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#11
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Isn't it the case, in an earth-centric reference frame, that an object moving in a straight line, when subjected to a horizontal force perpendicular to that motion, will move in a circle? If the force is in fact perpendicular to the MOTION (the course) and not to the HEADING. In the case of simply banking, the nose does not move (so the heading remains the same) while the course changes (due to the force you are talking about). But so long as the nose does not change direction, the force will not change direction either and the plane will simply continue in a straight line, in a slip. (Actually, if you simply bank the plane (with alerons), the nose will tend to move in the =opposite= direction, because of the drag induced by the alerons. If your plane uses spoilers, the reverse is true) Jose -- (for Email, make the obvious changes in my address) |
#12
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Now, if the plane goes into a shallow bank, the wing that is lowered will become more level, This is NOT how dihedral works. Dihedral depends on sideslip. No sideslip, no stabilizing force. Huh? I don't believe that is true, but explan further. Jose -- (for Email, make the obvious changes in my address) |
#13
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Huh? I don't believe that is true, but explan further.
First, understand that for an airplane to right itself from a bank, there must be a moment that rotates the aircraft around the longitudinal axis. Therefore, the total lift on one wing must be greater than the other. This extra force, times the moment arm, creates the necessary moment. The orientation of the wing lift to gravity is irrelevant. When the aircraft sideslips in one direction or the other, there is a component of the relative wind that moves parallel to the lateral axis of the airplane. Due to dihedral, parallel to the lateral axis is not parallel to the wing. When you resolve the vectors, you can see that the sideslip creates a component of the relative wind perpendicular to the surface of the wing, which increases its angle of attack. The reverse happens on the other wing. Most aerodynamics books contain detailed vector diagrams of this. The FAA's material get this wrong, as does most aviation training material. The point is, all dihedral effects depend on sideslip. In theory, an aircraft in a coordinated turn should experience no righting tendencies. |
#14
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Exactly right, dihedral has no effect in a coordinated turn.
On 2004-04-17 10:37:18 -0400, Greg Esres said: Huh? I don't believe that is true, but explan further. First, understand that for an airplane to right itself from a bank, there must be a moment that rotates the aircraft around the longitudinal axis. Therefore, the total lift on one wing must be greater than the other. This extra force, times the moment arm, creates the necessary moment. The orientation of the wing lift to gravity is irrelevant. When the aircraft sideslips in one direction or the other, there is a component of the relative wind that moves parallel to the lateral axis of the airplane. Due to dihedral, parallel to the lateral axis is not parallel to the wing. When you resolve the vectors, you can see that the sideslip creates a component of the relative wind perpendicular to the surface of the wing, which increases its angle of attack. The reverse happens on the other wing. Most aerodynamics books contain detailed vector diagrams of this. The FAA's material get this wrong, as does most aviation training material. The point is, all dihedral effects depend on sideslip. In theory, an aircraft in a coordinated turn should experience no righting tendencies. |
#15
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First, understand that for an airplane to right itself from a bank, there must be a moment that rotates the aircraft around the longitudinal axis. Therefore, the total lift on one wing must be greater than the other. This extra force, times the moment arm, creates the necessary moment. The orientation of the wing lift to gravity is irrelevant. Gee, now that you mention it, you're right. gobsmacked When the aircraft sideslips in one direction or the other, there is a component of the relative wind that moves parallel to the lateral axis of the airplane. Due to dihedral, parallel to the lateral axis is not parallel to the wing. When you resolve the vectors, you can see that the sideslip creates a component of the relative wind perpendicular to the surface of the wing, which increases its angle of attack. The reverse happens on the other wing. So the high wing is sort of "blown back down" by the sideways component of the relative wind. Makes sense to me. (the raised wing has a lower angle of attack, the lowered wing has a higher angle of attack). Never thought of it all the way through that way. Thanks. Jose -- (for Email, make the obvious changes in my address) |
#16
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"Teacherjh" wrote in message ... Isn't it the case, in an earth-centric reference frame, that an object moving in a straight line, when subjected to a horizontal force perpendicular to that motion, will move in a circle? If the force is in fact perpendicular to the MOTION (the course) and not to the HEADING. In the case of simply banking, the nose does not move (so the heading remains the same) while the course changes (due to the force you are talking about). But so long as the nose does not change direction, the force will not change direction either and the plane will simply continue in a straight line, in a slip. I'm having a hard time wrapping my brain around this. Maybe the different reference frames are confusing me. Let's say I put the plane into a 30 degree bank with ailerons yet maintain neutral rudder. To simplify things let's say I do this instantaneously. Right at that moment, the nose is pointing in the original heading. But what happens as this fairly large (earth referenced) horizontal lift force acts on the plane; clearly it moves it sideways. But what happens to the heading, that is messing me up. If the force continues, either it results in an acceleration which causes increasing velocity or it reaches a terminal velocity where the force is balanced by drag. Does the slip result in such large drag so quickly that the net result is similar to a crosswind, or does the plane act in the medium it is flying in (of course ignoring the earth below) and have the nose indeed change heading because the plane is now "climbing" sideways? I know from experience I need a lot of *opposite* rudder to counteract a 30 degree bank and keep the nose on the same heading (as in crosswind landings). |
#17
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Let's say I put the plane into a 30 degree bank with ailerons yet maintain neutral rudder. To simplify things let's say I do this instantaneously. Right at that moment, the nose is pointing in the original heading. But what happens as this fairly large (earth referenced) horizontal lift force acts on the plane; clearly it moves it sideways. But what happens to the heading, that is messing me up. The heading does not change. The force is pushing the aircraft sideways. Well, ok, the vertical tail will encounter some (sideways) resistance, causing the plane to weathervane a bit into the wind and changing the heading too, but that effect is small for small banks. If the force continues, either it results in an acceleration which causes increasing velocity or it reaches a terminal velocity where the force is balanced by drag. The latter, ultimately. And yes, the plane is then "climbing" sideways. Part of the reason for the heading change you would fight is the tail, which produces down lift, keeping the nose up against gravity. When banked, this results in a turn. But then less is available to keep the nose up, which is why you apply back pressure on the elevators. So all these things are interdependent, and become more so as the amount of bank is increased. Taken to the extreme (an aerobatics pilot might chime in here), if the wings are vertical (a knife edge) the only thing keeping the airplane up is the (once) vertical stabilizer and the fuselage. The (once) horizontal tail will want to keep the nose "up", which is sideways, and the plane will want to turn (do a loop horizontally). Ignoring other effects, of course. Jose -- (for Email, make the obvious changes in my address) |
#18
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Peter Gottlieb wrote: Let's say I put the plane into a 30 degree bank with ailerons yet maintain neutral rudder. To simplify things let's say I do this instantaneously. Right at that moment, the nose is pointing in the original heading. In my aircraft, the nose would swing in the opposite direction to the bank. The plane would continue to fly the original heading. Sideways. George Patterson This marriage is off to a shaky start. The groom just asked the band to play "Your cheatin' heart", and the bride just requested "Don't come home a'drinkin' with lovin' on your mind". |
#19
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On Sat, 17 Apr 2004 at 14:53:33 in message
, Teacherjh wrote: So the high wing is sort of "blown back down" by the sideways component of the relative wind. Makes sense to me. (the raised wing has a lower angle of attack, the lowered wing has a higher angle of attack). Don't think too much in terms of raising and lowering as it is a sideslip effect and can occur at any attitude. The sideslip can be generated by the rudder. This is why it is surprisingly easy to fly an RC model with rudder and elevators only. Plenty of dihedral and a powerful rudder and the slip generated by the rudder will create a roll and you fly around almost forgetting you have no ailerons. There are some things you cannot do tidily though. Not easy to do an axial role for example. -- David CL Francis |
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