If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#111
|
|||
|
|||
Backwash Causes Lift?
Le Chaud Lapin writes:
By the bottom part of the wing, right? Any displacement of mass downward will produce a matching upward forced. You could generate lift of a sort by launching rocks off the wing, but you'd soon run out of rocks so that's not very practical. But there's plenty of air mass, so if you can find a way to divert it, you can generate lift. If you have compression under a wing do to extended flaps and laminar friction of airflow, for example, then the lower surface of the wing forces air downward, and the air beneath the lower surface forces wing upward. In reality, the high pressure effects below the airfoil are almost insignificant. The lift is generated mostly by the diversion of air flowing over the wing downward. Why is this all so important? |
#112
|
|||
|
|||
Backwash Causes Lift?
Interested readers might enjoy looking at alphatrainers.com for a
discussion of lift. Mx's assertion that lift is mostly the result of downwash flies (pardon the pun) in the face of 'center of lift' analysis which in effect is that point on the wing where if for balance considerations the integrated upward forces were concentrated they could be considered to be operating at a point. If downwash, the center of action of which is somewhat aft of the following edge of the wing, was the major contributer of lift, one would expect the center of lift to be in that area -- aft of the wing. It's not. But what do I know, I'm just a psychologist -- with a minor in physics. |
#113
|
|||
|
|||
Backwash Causes Lift?
On Oct 4, 6:27 am, Mxsmanic wrote:
TheSmokingGnu writes: Then why do wings generate lift at negative AOA? They don't. That's a very common misconception, even among pilots. The effective AOA is always positive when the wing is generating lift. Once again, thou knowest not of what thou speakest. I just told you, in apost not long ago, that some airfoils will generate lift at up to -4° AOA. Here's a graph that shows lift being generated on some anonymous airfoil at -5°: http://en.wikipedia.org/wiki/Lift_coefficient Bernoulli at work. Newton, too, because there's downwash being generated. Dan |
#114
|
|||
|
|||
Backwash Causes Lift?
Tina writes:
Interested readers might enjoy looking at alphatrainers.com for a discussion of lift. Mx's assertion that lift is mostly the result of downwash flies (pardon the pun) in the face of 'center of lift' analysis which in effect is that point on the wing where if for balance considerations the integrated upward forces were concentrated they could be considered to be operating at a point. If downwash, the center of action of which is somewhat aft of the following edge of the wing, was the major contributer of lift, one would expect the center of lift to be in that area -- aft of the wing. It's not. I don't understand how you reached this conclusion. It's a bit like saying that all of the planet Earth must be massless except for a dimensionless point at its center, since that is where the center of gravity is. But what do I know, I'm just a psychologist -- with a minor in physics. Knowledge is more important than credentials. |
#116
|
|||
|
|||
Backwash Causes Lift?
wrote:
On Oct 4, 6:27 am, Mxsmanic wrote: TheSmokingGnu writes: Then why do wings generate lift at negative AOA? They don't. That's a very common misconception, even among pilots. The effective AOA is always positive when the wing is generating lift. Once again, thou knowest not of what thou speakest. I just told you, in apost not long ago, that some airfoils will generate lift at up to -4° AOA. Here's a graph that shows lift being generated on some anonymous airfoil at -5°: http://en.wikipedia.org/wiki/Lift_coefficient Bernoulli at work. Newton, too, because there's downwash being generated. Dan Don't bother. Believe me it's not worth it. You can talk this guy to death and all he will do is count you as another response. Angle of attack as we all know (with one exception it seems :-) can be both positive or negative. In fact, in high performance jets with a fuselage loaded IYMP, entering a coupled spin after a departure, it's extremely disorienting if the aircraft goes through PSG and stabilizes in an inverted spin mode where yaw is opposite to roll. g is a bad indicator as with a fuselage loaded IYMP you can get negative g either erect or inverted. The way we deal with this is through instrument interpretation rather than trying to eyeball what's happening, which can be next to impossible. The AOA indicator in the aircraft has a positive and negative side. If the AOA is stabilized at some value on the negative side, and the airspeed is stabilized at some mean low value, the spin is inverted. The turn needle will show spin direction either way. Again, don't waste your time. -- Dudley Henriques |
#117
|
|||
|
|||
Backwash Causes Lift?
Mr Dudley, once again you are confused. Most define angle of attack as
the chord line of a wing, and of course with that definition it can be negative and still generate lift. Mr Mx chooses a different way of defining it. It is some angle such that when it goes negative the airfoil can generate no lift. Do you remember the disbarred former president Clinton saying something about "It depends on what 'is' means"? In Mx's case, words change meaning so that he is NEVER wrong. It must be an interesting version of English he teaches. But he does offer amusement for some of us on otherwise humorless days, doesn't he? |
#118
|
|||
|
|||
Backwash Causes Lift?
Tina wrote:
Mr Dudley, once again you are confused. Most define angle of attack as the chord line of a wing, and of course with that definition it can be negative and still generate lift. Mr Mx chooses a different way of defining it. It is some angle such that when it goes negative the airfoil can generate no lift. Do you remember the disbarred former president Clinton saying something about "It depends on what 'is' means"? In Mx's case, words change meaning so that he is NEVER wrong. It must be an interesting version of English he teaches. But he does offer amusement for some of us on otherwise humorless days, doesn't he? AOA actually can be defined relative to any given reference datum, but normally it's considered in the industry as being the angle formed between the chord line of the wing and the relative wind as you have correctly stated. Quite frankly, I read what Mxemanic writes on occasion and can't figure out how he can be so close to getting it right and still manage to get it wrong. He's amazing, and an interesting study if nothing else. It's too bad he's taken this path on these groups. I've always felt he has a genuine interest in things aviation and would like to contribute, but he seems to be such a jerk that he gets in his own way. -- Dudley Henriques |
#119
|
|||
|
|||
Backwash Causes Lift?
On Oct 4, 6:39 am, Tina wrote:
Still waiting for the conservation of momentum derivation. My husband, also trained as an engineer, casually remarked he didn't think you could get from Newton's First Law to the that confirms my memory, but we are both willing to have that belief rebutted. My apologies for broaching the subject. Frankly, I would rather save it for the physicists. He also pointed out that how a CFI might explain how a VOR works would not satisfy an engineer. For that matter, the physics of flight as explained to a student pilot would not satisfy someone who might be interested in designing, as opposed to flying, an airplane, but I don't think the manuals you are looking at are in error. If the manuals are in error, then they are in error. If the manual issues a disclaimer, saying something, like, "this is not really what is happening, but this will suffice for us..." that would be ok. That's not what's happening. The manual mentions things like Bernoulli, Newtons laws of motion. It even uses vector notation for a few of the formula's. When one gets that close to the merchandise, they need to purchase it. I would point out that each field has its own language, and you denying the conventions used in aviation -- drag, lift and so on -- demonstrates an unbecoming trait for a student, and even a worse one for an employee. You may want to rething that attitude if you use it in real life. There is a difference between convention and errononeous information. I never discounted drag, lift, or so on...I discounted the explanations given some of my flight education materials. If it's wrong, it's wrong. If someone reading it gets comfort from thinking they understand, or whatever, that's fine for that person. But the writers of those manuals should know that their audience is broad, and should not publish erroneous information (after they know that it is erroneous). There is gross difference between explaining something in simple terms and being correct, versus explaining something in moderately-difficult terms, and being incorrect. I could probably explain VOR to a 10-year-old, without ever mentioning things like counters, angular frequency, anisotropic radiation, frequency bands, sub-carriers, convolution, etc....and my explanation would still be correct. "When you drink from a straw, there is no suction force." -Le Chaud Lapin- |
#120
|
|||
|
|||
Backwash Causes Lift?
Tina wrote:
Still waiting for the conservation of momentum derivation. My husband, also trained as an engineer, casually remarked he didn't think you could get from Newton's First Law to the that confirms my memory, but we are both willing to have that belief rebutted. You can't get conservation of momentum from F = m*a (or vice versa) since the latter is not a statement about anything being conserved. But you can get "For every force there is an equal opposite force" from conservation of momentum, and vice versa, with a small number of assumptions. You can use derivatives to derive one way and integration to derive the other. Here are several conservation laws that share a common derivation, starting with: Center of Mass is Conserved --------------------------- Center of mass of a closed system of particles of mass m1, m2, m3, ... mn must remain fixed for all time, which with a suitable selection of coordinate origins may be stated mathematically as: (a) m1*x1 + m2*x2 + m3*x3 + ... = 0 m1*y1 + m2*y2 + m3*y3 + ... = 0 m1*z1 + m2*z2 + m3*z3 + ... = 0 Note that this doesn't say that, for example, x1 can't vary with time. It only says that if it does then m1, m2, m3, x2 or x3 or other masses or positions must somehow change so the left hand side still remains zero. Momentum is Conserved --------------------- If position with respect to time is continuous (no discontinuities; e.g. no jumps) then we can take the time derivative of the above, yielding: (b) m1*dx1/dt + m2*dx2/dt + m3*dx3/dt + ... = 0 (And so on for the other coordinate axis.) This is of course just the conservation of momentum equations because dx1/dt = Vx1, a velocity. Note that d(m1*x1)/dt would have been more appropriate if the mass of particles varies with time. Force is Conserved ------------------ Given the continuity assumption above, then we can keep taking time derivatives of the above, yielding the next conservation statement: (c) m1*d^2(x1)/dt^2 + m2*d^2(x2)/dt^2 + m3*d^2(x3)/dt^2 + ... = 0 (And so on for the other coordinate axis.) This is of course just the old equal and opposite action statement in mathematical form because d^2(x1)/dt^2 = ax1, an acceleration. So if one claims any _one_ of the above conservation laws exists then the other two appear to follow with only a small set of (presumably reasonable) assumptions. |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
How much lift do you need? | Dan Luke | Piloting | 3 | April 16th 07 02:46 PM |
Theories of lift | Avril Poisson | General Aviation | 3 | April 28th 06 07:20 AM |
what the heck is lift? | buttman | Piloting | 72 | September 16th 05 11:50 PM |
Lift Query | Avril Poisson | General Aviation | 8 | April 21st 05 07:50 PM |
thermal lift | ekantian | Soaring | 0 | October 5th 04 02:55 PM |