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#41
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On Sun, 26 Dec 2004 at 21:58:05 in message
, Peter Duniho wrote: Thrust does contribute, yes. But the primary reason for requiring additional power is that, while the wing is capable of generating the necessary thrust at a lower airspeed, higher angle of attack (all the way up to the stalling AOA of course), the higher angle of attack results in higher drag, requiring higher thrust. I think Peter that an aircraft will climb if trimmed to the same angle of attack that it was using in level flight. It does this as long as the lift is slightly less and the speed drops to produce _less_ drag and lift, leaving more engine power and thrust to climb. When climbing extra work must be done against gravity. That extra work can come from increasing power or from reducing speed and therefore drag. Nitpicking point: wings do not create thrust! :-) You meant lift of course. -- David CL Francis |
#42
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Greg Esres wrote: The relative wind will be "coming from above", since that is the direction in which the aircraft is traveling. The relative wind doesn't ever "come from above" while the aircraft has a positive angle of attack..by definition. ;-) The relative wind always comes from the direction in which the aircraft is actually traveling. If the aircraft is climbing, the relative wind comes from above the horizon; ie. it is not horizontal. Nor will the aircraft stall with the relative wind "essentially horizontal." It certainly will if the aircraft is neither climbing nor descending, is not banked, and the pitch angle exceeds the stall angle of attack. Sounds like you think there is a zero angle of attack in that situation? No, I don't. George Patterson The desire for safety stands against every great and noble enterprise. |
#43
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CV wrote: "From above" is similarly meaningless, unless we specify whether we mean it in relation to the wing/aircraft or the horizon. Yes. I meant above the horizon and failed to say so. George Patterson The desire for safety stands against every great and noble enterprise. |
#44
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"David CL Francis" wrote in message
... I think Peter that an aircraft will climb if trimmed to the same angle of attack that it was using in level flight. Well, ignoring for a moment that I never meant to suggest anything about what happens if you simply increase power without changing anything else when just above stall speed... (my comments were simply about what additional power *allows*...not what it *causes*) You can't make that generalization. Changes in power affect elevator authority (affecting trim), as well as necessary rudder input (changing drag). It is entirely possible that when just above stall speed, an increase in power will result in an increase in angle of attack, an increase in drag, or both. What you can say is that if the pilot maintains the same angle of attack, but increases power, then the airplane will climb (I don't believe that added drag from rudder will ever be MORE than the added thrust, but I could be wrong about that). But that's not really what I was talking about. It does this as long as the lift is slightly less and the speed drops to produce _less_ drag and lift, leaving more engine power and thrust to climb. At an airspeed just above stall, a reduction in speed results in MORE drag. There is a reduction in parasitic drag, but there is a greater increase in induced drag, with a net increase in total drag (and that's ignoring drag caused by the rudder and any other control surfaces that require a change in position). When climbing extra work must be done against gravity. That extra work can come from increasing power or from reducing speed and therefore drag. The extra work comes ONLY from a net surplus of power. A reduction in speed is only guaranteed to produce a net increase in power available if the new airspeed is higher than Vbg. It can sometimes also produce a net increase, if the old airspeed was sufficiently higher than Vbg, and the new airspeed is close enough to Vbg, even if less than, but you need to know more about the old and new airspeeds in that case to say for sure what happens. More importantly, a reduction in speed is guaranteed to produce a net decrease of power available if the OLD airspeed is lower than Vbg (as it is when just above stall speed). Nitpicking point: wings do not create thrust! :-) You meant lift of course. Yes, of course. Thank you. Pete |
#45
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A reduction in speed is only guaranteed to produce a net increase
in power available if the new airspeed is higher than Vbg. I think you mean higher than the airspeed for minimum power, which is lower than Vbg. Even that's not really true, since the power available curve isn't flat. Vy is maybe closer to the truth. |
#46
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"Greg Esres" wrote in message
... A reduction in speed is only guaranteed to produce a net increase in power available if the new airspeed is higher than Vbg. I think you mean higher than the airspeed for minimum power, which is lower than Vbg. Yes...sorry, I use Vbg as a nice "landmark", since it's very close to the actual speed in question. I guess I should be more explicit about that, especially in this context. |
#47
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Yes...sorry, I use Vbg as a nice "landmark", since it's very close
to the actual speed in question. I guess I should be more explicit about that, especially in this context. I thought so. You couldn't be right about so much and get that one wrong. ;-) |
#48
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On Tue, 28 Dec 2004 at 16:55:05 in message
, Peter Duniho wrote: It does this as long as the lift is slightly less and the speed drops to produce _less_ drag and lift, leaving more engine power and thrust to climb. At an airspeed just above stall, a reduction in speed results in MORE drag. There is a reduction in parasitic drag, but there is a greater increase in induced drag, with a net increase in total drag (and that's ignoring drag caused by the rudder and any other control surfaces that require a change in position). HI Peter. How easy it is to get slightly confused on Usenet! What I was trying to say is that if you maintain the same AoA then the lift drag ratio remains the same, but because the lift required when climbing is less than when flying level you can climb at a reduced speed but with less drag. So under some conditions if you just raise the nose a little you can find a new steady state where speed is slightly reduced but with the same thrust you can climb at the same AoA.. There is a maximum lift drag ration at a modest angle of attack. Above _and_ below that angle of attack that ratio worsens. When climbing extra work must be done against gravity. That extra work can come from increasing power or from reducing speed and therefore drag. The extra work comes ONLY from a net surplus of power. I agree with that. However that net surplus can come from either more engine power or reduced drag. Because even in a modest climb the engine thrust vector plus the lift vector combine to match the weight. Put another way if you are flying below the maximum lift drag ratio and you increase the AoA to the optimum whilst keeping the same power the aircraft should climb. This is self evident if you are flying level at high speed and at a climbing power setting. Bring the nose up increasing the AoA and your aircraft will definitely climb. Agreed? A reduction in speed is only guaranteed to produce a net increase in power available if the new airspeed is higher than Vbg. It can sometimes also produce a net increase, if the old airspeed was sufficiently higher than Vbg, and the new airspeed is close enough to Vbg, even if less than, but you need to know more about the old and new airspeeds in that case to say for sure what happens. More importantly, a reduction in speed is guaranteed to produce a net decrease of power available if the OLD airspeed is lower than Vbg (as it is when just above stall speed). I think that is another way of saying what I have just said? I cannot remember if we started off with an assumption that the aircraft was only just above stall speed? If so then you are correct of course. Even in a steady glide the required lift is less than that needed in level flight! That is easier to see because the drag vector helps the lift match the gravity vector. -- David CL Francis |
#49
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"David CL Francis" wrote in message
... [...] So under some conditions if you just raise the nose a little you can find a new steady state where speed is slightly reduced but with the same thrust you can climb at the same AoA.. Your theory sounds wonderful, but I doubt it holds water in practice. Thrust does not reduce the required lift by much, especially not in the light planes we tend to fly. Steady-state pitch angles in climbs tend to be modest, meaning a tiny fraction of the thrust vector is the downward component. Just 17% of the total thrust, for a 10 degree pitch angle. Given how little thrust a light plane has in the first place (only a relatively small fraction of the total weight of the airplane in the first place, less than 10% in at least some cases, perhaps most cases), even taking 20% (or even 34%) of that and applying it to lift just isn't going to help that much. Even assuming an airplane with a thrust-to-weight ratio of 1.0 (a rare occurrance, but they do exist...some F-16s, for example), I'm not sure your theory holds up very well. You might think that you could simply increase thrust as you slow the airplane in order to allow a smaller AOA to suffice to provide the remaining necessary lift. But there's a problem with that idea. If the AOA is kept small, then the increased thrust will prevent the airplane from decellerating. To have a vertical component high enough to support the airplane will require a horizontal component so high that the airplane won't slow. If the AOA is allowed to increase, then what the increased thrust is actually allowing is for the wing to stall without the airplane being pulled downward by gravity (the wing WILL stall at the appropriate AOA, regardless of airspeed). It's not demonstrating anything about some "new steady state". The above thought experiment should also illustrate another problem with your theory: it ignores the change in the portion of lift actually contributing to counteracting gravity that occurs due to pitch changes. See below for more commentary on that. Of course, to me the biggest problem intuitively with your theory is that I am sure that aerodynamics involves only continuous functions. Given that, if you assume more than one steady state, you are claiming that there are multiple local minima/maxima between which are apparently "lower efficiency" areas. And of course, if there's more than one, I see no reason to believe that there are only two. This would then imply that the flight envelope has numerous of these local minima/maxima points. Given that in more than 100 years of study, this concept has never shown up as a noted element of the relationship between speed, drag, and lift, I'm inclined to believe that it's just not true (just as the idea of "cruising on the step" is not true). I admit that I have not brought out the equations and proved my point irrefutably. Someone like Julian Scarfe or Todd Pattist would probably do a better job discussing this, since they seem to be more "math oriented" (that is, they don't mind crunching some equations now and then ). But I still feel reasonably confident that there's no secondary "new steady state" one can achieve by increasing AOA and taking advantage of thrust. The extra work comes ONLY from a net surplus of power. I agree with that. However that net surplus can come from either more engine power or reduced drag. Because even in a modest climb the engine thrust vector plus the lift vector combine to match the weight. I assume by "the engine thrust vector" you really mean "the vertical component of the engine thrust vector". Assuming that, I agree with your statement, but I don't find it informative. The engine thrust vector has a non-zero vertical component even during level cruise flight, and yes it does contribute to counteracting gravity, reducing the lift required. But when you ask the question about HOW MUCH it does this, the answer is "not enough to change the fundamentals". Not for the airplanes we fly, and I think probably not for any airplane. Put another way if you are flying below the maximum lift drag ratio and you increase the AoA to the optimum whilst keeping the same power the aircraft should climb. This is self evident if you are flying level at high speed and at a climbing power setting. Bring the nose up increasing the AoA and your aircraft will definitely climb. Agreed? I never said anything to the contrary. If you are at an angle of attack lower than the best L/D AOA (and thus at an airspeed higher than the L/Dmax airspeed), increasing pitch angle without a change in power will result in a climb, yes. But that has nothing to do with what happens at an airpseed near stall, which occurs below the L/Dmax airspeed, and well above the best L/D AOA. I think that is another way of saying what I have just said? Well, since I seem to disagree with what you said, I sure hope not. I cannot remember if we started off with an assumption that the aircraft was only just above stall speed? If so then you are correct of course. Yes, this part of the discussion was entirely about the regime of flight near the stalling speed and AOA. Reducing drag by pitching up while above L/Dmax airspeed is uninteresting, since that's a direct consequence of slowing to an airspeed closer to L/Dmax airspeed. Even in a steady glide the required lift is less than that needed in level flight! That is easier to see because the drag vector helps the lift match the gravity vector. This seems like a good point at which to mention something else you've left out and which I alluded to earlier... Lift is always generated perpendicular to the wing's chord. Some people like to call just the vertical component of this force "lift", but the amount of force acting through the wing is the force perpendicular to the chord. However you label things, you cannot avoid the fact that when you change the angle of the lift vector, the portion of the force created by the wing used to counteract gravity is also changed. In particular, in the low-airspeed, power-on example, even as thrust is helping support the airplane, you are using your lift less efficiently, which means that the wing needs to generate more total lift just to provide the necessary vertical component. This is similar to the required increase in lift while in a turn, but due to redirecting the lift vector in a different way. Since the lift vector points slightly aft in level flight, even at high airspeeds when the angle of attack is low, it's easier to see how this negates at least some of whatever contribution thrust might make as the angle of attack is increased. However, in gliding flight, the vector is pointed forward, helping counteract the contribution drag makes to lift. I wrote "at least some" up there, but it should be apparent from the disparate magnitudes of the lift and thrust vectors that you get a much more significant change in lift than you do in thrust. The bottom line he even though thrust does contribute at least a little to counteracting gravity, it does not do so in a way significant enough to change the fact that, as you slow the airplane from any airspeed at or below L/Dmax airspeed, you experience increased drag. Pete |
#50
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relatively small fraction of the total weight of the airplane in the first place, less than 10% in at least some cases, perhaps most cases) Lift in a 10 degree climb should be reduced about 1.5%. I'm not sure your theory holds up very well. "His" theory is mentioned in a number of aerodynamics books. Although I agree that a small increase in AOA would not contribute enough vertical component of lift to overcome the initial increase in induced drag, there are ways to get into this regime of flight. If you had enough unused AOA left to generate a load factor, you could change the flight path then return the AOA to its original value. The aircraft may be able to stay on a steeper flight path due to the reduced parasite drag and reduced effective weight. Don't forget that thrust will increase slightly with a lower airspeed. To have a vertical component high enough to support the airplane will require a horizontal component so high that the airplane won't slow. Not really clear on what you mean by that. The only component of thrust that accelerates the airplane is that parallel to the flight path. If the angle of climb remains the same, then increasing thrust will obviously accelerate the airplane. However, as thrust increases, the angle of climb increases up to the point that the component of aircraft weight along the flight path is equal to the increase in thrust. Of course, to me the biggest problem intuitively with your theory is that I am sure that aerodynamics involves only continuous functions. Given that, if you assume more than one steady state, you are claiming that there are multiple local minima/maxima between which are apparently "lower efficiency" areas. And of course, if there's more than one, I see no reason to believe that there are only two. This would then imply that the flight envelope has numerous of these local minima/maxima points. All of the above is very vague. What I hear you say is "I don't want to believe you." ;-) There are an infinite number of steady states; every time I move the elevator, I create a new steady state. Given that in more than 100 years of study, this concept has never shown up I doubt you're familiar with even 1% of the 100 years of aerodynamic research and thought. I'm certainly not. You should realize that "I've never heard of it so it must be false" is a weak argument. Lift is always generated perpendicular to the wing's chord. No, for subsonic flight, it's perpendicular to the *local* relative wind, the relative wind that is modified by wingtip vortices. If lift were perpendicular to the chordline, you would have induced drag in a wind tunnel, and you don't. you cannot avoid the fact that when you change the angle of the lift vector, the portion of the force created by the wing used to counteract gravity is also changed. This is based on your mistaken notion above. Since the lift vector points slightly aft in level flight, Only because of induced drag. However, in gliding flight, the vector is pointed forward, helping counteract the contribution drag makes to lift. No, the lift vector is perpendicular to the local relative wind causing it. There is a rearward component (called "induced drag"), but there is no component forward along the flight path. |
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