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"Neil Gould" wrote in message
... [...] Any discussion about other forms of flying, whether it be in hovering Harriers or personal batwings are irrelevant to that context. Then perhaps you should take that up with the person who brought up such examples. Todd was not that person. Oh, wait...it was YOU that mentioned the F-18. (Minor nitpick: I don't recall for sure whether the F-18 actually has more thrust than weight; I believe that the F-16 does, and it's the only airplane I understood to have that characteristic. I will continue saying "F-18" in this post, with the assumption that you know for a fact it also has more thrust than weight...perhaps it's just one of the later models, like the Super Hornet, that does). I wasn't claiming that it does. It's just that the amount of lift after stall isn't sufficient to be relevant. That's a false claim. As the lift drops off in a continuous manner, there is a region "after stall" where the lift coefficient is just as high as usable regions "before stall". You may equivocate on whether a pilot can maintain the airplane at the angle-of-attack required to obtain that "after stall" coefficient of lift. But the fact remains that the lift is theoretically obtainable. As long as you don't want a lift coefficient very close to the maximum lift coefficient for the wing, it may not even be that hard to obtain the desired coefficient. There are, of course, other issues. The graph I'm talking about is actually the lift coefficient graph; actual lift depends on the lift coefficient (angle of attack) and airspeed. Drag increases dramatically at stall, and it would require a lot of extra power to maintain an airspeed sufficient to produce lift equal to the airplane's weight, flying just past the stalling angle of attack. But it certainly is theoretically possibly. Of course, if you have enough power. That's why my original reply stated, "Think F-18..." I fear we're back to square one. The F-18 has more power than is necessary. You only need enough power to overcome the drag. You don't need enough power to overcome weight, which is what you seem to be saying. This theoretical possibility isn't very relevant to the context of the post to which I originally replied, e.g., "full-stall greased landings" in a typical SEL. Sure it is. It discusses the actual aerodynamics, allowing someone to consider what would be required to make a "full-stall greaser". Physical characteristics of most airplanes preclude actually stalling the wing when in a safe landable position (mainly the issue of the tail winding up too low for a safe landing), but otherwise there's no obvious reason one could not only make a "full-stall greaser", but could actually *fly* the airplane onto the runway in the stalled condition. In fact, if anything (again, ignoring the geometry of the situation) the landing scenario is the most likely scenario in which a pilot could maintain the post-stall condition, since ground effect would dramatically reduce induced drag, induced drag being a primary reason that maintaining the airplane in a flying condition past the stall is so difficult. Of course, as Todd correctly pointed out, the physical geometry of most airplanes preclude stalling the airplane when in a position for a safe landing (ie just above the runway). But you incorrectly attempt to dispute that as well. The AOA is the "angle-of-attack". It's not a vector at all, never mind the one you describe. It is true that the AOA is relative to direction of travel through air (ie the "relative wind"). One doesn't have directional motion *without* a vector. ;-) I never said there were no vectors. I said the angle-of-attack is not a vector. "Vector...Etymology: New Latin, from Latin, carrier, from vehere to carry -- more at WAY 1 a : a quantity that has magnitude and direction and that is commonly represented by a directed line segment whose length represents the magnitude and whose orientation in space represents the direction..." When in doubt, post a definition? Seriously...what purpose was that supposed to serve? When one refers to the "angle of attack" (and, yes, I know that "AOA" is the acronym), one is definitely referring to motion having both direction and magnitude. No, they are not. The angle-of-attack is a specific angle, measured between the wing's chord and the relative wind. In fact, a motionless airplane can still have an angle-of-attack, just as long as there is some wind. "Relative wind" is just a non-technical way to state this. Actually, "relative wind" is a *technical* way to state the apparent wind relative to the chord of the wing. But angle-of-attack is something else entirely. Relative wind is indeed a vector. Angle-of-attack is not. However, it would have been better stated if I had said "... relative direction of _the wing's_ travel...", even though the typical SEL's wing pitch isn't drastically different from the rest of the aircraft. ;-) The angle of incidence (which is what you appear to be talking about now...that is, the angle between the wing chord and the longitudinal axis of the airplane) is yet again something else entirely different from angle-of-attack. The phrase you suggest as a replacement for angle-of-attack (that is, "relative direction of _the wing's_ travel") would not be a suitable replacement at all for "angle-of-attack", though it might serve as an synonymous phrase for "relative wind". The confusion here is not between the airplane's pitch angle and the wing's angle-of-attack. It's your insistence on calling the angle-of-attack a vector, when it's a scalar (and, it appears, your confusion between "relative wind" and "angle-of-attack"). My reply specifically separates the AOA from any ground reference. Actually, your reply implies that Todd doesn't understand that the angle-of-attack isn't measure relative to the ground. He does understand that, but the fact that angle-of-attack isn't measured relative to the ground doesn't change the fact that you can't stall most planes while in a position for a safe landing. And it is true that with most airplanes, the stalling angle-of-attack produces a pitch angle so nose-high that the tail will hit the ground before the main gear does. I responded to that. In the context of landing, if one flies slowly enough to stall, one can stall "flat" relative to the ground because the decrease in forward "relative wind" increases the AOA. That is what my remark addresses. Your claim is incorrect. As long as the airplane is flying just above the ground, the relative wind is parallel to the ground. No change in the angle-of-attack will occur from any decrease in speed, not directly. It is simply impossible to do what you suggest one might do. If one "flies slowly enough to stall", the angle-of-attack is at the stalling angle-of-attack, period. Furthermore, if one flies at a constant altitude (as one must do when landing an airplane, once over the runway), the relative wind is parallel to the ground, and thus the airplane's pitch angle is the same as the wing's angle-of-attack (ignoring the angle of incidence, of course). What WILL happen is that as the aircraft slows, the pitch angle of the aircraft will need to be increased, so as to continually increase the angle-of-attack of the wing. The increase in AOA increases the lift coefficient, compensating for the reduction in airspeed to maintain a lift force equal to the airplane's weight. If you do not increase the pitch angle, the airplane will simply descend onto the runway. You will not stall "flat" relative to the ground. Only one of two things can happen in the scenario you describe. You will either prevent the airplane from touching the runway by continually increase the angle-of-attack (which means no stall "flat" relative to the ground) , or the airplane will descend and touch the runway (again, no stall "flat" relative to the ground). In *either* case, the airplane will touch the runway before the wing stalls, assuming a safe landing. You are certainly correct that an airplane can be stalled in any attitude. But that in no way provides a basis for disagreement with Todd's statements. I think it does with regard to necessarily hitting the tail before stalling. You think wrong. Pete |
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