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Andrew Sarangan wrote:
Your analogy with driving tells me a little about your line of thinking. In that case, why does the car to slow down when it hits a steep hill? It is due to the inability of the engine to respond fast enough for the sudden demand in power. No, it is because there is always a lag in a real world feedback control system or it goes into oscillation. The lag is due to what is called the margin of stability. It is possible to design a control system that is virtually instantaneous. This is called a critically damped system. The problem with that is that if anything changes, like linkages wear, the system can easily go underdamped and it goes into oscillation. You don't want to be in a vehicle at 65 MPH with the cruise control going into oscillation. So for safety, cruise control systems are over damped, i.e. have a large margin of stability. For a fair explanation of control systems and stability, see http://en.wikipedia.org/wiki/Control_theory snip rest -- Jim Pennino Remove .spam.sux to reply. |
#2
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Andrew Sarangan wrote:
Your analogy with driving tells me a little about your line of thinking. In that case, why does the car to slow down when it hits a steep hill? It is due to the inability of the engine to respond fast enough for the sudden demand in power. Obviously, the cruise control does a pretty good job over small hills otherwise we would not be using them at all. If the engine were powerful enough and had a quick response, it should be able to maintain a constant speed over a steep hill. When you manually apply some extra throttle in anticipation of the approaching the hill, you are in fact 'helping' the cruise control do its job better. You are not doing something the cruise control is inherently incapable of doing. You are simply reducing the transient period. If left to its own device, the cruise control should eventually reach the set cruise speed over the hill, unless the engine is too small for the hill. No, it still isn't the same. No matter how large the engine, or how fast it responds, the end result is that a control system takes no action until an error is present. So at least SOME loss or gain in speed is required for the cruise to work, that is inherent in any feedback control system. Sure, if you can measure the error with greater resolution, and have a very large actuator with very fast response, you can make the amount of divergence from set point ever smaller, but you can't take it to zero. Consider an imaginary airplane with an infinitely large vertical fin. Would it need rudder to fly co-ordinated? I hope you would agree that the answer is no. The infinitely sized fin will generate an infinite restoring force, which really means the airplane will never deviate from co-ordinated flight. Now reduce the fin size to something smaller and practical. The restoring force will also scale down. In this case, the force may not be large enough to restore co-ordinated flight in all possible scenarios, such as slow flight and steep turns. In some cases it may experience a longer transient, and in some cases it may not reach co-ordinated flight at all. It all depends on how large the fin is, and how much air is flowing around it. In such cases where the fin can't do its job satisfactorily, the rudder is used to help it along. Same here. An infinitely large fin has infinite drag and thus the airplane would not fly so stability would not be an issue. :-) However, for any practical airplane with any adverse yaw forces during a turn, a fin alone will not maintain coordinated flight. A larger fin on a longer tail will get closer to be sure, but at least SOME yaw divergence is required for the fin to work. It is inherent in the way it works. There simply is not way to eliminate that fact. A rudder works differently since it gets its ability to act from other than aerodynamic forces (the pilot pushing on the rudder provides the actuation force). The rudder than thus provide yaw forces independent of any yaw displacement. The fin simply can't do this. So I still do not see your line of thinking. Well, I've given it my best shot, so I'll sign off now. I can't think of any other way to explain it. Matt |
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On May 30, 5:57 am, Matt Whiting wrote:
Andrew Sarangan wrote: Your analogy with driving tells me a little about your line of thinking. In that case, why does the car to slow down when it hits a steep hill? It is due to the inability of the engine to respond fast enough for the sudden demand in power. Obviously, the cruise control does a pretty good job over small hills otherwise we would not be using them at all. If the engine were powerful enough and had a quick response, it should be able to maintain a constant speed over a steep hill. When you manually apply some extra throttle in anticipation of the approaching the hill, you are in fact 'helping' the cruise control do its job better. You are not doing something the cruise control is inherently incapable of doing. You are simply reducing the transient period. If left to its own device, the cruise control should eventually reach the set cruise speed over the hill, unless the engine is too small for the hill. No, it still isn't the same. No matter how large the engine, or how fast it responds, the end result is that a control system takes no action until an error is present. So at least SOME loss or gain in speed is required for the cruise to work, that is inherent in any feedback control system. Sure, if you can measure the error with greater resolution, and have a very large actuator with very fast response, you can make the amount of divergence from set point ever smaller, but you can't take it to zero. Consider an imaginary airplane with an infinitely large vertical fin. Would it need rudder to fly co-ordinated? I hope you would agree that the answer is no. The infinitely sized fin will generate an infinite restoring force, which really means the airplane will never deviate from co-ordinated flight. Now reduce the fin size to something smaller and practical. The restoring force will also scale down. In this case, the force may not be large enough to restore co-ordinated flight in all possible scenarios, such as slow flight and steep turns. In some cases it may experience a longer transient, and in some cases it may not reach co-ordinated flight at all. It all depends on how large the fin is, and how much air is flowing around it. In such cases where the fin can't do its job satisfactorily, the rudder is used to help it along. Same here. An infinitely large fin has infinite drag and thus the airplane would not fly so stability would not be an issue. :-) However, for any practical airplane with any adverse yaw forces during a turn, a fin alone will not maintain coordinated flight. A larger fin on a longer tail will get closer to be sure, but at least SOME yaw divergence is required for the fin to work. It is inherent in the way it works. There simply is not way to eliminate that fact. A rudder works differently since it gets its ability to act from other than aerodynamic forces (the pilot pushing on the rudder provides the actuation force). The rudder than thus provide yaw forces independent of any yaw displacement. The fin simply can't do this. So I still do not see your line of thinking. Well, I've given it my best shot, so I'll sign off now. I can't think of any other way to explain it. Matt Matt. Consistently excellent explanations. It is amazing to me how people tend to view (in its simplist form) that a fixed torque can overcome consistently a variable one under "all" circumstances. Nice job. Robert |
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