were made of fabric – sometimes silk, as is the case with Dumont’s Demoiselle – held by very thin wooden ribs. Anything not necessary – like a windshield or comfortable seat – was not included. Structures were flimsy, so planes were easily damaged, even at slow speeds, if landings were less than perfect. Early airplanes were biplanes because the double wings could be supported by external cables and by each other, minimizing the weight of the structure necessary to hold the wings in place. (Many people think that early airplanes were biplanes because the higher wing area made it easier to fly at slower speeds. This is true in part, but the main reason is that a wing supported by cables is MUCH lighter than a cantilever wing of the same area). As engines became more powerful, planes could go faster, generated more lift, and thus became heavier. Most of the modern advances in aviation involve the use of light space-age materials which allow airplanes to be lighter than older airplanes, even when of similar size and shape. Modern airliners may look almost exactly like airliners 40 years ago, but they are lighter, so they can fly higher and faster, carry more fuel and cargo and people, be more efficient, etc etc. The 707 might look superficially like the A380, but the Airbus is made of much lighter materials (and also has more efficient turbofan engines).   - - - - - - - - - - - - - - - - - -   Summary: Less weight is good. More weight is bad. All other things being equal, a lighter airplane will -use less fuel -have a faster maximum speed -have a longer range -have lower take-off and landing speeds and distances -climb faster -have higher cruise and max altitudes -turn more tightly -carry more cargo -have a longer endurance (time in the air) (And obviously, a plane with too much weight will not get off the ground at all) The history of aviation is a battle against weight, especially in the early days. Airplanes became heavier as they became faster and more strongly built. Most modern airplanes incorporate space-age composite materials/plastics.