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#31
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Andrew Sarangan wrote:
Sure, but that still does not explain why the airplane turns. A horizontal component of lift will make the airplane side-slip, not turn. It is the stability (weathervane effect) that makes the airplane turn. By definition, the 'weathervane effect' occurs because air exerts more pressure on one side of the object (aircraft) than the other - same definition as slipping. Therefore, are you are asserting that an aircraft turns because it is slipping? More over, it will only turn if it is slipping? Hilton |
#32
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On Sun, 07 Nov 2004 06:13:43 -0500, Cub Driver
wrote: He wants to know why his pilot can't simply do rolls around the plane's longitudinal axis. Perhaps it would help to know what inputs are necessary to make such a roll? If you are rolling to your left, do you apply right rudder? It depends on the aircraft you're flying. I imagine jets require very little "top rudder" to do an axial roll. Some of the hot aerobatic planes scarcely allow time for rudder input, I imagine. Here's the sequence I learned for a slow, axial roll: Input aileron. Hold it through the maneuver for a steady roll rate. Input "top rudder" as the plane reaches knife-edge. If rolling to the left, right rudder will be on top first. Back to neutral on rudder as the plane goes inverted. Push on the stick to maintain level flight upside-down. Release the pressure on the elevator. Input top rudder as the plane reaches knife-edge. In the example of rolling to the left, this will now be left rudder. Release the pressure on the rudder as the plane rolls upright. Apply any necessary backpressure to maintain level flight. This is so much easier to do than it is to explain in writing. To do a rolling circle, add in appropriate inputs toward the center of the circle as needed. To do the rolls in a straight line, be careful to get the inputs in at the right time. Marty |
#33
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We are on the same page. The analogy with describing lift is spot on.
Never flown a 195 but I bet it rolls a bit with rudder too. I'd bet it has a little to do with fuselage blanking the trailing wing lift a bit while the leading wing works a little better. But all of that is a wild ass guess. I'll have to ask our resident 195 guy how it responds to rudder. Thanks. "Dudley Henriques" wrote in message ink.net... "Maule Driver" wrote in message om... I was always under the impression that in most light a/c, the dominate rolling effect from 'inside' rudder is the result of dihedral (or alternatively, sweep back). RC modelers are pretty adept at setting up aircraft for 'pure' yaw from rudder input - no dihedral, symmetrical layout, etc. Or setting up aircraft to bank and turn without ailerons - lots of didedral. Though I'd agree that the "forward motion of the outside wing" explanation accurately predicts the rolling effect from rudder input that occurs on most a/c - which is ok for training purposes. I would agree with this completely. Dihedral contributes heavily to the lateral stability of the aircraft if sideslip is present to be sure. The prime contribution of dihedral is in the development of a stable rolling moment with sideslip, which is consistent with what most of us are saying. The problem with answering many questions in aerodynamics is that there isn't one single example or answer that will suffice. (Lift is a PRIME example of this. ) Anyone trying to explain lift in a simple sentence will find a slew of missing data soon to follow :-) The problem in aerodynamics is that in much of what is happening, several explanations are in force physically together at one instant in time. The way we look at dihedral in the flight test community is primarily as it's effect on the lateral stability scenario which relates with sideslip present to relative wind, differential in angle of attack, changes in lift raising a windward wing producing stability. I think we're both on the same page, and dealing with the same effect since all of what we're discussing is present in complimentary yaw IF dihedral is present. Now, if we inject an airplane into this equation like a Cessna 195 for example.......... :-)))) Dudley Henriques International Fighter Pilots Fellowship for email; take out the trash |
#34
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"Hilton" wrote in
ink.net: Andrew Sarangan wrote: Sure, but that still does not explain why the airplane turns. A horizontal component of lift will make the airplane side-slip, not turn. It is the stability (weathervane effect) that makes the airplane turn. By definition, the 'weathervane effect' occurs because air exerts more pressure on one side of the object (aircraft) than the other - same definition as slipping. Therefore, are you are asserting that an aircraft turns because it is slipping? More over, it will only turn if it is slipping? Hilton Think of the space shuttle. If you fire rockets horizontal to the flight path, the shuttle will slide sideways. It will not turn the nose towards the direction of travel. An airplane turns because it wants to point the nose into the relative wind. Posted Via Usenet.com Premium Usenet Newsgroup Services ---------------------------------------------------------- ** SPEED ** RETENTION ** COMPLETION ** ANONYMITY ** ---------------------------------------------------------- http://www.usenet.com |
#35
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"Cub Driver" wrote in message ... On Sun, 07 Nov 2004 05:09:36 GMT, "Dudley Henriques" wrote: "why does the banking of wings by the use of ailerons not just roll an aircraft but also produces a turn He wants to know why his pilot can't simply do rolls around the plane's longitudinal axis. Perhaps it would help to know what inputs are necessary to make such a roll? If you are rolling to your left, do you apply right rudder? A slow roll is extremely difficult to visualize with verbal interpretation only. The best way to describe what you are asking..say a slow roll to the left is to first understand that a slow roll isn't exactly a roll dead on the longitudinal axis as the axis relates to a "level" roll. The visualization of a slow roll to the left can be made by envisioning a reverse capital letter D, which naturally isn't in my little bag of computer symbols, so a roll to the right is much easier to visualize. So to the right, you can envision a capital letter D. The reason for the D shape is because to do the roll properly, you will have to pass through exact inverted at the airplane's level flight inverted attitude to keep altitude in check during the roll. The tightness of the D shape will depend on the wing design of the airplane you're flying. A symmetrical wing on a Pitts, or an Extra for example will be a much tighter D than say a Citabria with a high lift cambered wing. Anyway, the vertical line of the D represents the raising of the nose during the first half of the roll until right past the full inverted position. At that point, the nose must again be lowered to a normal level flight attitude, and this is represented by the curve on the D returning the airplane back to upright level flight attitude. The roll can be done with inside rudder at roll initiation or without insider rudder. When flying airshow demonstrations, I seldom used inside rudder with inside aileron when initiating a slow roll. Reason for this is that adverse yaw will pull the nose outside, which if caught exactly right, will result in your going straight to the required top rudder you need to carry you through knife edge at the right spot on the roll axis, and if it's a point roll, you don't have to change feet at the first point, but to do this requires a fairly fast roll rate, so it's aircraft specific. In the P51 for example, the roll rate isn't fast enough to use the adverse yaw generated so that it negates the normal inside rudder you need to counter that adverse yaw, which in turn means you enter with aileron and insider rudder, and immediately go to top rudder as soon as a positive coordinated roll entry has been accomplished. Back to the Pitts, entering into the first knife edge, you have steady inside aileron and have blended in enough positive pitch with elevator to climb the vertical D line nose high for the inverted level transition.You hold top rudder through knife edge and then start blending in whatever forward stick you need to pint the nose at level inverted. You're also blending off the top rudder at this point at a rate that will neutralize it passing through inverted. Stick at this point is forward and in the corner for aileron and elevator blending. Passing through inverted, you switch to opposite side top rudder and reverse the procedure, blending in past the second knife edge, as you come back down the backside of the D curve whatever back pressure you need to return the nose to normal level flight attitude. What I've just described is much better learned in actual flight, where a slow roll can be done by rote, then understood afterwards...........and in far less words to boot :-) Dudley Henriques International Fighter Pilots Fellowship for email; take out the trash |
#36
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"Dudley Henriques" wrote in message ink.net... "Cub Driver" wrote in message ... On Sun, 07 Nov 2004 05:09:36 GMT, "Dudley Henriques" wrote: "why does the banking of wings by the use of ailerons not just roll an aircraft but also produces a turn He wants to know why his pilot can't simply do rolls around the plane's longitudinal axis. Perhaps it would help to know what inputs are necessary to make such a roll? If you are rolling to your left, do you apply right rudder? A slow roll is extremely difficult to visualize with verbal interpretation only. The best way to describe what you are asking..say a slow roll to the left is to first understand that a slow roll isn't exactly a roll dead on the longitudinal axis as the axis relates to a "level" roll. The visualization of a slow roll to the left can be made by envisioning a reverse capital letter D, which naturally isn't in my little bag of computer symbols, so a roll to the right is much easier to visualize. So to the right, you can envision a capital letter D. The reason for the D shape is because to do the roll properly, you will have to pass through exact inverted at the airplane's level flight inverted attitude to keep altitude in check during the roll. The tightness of the D shape will depend on the wing design of the airplane you're flying. A symmetrical wing on a Pitts, or an Extra for example will be a much tighter D than say a Citabria with a high lift cambered wing. Anyway, the vertical line of the D represents the raising of the nose during the first half of the roll until right past the full inverted position. At that point, the nose must again be lowered to a normal level flight attitude, and this is represented by the curve on the D returning the airplane back to upright level flight attitude. The roll can be done with inside rudder at roll initiation or without inside rudder. When flying airshow demonstrations, I seldom used inside rudder with inside aileron when initiating a slow roll. Reason for this is that adverse yaw will pull the nose outside, which if caught exactly right, will result in your going straight to the required top rudder you need to carry you through knife edge at the right spot on the roll axis, and if it's a point roll, you don't have to change feet at the first point, but to do this requires a fairly fast roll rate, so it's aircraft specific. In the P51 for example, the roll rate isn't fast enough to use the adverse yaw generated so that it negates the normal inside rudder you need to counter that adverse yaw, which in turn means you enter with aileron and inside rudder, and immediately go to top rudder as soon as a positive coordinated roll entry has been accomplished. Back to the Pitts, entering into the first knife edge, you have steady inside aileron and have blended in enough positive pitch with elevator to climb the vertical D line nose high for the inverted level transition.You hold top rudder through knife edge and then start blending in whatever forward stick you need to pin the nose at level inverted. You're also blending off the top rudder at this point at a rate that will neutralize it passing through inverted. Stick at this point is forward and in the corner for aileron and elevator blending. Passing through inverted, you switch to opposite side top rudder and reverse the procedure, blending in past the second knife edge, as you come back down the backside of the D curve whatever back pressure you need to return the nose to normal level flight attitude. What I've just described is much better learned in actual flight, where a slow roll can be done by rote, then understood afterwards...........and in far less words to boot :-) Dudley Henriques International Fighter Pilots Fellowship for email; take out the trash |
#37
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"Maule Driver" wrote in message om... We are on the same page. The analogy with describing lift is spot on. Never flown a 195 but I bet it rolls a bit with rudder too. I'd bet it has a little to do with fuselage blanking the trailing wing lift a bit while the leading wing works a little better. But all of that is a wild ass guess. I'll have to ask our resident 195 guy how it responds to rudder. I flew one years ago. Strong airplane...reminded me of something made out of solid aluminum :-)) It flew wonderfully...extremely stable. Sort of like a Beaver really. As for a turn resulting from pure rudder input on these airplanes....you still have that outside wing going faster than the inside wing irregardless of the existence or non existence of dihedral, so it will turn anyway eventually :-) Dudley |
#38
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I don't think anyone has hit on what is really going on here. The
explanations about the sideways force created by lift when the AC is banked cause it to move ( accelerate ) to the side, but not rotate. The dihedral wing explanation doesn't work either, because the raised wing's horizontal force is applied aft of the CG, thus causing rotation in the opposite direction of the turn. The major reason that a plane rotates about the vertical axis during a turn is wind vaning. You bank the plane, the lift is broken into horizontal and vertical components. The horizontal force causes the plane to accelerate to the side, but not rotate as others have stated. Now with the plane picking up speed in the lateral direction, the relative wind is now coming from one side of the plane, a forward quartering headwind! As anyone who has taxied on a windy day knows, planes have a natural tendency to face into the wind. This is caused by the big wind vane we call a tail. This rotates the plane into the wind. The relative wind is thus always leading the plane by a few degrees, causing a continued rotation. And as a side benefit, the horizontal acceleration is countered by the centripetal force of the turn, so we don't continue to accelerate to faster and faster horizontal speeds. |
#39
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Andrew,
Andrew Sarangan wrote: Hilton wrote: Andrew Sarangan wrote: Sure, but that still does not explain why the airplane turns. A horizontal component of lift will make the airplane side-slip, not turn. It is the stability (weathervane effect) that makes the airplane turn. By definition, the 'weathervane effect' occurs because air exerts more pressure on one side of the object (aircraft) than the other - same definition as slipping. Therefore, are you are asserting that an aircraft turns because it is slipping? More over, it will only turn if it is slipping? Hilton Think of the space shuttle. If you fire rockets horizontal to the flight path, the shuttle will slide sideways. It will not turn the nose towards the direction of travel. An airplane turns because it wants to point the nose into the relative wind. I understand that, but you never answered my question: "Are you asserting that an aircraft turns because it is slipping (weathervaning)?" Hilton |
#40
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