Actually, planes with wings with aerodynamic profiles are pulled up by vacu= um=20 (in a way to speak), more than pushing air down. Regards, Marcelo Fornaso ________________________________ From: Olin Lathrop To: Microcontroller discussion list - Public. Sent: Wed, January 26, 2011 5:32:46 PM Subject: Re: [TECH] How planes fly Vitaliy wrote: >> Planes already fly by "using the air". As the plane passes thru >> some air, it pushes it down, which in turn pushes the plane up to >> counter gravity. > > So you're in the Newton camp? :) You make that sound like a bad thing somehow. Since what we're talking about here is all much much less than the speed of light, Newton's laws apply. > I'd love to hear you debate my > Bernoulli friend. Bernoulli may explain how a wing causes the pressures around it and how it causes the air to move. However, none of the "how" can change the basic force ballance. To be clear, we're talking about a plane in steady level flight thru (originally) still air. Since there is no net accelleration, all the external forces must sum to 0. Taking the vertical component of that, there is gravity pulling the plane down. There must therefore be a equal and opposite force pushing it up since it's not falling out of the sky. The only thing the plane has to pus= h on is the air it flys thru. It follows that it is therefore pushing this air down. If you know the weight of the plane, you can go further and compute the downward momentum the plane is imparting on the air per unit time. That means you can figure out how many (Kg)(m/s) it must cause air t= o move dowwards every second. Note that Newton only says what the (Kg)(m/s) product must be, not any specific Kg and m/s combination. In fact, this is a one of the tradeoffs the airplane designer can make. While Newton doesn't care about the Kg versus m/s tradeoff, whoever is paying for the fuel does. The energy required to get this momentum is proportional to (m/s)**2. Ideally therefore you'd like to move lots of mass at low velocity. This basic physics explains why designs where power efficiency is important have wide wings. The wide wing pushes more Kg down, thereby not needing to push them down with as many m/s. Think of this another way. Planes must impart momentum on the air to stay up. The velocity component of that momentum however represents energy left in the air as a byproduct of producing the necessary momentum. Nothing in the basic free body analisys says a plane has to expend energy to stay up, just that we don't know of a way to avoid it in realizable airplanes. It is also interesting to look at that energy per unit time imparted on the air. Energy per time is power, and that is a minimum power that must be expended to keep the plane flying. Since power in airplanes is generally applied as a force in the direction of flight, this power can be converted to a force given the plane's speed. This force is opposite the direction o= f motion, and therefore looks just like drag. In other words, planes with finite wings have a minimum drag at each speed regardless of any actual friction with the air. Even if you could eliminate all lateral friction between the plane's surface and the air, this apparent drag would still be there. Anyway, back to your comment about Bernoulli. None of this says *how* the plane causes air to be pushed down, only that it must somehow do so. Bernoulli may be useful in understanding how a wing achieves this. In the end however, everything must happen within the basic force and momentum ballance according to Newton. Often it is useful to look at the same thing from different ways, but you still can't violate the basic principles I described above. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist =20 --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .