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![]() Jose wrote: The flying wing has some horizontal momentum which is secondary here, How much? mv The air thrown forward (or, if you will, the higher pressure ahead) tries to reduce that, the engine presumably makes up for it. Energy is 'pumped' into the air by the plane. Yes, and what form does that energy take? Heat. I maintain that it takes the form of a net increase in mv^2/2 over all the air molecules. Yes. Since m doesn't change, and 2 only changes in a pentium, that leaves v to change. This changes mv, thus momentum. Mass and energy are scalers but velocity is a vector. You can increase the average velocity of the air molecules without changing the momentum of the air mass. Indeed, that is exaclty what happens when you heat air. We agree that there is (microsocopic) momentum transfer at each collision. We disagree as to whether the net is zero, and I think that part of that disagreement has to do with just how much of the system we are looking at. More importantly we disagree on what causes lift. If there is lower pressure on the upper surface of a wing than there is underneath there will be an upward force on that wing. I think we agree on this. You argue that the presssure difference and resulting force is secondary, lift is actual caused by the reaction of the wing to the momentum change it induces in the air. But suppose the wing creates low pressure on the upper surface by throwing air sideways? You still have a pressure differential and the resultant force but the only downwash is the air flowing toward the upper surface of the wing from above to fill in the rarefied region. For that matter, consider the common demonstration using a notecard, thumbtack and a straw. Put the tack through the middle of a 3x5 index card or something similar. Put a drinking straw over the thumbtack. Hold the aparatus with the straw vertical and the notedard down. Blow through the straw and let go of the notecard. The notecard will be supported by the Bernouli effect. The only downwash is through the straw, directed at the notecard, pushing it down. There is no downwash from the card. The card does not deflect any air down, it deflects the air sideways. Yet the card is supported by the pressure differential created by the Bernouli effect. Horizontal flow accross the upper surface of the card creates that pressure difference. Downwash does not cause lift. Downwash is a secondary effect caused by the same phenomenum that causes lift. -- FF |
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