I am designing a "glider" of sorts, and I have some basic questions about the physics involved to get me started. How much lift is required to overcome the weight of an average person, say 150 pounds? Is the lift required simply 150 pounds?
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$\begingroup$ Also note that lift is quite diffecult to calculate, since it depends on the shape of the air-foil, air speed and angle of attack. And to be able to fly you also need to be able to maneuver, for example gain height. So your lift should also be able to exceed your weight (often by increasing the angle of attack, which also increases the drag). $\endgroup$– fibonaticDec 27, 2013 at 4:11
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$\begingroup$ Aircraft have a rating - wing loading - expressed in weight per unit area. A typical small plane is in the range of 10lb/square foot, which is achieved at a speed of roughly 60 mph. Lift is proportional to speed squared, so if you halve the speed, you need 4 times as much area. $\endgroup$– Mike DunlaveyDec 28, 2013 at 17:58
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2 Answers
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In steady flight, the sum of all the vertical forces has to be zero. This means that the glider has to lift its own weight, in addition to weight of its pilot. So 150 pounds would not be sufficient in this case.
answered Feb 27, 2018 at 22:59
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Yes, lift is nothing but the force in the upward direction. So to keep a 150 pound person aloft, you need a lift of 150 pounds (150 pound-force).
answered Dec 27, 2013 at 4:04
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1$\begingroup$ To be picky: "pound" is a word for a unit of mass and for a unit of force. An object with a mass of 1 pound will have a force of gravity of 1 pound acting on it on earth. Which means that if you try to use both of these units together, $F=ma$ doesn't work any more. Check out "slug" and "poundal", or stick with SI. $\endgroup$– DJohnMDec 27, 2013 at 18:40
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$\begingroup$ @User58220 Nope. Pound is a unit of weight, kilogram is a unit of mass. The metric system's corresponding unit of weight is kilogram-force, though you can of course use Newton. $\endgroup$ Dec 27, 2013 at 18:44
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$\begingroup$ @User58220 My bad, you're correct, the pound is a unit of mass. Strange; I was explicitly taught that the pound was a unit of weight and not mass. $\endgroup$ Dec 27, 2013 at 18:45
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$\begingroup$ Pound is definitely a unit of force, just as a foot-pound is a unit of torque. Any use of pound for mass is born out of students' misunderstanding, not out of scientific/engineering usage. $\endgroup$– user10851Dec 27, 2013 at 20:07
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$\begingroup$ There are several ancient, coherent versions of the Imperial system of units. One used pounds mass and poundals force. Another used slugs for mass, and pounds for force. As long as you stayed in one or the other, $F=ma$, with $a$ in $feet/second^2$ $\endgroup$– DJohnMDec 27, 2013 at 21:00