![]() ![]() The thrust to counter air resistance to give it the force to keep it moving forwards comes from sacrificing some height. The lift is created as air flows over the wings. Is the hang-glider really flying? The hang-glider pilot has both lift and thrust, even though there aren’t any propellers. The Hitchhiker’s Guide to the Galaxy, Douglas Adams. The knack lies in learning how to throw yourself at the ground and miss. There is an art, it says, or rather, a knack to flying. Because the hot-air balloon has no thrust, we cannot say that it is flying. It will change speed until there is no drag. In fact, the hot-air balloon will change speed until it is moving at the same speed as the air around it. However, the hot-air balloon has no thrust force to counter air resistance. The result is that the air inside the balloon is lighter than the air outside the balloon so the balloon will rise until the total weight force downwards is equal to the weight of the air that it is displacing. The hot air inside the balloon is less dense, which means that the air particles are not as closely packed together. In this case, the lift isn’t created by air moving over wings, but is created by buoyancy. Is a hot-air balloon flying? A hot-air balloon has lift to counter gravity. The stone is falling for the whole time that it is in the air. For the whole time the stone is in the air, the force of gravity is pulling it downwards. While it is moving through the air, it doesn’t have any forward thrust to counter air resistance – only its momentum keeps it moving forwards, and its forward motion will gradually decrease.Īlso, it doesn’t have any lift to counter gravity. What about a stone being thrown through the air? The stone gets its forward motion only while it is in contact with the person’s hand. Because they have neither lift nor thrust, this is definitely not an example of flying – it is an example of falling. The person falling from the hang-glider has no lift to counter gravity, so they fall towards the ground, and they also have no thrust to counter air resistance. Thrust to counter the force of drag (air resistance) – without thrust, an object will lose its forward momentum, which will also result in it falling to the ground. ![]() Lift to balance the force of gravity – without lift, an object will fall to the ground.To be classed as flying, an object needs two things: This leads into a useful definition of flight that includes hang-gliders as flying, but certainly does not include the person falling. (Don’t worry, the imaginary falling person in this example has a parachute!) Most people would agree that the person falling from the hang-glider is not flying but that the person still on the hang-glider is. He responded: “Come up on my hang-glider and I’ll drop you off and see if you think I’m flying!!” Some would use this to say that a hang-glider is not flying because it doesn’t have thrust, and it certainly excludes a stone being thrown through the air, because it has neither thrust nor lift.Ī lot of people disagree with the definition of flight as ‘anything moving through the air’, such as a stone being thrown.Ī hang-glider pilot was recently questioned about whether he was really flying. The traditional definition of flight is that it needs thrust and lift. So how can we do this? And which things are truly flying? ![]() But for some things it is not as easy to decide – a stone being thrown through the air, a hang-glider, a hot-air balloon…ĭouglas Adams once defined flight as the ability to throw yourself at the ground and miss. ![]()
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