Post by lyle on Feb 1, 2008 22:29:53 GMT -5
Hi All,
I’ve been thinking about this knife edge pitch coupling phenomenon for quite some time now and have come to some limited understanding of the problem that might help put together some of the pieces of the puzzle. First I should say that the pitch coupling isn’t dependant on whether you fly in a knife edge attitude but rather it is dependant on the airplane’s angle of attack, angle of sideslip and rudder deflection. I noticed that one person identified a situation in which the application of rudder caused the airplane to pitch toward the gear regardless of the airplane’s attitude. As most of us know the airplane doesn’t know if it’s upright, inverted, vertical, on its side, etc. Such is the case with the pitching moments that we experience in knife-edge flight in that they aren’t dependent on the plane’s attitude. The interesting part of all this is what physically causes the airplane to exhibit these cross-coupling tendencies. I’ve isolated at least three things that I feel are the main contributors to this pitch-coupling problem. I’m certain that there are more so I don’t want anyone to think that this list is exhaustive. Every time I looks at this problem from a different perspective I find another possible contributor to the pitch coupling phenomenon…that’s what makes it so interesting.
1) PITCHING MOMENTS DUE TO SIDESLIP
By sideslip angle I simply mean the angle of attack of the airplane in a directional sense. In the Aero world we typically refer to the angle of attack and the angle of sideslip as “alpha” and “beta” (greek symbols). With this in mind I should say that most aero types also believe that the airplane only cares about alpha and beta. We like to nondimensinalize things so we don’t have to confuse the issue with things like wing loading, airspeed, air density, g’s etc. Aerodynamically the airplane doesn’t really know anything about those dimensional things. In the real world we can’t, as R/C pilots, go around proclaiming that my airplane was flying yesterday straight and level at a lift coefficient (CL) of 0.001 and get all excited…people would think we were crazy. However, we can and do get excited when we say, “You should have seen my airplane, It was going 250 mph…that’s cool!”. Everything has its place even dimensional quantities, but from the airplane’s point of view the forces and moments that affect it are only a function of five things: 1) angle of attack , 2) angle of sideslip, 3) Reynolds number, 4) Mach number, and 5) the airplane’s geometry. For now, lets neglect Reynolds number and Mach number by saying that we aren’t going too slow or too fast for either of these to be big factors. Now that I’ve explained what I mean by sideslip, we’ll proceed to why it affects pitch coupling.
A) FUSELAGE SHAPE If one considers the fuselage or body by itself you can start to see how some shapes might be prone to producing an aerodynamic pitching moment when the body is in a sideslip. Let’s consider a fuselage in which the aft section of the turtle deck is rounded on the upper surface but flat on the bottom. Such a shape while sidesliping would accelerate the air more over the upper surface than over the lower. Consequently the pressure would drop over the top of the fuselage’s turtle deck and cause a nose down pitching moment to be generated solely by the fuselage shape. I’ve played around with this in the wind tunnel using an old Calypso (Prettner design) fuselage and found that this nose down pitching moment could be measured and was nearly equal to 2 degrees of up elevator at 15 degrees of sideslip. Don’t make the mistake of thinking that my Calypso had a knife-edge-pitching tendency because it didn’t. You have to consider the total package before you make that kind of statement. As a complete airplane one could easily set it up such that you wouldn’t see any noticeable pitch coupling until very large sideslip angles. A good example of an airplane that has a large disparity between the upper and lower aft fuselage cross-sectional shape is the Cap 232. It typically exhibits a noticeable nose down pitch coupling tendency in knife-edge that is partly due to its fuselage shape. You can see this effect if you put a “aerodynamic fence” so to speak down the centerline of the turtle deck. When you fly this new configuration you will usually see a decrease in the amount of nose down pitch coupling because the fence is reducing the air’s acceleration thus reducing the pressure drop.
B) VERTICAL TAIL (sweep, span, projected area) This is a more difficult effect to explain but never the less it can be a very dominate one especially if the vertical tail is very thick, and has a large sweep angle. I should say that any airplane that has a vertical tail that only sticks out of the top of the airplane (which covers 99.9% of all airplanes in existence) would exhibit this tendency to some extent. Let’s see if I can explain why… When you sideslip, the airplane’s vertical surface acts in the same manner as a wing and generates lift because of a change the vertical tail’s angle of attack. This lift is created by a change in the pressure distribution over the surface of the vertical tail. Typically the leading edge area (L.E. to 25% of local chord) of a lifting surface has a fairly large pressure drop over the leeward side. If you look at your airplane from the front and the top and then estimate how much projected frontal area your vertical tail has from each view you will get an idea of how much ability your vertical tail has to create a pitching moment while in sideslip. In other words, consider that a rather large pressure drop occurs over those projected areas while the airplane is sidesliping. This pressure drop when multiplied by the projected area gives you a force whose vector is above the airplane’s center of gravity (CG). If you think about it and draw a few pictures you can see how the vertical tail’s height and sweep can increase the moment arm and thus increase the nose down pitching moment due to sideslip. I found this out while working on an airplane that had a rather large vertical tail sweep angle. I then started to look for it in other airplanes and found that almost all standard airplane configurations have the same tendency to pitch down due to sideslip and that the vertical tail is a large contributor to this effect. If one were to design airplanes that had symmetric vertical tails (one top and bottom) we would alleviate a large amount of this while also getting rid of the need for right thrust.
C) ONLY THE WING AND HORIZONTAL TAIL Even though you can’t actually build just a wing and a horizontal tail and fly them together without some sort of fuselage, you can still consider the problem and draw some insight to what you have before you ever add a fuselage or vertical tail to the picture. Consider a wing and horizontal tail with symmetric airfoil sections both at zero angle of incidence relative to each other. If you place this combination in sideslip you will get zero pitching moment if the angle of attack of the system is zero. What if the combination is held at some angle of attack and you sideslip at this constant angle of attack…suddenly a small amount of pitching moment will appear that changes as a function of sideslip…Why? The answer lies in the downwash change that the tail sees as the sideslip angle changes. The spanwise downwash distribution for most planforms isn’t constant (theoretically it is for an elliptic planform…but in reality this isn’t exactly true) and this is even more true when the wing is in sideslip. This effect is a function of the main wing’s lift coefficient, the vertical and longitudinal position of the horizontal tail and the center of gravity. The purpose of pointing this out is to show that even with the simplest longitudinal model you can’t get completely away from pitch coupling. This sort of simple approach shows the importance of the downwash field and how the wing’s wake affects the tail’s ability to perform. Keep in mind though that everything’s relative and if you operate at very low CL’s (light wing loading/high speeds) these effects are minimized. You also want to get the tail out of the wing’s influence by having it be far away from the wing…kinda sounds like a pattern plane doesn’t it?
D) VERTICAL POSITION OF THE HORIZONTAL TAIL (This is the one that everyone’s so concerned about!) There is much debate as to what effect this has… The general consensus is that if you lower the tail it makes the airplane more prone to pitch to the canopy and if you raise the horizontal tail the airplane will certainly pitch to the gear…is this true? I can say that if you measure this in the wind tunnel you will find the opposite to be true…to a point. As the horizontal tail gets further away from the wing, the downwash it sees is certainly less. As Mike Nauman once eloquently wrote “You can’t get away from a whirlpool by swimming towards it!” this is the exact same situation that the horizontal tail deals with while the wing is producing lift. The main effect of downwash is that it reduces the static longitudinal stability of the airplane. Technically the downwash doesn’t reduce the airplane’s stability but rather how much the downwash changes per unit change of angle of attack. (If) a 10 degree change in angle of attack produced a 10.1 degree change in the downwash where the horizontal tail is located the tail would cause the airplane to be unstable. Yes you heard it right, If you put a horizontal tail on the airplane behind the wing it could become less stable than with the wing alone…but only if the rate of change of downwash with angle of attack was greater than 1. Luckily, downwash’s change with angle of attack is almost always less than unity thus a tail is a good thing. How could you possibly get a configuration where putting on a horizontal tail behind the wing is destabilizing? If you had a very low aspect ratio wing with the tail located very close to the wing’s trailing edge you could get this sort of situation…I know it sounds crazy but it’s been done. We don’t have this problem in pattern but sometimes this example it helps in the overall understanding of how things work.
Typically pilots like the feel of a certain amount of longitudinal stability which I will call “static margin” (“Static” because we are only talking about steady-state situations and “margin” because it represents how much CG margin you have before the airplane becomes neutrally stable in pitch). To keep the same amount of static margin you would have to move the CG forward as you lower the tail. If you keep the CG in the same place as you lower the tail you will need down elevator to retrim the airplane and the airplane will become less stable in pitch. If you continue to lower the tail the downwash’s effect would lessen just like it would if you raised the tail far above the wing. This means that at some horizontal tail position far above and far below the wing the required elevator trim would be identical if you neglect the drag moment you get from having the tail way up high or down low. (not to mention the drag you would get from the vertical tail that it’s attached to) This idea also points out that having a high tail will cause a slight nose up pitching moment from the drag of the tail (for a low tail the opposite would be true)
Another effect comes from the fact that the fuselage and wing have a wake and boundary layer associated with them that extracts momentum from the air. If the tail is forced to operate in this type of environment because of its relative position to the wing, it would certainly lesson the tail’s effectiveness. This may be what’s happening in practice when people lower the tail and the pitching tendencies reduce greatly…who knows.
E) POWER EFFECTS IN SIDESLIP As with any situation on a propeller driven airplane you must take into consideration the power effects… Just note that the propwash is skewed greatly behind an airplane that is flying with a large amount of sideslip such that the propwash will eventually align itself with the freestream. Since the propeller only spins in one direction, its effect will change sign whether you sideslip to the left or the right. This is a very esoteric and unquantifiable effect but theoretically it should be there. In one direction there should be upwash on the tail and on the other there should be downwash. The same goes for the P-Factor. If you see that the airplane pitches one way when you fly knife-edge with right rudder and the other way with left you might be led to the conclusion that power effects are causing the problem.
2) PITCHING MOMENTS DUE TO RUDDER DEFLECTION
This is a bit different than the pitching moments due to sideslip because in this case we consider that the airplane is at a zero sideslip angle and we are only deflecting the rudder. Why would this cause the airplane to pitch? What happens in this case is that the close proximity between the rudder/fin and the horizontal tail can cause them to influence each other. This is highly dependent on how close the tails are to one another so the geometry of the situation is critical. Let’s pose a simple example for illustration: Consider a T-tailed airplane where the rudder is deflected 30 degrees. The rudder deflection produces the same effect as a flap deflection on a wing. The trailing edge control surface alters the pressure distribution around the whole airfoil. Typically one will see that the suction side is much greater than the pressure side hence the idea that the wing is more or less sucked into the air rather that pushed. This is extremely poor terminology but I think the idea is conveyed…take a look at any airfoil data with a control surface deflected and you’ll see what I mean. Whenever the pressure drops the flow accelerates and if this happens close to the horizontal tail, it will be influenced also. In our example of the T-tail the pressure drops more on the underside of the horizontal tail than on the top thus one would expect to see a slight nose up pitching moment from a rudder deflection. You can see this in the wind tunnel and after you think about it for a while you realized that the reason this happens is because the suction drop on one side of the vertical tail is larger than the pressure rise on the other. This pressure drop is what alters the pressure distribution on the horizontal tail. One can imagine how sensitive this effect would be to things like the rudder and fin planform and the vertical tail height. The rule of thumb idea is that a high horizontal tail position will cause a slight nose up pitching moment with rudder deflection and a low tail will cause a slight nose down pitching moment…but only if the rudder hingeline isn’t swept.
If the rudder’s hingeline is swept the rudder will act sort of like an elevator. This effect could overshadow the effect of the rudder’s deflection on the horizontal tail’s pressure distribution. Now you see how the overall result you get while flying is a giant melting pot of small effects. Sometimes it’s hard to sort out the true “cause and effect”. If your airplane doesn’t pitch in knife edge you can pat yourself on the back and proclaim yourself a genius until you have to help your friend who has a coupling problem. If he’s a fellow competitor I guess you just keep your mouth shut and play dumb )
3) CENTER OF GRAVITY!!!
This is the most important thing to consider when you’re confronted with a pitch-coupling problem. Rest assured that there is a CG position that will cure your knife edge pitching problem but you may open up another can of worms by making the airplane’s handling qualities go to pot in other flight regimes. The designer who can balance all of these effects and make each phase of the aerobatic sequence easy to fly is the true “genius”.
The well-known effect that the CG has on pitch coupling is:
If you move the CG forward you will have to trim the airplane with more up elevator. When you fly in knife-edge at some sideslip angle, rudder deflection (which we know can produce a pitching moment) and angle of attack, you will have a counter pitching moment that comes from the elevator trim. This trim setting can cancel a nose down pitching moment, which comes from the sideslip, rudder deflection, etc.
The opposite is true if the CG is moved back. Many people have adequately explained this over and over so I won’t belabor the point, but I would dare to say that the CG is equally important if not more than the vertical position of the horizontal tail.
If you made it this far I certainly appreciate your patience. I hope what I’ve said hasn’t confused the issue too much. I’ve thought about this for years and I still find it interesting. I hope that everyone continues to strive for that perfect pattern design and will be better able to do so armed with a little better understanding of why things work. Nobody understands it all…that’s what makes it so challenging and fun at the same time.
George R. Hicks
I’ve been thinking about this knife edge pitch coupling phenomenon for quite some time now and have come to some limited understanding of the problem that might help put together some of the pieces of the puzzle. First I should say that the pitch coupling isn’t dependant on whether you fly in a knife edge attitude but rather it is dependant on the airplane’s angle of attack, angle of sideslip and rudder deflection. I noticed that one person identified a situation in which the application of rudder caused the airplane to pitch toward the gear regardless of the airplane’s attitude. As most of us know the airplane doesn’t know if it’s upright, inverted, vertical, on its side, etc. Such is the case with the pitching moments that we experience in knife-edge flight in that they aren’t dependent on the plane’s attitude. The interesting part of all this is what physically causes the airplane to exhibit these cross-coupling tendencies. I’ve isolated at least three things that I feel are the main contributors to this pitch-coupling problem. I’m certain that there are more so I don’t want anyone to think that this list is exhaustive. Every time I looks at this problem from a different perspective I find another possible contributor to the pitch coupling phenomenon…that’s what makes it so interesting.
1) PITCHING MOMENTS DUE TO SIDESLIP
By sideslip angle I simply mean the angle of attack of the airplane in a directional sense. In the Aero world we typically refer to the angle of attack and the angle of sideslip as “alpha” and “beta” (greek symbols). With this in mind I should say that most aero types also believe that the airplane only cares about alpha and beta. We like to nondimensinalize things so we don’t have to confuse the issue with things like wing loading, airspeed, air density, g’s etc. Aerodynamically the airplane doesn’t really know anything about those dimensional things. In the real world we can’t, as R/C pilots, go around proclaiming that my airplane was flying yesterday straight and level at a lift coefficient (CL) of 0.001 and get all excited…people would think we were crazy. However, we can and do get excited when we say, “You should have seen my airplane, It was going 250 mph…that’s cool!”. Everything has its place even dimensional quantities, but from the airplane’s point of view the forces and moments that affect it are only a function of five things: 1) angle of attack , 2) angle of sideslip, 3) Reynolds number, 4) Mach number, and 5) the airplane’s geometry. For now, lets neglect Reynolds number and Mach number by saying that we aren’t going too slow or too fast for either of these to be big factors. Now that I’ve explained what I mean by sideslip, we’ll proceed to why it affects pitch coupling.
A) FUSELAGE SHAPE If one considers the fuselage or body by itself you can start to see how some shapes might be prone to producing an aerodynamic pitching moment when the body is in a sideslip. Let’s consider a fuselage in which the aft section of the turtle deck is rounded on the upper surface but flat on the bottom. Such a shape while sidesliping would accelerate the air more over the upper surface than over the lower. Consequently the pressure would drop over the top of the fuselage’s turtle deck and cause a nose down pitching moment to be generated solely by the fuselage shape. I’ve played around with this in the wind tunnel using an old Calypso (Prettner design) fuselage and found that this nose down pitching moment could be measured and was nearly equal to 2 degrees of up elevator at 15 degrees of sideslip. Don’t make the mistake of thinking that my Calypso had a knife-edge-pitching tendency because it didn’t. You have to consider the total package before you make that kind of statement. As a complete airplane one could easily set it up such that you wouldn’t see any noticeable pitch coupling until very large sideslip angles. A good example of an airplane that has a large disparity between the upper and lower aft fuselage cross-sectional shape is the Cap 232. It typically exhibits a noticeable nose down pitch coupling tendency in knife-edge that is partly due to its fuselage shape. You can see this effect if you put a “aerodynamic fence” so to speak down the centerline of the turtle deck. When you fly this new configuration you will usually see a decrease in the amount of nose down pitch coupling because the fence is reducing the air’s acceleration thus reducing the pressure drop.
B) VERTICAL TAIL (sweep, span, projected area) This is a more difficult effect to explain but never the less it can be a very dominate one especially if the vertical tail is very thick, and has a large sweep angle. I should say that any airplane that has a vertical tail that only sticks out of the top of the airplane (which covers 99.9% of all airplanes in existence) would exhibit this tendency to some extent. Let’s see if I can explain why… When you sideslip, the airplane’s vertical surface acts in the same manner as a wing and generates lift because of a change the vertical tail’s angle of attack. This lift is created by a change in the pressure distribution over the surface of the vertical tail. Typically the leading edge area (L.E. to 25% of local chord) of a lifting surface has a fairly large pressure drop over the leeward side. If you look at your airplane from the front and the top and then estimate how much projected frontal area your vertical tail has from each view you will get an idea of how much ability your vertical tail has to create a pitching moment while in sideslip. In other words, consider that a rather large pressure drop occurs over those projected areas while the airplane is sidesliping. This pressure drop when multiplied by the projected area gives you a force whose vector is above the airplane’s center of gravity (CG). If you think about it and draw a few pictures you can see how the vertical tail’s height and sweep can increase the moment arm and thus increase the nose down pitching moment due to sideslip. I found this out while working on an airplane that had a rather large vertical tail sweep angle. I then started to look for it in other airplanes and found that almost all standard airplane configurations have the same tendency to pitch down due to sideslip and that the vertical tail is a large contributor to this effect. If one were to design airplanes that had symmetric vertical tails (one top and bottom) we would alleviate a large amount of this while also getting rid of the need for right thrust.
C) ONLY THE WING AND HORIZONTAL TAIL Even though you can’t actually build just a wing and a horizontal tail and fly them together without some sort of fuselage, you can still consider the problem and draw some insight to what you have before you ever add a fuselage or vertical tail to the picture. Consider a wing and horizontal tail with symmetric airfoil sections both at zero angle of incidence relative to each other. If you place this combination in sideslip you will get zero pitching moment if the angle of attack of the system is zero. What if the combination is held at some angle of attack and you sideslip at this constant angle of attack…suddenly a small amount of pitching moment will appear that changes as a function of sideslip…Why? The answer lies in the downwash change that the tail sees as the sideslip angle changes. The spanwise downwash distribution for most planforms isn’t constant (theoretically it is for an elliptic planform…but in reality this isn’t exactly true) and this is even more true when the wing is in sideslip. This effect is a function of the main wing’s lift coefficient, the vertical and longitudinal position of the horizontal tail and the center of gravity. The purpose of pointing this out is to show that even with the simplest longitudinal model you can’t get completely away from pitch coupling. This sort of simple approach shows the importance of the downwash field and how the wing’s wake affects the tail’s ability to perform. Keep in mind though that everything’s relative and if you operate at very low CL’s (light wing loading/high speeds) these effects are minimized. You also want to get the tail out of the wing’s influence by having it be far away from the wing…kinda sounds like a pattern plane doesn’t it?
D) VERTICAL POSITION OF THE HORIZONTAL TAIL (This is the one that everyone’s so concerned about!) There is much debate as to what effect this has… The general consensus is that if you lower the tail it makes the airplane more prone to pitch to the canopy and if you raise the horizontal tail the airplane will certainly pitch to the gear…is this true? I can say that if you measure this in the wind tunnel you will find the opposite to be true…to a point. As the horizontal tail gets further away from the wing, the downwash it sees is certainly less. As Mike Nauman once eloquently wrote “You can’t get away from a whirlpool by swimming towards it!” this is the exact same situation that the horizontal tail deals with while the wing is producing lift. The main effect of downwash is that it reduces the static longitudinal stability of the airplane. Technically the downwash doesn’t reduce the airplane’s stability but rather how much the downwash changes per unit change of angle of attack. (If) a 10 degree change in angle of attack produced a 10.1 degree change in the downwash where the horizontal tail is located the tail would cause the airplane to be unstable. Yes you heard it right, If you put a horizontal tail on the airplane behind the wing it could become less stable than with the wing alone…but only if the rate of change of downwash with angle of attack was greater than 1. Luckily, downwash’s change with angle of attack is almost always less than unity thus a tail is a good thing. How could you possibly get a configuration where putting on a horizontal tail behind the wing is destabilizing? If you had a very low aspect ratio wing with the tail located very close to the wing’s trailing edge you could get this sort of situation…I know it sounds crazy but it’s been done. We don’t have this problem in pattern but sometimes this example it helps in the overall understanding of how things work.
Typically pilots like the feel of a certain amount of longitudinal stability which I will call “static margin” (“Static” because we are only talking about steady-state situations and “margin” because it represents how much CG margin you have before the airplane becomes neutrally stable in pitch). To keep the same amount of static margin you would have to move the CG forward as you lower the tail. If you keep the CG in the same place as you lower the tail you will need down elevator to retrim the airplane and the airplane will become less stable in pitch. If you continue to lower the tail the downwash’s effect would lessen just like it would if you raised the tail far above the wing. This means that at some horizontal tail position far above and far below the wing the required elevator trim would be identical if you neglect the drag moment you get from having the tail way up high or down low. (not to mention the drag you would get from the vertical tail that it’s attached to) This idea also points out that having a high tail will cause a slight nose up pitching moment from the drag of the tail (for a low tail the opposite would be true)
Another effect comes from the fact that the fuselage and wing have a wake and boundary layer associated with them that extracts momentum from the air. If the tail is forced to operate in this type of environment because of its relative position to the wing, it would certainly lesson the tail’s effectiveness. This may be what’s happening in practice when people lower the tail and the pitching tendencies reduce greatly…who knows.
E) POWER EFFECTS IN SIDESLIP As with any situation on a propeller driven airplane you must take into consideration the power effects… Just note that the propwash is skewed greatly behind an airplane that is flying with a large amount of sideslip such that the propwash will eventually align itself with the freestream. Since the propeller only spins in one direction, its effect will change sign whether you sideslip to the left or the right. This is a very esoteric and unquantifiable effect but theoretically it should be there. In one direction there should be upwash on the tail and on the other there should be downwash. The same goes for the P-Factor. If you see that the airplane pitches one way when you fly knife-edge with right rudder and the other way with left you might be led to the conclusion that power effects are causing the problem.
2) PITCHING MOMENTS DUE TO RUDDER DEFLECTION
This is a bit different than the pitching moments due to sideslip because in this case we consider that the airplane is at a zero sideslip angle and we are only deflecting the rudder. Why would this cause the airplane to pitch? What happens in this case is that the close proximity between the rudder/fin and the horizontal tail can cause them to influence each other. This is highly dependent on how close the tails are to one another so the geometry of the situation is critical. Let’s pose a simple example for illustration: Consider a T-tailed airplane where the rudder is deflected 30 degrees. The rudder deflection produces the same effect as a flap deflection on a wing. The trailing edge control surface alters the pressure distribution around the whole airfoil. Typically one will see that the suction side is much greater than the pressure side hence the idea that the wing is more or less sucked into the air rather that pushed. This is extremely poor terminology but I think the idea is conveyed…take a look at any airfoil data with a control surface deflected and you’ll see what I mean. Whenever the pressure drops the flow accelerates and if this happens close to the horizontal tail, it will be influenced also. In our example of the T-tail the pressure drops more on the underside of the horizontal tail than on the top thus one would expect to see a slight nose up pitching moment from a rudder deflection. You can see this in the wind tunnel and after you think about it for a while you realized that the reason this happens is because the suction drop on one side of the vertical tail is larger than the pressure rise on the other. This pressure drop is what alters the pressure distribution on the horizontal tail. One can imagine how sensitive this effect would be to things like the rudder and fin planform and the vertical tail height. The rule of thumb idea is that a high horizontal tail position will cause a slight nose up pitching moment with rudder deflection and a low tail will cause a slight nose down pitching moment…but only if the rudder hingeline isn’t swept.
If the rudder’s hingeline is swept the rudder will act sort of like an elevator. This effect could overshadow the effect of the rudder’s deflection on the horizontal tail’s pressure distribution. Now you see how the overall result you get while flying is a giant melting pot of small effects. Sometimes it’s hard to sort out the true “cause and effect”. If your airplane doesn’t pitch in knife edge you can pat yourself on the back and proclaim yourself a genius until you have to help your friend who has a coupling problem. If he’s a fellow competitor I guess you just keep your mouth shut and play dumb )
3) CENTER OF GRAVITY!!!
This is the most important thing to consider when you’re confronted with a pitch-coupling problem. Rest assured that there is a CG position that will cure your knife edge pitching problem but you may open up another can of worms by making the airplane’s handling qualities go to pot in other flight regimes. The designer who can balance all of these effects and make each phase of the aerobatic sequence easy to fly is the true “genius”.
The well-known effect that the CG has on pitch coupling is:
If you move the CG forward you will have to trim the airplane with more up elevator. When you fly in knife-edge at some sideslip angle, rudder deflection (which we know can produce a pitching moment) and angle of attack, you will have a counter pitching moment that comes from the elevator trim. This trim setting can cancel a nose down pitching moment, which comes from the sideslip, rudder deflection, etc.
The opposite is true if the CG is moved back. Many people have adequately explained this over and over so I won’t belabor the point, but I would dare to say that the CG is equally important if not more than the vertical position of the horizontal tail.
If you made it this far I certainly appreciate your patience. I hope what I’ve said hasn’t confused the issue too much. I’ve thought about this for years and I still find it interesting. I hope that everyone continues to strive for that perfect pattern design and will be better able to do so armed with a little better understanding of why things work. Nobody understands it all…that’s what makes it so challenging and fun at the same time.
George R. Hicks
